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Sailboat ratios: SA/DISP; DISP/LOA

SA/Disp; Disp/LOA

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#1 Bull City

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Posted 10 July 2014 - 02:10 PM

I ran across this evaluation of a couple of common sailboat ratios. What do you all think?
 
SA/DISP
<16         Slow, underpowered
16 to 19 Reasonably good performance
20 to 22 High performance
>24         Super-high performance
 
DISP/LOA
<100           Ultra Light
100 to 200 Light
200 to 300 Moderate
300 to 400 Heavy
>400           Very Heavy


#2 kimbottles

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Posted 10 July 2014 - 03:13 PM

I ran across this evaluation of a couple of common sailboat ratios. What do you all think?
 
SA/DISP
<16         Slow, underpowered
16 to 19 Reasonably good performance
20 to 22 High performance
>24         Super-high performance
 
DISP/LOA
<100           Ultra Light
100 to 200 Light
200 to 300 Moderate
300 to 400 Heavy
>400           Very Heavy

Those were pretty much standard ratios back some years ago. They still will tell you a lot about a vessel, but not everything by any means.

I think Ted Brewer came up with a "comfort at sea" ratio that is also useful.

 

But nothing replaces just taking the boat out and sailing her in various conditions to really understand a boat's characteristics.



#3 Turd Sandwich

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Posted 10 July 2014 - 03:16 PM

That was what it was like in the 80's its not uncommon now for sport boats even tame ones to be 40+ up hill and well over 100 down



#4 MidPack

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Posted 10 July 2014 - 03:19 PM

I ran across this evaluation of a couple of common sailboat ratios. What do you all think?
 
SA/DISP
<16         Slow, underpowered
16 to 19 Reasonably good performance
20 to 22 High performance
>24         Super-high performance
 
DISP/LOA
<100           Ultra Light
100 to 200 Light
200 to 300 Moderate
300 to 400 Heavy
>400           Very Heavy

They seem somewhat out of date, especially the D/L descriptions. I'd guess there aren't many production boats built anymore with SA/D ratios under 16 or D/L ratios over 200, short of purpose built ocean passage boats. And to call everything with a D/L from 100 to 200 light seems awfully broad. I'd consider 100 light, and 200 pretty heavy - the difference is considerable. Just an opinion I guess...



#5 SpongeDeckSquareFoil

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Posted 10 July 2014 - 03:25 PM

I’ve found SA/DISP can sometimes be misleading, especially in the lower ranges. Too much of a boat’s ability to accelerate is dependent on hull shape.

Taken to extreme, say a boat with a SA/D of 15 has a sleek hull shape and fine keel, while a similar sized boat with a 17 SA/D is shaped like a shoebox with stub keel. The lower ranked boat will likely be quicker.

So the SA/D numbers aren’t something to look upon as absolutes.  PHRF ratings might be a better indicator of a boat’s performance.



#6 bloodshot

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Posted 10 July 2014 - 03:30 PM

yeah, SA/D is misleadinging IMO. 

 

For example, a J/22's SA/D is 24 and a J/70's is 25.  Real world performance and the eyeball test tells you these boats aren't in the same at all but the stats put them almost similar..



#7 SloopJonB

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Posted 10 July 2014 - 03:50 PM

Those ratios look like they are from the 80's. Ultra light would be under 50 D/L now. Sa/D ratios in the high teens were typical for IOR boats - my full race 1975 Quarter Pounder was 17. Now that range would be more typical of a cruising boat.

 

The dimensionless ratios are a significant comparator of relative design intent, like horsepower to weight ratios in cars, but by no means are they definitive.



#8 12 metre

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Posted 10 July 2014 - 04:31 PM

D/L ratio alone tells something about the ability to exceed "hull speed"

 

SA/D alone doesn't tell you much, unless comparing boats of fairly similar length and displacement.

 

Taken with D/L ratio, I believe it can tell you something.  For decent light air performance, the 2 numbers are inversely proportional, so a boat with lower D/L must have a higher SA/D.  As SJB mentions, boats like the old Peterson 35s had a typical SA/D of around 17, and are great light air perfomers, but had a relatively high D/L of around 250.

 

My rough tule of thumb for good light air performance is:

D/L >200 then SA/D > 16

D/L between 100 - 200 then SA/D >22

D/L between 50 - 100 then SA/D >27

D/L < 50, then SA/D >35



#9 SemiSalt

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Posted 10 July 2014 - 05:09 PM

The problem with SA/D is that it doesn't directly address the source of drag. At low speeds (say half of hull speed), drag is mostly from wetted surface. At higher speeds, form drag from the hull is more important. Displacement doesn't address either directly.

 

In general, a heavy displacement boat will have less wetted surface per unit of displacement than a light boat, so heavy boats may have a chance in light air. However, heavy boats also accelerate more slowly than light boats, so if there is some chop (like the powerboat wakes that roil up WLIS on calm day) to slow them down, the heavy boat can have a hard time. 

 

Around here, there is a Brit Chance boat with a SA/D about 15, and it kills in light weather. Few designers worked as hard as Chance to reduce wetted surface. The sections are very rounded, and the keel is a very high aspect ratio blade. 

 

PHRF numbers for popular racing classes are pretty good. PHRF numbers for uncommon boats, or old boats, or any design not raced at a high level, are not reliable. The sample size is too small, and most of the owners don't complain if they're not competitive.  

 

 

 

Ted Brewer's primer on design ratios is here: http://www.tedbrewer...achtdesign.html



#10 Alex W

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Posted 10 July 2014 - 05:15 PM

What do boats with low form drag look like, and what do ones with high form drag look like?



#11 12 metre

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Posted 10 July 2014 - 05:32 PM

Strictly speaking, a low FORM drag shape is narrow with a traditional cod's head & mackerel tail shape.  But there are a lot of other contributors to overall drag other than merely form drag (i.e. frictional drag, wavemaking drag, induced drag, interference drag, etc., etc)

 

A high form drag shape would be the opposite - wide with a pointy front end.



#12 Jose Carumba

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Posted 10 July 2014 - 07:11 PM

I ran across this evaluation of a couple of common sailboat ratios. What do you all think?
 
SA/DISP
<16         Slow, underpowered
16 to 19 Reasonably good performance
20 to 22 High performance
>24         Super-high performance
 
DISP/LOA
<100           Ultra Light
100 to 200 Light
200 to 300 Moderate
300 to 400 Heavy
>400           Very Heavy

 

It's not DISP/LOA, its DISP/L where L is DWL (Design Waterline), commonly abbreviated to D/L or DLR.  Using LOA will get you a wrong number.

 

DLR = Displacement in long tons (lbs / 2240) / (.01x DWL (in ft))3 .



#13 mrming

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Posted 10 July 2014 - 07:56 PM

One thing to watch out for with SA/D is it's usually calculated on the working jib, so IOR style boats with big overlapping genoas can often have a misleadingly low SA/D.

#14 MidPack

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Posted 10 July 2014 - 07:57 PM

While SA/D certainly has it's shortcomings, unfortunately wetted surface area is rarely provided, so pointing it out as a superior metric while true, is academic. So we're forced to look at the SA/D and guess wetted surface based on hull shape, and make a judgement - or better yet just sail the boat in question, or talk to trusted others who have.

PHRF or other ratings may also give clues RE: relative speed between similar LWL boats.

#15 Alex W

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Posted 10 July 2014 - 08:14 PM

One thing to watch out for with SA/D is it's usually calculated on the working jib, so IOR style boats with big overlapping genoas can often have a misleadingly low SA/D.

 

Yes, I found it interesting to compute the SA/D for my boat with my actual sails (not plain triangles as used in the calculations) and for different sail plans.  This is the resulting matrix:

 

sa-d.png

 

WJ = working jib, DR = double reefed

 

I'm still trying to guess how a boat might feel and perform based on it's hull shape.  I have a long way to go there.



#16 bmiller

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Posted 10 July 2014 - 08:19 PM

How much should one be concerned with capsize screening formula? In my continuing search for the next boat one in particular is very attractive but has a CSF of 2.22. I don't plan on crossing oceans but do want a solid coastal cruiser.



