Quote:
Originally Posted by GordMay  Wouldn’t you typically expect the lighter boat to have a higher “empty” SA/D ratio than the heavier boat, of otherwise comparable features? |
Bingo. Precisely correct. Most light boats have, in fact, higher SA/D ratios. Which is why they are faster--not because of the lighter weight. What we have here are confounding effects and the misattribution of causes.
But there is no law of physics that says heavy boat
must have lower SA/D ratios. It is harder to design a decent SA/D ratio for a heavy boat, for sure. Most times it's draft that is sacrificed. You need deep draft on a heavy boat to have the stability to stand up to a higher SA/D.
I used the above example to illustrate what happens to speed when SA/D is equalized. I think it's an eye-opener for some people, but I readily acknowledge that it does not represent what is typical. I do maintain, however, it is possible. My steel 36' has a SA/D of nearly 17; OTOH, it's draft is a tad over 6'. Thus, when I sail against other boats of similar size, I fair OK, unless the other boat has a much higher SA (which typically is a lightweight boat to boot). I am faster than plastic boats if they have lower SA/Ds; no surprise there.
I wish I had more draft, because my keel-to-total-draft ratio is less than lighter boats so I may not point as well, depending on the other boat. Obviously my hull, being heavier, sponges up more of my total draft--so for any given draft I have more hull and less keel than my lighter brothers.
The worst combination from a pure speed perspective, is shallow draft and heavy boat. That design will insure a low SA/D. So, when you combine the low horsepower from a low SA/D with the inability to point on account of the stubby keel, you end up with a slow boat. That model, however, describes many a steel sailboat, unfortunately.
To be sure, I don't like most steel sailboats. I don't think they have been designed well. They are undercanvassed. As a consequence they do poorly on speed metrics.