Jetski for Catamaran Inboard motor

[Fanie]

Would it work if one uses a jetski motor in a cat hull instead of an inboard or outboard ?

No exposed props...

[creepy93]

i think it would work u would have to have 2 jet ski engines. and i would literly have to be the jet skis but into the hull and then there is the turning problem but good idea

[eastcape]

Hi Fanie,

I did a small design a few years ago for a 16ft power cat with 2 jetski engines. The only problem at the time, was the engines were to expensive for the project. If I could have found a couple of second hand ones I would have built it.

[Fanie]

Thanks for responding guys. I know nothing about jetski's except there was a few occasions I wanted to strangle the owner

I could use the rudders to turn the boat

How well would these motors perform compared to an outboard or inboard in each hull ? The idea is to use them as inboards for a 10m sailing catamaran. Any advantages / disadvantages ?

[Pericles]

Fanie,

A touch uneconomical perhaps. Good article at http://www.woodenboat-digital.com/woodenboat/20070910/ Page 52 NIKI. Jet powered lobster boat.

Regards,

Pericles

[marshmat]

Jet ski pumps are, generally speaking, optimized for lightweight craft at high velocities. They are meant to absorb a lot of power at much higher RPM than the Hamilton or UltraJet pumps you generally think of when talking about catamarans of significant size. A PWC's pump has to be light, compact, and able to absorb the output of a very high-strung, 200 hp motor, while pushing only a few hundred kilograms of boat at 40-plus knots.

Thing is, jet ski pumps just aren't that efficient when compared against other pump designs, or against props. Witness the 400 hp, twin-jet runabouts we see from Yamaha, Sea-Doo et al, that, overall, perform about as well as their traditionally-propelled counterparts do with a single 250 hp V8. (This isn't to say that jets in general are inefficient- from what I can tell, a great many of them are as good as, or better than, their inboard or sterndrive counterparts- only that the small, high-revving style popular in PWCs usually doesn't fare as well in heavier, slower craft as a larger, slower pump will.) But at lower RPM, it seems they can give fairly decent thrust for the amount of fuel they use; if you're willing to cruise at less than half throttle, you could probably approach the gas mileage of a pair of outboards of similar thrust (although you won't get close to that of an inboard diesel).

Regarding Fanie's situation, I don't see a 10 metre cat needing nearly as much power as two of even the smallest jet-ski rigs will deliver. But then again, these things fare much better in terms of efficiency, longevity and noise at 1/4 to 1/2 throttle than they do at WOT. And you'd have power out the ying-yang for when you feel like coming up to the dock in a Force 5.

Generally speaking, you can't pull the drivetrain out of a jet ski and re-use it. Parts of the mounting and intake are moulded into the jet ski hull. You'd have to cut the entire underside out of the little beast and glass the complete unit in to yours, with appropriate structural reinforcing. But I think it could work. You'd have a somewhat unusual craft with engines that are reasonably quiet at idle, are not tremendously inefficient at lower speeds, and can still give you insane amounts of thrust for docking (or racing ahead of a storm) if you're willing to burn your fuel a lot faster.

[Fanie]

Couple of things I noticed. Less efficient than outboards. Heavy motor. Problems with raw sea water cooling corrosion. Mounting construction problems. Designed for light craft. Heavy on fuel. Hmmm...

[longliner45]

what would really be cool is to design the jetskis to be detached and used as jetskis after you got to were you are going,longliner

[marshmat]

what would really be cool is to design the jetskis to be detached and used as jetskis after you got to were you are going

Like so? (From http://www.shuttlecraft.com/deck.htm )