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What makes a SR 21 so fast?


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#1 kmcfast

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Posted 01 December 2012 - 12:27 AM

They avg. 165 PHRF nationwide.
Fast upwind or down? lite or hvy?
Spec numbers on the the boat are not that impressive.
What gives?

#2 dolphinmaster

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Posted 01 December 2012 - 12:48 AM

They avg. 165 PHRF nationwide.
Fast upwind or down? lite or hvy?
Spec numbers on the the boat are not that impressive.
What gives?


I had one. #69. In light air upwind, in flat water, we would have 5 degrees of point and 1knot more speed than a J-24 right next to us. I followed an Elite 30 around a navigational mark in 6-8 one day, relatively flat water. Went to a close reach and just walked away from them in minutes. They can surf a chop much sooner than a J-24 but don't go upwind so good into it. We went upwind 18 knots, different day, chop, rounded behind the J-24s, pretty close, and by the leeward mark, had caught the Cape Fear 38 at the Leeward rounding.

Not much volume up front so downwind in the blow, they do submarine. Not enough room in the back to really keep the nose up with transformed biscuits and gravy.

I think they are awesome well rounded boats. Go look at a Henderson 30 on the trailer and shrink it to 24 feet. That equals the M24. I think Glen was ahead of his time and drew some seriously fast boats. I also think the R-team who "drew" the M24, stole the lines from the Henderson 30.

Another race, was a single handed phrf pursuit race. 6 miles, 4-6 knots of breeze in the ocean here, with a gentle swell. Passed a very well sailed J-24 1/2 way up the first leg on al . Passed the S2 7.9 halfway along the 2nd leg upwind and light. No one else to pass, couldn't see the 2nd mark, didn't have a course heading, no chart, kept looking back to try to approximate direction and found it. Small white fishing bouy marking an artificial reef. Flew the chute on the third leg, gybed a few times and finished so far ahead of the Cape Fear 38, could barely see his chute.

Crazy good little boat. Most all of the ridiculousness of it occurred for me in displacement conditions.

Excellent question, Glen, ......... you here anywhere??????? Glen............. anyone seen him lately? Know how to find him?

#3 GMiller

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Posted 01 December 2012 - 01:17 AM

Dolphin, you race with Neil? I agree, they are fast boats, especially when Neil went to those new sails from the fella in Florida. Hope you guys come back up this way.

#4 dolphinmaster

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Posted 01 December 2012 - 03:51 AM

Dolphin, you race with Neil? I agree, they are fast boats, especially when Neil went to those new sails from the fella in Florida. Hope you guys come back up this way.


G, had a navy blue hull, Neil's was white. You in Oriental/New Bern?

#5 Steam Flyer

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Posted 01 December 2012 - 12:24 PM


They avg. 165 PHRF nationwide.
Fast upwind or down? lite or hvy?
Spec numbers on the the boat are not that impressive.
What gives?


I had one. #69. In light air upwind, in flat water, we would have 5 degrees of point and 1knot more speed than a J-24 right next to us. I followed an Elite 30 around a navigational mark in 6-8 one day, relatively flat water. Went to a close reach and just walked away from them in minutes. They can surf a chop much sooner than a J-24 but don't go upwind so good into it. We went upwind 18 knots, different day, chop, rounded behind the J-24s, pretty close, and by the leeward mark, had caught the Cape Fear 38 at the Leeward rounding.

... ...


Never sailed one but have sailed with/against them a bunch. I'd say that PHRF number averages about right; they can be pretty fast but I generally beat them boat-for-boat in a Lightning, generally lost to them in light air in the Johnson 18.

It would be interesting to see what a modernized one could do. I've also been eyeballing the SR-27 of which at least one has been converted to A-sail.

FB- Doug

#6 dolphinmaster

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Posted 01 December 2012 - 02:22 PM

to me, Glen's SR boats are like beautifully shaped breasts, regardless of the size, I want a taste every time I see one. :)

#7 HHN92

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Posted 01 December 2012 - 03:51 PM

As Glenn told me once, he's just a kid from the strawberry fields of Plant City, FL that happened in to working at Kiwi back in the day, probably when OH, and maybe Ron Holland, were there. With the variety of boats that went through there he probably saw quite a bit of shapes and ideas.

