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How to determine light air performance on paper?


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#1 MidPack

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Posted 17 November 2012 - 04:33 PM

Since there is no place to test sail every boat that might be of interest, and I don't want to fly all over the country to try boats, is there a way to objectively rank light air performance of a given design by looking at the numbers? Read further only if needed.

My first thought is SA/D (noting upwind and downwind SA/D), the higher the better. But I have seen boats with similar SA/D ratios (and the same sail config) where one is much better than the other in light air. So I turn to D/L, the lower the better for light air performance, and it seems to break the SA/D ties. And finally, I look at PHRF ratings (if available) for various boats, and for any two boats of the same waterline and it suggests to me that light air performance must be better for the lower rated boat. This seems to help, but there are some very knowledgeable people here, thought maybe there was a more reliable way to narrow the field when one can't test sail all the candidate designs.

#2 henry the navigator

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Posted 17 November 2012 - 04:53 PM

Midpack,

You could look at the polars.

#3 Christian

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Posted 17 November 2012 - 05:15 PM

MP - your method is somewhat on track - the numbers you mention all come into play. However - one of the most significant measures for light air performance is SA/wetted surface. Under hull speed that is what matters - waterline doesn't matter as you are not limited by hullspeed, Displacement has some influence mostly from a momentum perspective, i..e. starts and course changes and obviously a heavy boat will have a higher wetted surface than a lighter one.

Hullform naturally also has an influence but is harder to quantify in numbers. One factor is rocker - moderate is the best and allows you to move weight forward, nose in the water, ass out of the water thus reducing wetted surface - harder to do in boats with minimal rocker.

Also take a look at some light air venues and see which boats are most successful in those areas - LIS, Chasepeake are good areas to look at.

I'll take you sailing on one of the absolute best light air boats available if you come to naptown

#4 Bob Perry

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Posted 17 November 2012 - 05:19 PM

For light air performance you may want to look at wetted surface. If you have a "heavy" boat and a "light" boat and they have the same SA/D's the heavier boat may in fact have the better SA/wetted surface ratio. Maybe. Not sure though that you can get wetted surface numbers for the boats you are looking at. Plus, the light boat will be able to be heeled by crew weight easier and in heeling the boat wetted surface can be reduced and the Cp adjusted for better light air boat speed. You don't have that option to that degree in the heavy boat. If I were to generalize I'd say the despite the SA/D's being equal the lighter boat will most probably be the better boat in light air.

My 2 cents worth.

#5 SemiSalt

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Posted 17 November 2012 - 05:19 PM

You are certainly right that none of the commonly available specs or dimensions is really helpful. For example, around here we have a fairly heavy Chance design that must be 30 years old with a SA/D of about 14, and he cleans up in light air.

Are you talking "light" as in under 10, or as in a drifter?

It's possible to take LWL, Beam, and Displacement and make a guess at the wetted surface. That could be a help.

Here are some things you want to keep in mind. A tall rig may catch breeze that escapes a low rig. A deep narrow foil may have less wetted area that a short wide one, but keels are more efficient at high speeds than low, so a really skinny blade may not do the trick at low speeds.

#6 DRIFTW00D

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Posted 17 November 2012 - 05:43 PM

Its an equation boat / crew. Back to basics two identical boats both light air screamers. 1 cleans up and is fast . Difference CREW WORK / prep / good sails/ goes the right way.
We never reef or have twice in the last 10 yrs here. That is about 3 3dl type mains with just 1 reef point.
Look at Great Lakes results in your quest also. U could look at boats designed / developed for your area or light wind areas.

#7 MidPack

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Posted 17 November 2012 - 05:44 PM

Thanks folks. Some comments:
  • Polars are not always available, but it would be a good source when they are.
  • I thought wetted surface would be helpful, but while it may be helpful, I don't know if I've seen a production boatbuilder who published same.
  • It appears SA/D and D/L (plus PHRF when available) might be the best I'll be able to do. It usually works, but I have seen some boats that did not measure up in light air (fortunately I haven't bought any of them).
  • By light air, I guess I mean still moving/maneuverable down to about 3-4 knots TWS. Some days, that's all we get, and I don't like to sit on the mooring.


