Lester:
First let me thank your for your website. It has been a great help as I have explored the hobby. It is always good to read a engineer’s take (and the mathematics) on items that are usually “seat of the pants”.
RE: Leeway
Perhaps I should have been more specific. This boat will be raced in light and gusty conditions - the venue is a small pond surround by trees where not much wind gets down to boat level, but when it does it comes from every direction, including occasionally straight down! A friend and I have beat the length of the pond, he was on port and I was on starboard, and we were two boat lengths apart! We also occasionally race in “scientific drifting” conditions (0 - to 2 mph…3.3 Kph) My goal is to have a boat design (hull, foils and sails) that is biased to these conditions rather than the normal mid to top of A conditions.
I was using leeway in terms of more distance lost to leeward per unit of wind speed than in terms of absolute distance. My expectation is that when the boat is at very low speeds, the velocity of the gusts will be a larger percentage of the boats speed than if it were in mid-A range or higher conditions. With the increase of force on the sails, the boat will be out of balance and the fin stalling while it accelerates to the new higher boat speed. During the interim the boat will be unbalanced with the pressure on the sails exceeding the lift produced by the fin and therefore heeling dramatically, loosing distance to leeward. Since the difference in boat speed will be larger than if the boat were already moving rapidly the time to accelerate to the new speed will also be longer.
In just low wind speeds without gusts, wetted surface would be the critical factor.
I had always read that very fast boats tend to have thin high-profile foils, operating at low angels of attack, so I reversed the logic and concluded, perhaps erroneously, that a slow moving boat should have fin geared to a wider range of AOA and therefore should be thicker so it is less prone to stall. Since a thicker foil will have more form drag, I wondered whether it was better to stay with the traditional ~6% t/c or something else. The US One Meter class is very wide open regarding the foil parameters so I have lots of options, other than the maximum depth of the fin and bulb are limited to 14.25 inches (362 mm).
I was also further confused after speaking to Graham Bantock about which of his rudder profiles to use on a different project and he said that for the same surface area, rudders that where shorter and had a longer chord have less drag.
Perhaps you can bring an engineer’s perspective to this discussion.