#17 SloopJonB

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Posted 11 July 2014 - 01:11 AM

Not very - it's a VERY vague number - just enough to trigger further investigation.



#18 Bob Perry

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Posted 11 July 2014 - 02:07 PM

You guys and your silly numbers. Stand well back and take a long look at the entire picture. When that looks right you can play with some numbers. If you like.

 

Research this: Find a published magazine article that talks about SA/D and D/L before I started talking about it in 1974. Be specific.  I could give you the answer but I won't.

 

When Nathan and I went to the Long Beach Boat Show for the first time to promote the yet to be built V-40 I had all the numbers. I was well versed in them and their significance.

As I stood their on my soap box preaching SA/D's and D/l's people stopped, listened then told me they had never heard of those numbers before. The Westsail salesmen thought I had made them up!



#19 mrming

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Posted 11 July 2014 - 02:10 PM

Yes, I found it interesting to compute the SA/D for my boat with my actual sails (not plain triangles as used in the calculations) and for different sail plans.  This is the resulting matrix:

Mine comes out at 27 when I do that.

#20 Bob Perry

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Posted 11 July 2014 - 02:16 PM

But don't do that unless you just want to see what happens. In order to keep it apples to apples just use I,J, E and P. Forget overlap and roach. If you have a fat head main or one with lots of roach go ahead and include that. Sometimes I look and see what the SA/D is with the asym up, just for fun.



#21 Alex W

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Posted 11 July 2014 - 03:20 PM

I work in the "big data" niche of software, I just like looking at numbers and wanted to see how the calculations worked.  It was mostly an exercise to how the sail plans compared on my boat.  Empirically I knew that going to the working jib instead of reefing the main with the genoa made for a boat that handles better in moderately high winds.



#22 carcrash

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Posted 11 July 2014 - 03:24 PM

While hull shape seems like it should be important, its no where near as important as D/L, which is by far the dominant factor in drag at sub high speed planing. At high speed planing (about twice as fast as, say, a Volvo 70 has ever achieved) then its simply weight, length stops helping.

This is true for long, skinny, deep hulls like a multihull, for wide flat hulls like a scow, and everything in between.

The reason IOR was so silly was that it was predicated on the common but flawed theory that shape mattered more than weight.

The problem with comparing SA/D Ratios is that the SA used is generally not all that representative of the actual L/D of the sail plan, and that is what really matters. L/D is not dominated by area, but by luff length and aspect ratio. And the displacement used is not that of the boat when it is actually being used. The lighter and smaller the boat, the more erroneous the number.

Light air performance is all about sail area to wetted surface ratio, which is easy to come by today with CAD, but was rarely calculated before, and so rarely published as to be almost worthless for comparison.

In other words: better than nothing, useful for comparing otherwise fairly typical displacement boats.

#23 Bob Perry

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Posted 11 July 2014 - 04:28 PM

Alex:

I have used those ratios almost every day since I was 16 years old, 52 years ago. I may have called them "silly numbers" but I still use them. But In factor them in with a lot of other variables.

 

Crash seems to have a god handle on it.

Makes no sense to try and get reliable results from two formulae if you are comparing a Volvo boat with an Alberg 35.

 

I first learned about those ratios from an article by Ted Brewer in MOTOR BOATING and SAILING mag in about 1963. I was home, sick in bed and bored. I read Ted's article, pulled out a big stack of old yachting mags, got my 6", bamboo, K&E slide rule and went to work. I made an impressive list.



#24 kimbottles

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Posted 11 July 2014 - 05:29 PM

Alex:
I have used those ratios almost every day since I was 16 years old, 52 years ago. I may have called them "silly numbers" but I still use them. But In factor them in with a lot of other variables.
 
Crash seems to have a god handle on it.
Makes no sense to try and get reliable results from two formulae if you are comparing a Volvo boat with an Alberg 35.
 
I first learned about those ratios from an article by Ted Brewer in MOTOR BOATING and SAILING mag in about 1963. I was home, sick in bed and bored. I read Ted's article, pulled out a big stack of old yachting mags, got my 6", bamboo, K&E slide rule and went to work. I made an impressive list.


I have a large excel workbook full of those ratios for just about every sailboat I know of, I group boats together by type for comparison. I have FRANKIE in with the other long narrow vessels. Interesting stuff to study.

#25 Alex W

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Posted 11 July 2014 - 05:39 PM


The reason IOR was so silly was that it was predicated on the common but flawed theory that shape mattered more than weight.
 

 

Was the lower displacement that is now becoming the norm realistically available in the IOR days?  It seems like the innovations that have made this possible are good foam cores and vacuum bagged and higher tech resins.  Those things were cutting edge in the 70s but are pretty normal now.

 

It does seem like these numbers have some value when comparing similar displacement boats -- the 70s and early 80s designs that are inexpensive and flood the used market.  I'd like to sail more on lower displacement boats that have some cruising accommodations (the Pogo looks very interesting from that perspective), but wonder how long it will be before such boats fit into my budget.



#26 Alex W

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Posted 11 July 2014 - 05:40 PM

Alex:
I have used those ratios almost every day since I was 16 years old, 52 years ago. I may have called them "silly numbers" but I still use them. But In factor them in with a lot of other variables.
 
Crash seems to have a god handle on it.
Makes no sense to try and get reliable results from two formulae if you are comparing a Volvo boat with an Alberg 35.
 
I first learned about those ratios from an article by Ted Brewer in MOTOR BOATING and SAILING mag in about 1963. I was home, sick in bed and bored. I read Ted's article, pulled out a big stack of old yachting mags, got my 6", bamboo, K&E slide rule and went to work. I made an impressive list.


I have a large excel workbook full of those ratios for just about every sailboat I know of, I group boats together by type for comparison. I have FRANKIE in with the other long narrow vessels. Interesting stuff to study.

 

I harvested all of the data off of sailboatdata into a large spreadsheet that I'm willing to share if anyone wants it for non-commercial purposes.  5000ish boats.

I did it partially to find all of the data inaccuracies on SBD (for instance they list my Pearson 28-2 as a SA/D of 12, when it is more like 17).  I also did it to try and classify boats together and look for similarities.



#27 RKoch

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Posted 11 July 2014 - 05:54 PM


I ran across this evaluation of a couple of common sailboat ratios. What do you all think?
 
SA/DISP
<16         Slow, underpowered
16 to 19 Reasonably good performance
20 to 22 High performance
>24         Super-high performance
 
DISP/LOA
<100           Ultra Light
100 to 200 Light
200 to 300 Moderate
300 to 400 Heavy
>400           Very Heavy

Those were pretty much standard ratios back some years ago. They still will tell you a lot about a vessel, but not everything by any means.
I think Ted Brewer came up with a "comfort at sea" ratio that is also useful.
 
But nothing replaces just taking the boat out and sailing her in various conditions to really understand a boat's characteristics.

The "comfort factor" is interesting... I believe it compares disp to waterplane area? Lets take 2 40' boats, of equal displacement. One is fairly beamy and shallow, and somewhat higher freeboard to acheive headroom. The second is narrower, with a deeper hull (for equal disp), and can have a bit less freeboard to get the same headroom. My own experience is the latter boat will have a nicer motion.

#28 12 metre

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Posted 11 July 2014 - 05:56 PM


The reason IOR was so silly was that it was predicated on the common but flawed theory that shape mattered more than weight.
 

 

Was the lower displacement that is now becoming the norm realistically available in the IOR days?  It seems like the innovations that have made this possible are good foam cores and vacuum bagged and higher tech resins.  Those things were cutting edge in the 70s but are pretty normal now.

 

It does seem like these numbers have some value when comparing similar displacement boats -- the 70s and early 80s designs that are inexpensive and flood the used market.  I'd like to sail more on lower displacement boats that have some cruising accommodations (the Pogo looks very interesting from that perspective), but wonder how long it will be before such boats fit into my budget.

