This formula, as related to the wavelength of the water you are passing thru and your bow wave, will determine the maximum speed you can attain with a particular displacement hull design in a particular situation. A planing hull is a different ball game, and you can overcome the hull length at the water line max speed if you can get your yak on a plane (lotsa luck!). Now, there are several variations on this formula (and others) that sailboat sailors (mainly) have debated ad nauseam. Here is a link to some of this discussion, but I will leave this to the more mathematically inclined to figure out.
Well, the trouble with the hull-speed calculations and Hobie sailboats (and to a degree kayaks) is that the hullspeed only applies to full-displacement hulls. A Hobie 16's hull speed in displacement mode is a paltry 5.36 kts - ask anyone if the 16 can't go faster than this, even in light winds. Even the kayaks are not full-displacement by any means.
Hull speed is gravity-driven; the resistance from this has to do with the height of the bow wave, which depends on the weight (displacement) of the craft. While a kayak with an average paddler and full of gear might seem heavy at 250lbs, or a Hobie 16 at minium boat and crew weight at nearly 600 lbs, a 22-foot monohull could have 600lbs in ballast alone, plus another 800lbs of boat, plus 500 in crew weight, plus several hunderd pounds of gear, water, and fuel - the displacement of these are massive for their length and power and most ballasted monohulls are indeed limited by hull speed (exceptions being purpose-built ultralight ballasted racing monos like the Melges 24 which can plane and thus are not limited by hull speed) (completely off subject, this is a good part of why "monohulls point higher into the wind" - it is not some magical property of the single hull, it is just that they won't gain any more speed even with the additional power of heading off).
Most small, light, long craft (Hobies) are far more limited by drag. With the cats, upwind can lift a hull to reduce the drag from the water; however, the wind also creates drag on the boat. Downwind, the wind drag no longer hurts speed (coming at 90-degrees to the direction of travel or behind the boat), but both hulls are now in the water, creating more drag there (unless you are doing "The Wild Thing" on a spinnaker boat (Tiger) or very light boat (A-Cat) and flying a hull downwind for less drag.
Paddle/pedal yaks have minimum effect from wind drag without a large sail above them. While I have no personal experience with it, I'm betting that the speeds of all of these boats are fairly close. My guess is that the longer boats track better (giving better power) and the shorter boats have a bit less wetted area; in the end, the longer boats are a bit faster because of a little more power, and at similar displacement but longer length, they provide a more hydrodynamic shape.
Wow, that was a lot longer than I thought. Hope it was on-topic enough!
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