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Old 26-11-2021, 21:19   #1
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SA/Disp ratio myth or useful?

I haven't been able to find this thread anywhere here, if it already exists I apologize for asking the same question again;


I have read reviews of many fine looking boats, trolled more than a few forums and haven't come to any solid conclusions about the SA/Disp ratio.


Is it a valid measure of boat speed or is it like the salesman's "# of berths a boat has" only valid in a brochure type of information?


Thanks in advance.


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Old 26-11-2021, 23:14   #2
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Re: SA/Disp ratio myth or useful?

Sail Area/Displacement is just a simple ratio. As a predictor of displacement type boat speed, it is obviously only one factor and can be useful in the total context.

Given a fixed displacement, in general, more sail area is faster until the boat reaches hull speed and/or you need to reef.

Given a fixed sail area, a lighter displacement boat will be faster (in moderate conditions). In heavy conditions, a moderately heavier boat could be faster (and a lot more comfortable) than an ultra-light boat.

Of course hull shape enters into the equation too as boats of similar displacement can have very different hull shapes.

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Old 27-11-2021, 05:05   #3
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Re: SA/Disp ratio myth or useful?

The SA/D is analogous to an automobile’s horsepower-to-weight ratio, with sail area being a measure of power, and displacement being indicative of wavemaking drag.

SA/D is most indicative of performance, in moderate to heavy-air conditions.

In general terms, a boat having a sail area-displacement ratio under 15 would be considered under-canvased; values above 15 would indicate reasonably good performance; and anything above 18 to 20 suggests relatively high performance, provided the boat has sufficient stability and a low enough displacement-length ratio to take advantage of its abundant sail area.

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Old 27-11-2021, 05:19   #4
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Re: SA/Disp ratio myth or useful?

Sometimes looking at extreme examples helps.

The Hobie 16 has a SA/Disp of almost 75 and can hit speeds close to 26 knots.

Bristol 27 SA/Disp of 14 and with speed a little above 6 knots with PHRF of 240

Olson 30 (racing boat) SA/Disp about 26 with PHRF of 108 and speeds at times exceeding 10 knots
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Old 27-11-2021, 05:30   #5
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Re: SA/Disp ratio myth or useful?

Thanks everyone for chiming in.

I am familiar with the explanations regarding SA/Disp. and it's description of boat performance;

But a Westsail 32 has a ratio of 16.69 which would belie her nickname of Wet Snail, while other boats have a much lower ratio without any negative connotation.

Which is why I was asking is this ratio something for the brochures, not for the water.
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Old 27-11-2021, 05:45   #6
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Re: SA/Disp ratio myth or useful?

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Originally Posted by rexposeidon View Post
Which is why I was asking is this ratio something for the brochures, not for the water.
If you look at both large samples of passage times, and at the rating rules, it is pretty clear that:

(1) Waterline is the first driver of displacement monohull speed
(2) Sa/Dsp is the second-ranked driver and is meaningful
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Old 27-11-2021, 06:11   #7
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Re: SA/Disp ratio myth or useful?

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Originally Posted by rexposeidon View Post
Thanks everyone for chiming in.

I am familiar with the explanations regarding SA/Disp. and it's description of boat performance;

But a Westsail 32 has a ratio of 16.69 which would belie her nickname of Wet Snail, while other boats have a much lower ratio without any negative connotation.

Which is why I was asking is this ratio something for the brochures, not for the water.
Sometimes that's due to folks sailing them in light winds with no racing experience. It's a heavy boat and needs a breeze to get on it's feet.

A Westsail 32 is capable of hitting speeds of 7 knots in stronger wind conditions.
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Old 27-11-2021, 06:12   #8
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Re: SA/Disp ratio myth or useful?

Quote:
Originally Posted by rexposeidon View Post
Thanks everyone for chiming in.

I am familiar with the explanations regarding SA/Disp. and it's description of boat performance;

But a Westsail 32 has a ratio of 16.69 which would belie her nickname of Wet Snail, while other boats have a much lower ratio without any negative connotation.

Which is why I was asking is this ratio something for the brochures, not for the water.

Be careful when making comparisons of SA/D that the values were calculated the same way. Often, but not always, the "sail area" part is total area of the IJ and PE triangles not the actual sail area, which can be larger due to overlapping headsails and roachy mains. A Westsail 32 has a SA/D of 15.6 (according to sailboatdata.com) when using the IJ + PE method.


Either way, an SA/D value below 17 puts a boat in the "slow" category by my reckoning. In my mind that doesn't mean it moves slowly through the water, rather that it needs more wind to "get going". A boat's actual speed through the water is more related to length of waterline.
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Old 27-11-2021, 10:44   #9
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Re: SA/Disp ratio myth or useful?

So in an apples to apples comparison 2 boats similar waterline, displacement etc. The smaller the ratio the slower the boat.

But what if 2 boats, same waterline, one with a displacement of 12.5k, and a. SA/Disp. ratio of 14.5 the other a displacement of 19.5k, and a ratio of 15.5.