Depends on what you mean by lower displacement.  If you mean sport boat light, probably not, although they did have light boats back then, they were called dinghies if i recall. If you mean IRC "light", then sure, most boats were lighter than similar sized IRC boats today - it's a matter of what the rule favours more than anything.  Also, a lot of IOR boats had 50% bal/Disp ratios - try finding that on a modern boat.  Most modern boats have bulbs as a substitute for a high bal/Disp ratio.

 

If you mean light as in D/L under 150, there were a lot of cruisable and fast lightweights back then.  Hotfoot 31 or Ross 930 come to mind.  They were both in the 5000 lb range and had 6' or more headroom.



#29 Bob Perry

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Posted 11 July 2014 - 06:28 PM

Remember that using published displacements is always iffy. But it's usually all you can get. I get some boats for my SAILING  reviews with displ listed as "lightship" I have no idea what each builder considers "lightship".

I'm not sure that "lightship" is even relevant. No fuel? No water, no gear? No sense.



#30 RKoch

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Posted 11 July 2014 - 07:00 PM

I've weighed boats that turned out to be 20% heavier than advertised.

#31 mrming

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Posted 11 July 2014 - 07:32 PM

I was just thinking what a nice, polite thread this is for SA, when I remembered I was in Cruising Anarchy. :)

#32 Bull City

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Posted 11 July 2014 - 07:53 PM

A few things.

 

1) Jose is probably right about using DISP/LWL and not DOSP/LOA, however, what about the effect of overhangs which will lengthen the LWL as a boat heels?

 

It's not DISP/LOA, its DISP/L where L is DWL (Design Waterline), commonly abbreviated to D/L or DLR.  Using LOA will get you a wrong number.

 

DLR = Displacement in long tons (lbs / 2240) / (.01x DWL (in ft))3 .

 

 

2) What 12 Metre says sounds very interesting:

 

D/L ratio alone tells something about the ability to exceed "hull speed"

 

SA/D alone doesn't tell you much, unless comparing boats of fairly similar length and displacement.

 

Taken with D/L ratio, I believe it can tell you something.  For decent light air performance, the 2 numbers are inversely proportional, so a boat with lower D/L must have a higher SA/D.  As SJB mentions, boats like the old Peterson 35s had a typical SA/D of around 17, and are great light air perfomers, but had a relatively high D/L of around 250.

 

My rough tule of thumb for good light air performance is:

D/L >200 then SA/D > 16

D/L between 100 - 200 then SA/D >22

D/L between 50 - 100 then SA/D >27

D/L < 50, then SA/D >35

 

3) Is Length/Beam worth looking at, on the theory that narrow boat is faster?

 

 

 

 



#33 Alex W

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Posted 11 July 2014 - 08:00 PM

RKock: What did you weigh them on?  The Travel Lift load cells around here (Seattle) seem to disagree on weights on the same boat by more than 20%.  Seaview Bellingham had my boat come in at 12,000lbs, CSR had it come in around 7500lbs, design weight is around 7000lbs.  Canal in Seattle weighed my old Catalina 25 as 500lbs under weight (with crap, sails, engine on it), which also seemed unlikely.

 

A few months ago Bob Perry helped me figure out how to roughly estimate my boat's weight using the original drawings and measuring the freeboard.  It appears to be about 500-700lbs overweight, but that includes all of the crap that lives on the boat (100lbs in tools and parts, cushions, fuel, sails, etc...I did at least empty the water tank).  Of course this depends on the original drawings and the gelcoated in bootstripe being accurate, so it is also hit or miss.



#34 RKoch

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Posted 11 July 2014 - 08:14 PM

10,000 lb dyno scale, calibrated and certified. Weighed many boats for MORC ratings.

#35 Bob Perry

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Posted 11 July 2014 - 08:17 PM

Bull:

Making a blanket statement that overhangs increase the DWL as the boat heels is not accurate. Sometimes they do and sometimes they don't. There is a lot more at work than just the linear overhang. Besides, in a simple formula like D/L you can't factor in something as nebulous at best as effects of overhang. But you can pretend you can.

 

Alex:

My experience with travel lifts is that they are notoriously inaccurate. The only way to get accurate displs is with freeboards and the designer's drawings. Of this I am certain.

This is not hit or miss if you have accurate benchmarks on the hull for reference. There will always be some tolerance in the real world.



#36 Bob Perry

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Posted 11 July 2014 - 08:18 PM

Oh, I forgot. Fuck off MRM.

 

Is that better?



#37 RKoch

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Posted 11 July 2014 - 08:31 PM

Truck scales are also notoriously innaccurate. The source of the late 60s MORC "weight scandal" was all the boats being weighed on different scales. Some owners trailered the boat around to several scales, picking the "heaviest". Result was a few boats being 1000 lbs "heavier"than a sistership (worth about 12 sec/mile + or -), yet floating noticeably higher. Local MORC fleet bought a certified accurate scale and weighed all boats on it afterwards.

#38 mrming

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Posted 11 July 2014 - 08:32 PM

Oh, I forgot. Fuck off MRM.
 
Is that better?


Yep that's better. And anything with an SA/D of less than 25 can fuck off too.

#39 Bob Perry

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Posted 11 July 2014 - 08:33 PM

Yeah Mrms, I didn't want you to think you were lost.

 

If you have or can find a copy of my book I go into great detail with comparative VPP's on the effect of bow overhang. It is very revealing. In short, bow overhang doesn't do squat to extend the DWL when heeled. There is just not enough boat there to do any work. Sterns can use overhang more effectively but you have to take it case by case relative to the boat type and geometry of the stern.



#40 DDW

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Posted 11 July 2014 - 08:50 PM

A properly calibrated crane scale will give you weight accurate to +/- 20 lbs. 

 

OTOH, according to various travelifts, my boat is known to weight somewhere between 27,000 and 36,000 pounds. Approximately. 



#41 mrming

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Posted 11 July 2014 - 09:17 PM

While we're on this subject, I've been daydreaming about my next boat, and have come across the Dehler 28. There's little or no info about the boat out there with regard to racing, so I've been looking at the kinds of figures discussed here, along with the general hull shape etc, with a view to working out whether the boat would be any good under IRC.

I know IRC's not so big in the US, but maybe you guys could have a look at the boat in general and let me know your thoughts:

http://sailboatdata....p?class_id=6893

My feeling is that it looks right (for a cruiser racer) and so probably is right. The last rating I could find was from 2006 at 0.867.

#42 SemiSalt

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Posted 11 July 2014 - 09:24 PM

Per the web site, the Dehler is 5500 lbs and has 430 sq ft of sail. My Hunter 28 is listed at 7400 lbs and 400 sq ft, and is tender. So I'm thinking it's pretty sporty.



#43 RKoch

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Posted 11 July 2014 - 09:28 PM

A properly calibrated crane scale will give you weight accurate to +/- 20 lbs. 
 
OTOH, according to various travelifts, my boat is known to weight somewhere between 27,000 and 36,000 pounds. Approximately. 


Thats about the accuracy we felt we were getting.

#44 Bull City

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Posted 11 July 2014 - 09:40 PM

Wait a second,

 

D/L ratio alone tells something about the ability to exceed "hull speed"

 

SA/D alone doesn't tell you much, unless comparing boats of fairly similar length and displacement.

 

Taken with D/L ratio, I believe it can tell you something.  For decent light air performance, the 2 numbers are inversely proportional, so a boat with lower D/L must have a higher SA/D.  As SJB mentions, boats like the old Peterson 35s had a typical SA/D of around 17, and are great light air perfomers, but had a relatively high D/L of around 250.

 

My rough tule of thumb for good light air performance is:

D/L >200 then SA/D > 16

D/L between 100 - 200 then SA/D >22

D/L between 50 - 100 then SA/D >27

D/L < 50, then SA/D >35

 

So you're saying that the lower the D/L the higher the SA/D needs to be, but your rule of thumb seems to say that the lower the D/L the higher you want the SA/D.

 

I'm missing something.