#8 Bob Perry

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Posted 17 November 2012 - 05:52 PM

Semi:
Makes a good point. Back in the 60's I designed a 2 tonner HEATHER and I designed it for light air. I made it heavy so it could have a really tall rig for its rating. I gave it quite a bit of freeboard to get the tall rig even higher. That boat was unbeatable in it's day in light air. The Alan Holt designed half pounder, I forget the name, was similar, i.e. heavier than the other half tonners with a really tall rig. It was dominant in light to medium air. I used the same approach again on my half tonner UNION JACK, heavy, tall rig and high freeboard and it too was unbeatable in light air but a monster in a breeze off the wind.

Ted Hood was known for his heavy IOR boats that were very fast in light air.

But that was the 70's and just before the Kiwi light boats came on the scene.They changed everything.
I'd consider "light air" as any condition when you can't get the boat above .5 hull speed.

#9 Christian

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Posted 17 November 2012 - 05:56 PM

If you cannot get numbers for wetted surface (and I know it is tough to get for most boats) you can get a very good idea by looking at hull lines/shape. The absolute minimum wetted surface for a given displacement would be a ball shape. Hence what you would be looking for in terms of a minimum wetted surface shape would be a round bilge and a certain ampunt of rocker. any flat wide asses makes the wetted surface rise quite significantly and unless you can get that part of the boat out of the water (by weight forward and boat heel) you will have a sticky boat. Now - what you do get if you look for a round bilge boat is that it will be more tender as there is very little hull stability in such a shape so this is the kind of boat that needs to be hiked hard when it starts blowing.

Another important area to look at is a good tunable rig and a proper sailplan. Not all rigs/sail combinations work well in light air. Full battened mains are superior in very light air.

#10 SemiSalt

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Posted 17 November 2012 - 07:09 PM

On estimating wetted surface: I don't really have time to look this up right now, but if you search the Cruising Anarchy forum, you will find that I posted about a simple VPP project I had going. It was based on one done by Olin Stephens who had the idea of doing a VPP based only on easily available data. It included an estimate for WS that was basically hull plus keel plus rudder. The number for the WS of the hull was done by estimating the waterplane, and using the displacement to figure out how deep the hull body has to go. It was based on various technical papers, some of which were listed in Stephens' article. It was made somewhat more reasonable since he was working in the IOR era which implied that hull girths would be close to minimum for the displacement and beam.

This question is exactly the sort of thing that Stephens' felt could be explored. My version is no longer online. I hope to get back to it, but I got too busy at work to keep at it.

In his autobiography, Stephens explains how at the start of his career, he made boats as heavy as he could in order to get the most sail area he could for the light air of WLIS. (International rule, I think). This approach depends a lot on smooth water. If the water isn't smooth (e.g. kicked up by motorboat wakes like WLIS), the advantage may switch to lighter boats. The heavy boats are stopped less by a wake, but the light boat accelerates quicker, and that may be more important.

#11 MidPack

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Posted 17 November 2012 - 07:20 PM

Interesting post, as are the others.

The number for the WS of the hull was done by estimating the waterplane, and using the displacement to figure out how deep the hull body has to go.

As an engineer, that's an intriguing thought, I'll have to explore that further.

In his autobiography, Stephens explains how at the start of his career, he made boats as heavy as he could in order to get the most sail area he could for the light air of WLIS. (International rule, I think). This approach depends a lot on smooth water. If the water isn't smooth (e.g. kicked up by motorboat wakes like WLIS), the advantage may switch to lighter boats. The heavy boats are stopped less by a wake, but the light boat accelerates quicker, and that may be more important.

Sounds good, but a) no production builders seem to design heavy boats with sail area resulting in high SA/D that I know of ($$$) and B) I'd rather have smaller sails all else equal, lower cost and easier to handle. I've sailed on heavy boats with large sails for their size, they're a handful from a sail handling POV IMO.

#12 SemiSalt

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Posted 17 November 2012 - 07:58 PM

MidPack - as indicated by the Mr. Perry's comments, the heavy boat is not going to have high SA/D, only high SA/WS. However, most racing sailors are like you and prefer the light boat for the reasons you mention plus the livelier feel. They do put a bigger emphasis on having just the right number of crew aboard, however, which not a logistic problem I could deal with very well.

If you are going to get a boat that is too heavy to plane, get a C&C.

#13 12 metre

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Posted 17 November 2012 - 11:11 PM

Disp/L3 alone won`t tell you much about light air performance; it is mainly indicative of offwind potential in a breeze. Prismatic Coefficient tells you something about the potential of the hull in certain wind conditions, but doesn`t take into account horsepower.