The first time I saw an SRMax 21 sail (Bomb Shock) was going out into the gulf with 20-25 blowing, and I thought 'no way'. The little boat hung-in there and was quite impressive. I have seen an SRMax hang with a well sailed J24 upwind and then pull away once around the corner. Once during a heavy air pursuit race on Tampa Bay, with a long slog upwind, it was an SR33 first followed by the SRMax, and then OH sailing a Kiwi 35. We just could not reel-in the two SR boats.

Well sailed, an SRMax can sail well above its weight class.

#8 GMiller

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Posted 01 December 2012 - 04:12 PM


Dolphin, you race with Neil? I agree, they are fast boats, especially when Neil went to those new sails from the fella in Florida. Hope you guys come back up this way.


G, had a navy blue hull, Neil's was white. You in Oriental/New Bern?


I race with Pamlico Sailing Club, out of Washington. Essobee. Did he sell his?

#9 mustang__1

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Posted 02 December 2012 - 01:35 AM

I thought Snatch was still sailing around Sarasota? or are we talking way back in the day...

#10 dolphinmaster

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Posted 02 December 2012 - 02:10 PM

I had #69 back in 2000-01

don't know essobee.

#11 dolphinmaster

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Posted 02 December 2012 - 02:17 PM

They avg. 165 PHRF nationwide.
Fast upwind or down? lite or hvy?
Spec numbers on the the boat are not that impressive.
What gives?


I don't think we have really answered your question yet, only shared observations.

My guesses from years of looking at boats out of the water as if they were naked women on a beach,

She has no excess weight in her ends,

great foils,

great lines,

good balanced hull and rig,

She is a 21' perfect 10

#12 Christian

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Posted 02 December 2012 - 05:26 PM

Now you also have to put "fast" in perspective - compare it to a Viper 640 or Shaw 650 at the same length and you are looking at a rating difference of over a minute a mile - and those boats can sail to their ratings too. Not trying to diss the SR21 but fast is relative

#13 dolphinmaster

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Posted 03 December 2012 - 01:00 AM

Now you also have to put "fast" in perspective - compare it to a Viper 640 or Shaw 650 at the same length and you are looking at a rating difference of over a minute a mile - and those boats can sail to their ratings too. Not trying to diss the SR21 but fast is relative


That's like saying a Mclaren can beat a Mustang. Duh! That wasn't the question.

#14 Pete M

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Posted 03 December 2012 - 01:14 AM

That's like saying a Mclaren can beat a Mustang. Duh! That wasn't the question.



but that was the question

What makes a SR 21 so fast?



compared to what?

a j24?
a j70?
a shaw 650?
an 18 foot skiff?
a volvo 70?
vestas sailrocket?

#15 dolphinmaster

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Posted 03 December 2012 - 01:31 AM


That's like saying a Mclaren can beat a Mustang. Duh! That wasn't the question.



but that was the question

What makes a SR 21 so fast?



compared to what?

a j24?
a j70?
a shaw 650?
an 18 foot skiff?
a volvo 70?
vestas sailrocket?


Everything has context. The question wasn't a comparison question. What are the characteristics of any particular boat that make that boat perform with speed.

ie, each of the above has specific attributes that deliver in very specific conditions. The SR21 is a very unique design that goes and beats up on ordinary boats much larger than itself, ie. the Elite 30 mentioned above in light breezes on a close reach. The sr has much less water line but at different times and conditions and not in planing conditions. Is it purely Sa/D, flat run aft, foil shape, imitation whale flippers? Some of us non-pros like to think on these things too.

#16 jerseyguy

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Posted 03 December 2012 - 02:41 AM

Wha makes an SR 21 so fast? Sailing against slower boats.

:D

#17 dolphinmaster

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Posted 03 December 2012 - 03:54 AM

jersey, and Pete, you still don't get it.

When the Cal 40 came out, what made it so fast, when the J-24 came out, it was smokin everything, and on a good day, a good sailed J24 can still be competitive in a phrf fleet. A Viper can embarrass the shit out of a J-24, 27, 29, and probably 33. So what. Born and built in very different generations.

