Just how important is this ratio to bost speed and performance. I've noticed that most Beneteuas and Catalinas in the 40 foot range have rather unimpressive sa/d ratios of barely 16. All I've read is that 18 is most desirable for decent performance in a crusier/racer. This is what Tartan and Sabre offer. Any thoughts?
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Pretty much the same as Horsepower-to-Weight ratio in automobiles Bigger numbers are quicker.. Sail area is horsepower and displacement is weight.. The ultimate speed may be the about the same for similar waterline length but the amount of time to get to that speed (acceleration) may be very different, making the average speed very different when dealing with real world things like waves and puffy/gusty winds, which want to consume horsepower to re-accelerate the boat.. Lots more to it than just that and comparisons within about 5% are kind of moot.. but like in cars, the higher that ratio, generally the quicker the boat will be .. The ballast to displacement ratio is also important and gives a hint of how tender a boat might be, or whether it can carry all the sail that its SA/D ratio is based on..
Go to http://www.image-ination.com/sailcalc.html then go to Part 1 for direct calculations on a boat by boat basis.
Kloudie has given a good description ... but there are other parameters coupled to SA/D which indicate a boats 'tenderness', seakindliness, .... as well as the important 'acceleration'.
Kloudie has given a good description ... but there are other parameters coupled to SA/D which indicate a boats 'tenderness', seakindliness, .... as well as the important 'acceleration'.
Not like cars at all as the hull shape of a boat will skew this number a lot more than the body shape of a car. Bigger numbers only imply quicker.
You could build a square box that weighs 10,000lb. with 1,000 sq. ft. of sail area and I guarantee that the SA/Disp ratio will not accurately tell you how fast it is.
This number is a bad way to guess boat speed. PHRF, which can be very misleading, is more accurate because it is based on real world results.
You could build a square box that weighs 10,000lb. with 1,000 sq. ft. of sail area and I guarantee that the SA/Disp ratio will not accurately tell you how fast it is.
This number is a bad way to guess boat speed. PHRF, which can be very misleading, is more accurate because it is based on real world results.
SA/Disp should only be used as a 'relative' gauge. It really matters more when the winds are lighter too. A high SA/Disp does no good if you have to reef in winds too soon. You can have a very high SA/Disp and have to reduce sail in 10 kts of wind if its overdone. Another thing to note is that it depends on how the SA(Sail Area) is calculated - most times SA is calculated using only 100 foresail area, and many of the masthead rigged boats like a Catalina really very rarely go out sailing with 100 Jib and most are rigged with 135 or 150 % genny's.
The boats with high SA/Disp many times are fractional rigged with blade jibs and large mainsails (think Catamarans) so everybit of sail area is used in the calculation
So its
The boats with high SA/Disp many times are fractional rigged with blade jibs and large mainsails (think Catamarans) so everybit of sail area is used in the calculation
So its
Which brings up a question:
Which can carry more sail before unwanted weather helm and rudder action need applied: a) modern flat hull with 1/3rd weight in keel b) traditional deeper hull with near 1/2 weight in keel?
Considering that a keel's weight doesn't take effect until the boat starts to heel and that's when weather helm starts, wouldn't it make sense that a boat that has intial stability via the shape of the hull can carry more sail?
Which can carry more sail before unwanted weather helm and rudder action need applied: a) modern flat hull with 1/3rd weight in keel b) traditional deeper hull with near 1/2 weight in keel?
Considering that a keel's weight doesn't take effect until the boat starts to heel and that's when weather helm starts, wouldn't it make sense that a boat that has intial stability via the shape of the hull can carry more sail?
Too many other variables for that to be an answerable question. One important factor that is often overlooked that can be answered in that comparison is the rate at which steering characteristics change with heel. A steady and predictable increase in weather helm without any sudden changes in steering characteristics is a very nice feature and more likely to be found in the deeper traditional hull.
My E32 is a very good example. Helm pressure increases steadily as the boat heels but, even with the rail under water, it never reaches a point at which the rudder suddenly loses effectiveness and the boat starts to round up suddenly despite the helmsman's efforts. This is very nice when passing buoys closely in gusty conditions. Watching the knotlog, she doesn't slow down as heel increases either although leeway increases I'm sure. If she is going 6 knots at 15 degrees, she'll be going 6 knots at 25 degrees. I ease her up just to make a little to windward. Helm force may get very strong but, if you hold her to it, she'll go where you need her to go, also important when a lobster pot shows up and you're single handed and need to go downwind of it and there isn't time to ease sheets.
The wider, flatter boat, more typical of modern practice, will slow down and suddenly try to go her own way when pushed as far over as a boat like mine. Steering Strider to windward in puffy conditions can be work but it's a joy because of the feeling of predictable solid control. This does require keeping the speed up. If you get shy and let the speed decay or get hit by a gust when slow after a tack, she may round up on you but it's only happened a few times to me in four seasons.
I always had a preference for wide, powerful boats having grown up sailing catboats but this boat has really changed my thinking.
SA/D considerations
As stated above SA/D is a rough guide. In light air areas SA to wetted surface is an important consideration. Where the ballast is, has a lot to do with righting moment. Obviously the lower the ballast is the more effective it is. There are a lot of old designs with 40% ballast to displacement ratios, short keels and a lot of topside weight. They need a low SA/D ratio just to keep the boat upright when the wind picks up. Reaching machines they are, but just don't expect them to go to weather very well. Give me a slippery hull shape, efficient foils, and ballast carried low any day.
As stated above SA/D is a rough guide. In light air areas SA to wetted surface is an important consideration. Where the ballast is, has a lot to do with righting moment. Obviously the lower the ballast is the more effective it is. There are a lot of old designs with 40% ballast to displacement ratios, short keels and a lot of topside weight. They need a low SA/D ratio just to keep the boat upright when the wind picks up. Reaching machines they are, but just don't expect them to go to weather very well. Give me a slippery hull shape, efficient foils, and ballast carried low any day.
Fast
Just look at the real racing boats to see what makes a boat fast. They have very long foils of minimum weight attached to a massive bulb at the bottom about 12 feet down. Picture the Whitbread Boats or the BOC boats. So the ideal is minimum weight in the hull and maximum righting moment to keep the boat flat in the water to allow more sail area.
So sail to displacement and even ballast to displacement don't do the trick for me. How the ballast is carried has a much greater effect on performance than just how much ballast. Of course the racing boats don't care about creature comforts either. The boat is just an inconvenient thing to have in the way of the keel and the mast.
Just look at the real racing boats to see what makes a boat fast. They have very long foils of minimum weight attached to a massive bulb at the bottom about 12 feet down. Picture the Whitbread Boats or the BOC boats. So the ideal is minimum weight in the hull and maximum righting moment to keep the boat flat in the water to allow more sail area.
So sail to displacement and even ballast to displacement don't do the trick for me. How the ballast is carried has a much greater effect on performance than just how much ballast. Of course the racing boats don't care about creature comforts either. The boat is just an inconvenient thing to have in the way of the keel and the mast.
SA/Displacement is a good indicator of speed when comparing simular boats. Obviously hull speed is also a factor. But given two boats with the same waterline length and about the same weight the one with more sail area is probably faster. But when you are comparing a flatter hull light displacement boat to a deeper heavier narrower boat the comparison is more complex. Also wind strength and sea state come into play. Point of sail is also a huge factor. A boat can easily be faster under some conditions and slower under different circumstances. Even using handicapping numbers depends upon where the boats are racing.
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