Originally Posted by CPseudonym
Tad can you or someone else explain how to calculate hull speed on a catamaran design hull? Does it double the waterline length for that calculation?
With catamarans all bets are off.....
Most comparison formulas are only useful when comparing apples with apples. Though the same physical principals apply to catamarans and monohulls, different things are happening because the hull forms are so different. A mono hull may have a beam/length ratio of 3:1 or so, a catamaran demi-hull (one half the cat) may have a beam/length ratio of 10 or 12:1. We know that an important factor in resistance is the cross sectional area of any hull, and it's length. These are long and skinny.
Note from Vikings link that top speed is 29 knots and cruising speed 20 knots....but she's 132' long!
In fact things change so drastically once beam/length gets over 10:1, these hulls may not be planning at 29 knots....That is the boat is not lifting, but just sliding along through the water in displacement mode. There are big cats with displacement, semi-displacement, and planing hulls, and I'm not sure which this particular boat is.
You would not double the speed/length calculation for a catamaran unless the the hulls were joined end to end. As they are side by side and both must be pushed through the water the length of one hull applies.