#45 yogiwood

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Posted 11 July 2014 - 09:57 PM

basically you can increase displacement  and proportionally not add as much wetted surface



#46 kimbottles

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Posted 11 July 2014 - 10:01 PM

 

Alex:
I have used those ratios almost every day since I was 16 years old, 52 years ago. I may have called them "silly numbers" but I still use them. But In factor them in with a lot of other variables.
 
Crash seems to have a god handle on it.
Makes no sense to try and get reliable results from two formulae if you are comparing a Volvo boat with an Alberg 35.
 
I first learned about those ratios from an article by Ted Brewer in MOTOR BOATING and SAILING mag in about 1963. I was home, sick in bed and bored. I read Ted's article, pulled out a big stack of old yachting mags, got my 6", bamboo, K&E slide rule and went to work. I made an impressive list.


I have a large excel workbook full of those ratios for just about every sailboat I know of, I group boats together by type for comparison. I have FRANKIE in with the other long narrow vessels. Interesting stuff to study.

 

I harvested all of the data off of sailboatdata into a large spreadsheet that I'm willing to share if anyone wants it for non-commercial purposes.  5000ish boats.

I did it partially to find all of the data inaccuracies on SBD (for instance they list my Pearson 28-2 as a SA/D of 12, when it is more like 17).  I also did it to try and classify boats together and look for similarities.

How do I get a copy of that spreadsheet? I love studying that kind of stuff.



#47 mrming

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Posted 11 July 2014 - 10:11 PM

Per the web site, the Dehler is 5500 lbs and has 430 sq ft of sail. My Hunter 28 is listed at 7400 lbs and 400 sq ft, and is tender. So I'm thinking it's pretty sporty.


It also has a 40% ballast ratio. The one alarm bell for me is the draft at only 4' 5".

#48 12 metre

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Posted 11 July 2014 - 10:13 PM

basically you can increase displacement  and proportionally not add as much wetted surface

 

That sums it up pretty well.



#49 Alex W

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Posted 11 July 2014 - 10:14 PM

Per the web site, the Dehler is 5500 lbs and has 430 sq ft of sail. My Hunter 28 is listed at 7400 lbs and 400 sq ft, and is tender. So I'm thinking it's pretty sporty.


It also has a 40% ballast ratio. The one alarm bell for me is the draft at only 4' 5".

 

The one that I've seen in person also had sort of a bell bottom style keel that seemed to do a reasonable job (better than my boat) of putting the keel weight low and lowering the VCG.



#50 12 metre

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Posted 11 July 2014 - 10:22 PM

Wait a second,

 


So you're saying that the lower the D/L the higher the SA/D needs to be, but your rule of thumb seems to say that the lower the D/L the higher you want the SA/D.

 

I'm missing something.

 

Aren't "the lower the D/L the higher the SA/D needs to be" and "the lower the D/L the higher you want the SA/D" the same thing?  Trying to understand what you're trying to say.



#51 kdh

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Posted 11 July 2014 - 10:44 PM


The reason IOR was so silly was that it was predicated on the common but flawed theory that shape mattered more than weight.
 

 

Was the lower displacement that is now becoming the norm realistically available in the IOR days?  It seems like the innovations that have made this possible are good foam cores and vacuum bagged and higher tech resins.  Those things were cutting edge in the 70s but are pretty normal now.

 

It does seem like these numbers have some value when comparing similar displacement boats -- the 70s and early 80s designs that are inexpensive and flood the used market.  I'd like to sail more on lower displacement boats that have some cruising accommodations (the Pogo looks very interesting from that perspective), but wonder how long it will be before such boats fit into my budget.

 

And in the case of my boat, a recent Sou'wester 42, an IOR era design, all that technology, scrimp, and carbon and kevlar in the layup, was used. Saves 500 lbs which was added to the ballast.

 

I think "lightship" weight is overrated as relevant for a cruising boat. Give me waterline length, I'll keep all the shit I don't need out of the boat and I'll be happy.

 

Also, don't discount the idea that weight costs money--cheap builds are light, which factors into the modern notions.



#52 Bull City

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Posted 11 July 2014 - 11:22 PM

I am totally confused, but I think I see it now. In looking at these two measures across a range of boats, it seems that boats with lower D/L have high  SA/D, which is what you said should happen, an inverse relationship. Sorry to be a dumb fuck. Thank you.

 

Wait a second,

 


So you're saying that the lower the D/L the higher the SA/D needs to be, but your rule of thumb seems to say that the lower the D/L the higher you want the SA/D.

 

I'm missing something.

 

Aren't "the lower the D/L the higher the SA/D needs to be" and "the lower the D/L the higher you want the SA/D" the same thing?  Trying to understand what you're trying to say.



#53 SemiSalt

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Posted 11 July 2014 - 11:49 PM

I think the ratios are most useful when comparing boats designed in the same era for the same purpose. They don't explain the difference between an IOR boat and a sportboat. A sportboat doesn't have a high SA/D because it has wetted surface that will slow it in a drifter; it has it because of the high speeds that can be obtained in a strong wind if the hull is light enough.

#54 DDW

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Posted 12 July 2014 - 01:07 AM

Since this is Cruising Anarchy, cruise on over to this article by Starzinger. It was published in Cruising awhile back and he makes a pretty good case that average speeds over long periods can be explained by two numbers: SA/D and LWL. Vastly simplifying, LWL says how fast you can go and SA/D says how often you do that in real world weather.



#55 kimbottles

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Posted 12 July 2014 - 02:42 AM

interesting article, I have read other literature that said basically the same thing.



#56 Islander Jack

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Posted 12 July 2014 - 05:39 AM

Reading the article I was surprised that D/L and SA/D are not calculated as simple ratios.  LWL is cubed in the former and D is raised to the 2/3 power in the latter.



#57 SloopJonB

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Posted 12 July 2014 - 06:22 AM

That's because they are what is known as "dimensionless ratios" - the exponential functions serve to cancel out the differences in sizes of boats, allowing widely different sizes to be compared on a level playing field.



#58 12 metre

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Posted 12 July 2014 - 08:15 AM

They are supposed to be dimensionless numbers.  But technically, SA/D is, while D/L is not (but should be) 

 

In SA/D, you use SA in ft2 and D in ft3, so ft2/ft3^(2/3) = 1.  In other words, ft2 =ft3^(2/3), so the units cancel out, providing a dimensionless number.

 

However, in D/L you have D in lbs (long tons) and L in ft (multiplied by .010) so you have long tons/(ft)^3, and the units don't cancel out.  Properly, it should be D in ft^3 and L in ft^3 as well.  Problem is, in the case of a boat that has a D/L of 150 (using the accepted method of calculation) ends up with a D/L of 5282. which is cumbersome to use.  

 

My guess is that in the interest of simplicity, it was decided to effectively insert a constant.  For example, assuming a density of 64 lb/ft3, there are 35 ft^3 in a long ton.  So if you divide the 5282 (as calculated above) by 35, you end up with - 150!  Which is an easier number to utilize.  Because the difference is a constant (i.e. the results are directly proportional), it is just as valid to use the accepted method.  

 

However dimensionless numbers are supposed to yield the same result, no matter which system of units is used (SI or Imperial) and the accepted method yields different results if you tried to do it with SI units. However if you used m^3 for D and m for L, it works out the same as you would get using ft instead of m..

 

Dimensionless numbers are very common.  For example Froude Number and Reynolds Number. 



#59 Panoramix

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Posted 12 July 2014 - 11:09 AM

Thanks, I was trying to compute numbers unsuscessfully. and got to your post.

 

You are hurting my head with those hidden constants in your imperial units!



#60 kdh

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Posted 12 July 2014 - 11:24 AM

To me, SA/D is a measure of how likely the boat is to plane and its ability to accelerate. Useless to me as a cruiser. Give me a maximized waterline length for the accommodations I want and a rig size sufficient to drive the boat to near hull speed. Displacement doesn't say much about the latter.



#61 SloopJonB

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Posted 12 July 2014 - 02:36 PM

Sa/D is simply the horsepower to weight ratio. Lots of weight and lots of horses performs the same as low weight & low horses. I once knew a guy who had a '65 Impala with a huge hot engine that would turn 10 sec. 1/4 miles - for those who don't know what that means, a Posche Turbo does it in the 13's. That aircraft carrier Impala was dramatically faster.

