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Thread: Porposing issue

  1. #1
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    Porposing issue

    I recently finished my mini jetboat project, when I water tested it I found that it porposes pretty bad until it finally levels out. I read that I can add sand bags to the bow, doing so in small increments until the problem goes away. The tank is in the bow and currently only has 5 gallons in it, im going to try filling it first to see if thats an improvement, do you guys have any other suggestions? I also have trim tabs that I can add. This is a 12' boat that originally had a 3 cylinder yamaha mounted vertically, i changed it to a 1200cc polaris mounted horizontal. This required me at add a 8" bump on the tramsom to accommodatethe intake, and mounted the pump externally. All of this for sure changed the balance of the boat. Any help would be appreciated.

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    Last edited by beachcues; 08-03-2020 at 12:08 PM.

  2. #2
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    You have not shown pic of the bottom. Some times it can be helpfull to extend boat lifting strakes.

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    Picture added

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    I am not of very much help, just wanted to say nice work on the boat.

    Yes, I would try filling the tank to see what happens. I would however try adding trim tabs before I added any other additional weight to the boat, just because more weight = less speed/acceleration.

    I added a 6" setback/jackplate to my 17' Checkmate. The motor is a 1990 140 Evinrude, and is very heavy. After that it would porpoise especially when pulling skiers, or tubers. I had the spotter ride up front, open bow, and that helped but did not fix it. I use portable fuel tanks, so I kept 2 spares up front, and the spotter, helped more, but not gone. I changed props, experimented with motor height, and now it is all but gone.

    I know you are kind of limited to what you can do, so good luck!!!

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    Dont most jetskis do same thing?

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    Trim it down? Not familiar with that pump setup, but most ski's have some aftermarket pump wedges available, add wedges for more trim down.

    Add a more aggressive intake grate, top loaders was the term when I played with ski's, should grab the surface better and help keep the hull planted. Top end will suffer some.

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    Quote Originally Posted by Whaaaaat View Post
    Trim it down? Not familiar with that pump setup, but most ski's have some aftermarket pump wedges available, add wedges for more trim down.

    Add a more aggressive intake grate, top loaders was the term when I played with ski's, should grab the surface better and help keep the hull planted. Top end will suffer some.
    I may make a low water scoop, the pump is out of a yamaha

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    If it's a Yamaha pump then there should be plenty of grates and pump wedges to try.

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    Quote Originally Posted by Whaaaaat View Post
    If it's a Yamaha pump then there should be plenty of grates and pump wedges to try.

    Well taking the intake out would be the last thing I would do, the intake is mounted to the bilge floor, not mounted from the bottom so getting the intake grate off would be next to impossible. The pump i have has trim which I set all the way down. It was better but still doing it. I was thinking last night that the bumpout I made is 3/4 above the bottom of the boat. Could the boat be trying to ride on the pad and then the bumpout surface causing it to porpoise? Seems logical

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    Would definitely be a nice time for a go-pro rig. Answer that question pretty quick.

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