Catamaran hull speeds.

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spannerman

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Seeing Gludy's post got me thinking, its generally accepted that a displacement hull catamaran has a higher hull speed than a monohull. I forget the waterline length to beam ratio thingy, there was an explanation for this sometime ago something to due with hull interference but I can't find it, my question is would two long slender monohulls achieve the same if it were possible for them to travel side by side at the same distance apart as cat hulls, or do they have to be physically connected to achieve this higher speed.
The hi speed cats that operate out of Stavanger harbour easily do 36 knots +, and the hulls are slender canoe shapes in relation to their length with very little lifting surface under them, they climb up slightly with speed but nowhere near as much as true planing hull.
What do the boffins out there say?
 
Displacement cats are not slowed much by transverse waves as the fine bows dont create much of a wave. The formula for boat speed is misleading as the answer it provides is really the speed of the wave a boat creates with displacment monos normally sitting in the trough.

The limiting force for displacement catamarans is principally skin friction. They can be extremely fast, fuel efficient and comfortable in displacement mode. The only problem is making a decent catamaran look sexy.
 
The normal fomula 1.34 times square root of the waterline length in feet = speed in knots only applies to normal bean hulls.

So it would apply to a 60 foot 16 foot bean boat BUT as the beam becomes thinner the 1.34 number rises to give a higher displacement speed. This can be achived on a single hull - no need for a second one.

So efficient is this that a cat with two hulls will normally have a higher displacement speed than a mono of the same length. In effect the displacement speed is X knots for one hull and is still X knots for two hulls - so say you need 80 hp to drive one hull at displacement speed of ten knots- you simply need another 80 hp on the other hull to drive it at the same displacement speed of ten knots.

I see many changing over to SD cats - because they are so efficient. A 60 foot cat with 25 bean cruising at 89 knots at well over 2 mpg. Smaller ones giving 3 to 5 mpg and offering lots of room.

So the rules about limiting displacement speeds still apply to Cats its just that the slim hull gives then a much higher displacement speed.

A mono D hull with a displacement speed of 8 knots requires a huge amount of energy to do 10 knots the extra 2 knots are never worth it but a similiar length cat could offer 2 or 3 knots more and still use less fuel.
 
Sailors are conditioned to a factor of 1.34 being used in the formula. Catamaran designers use much higher coefficients to mathematically calculate “hull speed” on their designs. The underlying reason is that the narrow hulls do not make a significant bow wave and therefore the hull can slide through the water without the negative effect of transverse waves. For example, a hull to beam ratio of 10:1 will have a coefficient of 2.8 rather than 1.34 in Froudes law that will significantly increase the “hull speed”. The higher the coefficient the better the performance.

My displacement cat does 21 knots with 2x50 hp engines and 16 knots running with one engine turned off.
 
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So the rules about limiting displacement speeds still apply to Cats its just that the slim hull gives then a much higher displacement speed.


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I dont believe it is the same rules that apply - the formula is just fiddled to somehow apply to cats. The formula is ultimately the wave speed generated by a boat as it moves through the water. It has nothing to do with boat speed. My understanding is that the limiting factor for catamaran hull speed is not transverse waves - it is frictional resistance.
 
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