 

The other end of the spectrum would be something like a Lotus 7 - the automotive equivalent of a modern small sport boat.



#62 DDW

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Posted 12 July 2014 - 03:19 PM

To me, SA/D is a measure of how likely the boat is to plane and its ability to accelerate. Useless to me as a cruiser. Give me a maximized waterline length for the accommodations I want and a rig size sufficient to drive the boat to near hull speed. Displacement doesn't say much about the latter.

 

SA/D is a measure of planing ability mainly in racing boats. In cruising boats, it is a measure of how fast the boat will be in light wind, at  below hull speed, i.e, at what wind speed is it powered up. Since most cruising is done in too little wind, not too much, this is critical in real world average speeds. A short (typical) cruising boat will not plane, its speed limited by its LWL. A long cruising boat with limited sail area will spend too much time well below hull speed looking for more wind. Whatever LWL you can afford, you want to have enough sail to keep it moving well in 6 or 8 knots wind. That takes an SA/D above 20 at least. 



#63 Tom Ray

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Posted 12 July 2014 - 03:58 PM

Geez, I'm never returning to this thread.

 

All these numbers come pretty close to taking the fun out of sailing.



#64 Bob Perry

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Posted 12 July 2014 - 04:25 PM

Islander:

In simple terms, that is because you are trying to work with linear units, square units and cubic units. The square and cubic units have to be reduced so they work with the linear units. 

 

But I think Tom's latest take on this is the best. Just go sailing.



#65 miloman

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Posted 12 July 2014 - 04:42 PM

I'm not convinced that the average Ratio's have changed as much as some people suggest in this thread.  The changes have mainly come from the top end.  There are less boats sailing with very high DLRs and very low SA/Ds.  Are there boats sailing at very low DLR and very high SA/D ratios?  Yes.  But I think they're rarer than they seem.  Take a competitive 40 ft. offshore race boat:

 

LOA: 40.5 ft.

LWL: 39.5 ft.

BOA: 14 ft.

DISP: 8700 lb.

SA: 1302 sq.ft.

 

This gives a DLR of 62 and a SA/D of 49.  These are pretty incredible numbers.  

 

This boat carries 1500 lb. of water ballast, 4500 lb. ballast, has a 30 hp sail drive (350 lb.) and is meant to do offshore races, so she'll have a crew of at least 5 (900 lb.) and carry enough food and water for them for two weeks (another 600 lb, probably).  After you subtract those numbers you get a weight of ~800 lb. for her hull, deck, framing, rigging, joiner work, sails, deck gear, anchors, etc.  This is very improbable.  Most likely, the listed displacement is the light ship displacement.  

 

Once you add in the weight of all the gear, supplies, and equipment, it's easy to imagine her sailing at a displacement of ~14500 lb with water ballast and ~13000 without.  This gives a DLR of just over 100 with ballast tanks full and 92 with empty tanks.  Her SA/D ratios are 35 and 37 respectively.  Still pretty impressive numbers.  These numbers, by the way, were taken from a very competitive carbon-fiber 40 foot racer (the numbers were massaged a little to make them nice and round).

 

The point is, It's really hard to get a boat light enough to have an honest-to-god, as sailed, DLR under 70.  It can be done, but it isn't accomplished nearly as often as sales-brochures and sailing magazines would have you think.  I doubt there are that many cruisers out there who's boats, as sailed, have DLRs much under 100.

 

My memory is that the FRANCIS LEE's design DLR was about 50, and she came in just a bit underweight.  That shows you what's required to get really low DLRs.  The FL is very narrow and low (low hull and deck weight for her length due to less surface area), has the interior of a boat half her length, and benefited from an aggressive approach to weight savings in her build.  How many cruising boats do you think weigh only twice as much for their length (DLR of 100)?



#66 Veeger

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Posted 12 July 2014 - 05:41 PM

So back in the haze of 15+ years ago, I had a J 105 with a SA/D ratio around 22.7 as I recall.  This is considered pretty good but not excessive by any means.   Yet, when I see the crap in SA or JA, it sounds like this is a pretty poor light air boat.   Additionally, the 105's seem popular in the Bay Area because they must do pretty well in the 20-30kt stuff.

 

My question really is, Since when is 22.7 SA/D poor for light airs and why?

 

I also had a Yankee One Design that never really seemed to be to be particularly doggy in light airs but its SA/D was around 15.9.  There's just something I haven't been able to correlate very well.  Heck, maybe I just don't know what good light air performance is.....!



#67 miloman

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Posted 12 July 2014 - 07:17 PM

So back in the haze of 15+ years ago, I had a J 105 with a SA/D ratio around 22.7 as I recall.  This is considered pretty good but not excessive by any means.   Yet, when I see the crap in SA or JA, it sounds like this is a pretty poor light air boat.   Additionally, the 105's seem popular in the Bay Area because they must do pretty well in the 20-30kt stuff.

 

My question really is, Since when is 22.7 SA/D poor for light airs and why?

 

I also had a Yankee One Design that never really seemed to be to be particularly doggy in light airs but its SA/D was around 15.9.  There's just something I haven't been able to correlate very well.  Heck, maybe I just don't know what good light air performance is.....!

 

The SA/WS (Sail Area/Wetted Surface) ratio is much more telling about light air performance than the SA/D ratio since at low speeds friction is such a large percentage of overall resistance. And neither of those numbers tell the whole story.  If you look at the ratios that started the thread, 22 is considered high performance.

 

I think what you're seeing in SA or JA is partially crap, as you put it.  First off, the numbers themselves may not be all that accurate (as I pointed out above).  Secondly, everything is relative.  If you live in Pudget sound your idea of "light air" is different from someone sailing in the bay area.  If you're used to sailing high performance skiffs, you're idea of fast is different than if you're used to sailing a Rozinante (which is a good ghoster with a relatively low SA/D ratio, for that matter).  A 49er at 5 knots is boring.  An opti at 5 knots is exciting.  It's all part of the "fast is fun" attitude which tends to exagerate performance, and treat anything but the most high tech, high performance boats as hardly able to sail out of their own way.

 

Personally, I think the whole "fast is fun" mantra is wrong.  It took a long time before there were pleasure boats that are "fast" by todays standards.  Over the last few hundred years of pleasure boating, plenty of people somehow managed to suffer through a lot of miserable hours sailing and racing boring boats.  There are a lot of things that make sailing fun, the exhileration of high speed is just one of them.  By treating performance as the be all end all we turn sailing into something that's exspensive, elitist, and very very serious.  Speed, while exhilerating, can also be cold, wet, uncomfortable, and scary.  None of those are that much fun.  So I submit that going fast isn't fun, going fast for the effort you're putting in, is.  But that's just me.



#68 Veeger

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Posted 12 July 2014 - 07:33 PM

Thanks Milo.  I'd agree with you about that 'fast is fun' thing.  I had an F-28 that I enjoyed scooting around in.  Used to think, "Man, I just sailed 29  miles in a 3 hours sail!"  Then I realized that the guy in the Catalina 27 who 'only' went 10 miles for the afternoon, still had 3 hours of enjoyable sailing!

 

In the end, it's whatever you enjoy on whatever you choose to afford!   (Of course, I'm the guy sitting at my computer on a beautiful sunny day without a boat while everyone else is enjoying the summer....)



#69 Bull City

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Posted 12 July 2014 - 09:53 PM

Another ratio: Hours sailed/Hours Owned



#70 Alex W

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Posted 13 July 2014 - 12:01 AM

Another ratio: Hours sailed/Hours Owned

 

or a more balanced ratio (that doesn't take region into account) might be hours sailed/hours worked.

 

I think this has more to do with picking the right used boat (based on condition) than anything else, as long as cruisers are being considered.  