The Viper is so cool but so what, the Sailrocket can make it look like a stationary dot. You guys are missing the relativity of how did that boat, in the day, set the bar higher?

Still no real analysis of the relative design differences of the day that realized on the courses of yore.

If you don't know, you won't really understand your latest, fastest boat. You just know it goes fast but without a clue of why or how.

pete, why even bother, don't you have a tornado to sail?

#18 12 metre

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Posted 03 December 2012 - 05:13 AM

jersey, and Pete, you still don't get it.

When the Cal 40 came out, what made it so fast, when the J-24 came out, it was smokin everything, and on a good day, a good sailed J24 can still be competitive in a phrf fleet. A Viper can embarrass the shit out of a J-24, 27, 29, and probably 33. So what. Born and built in very different generations.

The Viper is so cool but so what, the Sailrocket can make it look like a stationary dot. You guys are missing the relativity of how did that boat, in the day, set the bar higher?

Still no real analysis of the relative design differences of the day that realized on the courses of yore.

If you don't know, you won't really understand your latest, fastest boat. You just know it goes fast but without a clue of why or how.

pete, why even bother, don't you have a tornado to sail?


I don't think a PHRF of 165 is that fast for a 22 footer, even for the era it came out in (1993 or thereabouts). Let's look at an even older 22 footer that rates about the same - the Pocket Rocket came out in 1983 (later morphed into the Rocket 22).

The Pocket Rocket was a bit sticky in the light stuff, but was fast upwind in a breeze. What made her so fast? Light boat for 1983 (2200 lbs), but much heavier than the SR-21, very beamy (9'6" or so), powerful hull (firm bilges and flat bottom, combined with flared topsides), long waterline with a plumb bow (very unusual at the time). Great Mull design - truly a design that was way ahead of its time, but could have used more sail area. Would be a great candidate to be turboed - oh wait, I forgot about the Rocket 22.

#19 TeamGladiator

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Posted 03 December 2012 - 06:14 AM


jersey, and Pete, you still don't get it.

When the Cal 40 came out, what made it so fast, when the J-24 came out, it was smokin everything, and on a good day, a good sailed J24 can still be competitive in a phrf fleet. A Viper can embarrass the shit out of a J-24, 27, 29, and probably 33. So what. Born and built in very different generations.

The Viper is so cool but so what, the Sailrocket can make it look like a stationary dot. You guys are missing the relativity of how did that boat, in the day, set the bar higher?

Still no real analysis of the relative design differences of the day that realized on the courses of yore.

If you don't know, you won't really understand your latest, fastest boat. You just know it goes fast but without a clue of why or how.

pete, why even bother, don't you have a tornado to sail?


I don't think a PHRF of 165 is that fast for a 22 footer, even for the era it came out in (1993 or thereabouts). Let's look at an even older 22 footer that rates about the same - the Pocket Rocket came out in 1983 (later morphed into the Rocket 22).

The Pocket Rocket was a bit sticky in the light stuff, but was fast upwind in a breeze. What made her so fast? Light boat for 1983 (2200 lbs), but much heavier than the SR-21, very beamy (9'6" or so), powerful hull (firm bilges and flat bottom, combined with flared topsides), long waterline with a plumb bow (very unusual at the time). Great Mull design - truly a design that was way ahead of its time, but could have used more sail area. Would be a great candidate to be turboed - oh wait, I forgot about the Rocket 22.

Which isn't even close to turboed enough; Donny stopped short on that one. Drop the bulb another 18" and increase the sail area. Hell the rig on the Rocket 22 will fit on a Martin 241 and that's only 2100# and was built in 1978!
SR 21 fast? Not really. Pretty much just the standard for the era.

#20 12 metre

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Posted 03 December 2012 - 07:56 AM

Yeah, the 241 is another old design that I think rates somewhere in the 170 range, and this was actually originally intended to be an IOR 1/4 ton. When the IOR changes came out in 1978 or thereabouts, there was no way it would still rate 1/4 ton, so the stern crease was removed. What made the 241 so fast is that it bears a strong resemblance to Magic Bus IMHO.

I ignored 24 footers since the original post is about a 21 footer, but there are many 24 footers of that era, or earlier that rate in the neighbourhood of 165 (Martin 242 as another example, and it has something that actually resembles an interior).

