A little baffled by sailboat performance numbers...

[rgranger]

So I've been spending my quarantined time diving down various rabbit holes and today's obsession is sailboat performance numbers. A while back I created this cheat sheet for my own use...Most of these numbers I got from the "definitions" link on Sailboatdata.com

I am not sure how useful the S# is. On many of the cruising boats I've looked over, the S# is below 2... but many of the cruising boats have very respectable Sail Area/Displacement numbers. Also, very few boats have an S# much above 2.

Is this rating system biased towards racing and I should rethink how to interpret the S#? Do you have to be a serious racer to even worry about S#'s

For example, here is the cape dory 33

The S.A./Disp says it it is reasonably canvased but the S# would have me believing this thing is a turtle.

Here is the pearson 33... similar story.

Here is the Ericson 33

By sailboatdata.com's rankings we are now getting into the "cruising" class boat which are (in my mind) on the slower side of respectable performance. If you keep digging you see that S.A./Disp and S# do not always track ... for example...

Here is Endeavor 33

Here is Hunter 33 with almost the same S# but 0.62 lower S.A./Disp.

.

Even when I put in specs for a Beneteau 325 I still only get into the low 3s for an S# and I have always thought of the Bene's as being on the racing side of the cruising spectrum... but by sailboatdata.com's ranking this is right over the line between cruiser and racer/cruiser

Is the S# worth looking at? Or is S.A./Disp a better indication of performance.

I'd love to hear @Jackdaw 's take on this.

r

 

[shemandr]

Man, you are bored! I was watching Spanish lessons on TV yesterday. Como esta usted?
This has the potential to be a good thread. Unfortunately, I'm not familiar with the S# (But I'll read up on it). But your reading of the Cape Doggie is correct. The SA/displacement of only 15.62 is low. And that whopping displace to length ratio of 403 denotes obesity. So under canvassed, wide and heavy - not fast. It's a great cruising boat however. Water tanks, fuel tanks, stowage galore.
The Ericson 33 is a lot more like a '70's racer/cruiser with a greater SA/Displacement of almost 20 and a Disp/Length of 227. In PHRF club racing a reasonable performer. We won't talk about the keel which isn't a good foil and doesn't get ballast down low. My dockmate had one and he would roll out that jib and reach to the South Fork, turn around and reach back. He was all smiles and he's in his 80's. Great boat for him.
Let's see what others have to say...

 

[dlochner]

And that whopping displace to length ratio of 403 denotes obesity. So under canvassed, wide and heavy - not fast.

One thing to consider beyond just the numbers is the hull design. A Cape Dory tends to have shorter waterline length for compared to other boats of a similar LOA. It also has a narrower beam than most boats its size. These factors also contribute to performance.

 

[BarryL]

Hey,

IMHO the best guide for 'relative' performance is the PHRF number.

The CD 33 rates around 180, which is slow.
The Pearson 33 is at 177 (For the oldest 33 model). Still slow, but better.
The Ericcson rates 126, which is fast.

SA/D is pretty good but you are off with your classifications. Really, anything under 16 is under canvassed. 16-20 is reasonable. Race boats are way higher than 20. For example, my C&C 110, which no one would call a true race boat, has an SA/D of 23.

Barry

 

[PaulK]

Barry has the right idea. None of the six boats you give as examples are racy. I sail a J/36, which shows S# 3.78 on Sailboatdata, and has a SA/D of 22.7 (similar to Barry’s C&C 110). We rate 87 PHRF with a R/F, and are considered to be an old warhorse by the racers at my club who go at it in J/109’s, J/120’s, J/133’s, FT10’s and SC37’s that hoist asymmetricals on their sprits.

 

[shemandr]

Yeah, you run into the sprit revolution. That's why there are so many "Discussions" about this. Our PHRF rating committee stepped into this a decade or so ago and it didn't go well. They tried to separate racing vs, crusier/racers, and maybe a few cruisers with various designations in PHRF. It mostly didn't work. Turns out a Hobie 33 doesn't want to be a racer when they are cleaning up in the cruising fleet. So the system pretty died from lack of vision. I've posted this before, but the venerable John Lockwood once said, "If the same three boats win every race than eventually they will be the only three racing." A lot of wisdom there.

 

[Jackdaw]

I'll chip in later, HH now!

 

[DArcy]

If you are talking about speed, look at PHRF ratings. PHRF is subjective and varies from one location to the next for identical boats but in general is a good indication of how fast a boat can sail. SA/D, Disp/L are OK for general comparison of similar boats but won't tell you much between the speed of a 20 footer and a 40 footer. Water line takes over so a relatively slow 40 footer will easily outpace a relatively fast 20 footer. Now a 21 foot Viper 640 will leave an Island Packet 40 in it's wake - but that's not a fair comparison.
Your definition of SA/D > 18 being a high performance racer is a little optimistic. I'd push that to 21 or maybe even higher. Even the Island Packet 40 has an SA/D of 18 - The Viper SA/D is 44
There are lots of PHRF lists around but here is one

PHRF New England - Handicapping - Base Handicaps

PHRF New England is an independent handicapping authority whose handicaps are used by fleets sailing on Buzzards Bay, Massachusetts Bay, the Gulf of Maine, and Lake Winnipesauke.

www.phrfne.org

Oh, and Vipers make their own wind

 

[PaulK]

Oh, and Vipers make their own wind

Are you sure he’s not sculling? Sailed with an Olympic coach at Hyères once who was really good at it.

