outboard powering a cat

I have built a 14' power catamaran with a planing hull. I have noticed that cats with a single outboard have a third smaller sponson in front of the outboard. Is this necesary? Is the purpose of this a larger planing surface at the stern? Can an outboard just be extended down behind the tunnel with the cavitation plate level wth the two keels?
If I were to use dual outboards lined up with the two sponsons would they have to be of the same size? If a 6hp and a 15hp engine were ran together would the 6hp actually add to the thrust or would it be drag? I know the boat would want to turn a bit but the 6hp could then be used for trolling. Would the prop pitch be the same? Any help would be appreciated.

 

The wake between the two hulls is the issue, it can create numerous problems like cavitation and poor efficiency etc... You COULD mount it IN FRONT OF THE BOAT in a "tractor" configuration.

As for your second question, they would have to be the same size. The net thrust must be through the centre of mass for the two hulls together. In theory you could run a small motor with a higher pitched prop and a larger motor with a lower pitched prop, but in reality, you'll ruin one of the two motors very quickly (if not both). The rpm ranges and gear ratios available won't let you do this anyway.

 

Thanks for your reply Thunderhead19. So if I get this right, to mount a single outboard on a catamaran, there has to be something in the tunnel area to "clean up" the water so that the prop doesn't cavitate or ? That makes sense and explains the center sponsons I have observed on other boats. Thanks again!

 

What plans did you build the Cat from?

 

For good operation on plane, an outboard needs a smooth uninterrupted flow to the prop. The centre pod on single-prop cats serves largely to ensure uniform flow to the prop; ie. the prop is not going in and out of wave troughs. So for running single-engined you really do need one for it to work out.

Running on two OBs of different types doesn't work too well. I've seen it done on displacement hulls but on plane it's very hard to get the result to be easily controllable, and the boat will always pull to one side or the other depending on speed. It will also cause one of the motors to be overloaded and likely destroyed. So if you want to plane, use a single, or 2 motors of the same type. (Even two 15hp engines of different types is a bad idea, as the torque curves will not match and you will get the same handling and overloading problems.)

 

It really depends on the hull spacing. If they're far enough apart, then there wont be any colliding wake issues for the motor to deal with.
Also, are the pods you're talking about actually in contact with the water? if not then they may just be there to protect the o.b from any waves that pass under the bridgedeck

 

To JEM:
Plans?! If I started using plans and reading directions my wife would think I was an imposter. It's my own design, well I borrowed a little bit here and there. I started with the stems and keels and muddled my way through it. Was kind of an exercise of "Can I do it". Not a recomended way of doing things unless you've got a lot of time on your hands and enjoy puzzles.(I am neither). But I really enjoyed the experience. I'll try to include a picture in case you're interested.

To Willallison:The pods I have seen seem to be just below the water line but I have only observed them out of the water

I don't think the pictures worked so look in my gallery

 

from the pics you've posted, this is obviously a planing hull form. The hulls are too close together the get the kind of undisturbed water that I was talking about. And if the pos you've seen are below the waterline, then I'd have to concur with the others - they'd be to help a single 0/b get clean water

 

Plans? We don't need no stinkin plans!

Looks like it was a fun project. Don't want to hi-jack the thread so would you send me an e-mail? I have some performance questions (not motor related). Small cats have always intrigued me.

 

If you want to run a single motor on this rig, a fairly simple pod with a V-shaped entry and flat bottom should suffice to smooth out the water flow to the motor. In keeping with your design philosophy, you could use a shaving mirror taped to a stick to observe the water flow back there while a friend drives. Try to get the pod at a height where it's always in the water but not so deep as to cause a whole lot of drag. Set the motor's antiventilation plate level with the pod bottom and you should be set. Perhaps a pod mounted on threaded rods that you can move up and down a bit until you find the ideal position?