Forgot to mention the Hotfoot 20, which is a foot shorter and was first built around 1985,

#21 dolphinmaster

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Posted 03 December 2012 - 10:30 AM

Yeah, the 241 is another old design that I think rates somewhere in the 170 range, and this was actually originally intended to be an IOR 1/4 ton. When the IOR changes came out in 1978 or thereabouts, there was no way it would still rate 1/4 ton, so the stern crease was removed. What made the 241 so fast is that it bears a strong resemblance to Magic Bus IMHO.

I ignored 24 footers since the original post is about a 21 footer, but there are many 24 footers of that era, or earlier that rate in the neighbourhood of 165 (Martin 242 as another example, and it has something that actually resembles an interior).

Forgot to mention the Hotfoot 20, which is a foot shorter and was first built around 1985,


How about the Moore 24 that rates 150 from that early era. Like you did with the pocket rocket, what makes it so fast?

oh yea, I just remembered, a viper rates 99 and is shorter so the Moore really isn't fast.

but really, talking about boats is like talking about breasts. They are mostly all beautiful and yes, not all are 36d's.

#22 kmcfast

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Posted 03 December 2012 - 09:14 PM

Ahh nothing like a shit fight in the morning.
One should use boats with similar sailing lenght or LWL. for a proper analysis.
SR 21 has 18' LWL
Pocket Rocket 22' LWL
Martin 241 21' LWL?
Conclusion: The SR has a very low drag setup for 1993. 3.6 rig aspect ratio, high aspect thin foils, very little rocker, high SA/D., hiking ramped 8' beam.
Dolphinmaster is right on again with his observations.

#23 Beau.Vrolyk

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Posted 03 December 2012 - 10:29 PM

Horses for courses:

For the race from San Francisco to Hawaii, it'd be hard to beat a Moore-24. Race from Hawaii to San Francisco, you'd get killed

I used to own an IOD (7000 lb thing that looks like a 6 meter without an overlapping jib) it also rated 150 PHRF, the same as my Moore. Upwind it would destroy the Moore, down wind it would get picked off. On a windward leeward course the two boats were almost exactly the same elapsed time even though they never sailed next each other.

It's really silly to talk about how "fast" a boat is without saying in which direction it's sailing, what the wind speed is, the sea state, etc.... Thankfully, there are horses for courses and even an 8-meter like YUCCA gets to win a "downwind" (the Ditch) race once in 20 years - when the wind blew from the wrong direction at 25k.

BV

#24 12 metre

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Posted 03 December 2012 - 11:32 PM

Ahh nothing like a shit fight in the morning.
One should use boats with similar sailing lenght or LWL. for a proper analysis.
SR 21 has 18' LWL
Pocket Rocket 22' LWL
Martin 241 21' LWL?
Conclusion: The SR has a very low drag setup for 1993. 3.6 rig aspect ratio, high aspect thin foils, very little rocker, high SA/D., hiking ramped 8' beam.
Dolphinmaster is right on again with his observations.


So, it seems to me like you were intending to start a shit fight, since in your original post you ask why the SR 21 is “so fast” even though it’s specs are not that impressive, yet now you go on to list a bunch of specs that leads to your conclusion about what makes the SR 21 “so fast”.

Low drag setup? I’m not really sure what you mean by that, but I’ll play along anyway.

So, lets go through your list:

High aspect thin foils? Most light high performance boat over the past 30 years have high aspect thin foils. What section are they? NACA 4 digit (not low drag) or 6 series (laminar low sections)? How do you know it is thin, from eyeballing? What is t/c ratio?

Very little rocker? It is almost impossible to design a 1300 lb boat with 18 feet of waterline with much rocker. Besides, low rocker is not a low drag shape in light air, but is better in a breeze. Rocker and Cp are usually inversely proportional – low rocker usually results in higher Cp while more rocker tends to result in lower Cp and improved light air performance.

High SA/D? A light boat has to have a high SA/D since they generally have proportionately more wetted surface for their displacement. The 2/3 term in the calculation is intended to compensate for that, but is really only useful in comparing boats of similar size and displacement. Now, if you had the SA/WS ratio, that would be useful.