 

[DArcy]

No, they were sailing. It is amazing how that boat moves in light air. And with enough wind and crew weight they can plane up wind.

 

[Benny17441]

Do not forget that "ratios" are mostly good for comparing the performance of similar boats to each other. When trying to compare widely different size boats the ratios usually fall short. For example an h27 Cherubini has a Capsize Screening number of under 2 deeming it on the safe side and if you compare it to some of the 30' 32' or 34' boats which may have a Capsize ratio of over 2 it would be wrong to draw the conclusion that the 27' boat would be the best choice to take offshore. Ratios also do not disclose the quality and reliability of individual boats to maintain a ratio over time.

 

[dlochner]

Most issues of Good Old Magazine includes a review of an older boat and an analysis by Rob Mazza in which he compares the reviewed boat to 2 or 3 other boats of similar size and purpose. In the March/April issue the Gulfstar 39 was the reviewed boat and it was compared to a Mariner 39, and a Pacific Seacraft PH 40.

 

[TomY]

I pay attention to SA/D numbers and PHRF ratings for boat comparisons.

My last boat had a SA/D of less than 15. My present boat, older design, has a SA/D of nearly 18. There is little comparison when it comes to light air sailing in the higher SA/D boat. That comes alive in 5 knots of wid whereas the lower SA/D boat would only move if flying a full spinnaker in those conditions.

I find PHRF useful not so much for comparing speed on a triangular race course (which is where the PHRF # is intended), but to compare two boats in general coastal sailing. What is their range? On the average, we sail a few hours between anchorages. We've sailed with friends in faster boats, one a J 35 (a minute a mile faster), and slower boats (a minute a mile slower).

In real use, our speed difference - while huge in a race - doesn't amount to much change in range or speed in actual coastal sailing.

In an overnight, you could see a more significant time savings.

I look more for a boat that can sail those coastal miles and forget who gets there first, by a few minutes. In a race, seconds per mile are everything hence, PHRF ratings of well known boats are useful for boat comparisons.

 

[Hayden Watson]

You can get high S#'s in production cruisers. C425 has S# = 4.19
I have raced on a J90 which is full-on racer listed with a S# = 7.24 and SA/Disp = 32.35 but it was highly modified incluiding a new 5' taller rig so the SA/Disp ware really about 46. Lots of fun!

 

[DrJudyB]

Can someone please tell me the formula for S#? Thanks.

 

[rgranger]

From Sailboatdata.com

S# first appeared (that we know of) in TellTales, April 1988, “On a Scale of One to Ten” by A.P. Brooks . The equation incorporates SA/Disp (100% fore triangle) and Disp/length ratios to create a guide to probable boat performance vs. other boats of comparable size. For boats of the same length, generally the higher the S#, the lower the PHRF.
Under 2 - Slow, under powered.
2-3 - Cruiser
3-5 - Racer Cruiser
5+ - Fast/Racing

 

[Jumpstart]

The numbers quoted in brochures, sailboat data, and even the US Sailing rig database are way off. As a former IRC measurer I physically weighed several boats. Boats had to stripped of anchors, rode, sails and anything that wasn't bolted down. Fuel, water, and holding tanks were empty or weight of contents deducted. Bottom line is that advertised weights of 10-15,000 lb boats were off by 500-2000 lbs. It seems design weights and brochure weights are 2 different things. Real world weights are another thing altogether. SA/D and ballast to displacement, capsize ratios are all influenced by this and are not that accurate. Only boat that I ever weighed that was lighter than listed weight was Schock 35 and that was only 200# on a 10,000 lb boat.

 

[DrJudyB]

i found the definition of S#.

S# = 3.972 x 10^[-DLR/526 + 0.691 x (log(SAD)-1)^0.8]
DLR in the formula is Displacement-Length Ratio and SAD is Sail Area/Displacement Ratio.

That is one flippin’ kluge of a formula. I understand DLR and SAD and how they relate to physical attributes, but S# is absurdly distanced from physical attributes.

 

[dlochner]

i found the definition of S#.

S# = 3.972 x 10^[-DLR/526 + 0.691 x (log(SAD)-1)^0.8]
DLR in the formula is Displacement-Length Ratio and SAD is Sail Area/Displacement Ratio.

That is one flippin’ kluge of a formula. I understand all the other ratios and how they relate to physical attributes, but is absurdly distanced from physical attributes.

It is an absurd attempt to condense everything about a boat's performance into one number. As much as we would like to have a single number that accurately describe's performance, whether it is boat, a car, or a human. There are just too many factors that affect performance in different situations.

Even PHRF numbers, which purport to be a measure of boat speed relative to other boats, is fraught with faulty assumptions and biased ratings.

Don't get me started.

 

[DArcy]

i found the definition of S#.

S# = 3.972 x 10^[-DLR/526 + 0.691 x (log(SAD)-1)^0.8]
DLR in the formula is Displacement-Length Ratio and SAD is Sail Area/Displacement Ratio.

That is one flippin’ kluge of a formula. I understand DLR and SAD and how they relate to physical attributes, but S# is absurdly distanced from physical attributes.

And it is really a guesstimate of how a boat will compare to boats with similar waterline length. It doesn't predict the actual speed potential of a boat.