 

I just came back from an enjoyable weekend of sailing.  I was anchoring near Francis Lee and got to meet Kim and tour the boat, that was the highlight of the weekend.  It's a really special project.  After that I sailed off of the anchor (it's nice leaving a bay without using the motor), and had lots of fun sailing home too.  Near West Point I did my best to hold off a Morgan 38-2 while beating north, but he finally passed me after about 20 or 30 minutes (waterline wins).  It's an amazing day on the water on Puget Sound and I think everyone is out...I've never seen the docks so empty at Shilshole.  

 

That was an awesome 24 hours in my 5ksb (Pearson 28-2).



#71 Bull City

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Posted 13 July 2014 - 12:21 AM

Today, we had very light wind, and lots of power boats. I motored to a swimming spot, anchored, had my lunch, swam, and watched some bikinis.



#72 Bull City

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Posted 13 July 2014 - 12:24 AM

Light air is OK, so long as there are no power boat assholes zooming around.



#73 kdh

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Posted 14 July 2014 - 01:09 PM

Sa/D is simply the horsepower to weight ratio. Lots of weight and lots of horses performs the same as low weight & low horses. I once knew a guy who had a '65 Impala with a huge hot engine that would turn 10 sec. 1/4 miles - for those who don't know what that means, a Posche Turbo does it in the 13's. That aircraft carrier Impala was dramatically faster.

 

The other end of the spectrum would be something like a Lotus 7 - the automotive equivalent of a modern small sport boat.

 

But remember that horsepower relates directly to acceleration, not to speed.



#74 SloopJonB

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Posted 14 July 2014 - 02:12 PM

Sa/D is simply the horsepower to weight ratio. Lots of weight and lots of horses performs the same as low weight & low horses. I once knew a guy who had a '65 Impala with a huge hot engine that would turn 10 sec. 1/4 miles - for those who don't know what that means, a Posche Turbo does it in the 13's. That aircraft carrier Impala was dramatically faster.

 

The other end of the spectrum would be something like a Lotus 7 - the automotive equivalent of a modern small sport boat.

 

But remember that horsepower relates directly to acceleration, not to speed.

If you check you'll find that torque relates to acceleration, horespower relates to speed.



#75 Thorvald

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Posted 14 July 2014 - 02:36 PM

Another ratio: Hours sailed/Hours Owned

Probably the most important one!

#76 12 metre

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Posted 14 July 2014 - 03:42 PM

 

Sa/D is simply the horsepower to weight ratio. Lots of weight and lots of horses performs the same as low weight & low horses. I once knew a guy who had a '65 Impala with a huge hot engine that would turn 10 sec. 1/4 miles - for those who don't know what that means, a Posche Turbo does it in the 13's. That aircraft carrier Impala was dramatically faster.

 

The other end of the spectrum would be something like a Lotus 7 - the automotive equivalent of a modern small sport boat.

 

But remember that horsepower relates directly to acceleration, not to speed.

If you check you'll find that torque relates to acceleration, horespower relates to speed.

Yes, an all too common misconception.



#77 MidPack

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Posted 14 July 2014 - 04:00 PM

Personally, I think the whole "fast is fun" mantra is wrong.  It took a long time before there were pleasure boats that are "fast" by todays standards.  Over the last few hundred years of pleasure boating, plenty of people somehow managed to suffer through a lot of miserable hours sailing and racing boring boats.  There are a lot of things that make sailing fun, the exhileration of high speed is just one of them.  By treating performance as the be all end all we turn sailing into something that's exspensive, elitist, and very very serious.  Speed, while exhilerating, can also be cold, wet, uncomfortable, and scary.  None of those are that much fun.  So I submit that going fast isn't fun, going fast for the effort you're putting in, is.  But that's just me.

And I'd respectfully counter with, you can always shorten sail (earlier) on a faster/performance boat, making the boat more useful over a broader range of wind speeds. I am not talking about leading edge Grand Prix boats, but a J/Boat vs a Catalina (please don't get hung up on the example brands, not the point). If you buy a boat with a 16 SA/D and 200 D/L, odds are it will be horrible in light air, and there's usually nothing within reason you can do about it. If you have a boat with a 24+ SA/D and a 120 or less D/L, you can use it in light air, reef when things are rough or choose to go fast if you have the crew & desire. There are plenty of performance boats with reasonable creature comforts, so I can't see buying a floating condo that's stuck in anything less than 12 kts. But that's just me.



#78 Bull City

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Posted 14 July 2014 - 04:41 PM

Horsepower determines Speed; Torque determines Acceleration.  

 

Can we say that Boats A and B (having same SA, LWL and wetted surface area, but different DISP) will attain the same speed in the same conditions? 

 

Can someone relate this to sailboat measures?



#79 SemiSalt

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Posted 14 July 2014 - 06:07 PM

If I may ask, where do you buy a boat with a SA/D of 25 and D/L of 120 with the accommodation of a Catalina 30 for the $20-30K that the Cat 30 costs in today's market?

#80 12 metre

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Posted 14 July 2014 - 06:26 PM

Personally, I think the whole "fast is fun" mantra is wrong.  It took a long time before there were pleasure boats that are "fast" by todays standards.  Over the last few hundred years of pleasure boating, plenty of people somehow managed to suffer through a lot of miserable hours sailing and racing boring boats.  There are a lot of things that make sailing fun, the exhileration of high speed is just one of them.  By treating performance as the be all end all we turn sailing into something that's exspensive, elitist, and very very serious.  Speed, while exhilerating, can also be cold, wet, uncomfortable, and scary.  None of those are that much fun.  So I submit that going fast isn't fun, going fast for the effort you're putting in, is.  But that's just me.

And I'd respectfully counter with, you can always shorten sail (earlier) on a faster/performance boat, making the boat more useful over a broader range of wind speeds. I am not talking about leading edge Grand Prix boats, but a J/Boat vs a Catalina (please don't get hung up on the example brands, not the point). If you buy a boat with a 16 SA/D and 200 D/L, odds are it will be horrible in light air, and there's usually nothing within reason you can do about it. If you have a boat with a 24+ SA/D and a 120 or less D/L, you can use it in light air, reef when things are rough or choose to go fast if you have the crew & desire. There are plenty of performance boats with reasonable creature comforts, so I can't see buying a floating condo that's stuck in anything less than 12 kts. But that's just me.

Nonsense!

 

By your criteria, one of the most talked about boats in SA as being a light air weapon (CF 27) should be a dog in light air with a SA/D of 22.5 and D/L of 154.  Similarly, most mid 70's IOR boats should be absolute turds in the light with a typical SA/D of only 17 and D/L around 230 (Peterson 35 for example)



#81 Panoramix

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Posted 14 July 2014 - 06:32 PM

Horsepower determines Speed; Torque determines Acceleration.  

 

Can we say that Boats A and B (having same SA, LWL and wetted surface area, but different DISP) will attain the same speed in the same conditions? 

 

Can someone relate this to sailboat measures?

 

Definitely no.

 

First of all it will be hard to increase displacement without increasing wetted surface, secondly at hull speed, the higher the displacement, the more power you need. So if you got more displacement by sticking more weight in the keel (increasing power), your boat will reach upwind top speed for a higher wind speed, if you got more displacement without increasing power, it will just be slower.



#82 boat_alexandra

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Posted 14 July 2014 - 07:39 PM

I am writing a weather routing program and I need access to polars for many boats.  The user can supply a polar file, but it also would be nice to automatically compute polars from various parameters.   I have applied this in the form of the sailboat transform, but unfortunately it seems unrealistic for cruising boats.

 

I am computing two types of drag (which ultimately determine the steady state speed of the boat)  This is skin drag based on a constant and velocity squared, and wave drag which is based on a constant and a very steep exponent based on waterline lenth.  I did also use the actual formula for this one (which allows breaking through into planing) but it turned out to give polars diagrams cases where planing is barely achieved making very rough looking polars.

 

I believe another way would be to enter SA/D and D/L ratios, as well as several additional coefficients.