Hiking ramped 8’ beam? No drag advantage there, but certainly useful upwind in a breeze. FYI the Pocket Rocket has highly flared topsides and 9’6´beam.

Your LWL comparisons? If you want to use that, here they are from the US Sailing web-site:

SR 21 – 18’
Pocket Rocket – 21.5’
Martin 241 – 20’
Martin 242 – 20.25’ (listed as 19.25’ on Sailboatdata.com, so take your pick)
Hotfoot 20 – 18’

As to your comment that “Dolphinmaster is right on again with his observations”, which observations are you referring to other than some rather general and subjective ones like “great foils, great lines, and good balanced hull and rig”,

Don’t get me wrong, I think the SR 21 is a great little boat, but I’m not a cheerleader either

#25 kmcfast

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Posted 04 December 2012 - 02:53 AM

Very little rocker allows a longer keel span and stay within max. draft requirements.
As for the keel & rudder foil sections & thickness/chord, they look like NACA series 6 sections in the pics but hard to tell. Ramped hiking can allow for a narrower waterline beam for similar righting moment?
Your comments are thoughtful and scholarly.
PS I raced on a turboed Mull Pocket Rocket 22 "Wedge" for years.

#26 Christian

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Posted 04 December 2012 - 04:37 AM

Ahh nothing like a shit fight in the morning.
One should use boats with similar sailing lenght or LWL. for a proper analysis.
SR 21 has 18' LWL
Pocket Rocket 22' LWL
Martin 241 21' LWL?
Conclusion: The SR has a very low drag setup for 1993. 3.6 rig aspect ratio, high aspect thin foils, very little rocker, high SA/D., hiking ramped 8' beam.
Dolphinmaster is right on again with his observations.


Why are you hung up on LWL? This is a static waterline length at 0 degrees heel - doesn't mean much in comparison to when the boat is sailing with crew.

The V640 LWL is 19'1" BTW - and it still would beat the SR21 by a country mile. The SR21 may have been an ok boat 20 years ago but it certainly is a bit long in the tooth - in comparison, the Viper came out in 1996/97 (only 3 years later) and is still going strong

#27 Christian

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Posted 04 December 2012 - 04:41 AM

you can also compare it to the Ultimate 20, which is of similar vintage and specs but is also a good bit faster than the SR though not as fast as the Viper or the Shaw

#28 dolphinmaster

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Posted 04 December 2012 - 05:10 AM

you can also compare it to the Ultimate 20, which is of similar vintage and specs but is also a good bit faster than the SR though not as fast as the Viper or the Shaw


To keep it simple for folks like myself, I like to use analogies. I'd call the Ultimate 20 strengths are based on basic sailing HP, the SA/D ratio.

The SR 21 strengths are based on the hydrodynamics of the hull shape. I don't just want to know ratios, I want to look at a hull and like a well proportioned breast, automatically know that boat will be fast.

Some cars get their speed from the HP they have available, some from being slippery in the air, some blend both.

One of my additional questions is .............. where did Glen go? .............. Not physically, appears Hunter design team, but mentally and creatively?

It "appears" mucho design strengths, lost in Gainesville.

#29 kmcfast

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Posted 04 December 2012 - 05:34 AM


Ahh nothing like a shit fight in the morning.
One should use boats with similar sailing lenght or LWL. for a proper analysis.
SR 21 has 18' LWL
Pocket Rocket 22' LWL
Martin 241 21' LWL?
Conclusion: The SR has a very low drag setup for 1993. 3.6 rig aspect ratio, high aspect thin foils, very little rocker, high SA/D., hiking ramped 8' beam.
Dolphinmaster is right on again with his observations.


Why are you hung up on LWL? This is a static waterline length at 0 degrees heel - doesn't mean much in comparison to when the boat is sailing with crew.

The V640 LWL is 19'1" BTW - and it still would beat the SR21 by a country mile. The SR21 may have been an ok boat 20 years ago but it certainly is a bit long in the tooth - in comparison, the Viper came out in 1996/97 (only 3 years later) and is still going strong

Yeah most boats have a longer sailing lenght when heeled that's true.
The Viper 640 #550 lighter, barely self righting, and not a fair comparison.
U 20's have a big asail these other boats don't...

