Glen-L.com

I'm going to start building a Cracker Box as soon as the plans arrive. But I will need a transmission. I'm sure I can figure out a way of getting a conventional motor and tranny to fit but... I'm also kicking around the idea of installing dual jet ski engines and jet drives. I'd prefer the dual because my usual boating (jet ski) is touring on Lake Michigan. Losing my only engine 20 miles from shore doesn't seem appealing.
So... what do you think of this change? Will these engines and drives create enough thrust to give good performance to a Cracker Box? How about cockpit configuration considerations? Thanks for any input.

Larry

I am glad to see I am not the only lunatic that thought about dual jet pumps on a CB.

My only points of concern were excessive rear weight bias, likely the cockpit would need a significant relocation. Secondly, and more of a concern, the hull could be too flat to assure good clean waterflow into the suction piece. If the pumps were placed as close together as possible, and fitted with a decent shoe it just might be ticket.

Is it possible to put the engines forward of the cockpit, the way the CB is designed and use longer drive shafts to connect to the pumps?
Chris Craft built a fishing boat with a center mounted engine and a"jackshaft" to an outdrive.

Caber,

This is the second time you've mentioned a "shoe" on a jet installation. In the Kawasaki jet ski world I live in, I have not heard the term before. Kawa jets have a grate (just two bars, really) that go fore-and-aft inside the jet inlet, designed to keep rocks from getting sucked up into the jet pump. There is also a cover over the bottom of the jet pump (Kawa refers to it as a "ride plate") that makes the bottom of the jet ski continue over the pump area.

I've also seen some aftermarket grates for jet skis that incorporate slanted plates across the grates to drive water upward at an angle to the pump.

Either one fo these what you are refereing to as a "shoe"?

TRM

Slanted plates are more of a loader than a shoe. The idea of a loader is to get water into the top of the impeller eye. Since the actual distance the water must travel from the suction-piece opening to the top of the impeller is longer than to the bottom, it helps to get a bit of a lift. They aren't much help unless you are up around the speed where the impeller is starting to run out of water.

A shoe is the single biggest improvement (IMHO) to pump efficiency. Allot of times they are integrated into a ride plate. The shoe is a hydrodynamicaly smooth extension of the back of the suction piece. It hangs down past the lowest portion of suction piece, a bit like a scoop. It helps make up for irregularities in the hull, or in cases where the suction piece may actually be recessed below the bottom surface of the hull. At speed the ram-effect feeding the pump can be dramatically improved by shoeing. The trick is hitting the perfect balance between drag, and starving the pump!

Granted, the faster it moves, the more critical the balance becomes, but on my 13' sprint it runs 1.2PSI or so in the suction piece with a clean intake, and with the ideal shoe it will run 3.5-4PSI in the mid-speed area. Actual top speed difference is nearly 15MPH.