 

Currently the Displacement Length is computed as:

displacement_long_tons / (.01 * length_water_line_ft)^3

 

For Sail Area to displacement, I don't actually have a proper dimensionless formula (if that even makes sense because after all physics is a bit different at different scales)

 

I have a large excel workbook full of those ratios for just about every sailboat I know of, I group boats together by type for comparison. I have FRANKIE in with the other long narrow vessels. Interesting stuff to study.

 

Where exactly can I download this workbook?

 

I harvested all of the data off of sailboatdata into a large spreadsheet that I'm willing to share if anyone wants it for non-commercial purposes.  5000ish boats.


I did it partially to find all of the data inaccuracies on SBD (for instance they list my Pearson 28-2 as a SA/D of 12, when it is more like 17).  I also did it to try and classify boats together and look for similarities.

 

Do you have more information?  Where can I find this?

 

They are supposed to be dimensionless numbers.  But technically, SA/D is, while D/L is not (but should be) 

 

In SA/D, you use SA in ft2 and D in ft3, so ft2/ft3^(2/3) = 1.  In other words, ft2 =ft3^(2/3), so the units cancel out, providing a dimensionless number.

 

However, in D/L you have D in lbs (long tons) and L in ft (multiplied by .010) so you have long tons/(ft)^3, and the units don't cancel out.  Properly, it should be D in ft^3 and L in ft^3 as well.  Problem is, in the case of a boat that has a D/L of 150 (using the accepted method of calculation) ends up with a D/L of 5282. which is cumbersome to use.  

 

My guess is that in the interest of simplicity, it was decided to effectively insert a constant.  For example, assuming a density of 64 lb/ft3, there are 35 ft^3 in a long ton.  So if you divide the 5282 (as calculated above) by 35, you end up with - 150!  Which is an easier number to utilize.  Because the difference is a constant (i.e. the results are directly proportional), it is just as valid to use the accepted method.  

 

However dimensionless numbers are supposed to yield the same result, no matter which system of units is used (SI or Imperial) and the accepted method yields different results if you tried to do it with SI units. However if you used m^3 for D and m for L, it works out the same as you would get using ft instead of m..

 

Dimensionless numbers are very common.  For example Froude Number and Reynolds Number. 

Yes, so where is your final formula?



#83 12 metre

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Posted 14 July 2014 - 07:59 PM

SA/D is traditionally calculated as SA in ft2 and D is Disp in cubic feet to the 2/3 power  or ft3(2/3).  This results in ft2/ft2 (just realized you can use superscripts here) so non-dimensional

 

D/L is traditionally calculated as D in long tons (Disp in lbs divided by 2240) and the divisor is LWL in feet, multiplied by .01 cubed or (.01 x LWL) 3 . So you end up with a number that is a function of long tons / ft3 .  These terms do not cancel out, so it is dimensional, although it would become non-dimensional if you expressed D in ft



#84 boat_alexandra

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Posted 14 July 2014 - 08:47 PM

Would that make it non-dimensional since displacement in pounds / 64 is displacement in ft?



#85 SemiSalt

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Posted 14 July 2014 - 08:55 PM

Boat_Alex: calculating a polar is basically calculating a VPP. This is difficult to do from brochure numbers. I know because I did it and the numbers weren't convincing.

Based on studying PHRF ratings, I think that if you have two boats of about the same size designed at about the same time for the same purpose, the ratings, and by extension the polars, will be very similar. Perhaps if you have a range of sizes, you can interpolate.

A pure cruiser will probably give up some speed, especially in light winds, in exchange for docile handling, or good directional stability, or some other characteristic, based on the designer's views.

#86 12 metre

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Posted 14 July 2014 - 09:03 PM

Would that make it non-dimensional since displacement in pounds / 64 is displacement in ft?

No - unless you raise the displacement in ft3  to the 2/3 power, yielding ft2  .

 

Non-dimensional numbers allow for the same result no matter what system of measurement is used.  So SA/D could just as easily be expressed as SA in m2 and D in m3(2/3), and you would end up with the same result. 

 

This doesn't work out if you try the same thing with D/L ratio, because it is not non-dimensional.  Now, if you expressed D in ft3 then it would be non-dimensional if L was in ft (or m3 if L was in m)



#87 kdh

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Posted 14 July 2014 - 09:15 PM

 

Sa/D is simply the horsepower to weight ratio. Lots of weight and lots of horses performs the same as low weight & low horses. I once knew a guy who had a '65 Impala with a huge hot engine that would turn 10 sec. 1/4 miles - for those who don't know what that means, a Posche Turbo does it in the 13's. That aircraft carrier Impala was dramatically faster.

 

The other end of the spectrum would be something like a Lotus 7 - the automotive equivalent of a modern small sport boat.

 

But remember that horsepower relates directly to acceleration, not to speed.

If you check you'll find that torque relates to acceleration, horespower relates to speed.

 

I checked. I meant only that power is related to force which is related to acceleration by F = ma. An object at rest or at constant velocity stays so unless forces are at work.

 

At a constant speed which is less than hull speed sail area is used to overcome friction, determined mostly by wetted surface area. This friction is not easily related to displacement.

 

Discussions of torque should be reserved for things like spinning engines.

 

I found myself much faster than a similar length boat beating into about 12 knots this past weekend. I had a reef in the main and a few rolls in the jib. I was sailing with my daughter and a friend who don't like "tippy" boats. We had enough power to keep the boat moving at near hull speed. With the autopilot on. Our "competition" was on their ear and frustrated.



#88 boat_alexandra

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Posted 14 July 2014 - 09:26 PM

 

Boat_Alex: calculating a polar is basically calculating a VPP. This is difficult to do from brochure numbers. I know because I did it and the numbers weren't convincing.

Based on studying PHRF ratings, I think that if you have two boats of about the same size designed at about the same time for the same purpose, the ratings, and by extension the polars, will be very similar. Perhaps if you have a range of sizes, you can interpolate.

A pure cruiser will probably give up some speed, especially in light winds, in exchange for docile handling, or good directional stability, or some other characteristic, based on the designer's views.

How did you do this?

 

I am using the sailboat transform, but it clearly doesn't take enough parameters into account.  It can work for ice sailing.

 

Interpolate between boats?  How does this work with wave drag?

 

There are many variables at play, and I would only consider 5 or 6.  Available online the various ratios and boat parameters for most production boats is known.  This allows a convenient starting point for a polar diagram.  From there, additional measurements of speed and from wind sensors may refine the polar diagram.

 

 

polar



#89 SemiSalt

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Posted 14 July 2014 - 09:46 PM

Pray tell, what is the sailboat transform?

#90 DDW

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Posted 14 July 2014 - 09:57 PM

Discussions of torque should be reserved for things like spinning engines.

Similarly, discussions of HP should be limited to spinning engines. Sails (in the speed ranges we are interested) produce force. The force they produce is (mostly) independent of boat speed. Since power (horsepower, if you must) is force x speed, a fully powered sailboat rig set on a boat at anchor produces no power at all, while the same rig sailing at 7 knots might be producing quite a bit of power. It is a bit unintuitive, and similar to jet engines, which are typically rated in thrust - not HP - for the same reasons. They produce no power standing still and phenomenal power at 500 knots. The same rig, on a much faster hull in the same wind will produce more power. 

 

It is less confusing to talk about the driving force of a rig.

 

 

Can we say that Boats A and B (having same SA, LWL and wetted surface area, but different DISP) will attain the same speed in the same conditions?

 

Changing that to SA/D, adjusting wetted area as required, and staying within the displacement realm, it will be pretty close. What you will find is the heavier boat will be a little slower in a lot of wind because its displacement makes the wave making drag increase faster, but it might be faster in light wind because the SA increased faster than the wetted area.



#91 RKoch

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Posted 14 July 2014 - 10:53 PM

Torque is how much work an engine can do. HP is how fast it can do it. Example: Crewmember A can crank a 150lb bowman to the top of a 50' mast in 2 minutes. Crewmember B can crank the same bowman up the same mast in 1 minute. They both do the same work, but crewmember B has twice the HP. Crewmember B can crank the 300# owner up the mast in the same 2 minutes crewmember A can crank up the 150# bowman. In that case, B has twice the torque.