#30 Christian

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Posted 04 December 2012 - 05:51 AM



Ahh nothing like a shit fight in the morning.
One should use boats with similar sailing lenght or LWL. for a proper analysis.
SR 21 has 18' LWL
Pocket Rocket 22' LWL
Martin 241 21' LWL?
Conclusion: The SR has a very low drag setup for 1993. 3.6 rig aspect ratio, high aspect thin foils, very little rocker, high SA/D., hiking ramped 8' beam.
Dolphinmaster is right on again with his observations.


Why are you hung up on LWL? This is a static waterline length at 0 degrees heel - doesn't mean much in comparison to when the boat is sailing with crew.

The V640 LWL is 19'1" BTW - and it still would beat the SR21 by a country mile. The SR21 may have been an ok boat 20 years ago but it certainly is a bit long in the tooth - in comparison, the Viper came out in 1996/97 (only 3 years later) and is still going strong

Yeah most boats have a longer sailing lenght when heeled that's true.
The Viper 640 #550 lighter, not self righting, and not a fair comparison.
U 20's have a big asail these other boats don't...


The Viper is self righting. Sure it is a lot lighter and doesn't have a cabin.
I find it funny that you are talking about the SR21 and are thinking about why it is "so fast" and when other boats of similar size and vintage are brought up (much faster boats mind you) - you start explaining "why" a comparison is not "fair". Off course these other boats are different - as in lighter and more powered up - there is a reason they are much faster. Hense my original post about "fast" being relative............

#31 kmcfast

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Posted 04 December 2012 - 06:52 AM

The Viper is self righting
http://www.norcalsai...t_Alcatraz.html

#32 Beau.Vrolyk

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Posted 04 December 2012 - 07:25 AM

kmcfast,

Great picture! You'll notice the blue boat in the background that is "nominally" slower than a viper. But much faster when the viper is busy self-righting. (That's not even a particularly windy day for SF Bay)

Posted Image

Like I said above, horses for courses. Take a viper to Hawaii - NOT HARDLY. But there are plenty of 20 to 24 foot boats that are quick, stay right side up and seaworthy enough to make it to Hawaii (Like the blue one in the background that won the single handed transpac this year.)


Dolphinmaster,

Look, it takes really excellent race boat designers years to develop an "eye" for what's fast and even they will admit readily that they get it wrong a LOT. The process of yacht design, and especially optimization for performance of a certain type (like upwind or downwind or to a handicap rule) requires lots and lots and lots of math. It is entirely about ratios and optimizations of hundreds of variables. It is not, as I understand it, some romanticized "eye for speed". Like every competitive sport at the highest level - it takes great engineering to deliver great performance. This stuff isn't art it's science. So, when you say you just want to look at a hull and be able to tell how fast its going to be - there's a pretty good chance you'll be disappointed forever. The basics are easy, performance at the highest level is extremely subtle.

BV

#33 12 metre

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Posted 04 December 2012 - 08:59 AM

Okay, I’m willing to give the OP the benefit of the doubt and surmise that the original question was poorly framed. A better way of asking the question that may have given you more constructive answers would have been along the likes of “I’ve noticed that nationwide, the SR 21 rates around 165 – any ideas why it would be rated so fast?

Going on the assumption the SR 21 is indeed a fast boat, here is my list of reasons why or why not.

I’ll rule out the SA/D of 26.7 right off the bat. It may seem high compared to keelboats of more moderate displacement, but it is not that high for a very light boat, for reasons I gave in an earlier post. The Hotfoot 20 (which was the basis for the U-20) has a much higher ratio at 36, but that number is probably a bit distorted since they sail with a non-overlapping jib. Comparing the Hotfoot 20 to its bigger brother, the Hotfoot 27 (which weighed 3600 lb), the HF 27 had a SA/D of only 23.7 but in my opinion was a better light air boat. In fact I would go so far as to state that light air was the Achilles Heel of the HF 20 as they were seldom a threat to the HF 27s in those conditions Next, I’ll trot out that renowned light air flyer known as the CF-27, which tips the scales at a somewhat hefty 4200 lbs and yet has a SA/D of only 22.6

Low Disp/Length ratio. I think the SR 21 has a Disp/Length of 104 which should make it quite fast downwind in a breeze. The HF 20 on the other hand has an even lower D/L of 76, so the HF 20 should be even faster downwind in a breeze (which I suspect it is)

Now, since the PHRF ratings of the SR 21 and HF 20 are similar, I would contend that the HF 20 is likely faster in a breeze which means to me that the SR 21 is likely faster in light air.

