Hull Speed

[mikewilkes]

Technical question for some bright spark please. Does anybody have a formula forcalculating the hull speed of a catamaran. I found one for mono's but then it went on to say that it was irrelevant cos cats were different!!
Would appreciate any help,
Mike

 

[kgi]

As you have posted this in the liveaboard section i assume you do, or will be livingaboard, if so then hull speed is irrelevant, as i own a cat and can testify that as soon as you put gear on it for cruising it slows down, (the smaller the cat the greater the effect), the only thing i found to be effective was to be pretty brutal when it came to keeping the boat light, even to the extent of fitting smaller water tanks, and fitting plastic fuel tanks, it has worked but it was a lot of effort,.......keith

 

[AndrewB]

Theoretical 'hull speed' is determined by the distance apart of the bow and stern wave, which is the hull waterline length. It's the same for a catamaran as for a monohull.

However, in practice the theoretical hull speed is irrelevant for cats. Any boat that can readily ride its own bow wave, i.e. plane, can break through the theoretical maximum and then there is no upper speed limit set by the hull.

 

[snowleopard]

beating Hull Speed

hull speed (normally 1.4 x sq root of lwl in feet) is not relevant to boats with length:beam ratio above about 8:1. my cat for example (lwl 37ft) should theoretically dig itself a hole in the ocean and squat in it at 8.5 knots but in fact goes on accelerating till i run out of bottle at around 15 knots. right now it's overloaded with a year's stores and it takes a lot of wind to achieve that speed but still no sign of any hull speed limit.

note that some cats do have hull speed limits because the hull form creates the bow & stern waves, the catalac is a notorious example.