Boating How-To’s - Some help with a motor...

I am building a wakeless launch for my son's rowing club. It is just an old hobie cat with a deck mounted on it.
http://www.frontiernet.net/~toller/WL.jpg
http://www.frontiernet.net/~toller/mount.jpg
I water tested it today with a 9.9hp short shaft motor on it. It is obviously missing seats and a console, but I had an extra person on it, so it weighed 10% more than it would weigh in use. It ran fine up to about 10mph, then water started coming straight up from around the motor. Full throttle was about 13 mph, and the fountain of water was pretty impressive. The boat was level the whole time.

Any idea what caused the fountain and what can be done about it? Someone suggested the prop wasn't low enough and we had to go to a long shaft, but it is about 11" from the top of the motor mount to the water line, so the motor should have been submerged enough.

Also, the shells the boat will have to keep up with go about 12mph max, so while a 9.9 can do it, I question if it is good idea to keep them running at full throttle; maybe going up to a 15hp would be a good idea. What kind of performance do you think we might get out of it then?

Thanks much.

The cat raises as the speed increases.

agree with above post... Try getting a jack plate and lowering the motor.

Either your motor is rising as the hull speeds up and lifts or you need to trim it under more.
My 9.9 on a 15' fiberglass Gheenoe pushes it 22 miles per hour loaded with fishing equipment and a 12 volt battery so unless that little hull is heavier than it looks you should be good with that motor.

I originally had a different motor mount 6" lower, but the mount was hitting the water, so I raised it up.
At lower speeds it is well trimmed. At high speeds I was mainly paying attention to the speed on the GPS, so I don't know exactly what the trim was.

Would a long shaft do the same as lowering the motor?

I think the boat as photographed with motor, gas, and three people weighed about 1100 pounds. We only intend on having two people in it, but have to add seats, a console, and safety equipment.

That's a lot of weight...

That's a lot of weight...

Three men are 550 pounds; two sheets of plywood at 50#/sheet, 12 aluminum tubes at 8#/tube, the motor is 100 pounds, basic boat is 200 pounds. Then there's the gas and all the fasteners.
It will be used with just two people on it, probably lighter than the people testing it.
It has a maximum capacity of 400 pounds and I have stripped about 150 pounds off it (mast, rudders, etc.) so I am overloaded, but I will never have it on one hull like cat sailors usually do.
But of course all that weight has to be propelled.

I have exchanged emails with someone who built a wakeless launch from scratch.
He also has the fountain, though not as bad as mine.
Apparently commercial boats have deflectors to keep water away from the shafts (above the props obviously) and that reduces the fountain.

I now think that I have the prop in the water too far; apparently anymore than the minimum just makes it worse. I am going to try running with one less person (which should raise the boat up an inch or two) and put an inch shim on top of the motor mount. If it reduces the fountain, then I will have found the problem.