SA/Dspl (2/3) is probably the best indicator if you take into account it`s limits. The difficulty is that for a given length, displacement and wetted surface do not vary in proportion, i.e. if you halve the displacement, wetted surface will be reduced, but not by half. For example, take a floating 4` x 4` box with a depth of 1`. This will have a wetted surface of 32 sq ft. If you reduce the depth to 0.5 ft, displacement will be halved, but wetted surface will now be 24 sq ft, or only 25% less. While that is what the (2/3) term is supposed to account for, it really only allows for a valid comparison between boats of similar length and displacement. This partly explains why heavier boats with seemingly inferior SA/Dspl (2/3) ratios are often better light air performers than their much lighter counterparts.

Having said all this, I propose a simple set of numbers for a 27-30 ft R/C:

For Disp/L3 over 200, the min SA/Dspl(2/3) should be 20 or more
For Disp/L3 of around 150 (the most common), the min should be 22 or more
For Disp/L3 under 100, the min should be 25 or more

Now keep in mind this is a quick and dirty analysis using numbers pulled from the US Sailing web-site. The boats used include the J family of early 80s R/C such as J-24, J-29, J-30, Santana 30/30, CF-27, Olson 30, and Ross 930. Also, in calculating SA/Dspl(2/3) the traditional method of using 100% SA was used, which probably makes M/H boats look less powerful than they really are, so you may want to adjust the SA to include the jib overlap area. If so, the above minimums will need to be adjusted upwards, say to 33 for Disp/L3 under 100.

Following are the SA/Dspl(2/3) of some of the boats used:

CF-27 (22), J-24 (20), J-29 (23), J-30 (19), O30 (26), R930 (27)

Some may say the O30 is better than the R930 in light air, which I believe is true, but the O30 is quite slack bilged (trading off stability for less wetted surface) while the R930 has quite firm bilges. In other words, the R930 is an inherently higher wetted surface design hence the need for relatively more HP even though they are of similar Disp and fairly similar Dspl/L3. So while there are many other factors that come into play, I think an analysis like this is a good starting point to separate the wheat from the chaff so to speak.

Again, a fairly simple analysis, but in an afternoon, the OP could go through the US Sailing web-site and pull off the numbers for boats in the size range he is looking for as known yardsticks and do a similar analysis in Excel.

#14 wanchaibelle

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Posted 18 November 2012 - 05:09 AM

The ORC website enables you to get VPP-based performance predictions for a massive number of boats - there's a high chance that inside their database you can find sisterships of the boats you would like to assess. It's about ten bucks a go...

#15 Gouvernail

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Posted 18 November 2012 - 06:59 AM

Since there is no place to test sail every boat that might be of interest, and I don't want to fly all over the country to try boats, is there a way to objectively rank light air performance of a given design by looking at the numbers? Read further only if needed.



PHRF numbers vary by locality. Boats that do well in light air probably have lower ratings around teh Chesapeake and in austin. Boats that suck in light air but do well in a blow probably have lower numbers in Corpus and San Francisco.

#16 CJV

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Posted 18 November 2012 - 05:18 PM

Great thread guys. Thanks for the quality contributions.

#17 12 metre

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Posted 18 November 2012 - 10:45 PM


Since there is no place to test sail every boat that might be of interest, and I don't want to fly all over the country to try boats, is there a way to objectively rank light air performance of a given design by looking at the numbers? Read further only if needed.



PHRF numbers vary by locality. Boats that do well in light air probably have lower ratings around teh Chesapeake and in austin. Boats that suck in light air but do well in a blow probably have lower numbers in Corpus and San Francisco.


Yeah, while PHRF is far from perfect, personally I'd take PHRF numbers in known light or heavy air regions over ORC polars. In theory, the polars should be better, but I've seen too many TCFs for various boats in various conditions that are completely out to lunch in my opinion. If it can't get a grasp on TCF, how can it be expected to produce reliable predictive polars? Polars are useful for sailing angles, but used as a tool to compare one boat to another, I don't have so much faith.

#18 Amati

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Posted 19 November 2012 - 04:08 AM

FWIW, when we did Amati, which is a light air boat, I wanted D/L under 100, SA/D working sail
above 30, which I didn't get- its 24- which is very nice, but I'm working on a 165% genoa, and SA/WS over 3, working sail (blade and main).

Light airs-:)

#19 SemiSalt

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Posted 19 November 2012 - 03:05 PM

Polars are useful for sailing angles, but used as a tool to compare one boat to another, I don't have so much faith.


