That formula doesn't really apply to modern sailboat design.
I don't mean to sound abrupt or come across as a know it all. When I read Jackdaw's reply, I was standing at the register at Lowes waiting for them to track down an item number. The bold test was a hast cut and paste job from Google, not some expression of feeling, if that was even considered by anyone. Jackdaw seems like someone who gives it straight and likes it given straight in return.
My point was that the term hull speed had a very specific meaning and no appendage or change of shape will change that. The number or ratio expressed in the formula actually misses the whole point of the concept because it sanitised it to math without context. Hull speed is still very much important in modern yacht design because of what it really represented, an expression of fluid dynamics and the application of Bernoulli equation to sailing. I'm sure most of you know this but I'll type it out loud for posterity.
As a body begins to move through the water it displaces and carries a volume of water forward with it. Fluids being what they are, try to return to their closest original state so, after plowing up on the bow it RACES to get back to where it was before. The racing water falls down the bow wave. The water moves against the boat faster in the resulting trough until it slows back down as the pressure caused by the bow's movement equalises to normal this causes the second peak in a wave and a wake is the result. When a boat is moving slowly, there are multiple waves that form along the waterline of the hull. The faster the boat moves, the more water is carried forward and displaced by the boat. The water in front is forced to change its inertia by the hull moving into it and then again by the hull moving out of it, the stern leaves a hole and the water races to fill it. Another way to see it is a movement from high to low pressure systems.
When the vessel increases speed the waves of the wake lengthen because more water is being moved aside faster and it takes more distance for the return to normal. Hull speed, conceptually, results when the length of a vessel's wake equals the length of her waterline. It is not nor was it ever a limit. It just means, after that point, any increase in force to increase speed has to also lift the boat upwards onto the bow wave. If there is enough force to drive the boat up that wave you get a planning boat.
The shape of a hull and the trim as well as amount of wetted surface are factors in determining the ease with which these various states are achieved.
When I was a boy, my father taught me all this and said it's all about wetted surface. A round bottom had less wetted surface than any other shape. About my high school years I heard that a sailboard held the world's speed record for mono-hull under sail. The speed was amazingly fast compared to the 3-9 knots I was use to. This and my college physics classes lead me to believe that flat bottoms with no rocker would actually be better because even though there was theoretically more wetted surface to cause drag, they could actually be lighter and a lot shallower meaning the distance the displaced water had to move to get out of the way was a lot less. That equates to less force, a shallower wake, less incline to climb. Wide flat hulls would be inherently more more stable so less weight in the keel translating to less displacement. This all means an easier boat to push through the water. Couple that with a natural stiffness from both wide and flat hulls and you get more power to push an easier to push hull.
As I've said, I'm not a racer, you all will know much better than I will but, I understand that a lot of races occur when the wind is less than optimal for a planing hull. Understanding the concept of hull speed might make a difference to winning those races.
As for the application of hull speed in modern yachting, there are a lot of threads here that discuss older, refurbished boats that both cruise and race.
I am happy to read and try to understand any points, objections or corrections any of you might have with my explanation of my understanding of hydrodynamic. I think I know what I think I know and I know I don't know most of it.
- Will (Dragonfly)