#92 kdh

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Posted 14 July 2014 - 11:30 PM

It is less confusing to talk about the driving force of a rig.

 

 

Can we say that Boats A and B (having same SA, LWL and wetted surface area, but different DISP) will attain the same speed in the same conditions?

 

Changing that to SA/D, adjusting wetted area as required, and staying within the displacement realm, it will be pretty close. What you will find is the heavier boat will be a little slower in a lot of wind because its displacement makes the wave making drag increase faster, but it might be faster in light wind because the SA increased faster than the wetted area.

 

DDW, to me this is the point. SA/D is only useful to the extent that the additional force from increased sail area matches the increased frictional wetted-area force caused by increased displacement. 

 

It doesn't, so we have to qualify the model with statements like "within the displacement realm."

 

The basic situation: at less than hull speeds for a given waterline length a heavy boat is faster than its SA/D would imply relative to a light boat. Light allows faster acceleration, but this is not important for cruising purposes.



#93 DDW

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Posted 15 July 2014 - 12:01 AM

OK, torque isn't how much work an engine can do. Torque times RPM is. An object can apply torque and do no work at all. Torque is a force, force is not work. It isn't energy either. I have an iron bar here that can produce 1000 ft lbs of torque, but practically no work at all. 

 

DDW, to me this is the point. SA/D is only useful to the extent that the additional force from increased sail area matches the increased frictional wetted-area force caused by increased displacement. 

 

kdh,

 

I was mainly being pedantic for levity. 

 

However,  SA/D is useful by your criteria. If I hold LWL, SA/D, and general form the same, and increase displacement, what happens? It's a little nebulous, but consider a cylindrical hull form: length of the cylinder stays the same (LWL), radius goes up to get the extra volume (displacement). The area of the cylinder shell (wetted area) goes up proportional to the radius, volume (displacement) goes up proportional to the square of the radius. In other words wetted area goes up proportional to the square root of displacement. But to keep SA/D constant as defined, we must add sail area proportional to disp 2/3. That increases faster than wetted area which only increases at disp 1/2 . With increasing displacement then, sail area increases faster than wetted area, if we keep other things constant. In conditions  where wetted area dominates drag then, the heavier boat will be faster.

 

 

Pretty sure Tom Ray will be gone by now :)



#94 boat_alexandra

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Posted 15 July 2014 - 08:19 AM

We already know scaling a boat up in size makes it faster in light and heavy winds (for different reasons)   The same is true for solar powered electric drives.

 

It is funny that people are always claiming that I have some advantage in a bristol 27 over their much larger boat in light winds so they are forced to use an engine while I don't need one.  The fact,  is they are just bad at sailing, too lazy to deal with a spinnaker, and impatient. 

 

For sailboat transform, try first link on google search "sailboat transform"  I would be very interested in alterate VPP calculations, especially ones based on SA/D and D/LOA ratios (as well as other parameters) 



#95 kdh

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Posted 15 July 2014 - 12:14 PM

OK, torque isn't how much work an engine can do. Torque times RPM is. An object can apply torque and do no work at all. Torque is a force, force is not work. It isn't energy either. I have an iron bar here that can produce 1000 ft lbs of torque, but practically no work at all. 

 

DDW, to me this is the point. SA/D is only useful to the extent that the additional force from increased sail area matches the increased frictional wetted-area force caused by increased displacement. 

 

kdh,

 

I was mainly being pedantic for levity. 

 

However,  SA/D is useful by your criteria. If I hold LWL, SA/D, and general form the same, and increase displacement, what happens? It's a little nebulous, but consider a cylindrical hull form: length of the cylinder stays the same (LWL), radius goes up to get the extra volume (displacement). The area of the cylinder shell (wetted area) goes up proportional to the radius, volume (displacement) goes up proportional to the square of the radius. In other words wetted area goes up proportional to the square root of displacement. But to keep SA/D constant as defined, we must add sail area proportional to disp 2/3. That increases faster than wetted area which only increases at disp 1/2 . With increasing displacement then, sail area increases faster than wetted area, if we keep other things constant. In conditions  where wetted area dominates drag then, the heavier boat will be faster.

 

 

Pretty sure Tom Ray will be gone by now :)

 

Thanks, DDW. This is the sort of mathematical argument that's useful to me. It sounds like then for SA/D (really SA/D^2/3) the implicit hull model is a sphere, where volume (displacement) goes as the cube of the radius and wetted surface as the square.

 

To me, along with assuming the boat is shaped like a sphere, the important assumptions are that we get increased displacement by making the hull bigger rather than using heavier materials, and that the relationship between sail area and driving force is functionally similar to that between wetted surface and the associated frictional force.

 

Not a terrible model, really, if we want to look at things simply.



#96 kdh

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Posted 15 July 2014 - 12:26 PM

Here's where I end up with all of this. For cruising, a model of speed is hull speed, which goes as the square root of waterline length. This assumes that you have enough sail area to drive your boat to near hull speed given the drag of your hull and wind conditions.



#97 Tom Ray

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Posted 15 July 2014 - 12:26 PM

Pretty sure Tom Ray will be gone by now :)

 

I was bored and came back. Got more bored, now I'm leaving again. Though I did enjoy Alex's post about trying to hold off a Morgan 38 with his Pearson 28. Possibly because it was the only part of the thread I understood.

 

Torque?



#98 Py26129

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Posted 15 July 2014 - 01:34 PM

I'm with Tom.  I'm out of my depth in a puddle in this discussion. 

 

For me, the only ratio that matters is Smiles / Mile.



#99 SemiSalt

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Posted 15 July 2014 - 03:01 PM

We already know scaling a boat up in size makes it faster in light and heavy winds (for different reasons)   The same is true for solar powered electric drives.
 
It is funny that people are always claiming that I have some advantage in a bristol 27 over their much larger boat in light winds so they are forced to use an engine while I don't need one.  The fact,  is they are just bad at sailing, too lazy to deal with a spinnaker, and impatient. 
 
For sailboat transform, try first link on google search "sailboat transform"  I would be very interested in alterate VPP calculations, especially ones based on SA/D and D/LOA ratios (as well as other parameters) 



From the link:

"Comment: The above approximation is inaccurate for a displacement hull (with no planing capabilities) in speeds close to and above the hull speed (which are typically out of the range of polar diagrams). I.e., it should be used only within a circle around the origin in the diagram, with a radius that is a little smaller than the hull speed."

So, if you have one model that works below hull speed, and you have hull speed as another model, you've got it knocked.

BTW, I would think its closer to say that a Bristol 27 never has an advantage over any other boat of comparable size. Well, any marconi sloop of comparable size.

#100 DDW

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Posted 15 July 2014 - 04:25 PM

Thanks, DDW. This is the sort of mathematical argument that's useful to me. It sounds like then for SA/D (really SA/D^2/3) the implicit hull model is a sphere, where volume (displacement) goes as the cube of the radius and wetted surface as the square.

 

To me, along with assuming the boat is shaped like a sphere, the important assumptions are that we get increased displacement by making the hull bigger rather than using heavier materials, and that the relationship between sail area and driving force is functionally similar to that between wetted surface and the associated frictional force.

 

Not a terrible model, really, if we want to look at things simply.

 

Theses ratios are meant to compare boats of different sizes, and so reduced to unit length. the ^1/3 term is simply because displacement increases as the cube of length (for the same form), the ^2 term because sail area increases as the square of length. In my example, it was assumed that the length does not change, thus the cylinder rather than sphere, and ^2 rather than ^3. Growing the boat in proportion (including length) sail area and wetted surface track at 1:1 (both by length^2 and displacement ^2/3). But growing only sideways, sail area increases faster than wetted area. Roughly then, a boat with a higher SA/D ratio likely also has a higher sail area to wetted area ratio (hull forms the same). Pending the details of course!

 

The formulae suggests that sail area cannot keep up with displacement drag as the boat gets larger which seems to be true. Hard to hang enough sail on a really large boat. On the other hand it becomes much easier to reduce the D/L ratio which makes up for that to some extent. And no one can argue with LWL.






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