I really don't have the experience to back this up, but I suspect that you're right. Sailing has gotten to be a very precise sport, so tiny differences in performance become conspicuous on the race course. While a VPP that is accurate to 1% might be a huge success from a math modeling point of view, that one percent is big on the race course. And the VPPs are assuming a "normal" boat but not all boats are completely normal. They are also aimed at usual conditions and will become less accurate in light or heavy air.

VPPs shine in comparing boats that are very similar, like different prototypes for the AC cup. They excel in questions like "what happens if we take two feet off the boom and add one foot to the mast," where the question is not "how fast", but "which is faster".

#20 Bob Perry

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Posted 19 November 2012 - 03:28 PM

I'm with 12m. Don't underestimate the value of local PHRF ratings.

#21 12 metre

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Posted 19 November 2012 - 08:59 PM


Polars are useful for sailing angles, but used as a tool to compare one boat to another, I don't have so much faith.


I really don't have the experience to back this up, but I suspect that you're right. Sailing has gotten to be a very precise sport, so tiny differences in performance become conspicuous on the race course. While a VPP that is accurate to 1% might be a huge success from a math modeling point of view, that one percent is big on the race course. And the VPPs are assuming a "normal" boat but not all boats are completely normal. They are also aimed at usual conditions and will become less accurate in light or heavy air.

VPPs shine in comparing boats that are very similar, like different prototypes for the AC cup. They excel in questions like "what happens if we take two feet off the boom and add one foot to the mast," where the question is not "how fast", but "which is faster".


Absolutely. I do think they are also probably very useful in "what if" scenarios and for comparing very similar boats, say perhaps a J-35 vs Schock 35.

Another issue I had with them is their notion of light, medium, and heavy airs. I can't remember which rule it was, but the wind speed range for light/medium/heavy all seemed medium to me. .

#22 SilverSurfer

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Posted 19 November 2012 - 10:05 PM

Wetted surface area can be taken from ORC or IMS certificates. Only for the canoe body I think....

#23 henry the navigator

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Posted 20 November 2012 - 02:47 AM

Full marks for the comments regarding the value of a polar for making a comparison. The PHRF numbers compared and contrasted from heavy and light air venues sounds like genius.

MidPack, when your analysis has been reduced to a graphic could you share it?



#24 Elegua

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Posted 20 November 2012 - 08:33 AM

Someone much smarter than me said look at sail area, wetted surface and prismatic coefficient.

I'm slow, so I look at PHRF for a directionally correct as opposed to specifically wrong answer.

#25 HWP

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Posted 21 November 2012 - 12:08 AM

One thing you can look at is the relative size of the foils -- low aspect ratio foils have more surface area. A boat with daggerboard/centerboard that can be legally retracted (i.e., is self-righting with board up) you can do very well off-wind compared to similarly rated competitors. A good example is the S2 7.9.

#26 barefoot children

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Posted 21 November 2012 - 05:14 PM

For light air performance you may want to look at wetted surface. If you have a "heavy" boat and a "light" boat and they have the same SA/D's the heavier boat may in fact have the better SA/wetted surface ratio. Maybe. Not sure though that you can get wetted surface numbers for the boats you are looking at. Plus, the light boat will be able to be heeled by crew weight easier and in heeling the boat wetted surface can be reduced and the Cp adjusted for better light air boat speed. You don't have that option to that degree in the heavy boat. If I were to generalize I'd say the despite the SA/D's being equal the lighter boat will most probably be the better boat in light air.

My 2 cents worth.

Somewhere I have an IMS publication listing the north American fleet. One of the columns is Sail area to wetted surface ratios
(the stickness ratio), course it only covers boats IMS measured

#27 _DB

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Posted 21 November 2012 - 08:21 PM

You could just look up (for free) the ORR ratings for the boats you are interested in and compare the W/L numbers at 8 knots. If you boat isn't on the list, then you can always build a quick curve fit using the data and make your own 'vpp.'

http://offshore.ussa...eward_50_50.htm

#28 MidPack

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Posted 21 November 2012 - 08:45 PM

You could just look up (for free) the ORR ratings for the boats you are interested in and compare the W/L numbers at 8 knots. If you boat isn't on the list, then you can always build a quick curve fit using the data and make your own 'vpp.'

http://offshore.ussa...eward_50_50.htm

I wasn't aware of that resource. Just spent some time looking at that, very interesting, thanks! Matches up with what I expected in most cases, though not all (which is what I was looking for)...




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