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  1. #1

    custom boat using 2008 kawasaki stx 15f motor, questions about 148mm vs 155mm pump

    I built a river boat for cruising shallow rivers. I used the kawi stx 15F, Im still tweaking it, Ill try a new impeller next week with more aggressive pitch, the last one was slipping. I could only hit half throttle

    Im wondering if the kawi ultr 155mm pump would fit the boldt patten on the kawi 15f???? How much performance difference will I see between 148mm pump and a 155mm pump.Any thoughts?

    Im going for hole shot, i need to get on plain quick in some spots. Id like to hit 45mph or faster even though I usually on go about 20-30mph. If i can cruise with less effort i get better gas mileage.

    Heres a video of the boat. My intake it 6.5x12, so no water restrictions there.



  2. #2

    Join Date
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    badass, i assume the porpoising is caused by motor not big enough to keep the boat on plane easily? same as the issues with the gp760 (besides hull design)

  3. #3
    Since I've posted that video I've learned the impeller I was using has less pitch than stock. I've ordered a new one with more aggressive pitch to bring RPMs down, my max is 7000, I can hit that at half throttle. When my new impeller gets here ill do another test video. Hopefully with much better results. The porpoising I think is because I have a long ride plate covering the nozzle on the bottom so it wont hit rocks. I think its causing the boat to rock back and fourth, I think shimming the nozzle down slightly, or adding trim will resolve that issue.

    What I'm wondering....will a 155mm pump from the kawi 250x bolt right on the 148mm that's currently on my boat now, and how much affect would this make?
    Last edited by Ninjamaster; 07-23-2014 at 07:25 PM.

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  5. #4

    Join Date
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    the question about the pump is above my pay grade. either way, that is an awesome job. i'd take it in a heartbeat

  6. #5
    RLACEMAN's Avatar
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    This is a unique application of this powerplant, I think you should stay with the 148mm pump as you thought you might not be pulling in enough water with your intake design, adding the larger pump would probably make it worse. I think what is happening is in the 15F the faster speed of the ski forces more water into the intake, having the right pitched impeller should help you, lets us know what you find. I like that you can drive this boat in very little water, a very good designed project, now you just need to fine tune it.

  7. #6
    steve45's Avatar
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    I've got a neighbor that played around with building an aluminum hull boat with some sort of PWC engine. He came over and looked at my 750s for donor 'Skis, but wanted larger engines for what I wanted for the boats.

  8. #7
    my new impeller comes next week, ill do another test video. I dont think my intake will be a limiting factor, I had it made way over sized so it shouldnt have any water restriction issues.

    The questions I have, does the 155mm kawai ultra 250x pump bolt right up to the 148mm; and what kind of gains could i reasonably expect to see, hole shot and top end?

  9. #8
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    You will only see gains if your tunnel will support the extra water. What does the tunnel and shoe look like? Realistically you will only see a gain on the bottom but more than likely it will be a loss everywhere. Unless you make more power you can still only move 148mm of water. You could bore the exit nozzle out on the 148 but in doing so you will lose top end.

    Get the right pitch impeller and call it a day.

    Sean

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