# determining CATamaran hull speed

Discussion in 'Boat Design' started by SCATAR, Mar 22, 2010.

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### SCATARNew Member

we have a very rigid rather over-engineered heavy (30ton) 18m catamaran. the original owner is deceased and boatyard lost all its records in hurricane katrina. from what I may make of the four engineering drawing found, she sits 8” (±2”) low as a result of the extra alloy. the result is a cat that struggles at 6-7 knots in 20-30 knot winds. we are considering a new sail plan but wanting, feel we are needing, to have a calculation of the hull speed. anyone who would be able to recommend someone, or may themselves be able, to do the requisite calculations to determine hull speed? we are also considering adding 1.5-2m stern with an extended swim platform but wonder about the impact on handling...

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### marshmatSenior Member

Hi Scatar,

There are a few naval architects around this community who specialize in multihulls, although your particular boat might be a rather rare one. We'll see if any of them chime in here....

The short answer is- from my perspective- that I agree with you, 6-7 knots for a 30 ton 18 m cat in 20-30 knot winds is not good sailing performance. (But wow, is that heavy for a sailing cat- is she more of a "charter cruiser" type than a "blue-water" type?)

However, knowing "hull speed" is almost irrelevant in this case. The concept of "hull speed" (v = 1.34 * sqrt(LWL) for waterline in feet, v in knots) assumes that the boat is essentially limited to the speed of a wave that's as long as the hull- a rough (at best) approximation for monos, and almost meaningless for cats.

Your naval architect, once you find one, will have access to much more sophisticated performance prediction techniques that take into account the actual hull shape, weight, windage, sail plan, sail trim for different points of sail, etc. This is a fairly involved process, as it involves the NA reviewing all the drawings and known data about the boat and preparing computer simulations to determine the effect of various modifications on her performance.

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