Displacement Cat performance

[andis]

I've been reading the posts on cats, but so far only vague statements are made on their performance. Hence, does anyone have any clue as to how large a disp cat needs to be to cruise at 25 knots?
What should the consumption be at such a speed?

 

[hobiecat]

The displacement formula just calculates the speed at which the distance between the crests of the bow wave and stern wave are equal in distance to the waterline length. In that sense "hull speed" is identical for a monohull or a cat of equal waterline. This speed in knots is 1.34 X square root(waterline in feet).

This is also maximum speed for most displacement mono boats. However, a very narrow hull makes a small bow wave and thus the power needed to exceed "hull speed" is reduced. The fineness ratio of displacement cat hull is important in speed calculations.

Why bigger displacement cats do not increase in speed easily due to weight (volume). This is why buying displacement cats is opposite thinking to monos - the better cats use more expensive light composite materials. Monos are sold on basis that heavier build is better. Cat thinking is the exact opposite.

Cat displacement cruising is therefore possible at 25 knots in many sized boats. Fineness of hulls and weight are more important criteria and consumption will be related to this.