Engine sizes

[Cheechako]

30hp X 2 on a 42 cat was not enough in some situations. OF course that was the Yanmar high rpm HP rating which really is BS. Consider what HP you are really getting by looking at the HP curve at about 2600 rpm...

[C--man]

Mark-

when you say "hull speed" how many knots are you talking?

on the Winwndspeed 40 with fine hulls and daggers the pair of 27s

would push it easily to about 7knots even with one engine without much throttle

but when I increased the throttle more all it did was force the stern down

and did not go any faster, is that normal?

By the way, Catana is a very nice Cat.

C-man

[Cheechako]

The problem is not flat water speed, but when the wind pipes up with a cat's windage, you need plenty of power. Some squat is normal on the upper end of the hull speed curve, cat or mono. Actuall, the cat I mention above was I think 27 hp not 30. All in all it was OK, but if reoplacing the engines I would have went to the 35 that was available. I was a much more robust engine I understand.

[feelsgood]

Hull speed is deturmined by the square root of the water line length To push a boat above this speed takes alot of hp and burns more fuel. But it doesnt mean the a cat has 2 hulls so will go twice as fast. I agree that the 35 hp engines would be best

[mark_morwood]

Originally Posted by C--man

Mark-

when you say "hull speed" how many knots are you talking?

on the Winwndspeed 40 with fine hulls and daggers the pair of 27s

would push it easily to about 7knots even with one engine without much throttle

but when I increased the throttle more all it did was force the stern down

and did not go any faster, is that normal?

By the way, Catana is a very nice Cat.

C-man

Around 9 knots. Once I get to around 9 knots, more throttle doesn't make much difference to speed on our boat.

The formula (1.34 * square root(waterline length)) suggests closer to 8.5 knots with a 40' waterline length on fine hulls.

I agree with comment that punching into wind and sea is when you really need effective power (prop and engine).

Mark.

[C--man]

that sounds about right, Thanks!

[Esprit Marine]

The needed power mainly depends of the displacement.

Most of the production catamarans have a ratio of 6 to 7 hp per 1000kg maximum displacement, or 8 to 10 hp per 1000kg lightship weight, and this is rather on the high side. This is the total for both engines.

Hull speed is the limit over which the energy needed to displace the yacht is considered to increase dramatically. But the 1.34 *squareroot(waterline*length) limit was calculated by Froude for boats typically having a length:beam ratio around 4:1 (monohulls), and it is of much less importance with multihulls. See explanation here.