# Jet boat build



## KGouty (Jul 31, 2013)

Building my own jet boat......using twin 700 cc yamaha jet ski engines. Boat is a 1962 Texas Maid ski boat. Converted to a center console fishing boat, and now to a jet boat. Looking for input on installing the water intake.....should i mount flush? or below the hull? bottom of boat is not flat either. Should i mount "pump" outside of boat?


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## KGouty (Jul 31, 2013)

here is a link to a jet jon boat build that has 700cc yamaha and it shows pump mounted outside the boat........what are everyones thoughts?? Good? Bad? ???

https://www.youtube.com/watch?v=4J31xB45GVk


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## PSG-1 (Jul 31, 2013)

Actually, the last 2/3 of the pump is outside the transom. Specifically, the wear ring, stator, thrust nozzle and steering nozzle. 

In this configuration, the intake would be positioned so the rear flange joining the wear ring, would be about where the transom meets the hull. Doing this sets the pump back about another 12", which means you can set the engine back that much farther, to keep from eating up deck space, and to put more weight at the stern so the pump has better hookup with the water.

This will work fine, you just need to fabricate support brackets that extend past the transom, for the wear ring, stator, and thrust nozzle to bolt up.


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## KGouty (Jul 31, 2013)

OK thanks......just trying to get the best idea before i cut into my boat. Thanks


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## PSG-1 (Jul 31, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=324379#p324379 said:


> KGouty » 16 minutes ago[/url]"]Building my own jet boat......using twin 700 cc yamaha jet ski engines. Boat is a 1962 Texas Maid ski boat. Converted to a center console fishing boat, and now to a jet boat. Looking for input on installing the water intake.....should i mount flush? or below the hull? bottom of boat is not flat either. Should i mount "pump" outside of boat?




You want the intake to be right at flush with the hull. Too far up, and it cavitates. Too far down, it drags. Make sure to position it so there is a smooth transition from the boat hull to the pump.


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## PSG-1 (Jul 31, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=324390#p324390 said:


> KGouty » less than a minute ago[/url]"]OK thanks......just trying to get the best idea before i cut into my boat. Thanks



I know what you mean! Once you cut the hole, it's the point of no return. With this type of operation, it's measure several times, cut undersized, then take off small increments to fine-tune it.


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## KGouty (Jul 31, 2013)

ok great thanks a ton!!......so flush is my best bet and back as far as possible in the boat. And yes if i move most of the pump outside the boat i gain 12". Thanks for the help


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## PSG-1 (Jul 31, 2013)

One other trick you can do to gain some set-back....cut down the drive shaft, and re-thread the end for the coupler (if using a yamaha or kawasaki type spartan jaw coupler) Just be sure to leave enough length on the driveshaft beyond the pump bearing sleeve where you can slide the coupler back far enough to disengage it from the engine coupler, for pump disassembly.

Cutting the driveshaft shorter also reduces deflection and vibration, especially when the boat cavitates in sharp turns or heavy chop.

However, this only works if you're able to set the distance between the motor mounts and pump. If you're cutting it all out as one unit, leave it as it is, and set the whole thing back as far as possible.


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## KGouty (Jul 31, 2013)

Great idea and thanks! The yamaha has a carrier bearing between the motor coupler and the pump shaft so yea it would be great to eliminate that part too.


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## PSG-1 (Jul 31, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=324399#p324399 said:


> KGouty » less than a minute ago[/url]"]Great idea and thanks! The yamaha has a carrier bearing between the motor coupler and the pump shaft so yea it would be great to eliminate that part too.



Right, that's what I was referring to. Yamaha uses a REALLY long driveshaft, especially on the XL1200W. The longer the shaft, the more deflection you have. And since max allowable deflection is just .020", you see the advantage of making the shaft shorter.

I cut mine down where there is just enough room to unbolt the pump, and slide it back to disengage the engine coupler from the driveshaft coupler, then unthread the driveshaft coupler and remove the pump.


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