Jet Jon: Additional Notes
The story behind the Seadoo
I went to a state surplus sale and there were four 2001 Seadoos - former rentals at state parks. All were in varying degrees of "distress". I bought one (for $1K) went home thought about it and raced back and bought the other three for $1K each. I was looking for a Kawasaki 650 to upgrade Jetfish and even bought one that turned out to be ruined (submerged in salt-water). I figured a Seadoo would do - even if it meant replacing the bottom in my boat. So, I ordered the parts to restore the three best Seadoos (one of them "finished" is in the photo on the opening page). I passed on my savings to three very happy buyers. The one I cut up was actually structurally damaged in the bow area (I never took a photo from the front) from high speed contact with a pylon. It could have been repaired, but what a great opportunity to repower my boat! Interestingly, when I pulled the motor it made a strange gurgling noise - it was full of water! I had already sold the other three Seadoos - I was sick. Then I discovered the water in the motor was fresh, and although it required a complete disassembly, the motor was unharmed. I still maintain receipts for the Seadoos - just in case someone who sees the cut-up thanks there was some "funny business" going on. Nope - just a lucky find.
In a nutshell here is what I know about jetksi power in full size boats.
Jetski pumps do not like air, and they do not like rocks. The air issue can be avoided by selecting a hull with a deadrise that approximates the deadrise of the donor jetski. Avoid putting strakes or other obstructions within three feet of the front of the intake. A flat bottom will work, but it will cavitate (aerate) in very minor chop and be nearly unusable in typical weekend lake chop (stick to small rivers).
Jetski pumps are touted as linear flow - not exactly true. The only true linear jet pump is those developed for use by Navy Seals and Army Special Forces and they are highly modified outboard lower units (sometimes on ebay). Water going through a jetski pump in a regular sized boat is lifted a bit as it goes through the pump. This puts a significant downward pull on the aft of the hull and can result in porpusing as you near cruising speed. I've tried a VTS (variable trim system) but due to the length of the hull benefits were limited. Then I tried 6"x12" manually adjusted trim tabs - eureka! A nice smooth ride.
Rocks - as I said pumps do not like rocks. I have replaced several Kawasaki pumps and one Seadoo pump (much more expensive and harder to find than Kawasaki 550) because I did not know how to run my boat in shallow rocky water. In one instance a pump ingested a granite rock, actually sawed the pump in half before I knew it was there! In water under 18" the pump is strong enough to lift rocks off the bottom and pull them through the pump with disastrous results. At speed, this isn't a problem - rocks are lifted, but the intake has moved before they can get into the pump. The problem occurs when you start the engine in water less than 18" deep. There are two things you can do to prevent rock ingestion - one (risky) is to drift down river until you are in deeper water before starting the engine. The other solution is the fabrication of a "rock grate" which is a steel intake grate with metal bars on 1/4" spacing running lengthwise (crosswise will cause pump cavitation). You can still damage the pump ingesting small rocks - but at least you won't have to "walk" home.
Very sandy (muddy) water can fill the engine's cooling cavities and cause the engine to overheat. Never let the motor idle on a sand bar or while beached. An aftermarket hose attachment will let you run the motor with the boat on the trailer - allowing you to flush the engine if you operate in less than ideal waters.
Adding a second cooling line to the pump/engine is OK if you always operate in warm waters. Trout rivers are too cold for this and cooling the engine with too much cold water can cause it to cease.
Weight is critical with jetski powered boats. 750cc and up should be used for full sized boats (14'-18'). A variable pitch impeller or a impeller designed for towing skiers will work the best for heavier boats (500 lbs up). Speed impellers are fine for the little 11' hydroplane type hulls. Also, proper alignment and shimming is critical for all these pumps. It is worth the expense and time to take a pump to the pros (hopefully in off-season) for proper set up. A Seadoo pump alignment tool is over $300 - and a improperly aligned, the pump will give your boat a severe teeth rattling vibration.
Although my focus was on Kawasaki and Seadoo donor skis there are lots of other brands out there that work just as well. I know that Polaris is very similar but Yamaha has an aluminum intake and will need an totally different installation process.