Considering 2 jet ski engines for an AQUA CAT

About using jet ski motors to power small boats

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Considering 2 jet ski engines for an AQUA CAT

Post by pirate » Sat Oct 27, 2007 10:18 am

Any input from you guys would be appreciated. I am considering putting a jet in each sponson of the Aqua Cat. The boat is built but not powered.
I don't know anything about the jet ski engines and pumps. If you have an idea on size that would be helpful also.

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Post by terrymc » Sun Oct 28, 2007 7:02 pm


Of course it can be done - nothing's impossible...

To a first approximation, 2-stoke engines put out about 1 hp per cubic inch displacement, and a liter is about 60 cubic inches. Thus, my 550 Kawasaki has a 35 cu in displacement and puts our 36 HP. The Aqua Cat is a pretty big boat - maybe a couple of 750s...

That being said (and I don't want to talk you out of this), do know what you are getting into. Add a year of build time to put in the's just a lot of work - most everything needs to be designed as well as built. I think the biggest negative in this is that a boat this size needs a F-N-R transmission - and jets don't have that. You can rig a reverse bucket, but "neutral" is turning off the engine - and when you do that you don't hav eany steering. My "real" boat is a 27 ft cruiser, and I couldn't conceive trying to dock her without a F-N-R.

Ask anything specific, and I'll try to give you an answer...



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steve crawford
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Post by steve crawford » Mon Oct 29, 2007 6:37 pm

I'm with Terry a heck of a lott of work. and unless you go with big motors,
you will be disapointed.
I personaly thing these jet ski mods are best suited for 12' 2 seater boats.

Still wating for some one to put a 900 or bigger in one.These hulls can be beefed up for the abuse look at the racing hydros that are 12' 60mph plus.

The 14' Jet Cat sponsons could handel it also but the hull is still big and heavy. A pair of these big gas guzzeling motors to push a catamaran I couldn't aford the gas.

You say the boat is built let's see it.

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Post by pirate » Tue Oct 30, 2007 5:14 pm

I have found a 18' Sea doo Challenger boat with twin 110's. Once again I'm not sure what these engines equate to in regular boat motors
These Sea doo boats weigh around 1800 -1900 lbs and I'm thinking my boat hull weighs 500 - 600 lbs now. The inside is not finished out so that weight has to be added plus fuel and the rotax engines. I already have 2 4 stroke Homelite engines for this. Yea I said Homelite, as in 1972 models. These are the first 4 stroke marine motors.

Terry and Steve, you both have said it will be quite a task to put these in.
Is it the hull fitting or the controls that you are refering to.

Thanks again for your help

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steve crawford
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Post by steve crawford » Wed Oct 31, 2007 3:28 am

It is cutting the holes. You have to decide if you are going to do like Terry,and not graft in part of the donner ski or like I did. If you just
strip out the parts needed you can sell the seado hull.

Should be an interesting project ( go for it).

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Post by Dale Mattson » Thu Nov 01, 2007 10:42 pm

Sea Doo Speedsters from 1995-1999 were equipped with twin rotax engines. They can be heavily modified to produce a lot of horsepower.
I have seen wrecked boats on ebay in the $4k range. That's a lot of hardware for the money. The brain, all the drive train, all the electrical, even the gas and oil tanks, etc.
Plus, Rotax is a good company with available parts.
I looked at putting this set-up in a Flying Saucer, but honestly, It's an engineering nightmare for a guy like me. In my case, it's just better to bolt a new honda outboard on it and GO!
But...I would love to see somebody put this very cool twin set-up on a wooden boat.

Good Luck!!!!

Don't believe the hype.

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Post by Caber-Feidh » Fri Nov 02, 2007 12:00 pm

I think a twin-jet powered cat would be cool, the design solves one of the biggest twin-pump problems - sucking air in turns. I have bounced the idea around many times, just never was able to put the right power/pump/hull together.

That said, there are a couple of critical considerations.

With twin-anything, weight becomes an issue. The power:weight advantage of 2-smokers makes them the ideal mills.

If a pump is tuned to, say 5500 revs and 45HP, it's never going to work well at 3500, regardless of the shaft HP. Finding an impeller tuned to a low-reving 4-stroke is going to be a u-weld-it lesson. At a lower speed the swept area of the impeller disk of a given amount of time is reduced, the disk becomes "leaky", inertia and pressure are lost.

the farther between the pumps, the more critical the power balance is going to be. Easy fix, double throttles.

Wide spaces between the pumps is going to make the helm a bit bizarre. In turning the boat is going to try to go straight ahead Even though you are vectoring the thrust to the side, the thrust is still just exactly that... thrust. The side to the inside of the turn is going to be ejecting a water mass of nearly equal speed and volume as the outside, the tendency is going to be toward a funky-feeling under-steer condition. The bigger the pump, the smaller the hull, and the farther between outlets, the worse it will be.

Secondly, keep in mind that jet pumps, by their very nature, are self-speed limiting. In performance boats mixed-flow pumps have all but replaced axials on lake boats. Their design makes them a high-pressure pump, whereas axial flow is more volume. Your boat will never go faster than the water stream out the back, no matter how many pumps you put on it. (in the real world the best hulls are good to about 65% of the nozzle water velocity) Most of the older PWC pumps are axial flow, and can not develop high pressures no matter how much you restrict the flow. On the + side, high volume pumps are GREAT for holeshot acceleration, exactly what PWC riders want... and perfect for 12' boats.

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