In the world of Monohulls, with all things being roughly proportional except that one boat has a longer waterline length, the boat with the longer waterline (as a percentage of its speed) will be faster approximately by the square root of the longer waterline length divided by the square root of the smaller waterline. Of course that is only theory, in reality, far more important than theoretical hull speeds is the percentage of time that a boat sails at or near its theoretical hull speed. Here there are huge variations in the percentage of time that boats are near theoretical hull speed which is the reason that modern boats generally turn in dramatically faster passage times than more traditional boats with similar hulls speeds.

All bets are off when you talk about a mathematical relationship of length to speed in multihulls. While there is some relationship, most multihulls behave as semi-displacement hulls (rather than planing or displacement) and so their speed has very little direct relationship to thier length. Just to be clear, while it is true that specialized very high performance cats can hit enormous speeds, the normal 40-42 foot cats very rarely make 10-15 knots and most cruising cats never make the upper end of those kinds of speeds.

The reality is that in windspeeds under 20 knots or so, a modern monohull will actually be faster than most cruising cats on almost all points of sail.

Respectfully,

Jeff