is this true about jet ski engines...... if so then what?

About using jet ski motors to power small boats

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jason vandeusen
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is this true about jet ski engines...... if so then what?

Post by jason vandeusen » Wed Mar 11, 2009 6:19 pm

Hello I am thinking about building and designing a small river sled that may possibly be powered by a jet ski motor and pump. so far the applications I have seen here have been installed in small runabouts that operate in lakes. I came across this question and answer on the internet, yahoo I believe. I want to know if this answer is true, jet ski motors will not effectively operate in river environments. If this is true then its back to the drawing board because i will be out of options.

Question:
Who makes a two person, fast jet drive fishing boat for shallow, stump infested waters? Are there any? Or should I just put a jet drive outboard on a heavy duty john boat? I wanted a pwc hull design for the softer ride enroute across the lake.

Answer:
You NEVER want to get a PWC or mini-jet boat intake ANYWHERE near the bottom! There are very efficient jet drive outboards with large intake filters designed to keep the bottom out of their pumps. But, the PWC and small jet boats AIN'T IT! You'd destroy the PWC pump in an hour trying to run it in shallow water. It's ONE BIG VACUUM CLEANER that sucks EVERYTHING IT CAN into the pump....NO!

I was skeptical about this answer because my home river is used by jet ski's occasionally. Of course I have only seen this a few times and they may have in fact been destroying their motors if this were true.

so is this true or not?

I hope not or my project is dead before I even start.

davefroriep
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Re: is this true about jet ski engines...... if so then what?

Post by davefroriep » Wed Mar 11, 2009 7:30 pm

That reads 100% true - they are vacuums and, depending on the size of the engine, could suck up pretty much anything closer than 12"... The 400 hp Jet Boats may even double that "draft" if you mosh on the throttle from a dead stop... Think about how much water comes out the other end... it has to come from somewhere!

On the other hand, if you're cruisin', you'll be on a plane with a shallower draft and moving forward, so you may be out of the way before the "stuff" you suck up gets to the intake... Big maybe though.

In my opinion, Jet drives are only OK for 12"+ open water below the intake. Also, the intake "filter" (screen / vent / grill) is a key part here... gravel / sandy beaches are going to make some noises and potentially trouble, but for the most part, you will get a clogged up vent or some "stuff" wrapped up in your prop before you cause serious damage...

It all comes down to how deep it is and how fast you're moving - you said you see ski's there already, so you're halfway ok, but keep in mind you will draw more and move slower :?

upspirate

Re: is this true about jet ski engines...... if so then what?

Post by upspirate » Thu Mar 12, 2009 6:42 am

I would ask the people you see running them what trouble they may have.

Also,if you PM Caber-Feith on this forum,he is kinda the resident jet expert guy

jason vandeusen
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Re: is this true about jet ski engines...... if so then what?

Post by jason vandeusen » Thu Mar 12, 2009 8:37 am

Thanks for the replies that makes sense.

So the basic principal would be to start the jet motor in the mid depth water and power through the shallow areas on plane. Jet rigs are becoming hugely popular on my home river so they must do something right.

I was thinking about a simple prop outboard but the water gets so skinny here and this local jet boat explosion had me thinking.

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Caber-Feidh
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Re: is this true about jet ski engines...... if so then what?

Post by Caber-Feidh » Thu Mar 12, 2009 10:35 pm

Dirty water needs an axial flow pump. Not a mixed-flow... Sand and debris ears them up fast. Once the clearance open up, the pump is hosed.

This is an axial flow pump http://www.cpperformance.com/products/J ... rawing.htm
Note that the impeller is a bit like a cross between a fan and a propeller. (part 27) This design is generally lower speed, but stronger launch/tow abilities

This is a mixed flow design... http://www.cpperformance.com/products/J ... rawing.htm Note the impeller is a bit more of a cross between a centrifugal pump, and a very high pitch propeller. (part 29) With this design the clearance between the impeller skirt, and the wear ring (part 3) is critical. Since the flowrate is lower on a mixed flow design, relying more on pressure than mass, the design has little room for wear. With typical "good" clearances in the .025" range, and wasted being .050" there is not much room, for sand-pumping.

PWC use both designs. An axial with a good rock grate is pretty bullet-proof, a mixed will be faster, but more prone to damage, and rapid wear from abrasive conditions.

(Not intending to pimp CPP, they just have good diagrams online. :P )
Some plan to stroll through Saint Peter's Gates, I plan to go through them at 150mph... backwards... in a screaming ball of flame, with a glimmer belt wrapped around my head, and a NOx button in my hand.

jason vandeusen
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Re: is this true about jet ski engines...... if so then what?

Post by jason vandeusen » Sat Mar 14, 2009 2:22 am

thanks for the great information that helps a ton!

One last question. Are the jet pumps in jet skis lighter and smaller than other jet pumps or are they in the 80-100lb range like most?

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Dave Grason
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Re: is this true about jet ski engines...... if so then what?

Post by Dave Grason » Sat Mar 14, 2009 2:29 am

Jason, what river are you on?
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

jason vandeusen
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Re: is this true about jet ski engines...... if so then what?

Post by jason vandeusen » Sat Mar 14, 2009 3:31 pm

im on the Muskegon river, in western Michigan. Its a big tailwater that behaves much like a spring creek. Generally runs clear and low most of the year expect during the spring like right now. Lots of boat traffic and few wading opportunities. In low water situations there are areas of this river that are inches deep the whole width of the river.

This river could handle the larger jet sleds in the 17-20' range but I am not into larger and especially heavier boats. I want a smaller sled that I can trailer with my car and is less than 800lbs, any heavier than that and its a brick to me.

since no one manufactures a sled like this and also since I build boats I have decided to build my own smaller lighter design. hence the need for a smaller lighter jet drive and engine but one that is river worthy.

sorry for the book but thats why I am asking so many questions.

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Cranky Badger
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Re: is this true about jet ski engines...... if so then what?

Post by Cranky Badger » Wed Mar 18, 2009 10:18 pm

Just curious here since I don't know much about jets, but couldn't you draw the water through a hollow skeg that would act like a plenum ? That way, you are drawing water from in front of the boat rather than below it. You might also get a ram effect from forward speed as well, especially if you shape it like a venturi. A venturi ought to prevent cavitation inside the plenum/skeg as well, no ?
-Brian

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