The "constant" in the equation for displacement hulls is dependent largely on the fineness of the hull in question. The oft-quoted 1.34 constant is pretty good for hulls 1/3 to 1/2 as wide as they are long (fineness of roughly 2:1) but terribly wrong for hulls 1/10, 1/12, or even narrower in beam compared to length. The formula is still good, it's just that the constant changes dramatically. It's something like 4 or 5 for most beach cats; this works well with empirical data of maximum speeds such as Carl posted.

sail fast, y'all!

PS: My abilities (and lack of them) limit my top speed, too.