Which outboards for me ?

[Scaramanga F25]

Catamarans do not follow monohull speed length ratios.

[brianlara 3]

My mistake. Thanks.

[Jim Cate]

Displacement speed is limited to 4/3 Pi x R squared right?

You've got formula dyslexia, Brian!

4 pi R^2 is the surface area of a sphere
4/3 pi R^3 is the volume of a sphere

nominal hull speed of a traditional displacement hull is ~1.3 sqrt LWL (speed in knots, LWL in feet). Long, skinny hulls as used in most cats don't fall under that rule, so Dave's boat isn't limited in that way.

Jim

[Cadence]

I fail to see it's being relevant.4 pi R^2 is the surface area of a sphere
4/3 pi R^3 is the volume of a sphere? I agree 1.3X the squire root of the waterline is a rough guestament on a mono but it is rough.

[Seaslug Caravan]

Dave-S, how are you outboards mounted?

Through hull or in a Nacelle arrangement?

Are your current motors "long shaft" or XL shaft?

Does the existing setup cavitate in a seaway?

Going bigger isn't necesarily going to provide more grunt in adverse conditions if the props aren't in the water long enough.

I've always found the XL shaft outboards are better on a cat if you can get them..

[Jim Cate]

Originally Posted by Cadence

I fail to see it's being relevant.4 pi R^2 is the surface area of a sphere
4/3 pi R^3 is the volume of a sphere? I agree 1.3X the squire root of the waterline is a rough guestament on a mono but it is rough.

they are relevant because in his post, Brian used the formula " 4/3 Pi x R squared" as the formula for calculating hull speed. That seemed to me to be a combination of the two spherical formulas, neither of which were appropriate.

I thought he might like to know what the origin of his might have been.

Jim

[brianlara 3]

Your right Jim! I have volume of a spheere on the brain.
1.34 times sqrt w/l length.
I ain't dyslexic.......mentally inept !!
Cognative dysfunctional.
Ptsd ad nauseum .

[clockwork orange]

Originally Posted by brianlara 3

Displacement speed is limited to 4/3 Pi x R squared right? So at 8.5kt you are getting close to that number.
The only reason your boat "sails" much faster than that is because you're planing.
And even 40 HT motors may not enable planing. (surely not ??).
A lot of money for another knot perhaps??
At 8.5 you're already motoring faster than a Maritimo 50 if at displacement speed.
Have I got it wrong?
Schoinning will know.

Cats do not plane, even when they are doing 20 knots they are still in displacement mode, they don't have a hump to climb over because of their slender hulls so more hp should result in more speed within reason but 8.5knots is not bad.

[brianlara 3]

Originally Posted by clockwork orange

Cats do not plane, even when they are doing 20 knots they are rstill in displacement mode, they don't have a hump to climb over because of their slender hulls so more hp should result in more speed within reason but 8.5knots is not bad.

Well explained. Thanks.

[SteveInMD]

I've heard Suzuki's have a warranty far better than any other, like 6 years, unlimited hours. Perhaps that was a special deal at the boat show this year, but it's worth looking into.

[Dave_S]

Originally Posted by SteveInMD

I've heard Suzuki's have a warranty far better than any other, like 6 years, unlimited hours. Perhaps that was a special deal at the boat show this year, but it's worth looking into.

Might have been a special. They are now 5 years. The Honda warranty is 7 years - you have to have them serviced annually by an authorised dealer @ $350-$400.

[Dave_S]

Originally Posted by Seaslug Caravan

Dave-S, how are you outboards mounted?

Through hull or in a Nacelle arrangement?

Are your current motors "long shaft" or XL shaft?

Does the existing setup cavitate in a seaway?

Going bigger isn't necesarily going to provide more grunt in adverse conditions if the props aren't in the water long enough.

I've always found the XL shaft outboards are better on a cat if you can get them..

1. Through Hull. They slide straight up and down on rails and are on centre line of each hull.
2. Long shaft not XL
3. Doesn't cavitate but can draw air crossing bars otherwise they are no issue

[Dave_S]

Originally Posted by SV DestinyAscen

Suzuki outboards seem to be A great pick. Fuel injection and great fuel management.

I'm starting to waver on the Honda when I read the specs on the Suzuki.

The Suzuki's are about $500 per side cheaper than the Honda and have double overhead cams with 4V/cylinder.

The Honda has single cam with 2V/cylinder which would make me think this is an older design and not as efficient. Honda has 2 years more warranty and weighs a little lighter (6kgs).

My problem is I have some history with Honda and its all good, reliability and fuel consumption. I don't know much about the Suzuki at all.

[Scout 30]

What about Etecs? More torque & would be lighter.

[Dave_S]

Originally Posted by Scout 30

What about Etecs? More torque & would be lighter.

I would prefer to use 4 strokes, I have a 2 stroke on the tender which I'm hoping I can make a deal on and swap it for a 4 stroke while I'm at it.

I did watch a few reviews on them and they seem quite good.