Catamaran Hull Speeds

[mikewilkes]

A query for the more technical does anybody have a formulae to calculate the hull speed for a catamaran, please? I have tried this one on Liveaboard link and I got the impression that I should not have used that particular forum, from the reply I got.
I got a formulae out of a cat book but then it went on to tell me that it was irrelevant cos cats were different!! As i live on my cat in the med and know that the wind is not really for "sailing" then if I can find out what the hull speed should be then increase my engine to suit.
Many thanks,
Mike

 

[Boathook]

There are many variables for a Cat hull speed and owning one I vary the speed to suit the conditions and my mood!, also I have never seen a formula for calculating speed for the reasons you quoted.
My 9m Catalac cruises at 5 knots under power and sails at up to 7 knots. There are a lot of variables that will effect this though.

 

[vyv_cox]

A tricky one, I always thought. Does a catamaran plane, or not? A Hobie 16 will certainly do well over 20 knots on a lwl of about 13 ft but it's hardly a planing hull. So how does it do it?

 

[Jacket]

Racing cats far exceed because their slim, low volume hulls don't create large waves, which are what cause monohulls to reach hull speeds. However, your average cruising cat will have a larger, bulkier underwater profile to carry its displacement, and so will reach a hull speed in the same way as a monohull. This speed is likely to be not much more than that of a similar sized monohull, the difference being that a cat reaches top speed easier.

I was once told that the "square root of the waterline length in feet" formula works just as well for cats as mono's, multiplied by about 1.5 for heavy displacement cats, and about 2.0 for the performance cruising cats.

If you're interested in hull speed under engine, try talking to the engine manufacturers - they probably have experience in this sort of problem.