Now I’ll get round to why I suspect it may good in light air, if that is indeed the case. I’ve pretty much ruled out the SA/D already. This leads me to suspect it has something to do with the amount of topside flair, or more specifically, the attendant narrow waterline beam. This would reduce wetted surface significantly, so it may well have a very good SA/WS ratio. Another benefit to reduced waterline beam is that it tends to reduce waterplane area as well, which at speed generally leads to the generation of smaller hull waves, and in turn lower resistance. A Further benefit to reduced waterline beam is that the hull tends to become less asymmetrical when heeled, which tends to result in less drag.

The only real downside to reduced waterline beam is that form stability is reduced so heavy air performance upwind and reaching tends to suffer. This can be countered by slinging a bulb on the keel. I would also contend that a bulb really works best with narrow boats and are much less useful on beamy boats, which have to heel so much (inducing asymmetry) before the bulb begins to really work.

Anyways, those are my thoughts, make of them what you will.

#34 kmcfast

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Posted 04 December 2012 - 03:00 PM

Okay, I’m willing to give the OP the benefit of the doubt and surmise that the original question was poorly framed. A better way of asking the question that may have given you more constructive answers would have been along the likes of “I’ve noticed that nationwide, the SR 21 rates around 165 – any ideas why it would be rated so fast?

Going on the assumption the SR 21 is indeed a fast boat, here is my list of reasons why or why not.

I’ll rule out the SA/D of 26.7 right off the bat. It may seem high compared to keelboats of more moderate displacement, but it is not that high for a very light boat, for reasons I gave in an earlier post. The Hotfoot 20 (which was the basis for the U-20) has a much higher ratio at 36, but that number is probably a bit distorted since they sail with a non-overlapping jib. Comparing the Hotfoot 20 to its bigger brother, the Hotfoot 27 (which weighed 3600 lb), the HF 27 had a SA/D of only 23.7 but in my opinion was a better light air boat. In fact I would go so far as to state that light air was the Achilles Heel of the HF 20 as they were seldom a threat to the HF 27s in those conditions Next, I’ll trot out that renowned light air flyer known as the CF-27, which tips the scales at a somewhat hefty 4200 lbs and yet has a SA/D of only 22.6

Low Disp/Length ratio. I think the SR 21 has a Disp/Length of 104 which should make it quite fast downwind in a breeze. The HF 20 on the other hand has an even lower D/L of 76, so the HF 20 should be even faster downwind in a breeze (which I suspect it is)

Now, since the PHRF ratings of the SR 21 and HF 20 are similar, I would contend that the HF 20 is likely faster in a breeze which means to me that the SR 21 is likely faster in light air.

Now I’ll get round to why I suspect it may good in light air, if that is indeed the case. I’ve pretty much ruled out the SA/D already. This leads me to suspect it has something to do with the amount of topside flair, or more specifically, the attendant narrow waterline beam. This would reduce wetted surface significantly, so it may well have a very good SA/WS ratio. Another benefit to reduced waterline beam is that it tends to reduce waterplane area as well, which at speed generally leads to the generation of smaller hull waves, and in turn lower resistance. A Further benefit to reduced waterline beam is that the hull tends to become less asymmetrical when heeled, which tends to result in less drag.

The only real downside to reduced waterline beam is that form stability is reduced so heavy air performance upwind and reaching tends to suffer. This can be countered by slinging a bulb on the keel. I would also contend that a bulb really works best with narrow boats and are much less useful on beamy boats, which have to heel so much (inducing asymmetry) before the bulb begins to really work.

Anyways, those are my thoughts, make of them what you will.


So 12 metre, have you ever thought of a career in Yacht design?
I think you nailed it on the WL beam. thx

#35 kmcfast

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Posted 04 December 2012 - 03:07 PM

Hey I wonder if any of that Viper crew got a gig on the Oracle AC 72?




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