Which one is faster? Obviously a lighter boat would move faster than a heavier boat but these ratios are very close.

Is the ratio relevant in this case or is it now apples to oranges?
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Old 27-11-2021, 10:59   #10
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Re: SA/Disp ratio myth or useful?

Quote:
Originally Posted by rexposeidon View Post
So in an apples to apples comparison 2 boats similar waterline, displacement etc. The smaller the ratio the slower the boat.

But what if 2 boats, same waterline, one with a displacement of 12.5k, and a. SA/Disp. ratio of 14.5 the other a displacement of 19.5k, and a ratio of 15.5.

Which one is faster? Obviously a lighter boat would move faster than a heavier boat but these ratios are very close.

Is the ratio relevant in this case or is it now apples to oranges?

In light air, the heavier boat (with higher SA/D) should move a little faster (assuming wetted area hasn't increased disproportionately to weight and sail area). In heavy air, both will be limited by waterline length. However, the light boat will be more responsive and accelerate faster in a gust or out of a tack.
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Old 27-11-2021, 11:16   #11
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Re: SA/Disp ratio myth or useful?

If you want a better understanding of boat speed, buy a copy of Gerr's "The Nature of Boats". Very simple to read, clear, easy to understand. Gerr is the director of the Westlawn Insititute of Yacht Design and has his own design firm. He understands the subject.

Rslfkin's answer above is accurate.
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Old 27-11-2021, 11:21   #12
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Re: SA/Disp ratio myth or useful?

Quote:
Originally Posted by GordMay View Post
The SA/D is analogous to an automobile’s horsepower-to-weight ratio, with sail area being a measure of power, and displacement being indicative of wavemaking drag.

SA/D is most indicative of performance, in moderate to heavy-air conditions.
My thinking is that SA/Disp is more indicative of performance in LIGHT AIR conditions.

The logic is that the horsepower applied to the boat is a combination of sail area and wind strength.

In moderate to heavy air you get plenty of horspower. In fact reduction of sail area (reefing) is often appropriate.

In light air however you can often use as much sail area as possible to extract what power there is in the lower wind speed.

Performance in moderate to heavy air is more a function of hull shape. When there is plenty of power available it is the hull characteristics which determine how fast you will go.
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Old 27-11-2021, 11:43   #13
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Re: SA/Disp ratio myth or useful?

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Originally Posted by wingssail View Post
My thinking is that SA/Disp is more indicative of performance in LIGHT AIR conditions.

The logic is that the horsepower applied to the boat is a combination of sail area and wind strength.

In moderate to heavy air you get plenty of horspower. In fact reduction of sail area (reefing) is often appropriate.

In light air however you can often use as much sail area as possible to extract what power there is in the lower wind speed.

Performance in moderate to heavy air is more a function of hull shape. When there is plenty of power available it is the hull characteristics which determine how fast you will go.

Agreed. In heavy air, SA/D is only an indicator of how soon you'll need to start reefing. Or for a modern, flat hull, how much wind it'll take to get enough drive to start planing.
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Old 28-11-2021, 07:28   #14
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Re: SA/Disp ratio myth or useful?

Use it as a possible indicator to limit your searches. Obviously a motor sailor will have a very low ratio and a ULDB will have a high number. Real world is that unless the boat is used purely for racing, owners tend to fill up their boats with "essentials" and the actual number gets meaningless. Parts and tool redundancy aboard will factor in heavily. Bottom growth and blown out sails can be very heavy contributors to a boat slowing down over time. A well sailed "slow boat" can frequently out perform a poorly sailed "fast boat". Being comfortable and not exhausted on a long passage means better decision making.
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Old 28-11-2021, 10:27   #15
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Re: SA/Disp ratio myth or useful?

Quote:
Originally Posted by rexposeidon View Post
I haven't been able to find this thread anywhere here, if it already exists I apologize for asking the same question again;

I have read reviews of many fine looking boats, trolled more than a few forums and haven't come to any solid conclusions about the SA/Disp ratio.

Is it a valid measure of boat speed or is it like the salesman's "# of berths a boat has" only valid in a brochure type of information?

Thanks in advance. Rich
SA/Disp is just one of a series of ratios that when compared between boats gives some indication of performance tendencies...all together they begins to tell you what kind of boat you are buying. Do you want a fast boat or do you want one that is more comfortable offshore (COMFORT ratio) with perhaps less of a chance of capsizing (CAPSIZE ratio)?

However, I always caution using the SA/Disp ratio given by manufacturers because they often inflate the SA/Disp ratio by including a genoa into the equation rather than the 100% forward sail area; if in doubt compute it yourself using the boat's I, J, P, E numbers. If comparing a cutter rig the total forward sail area is the number to use, not the total area of both forward sails.

I have also noticed that sailboats that are offered with various keel options don't list the different total displacements despite the fact that the keels can vary considerably.

As previously mentioned you can go to saildata.com to get sailboat specifications and some of the ratios or perhaps Sail Calculator Pro for a direct boat to boat comparison for many of the ratios.

Knowledge is power.

Good Luck.

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