Waterjet handling problem

Discussion in 'Powerboats' started by SparkyDog, Jul 21, 2004.

  1. SparkyDog
    Joined: Jul 2004
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    Location: Oregon

    SparkyDog Junior Member

    I have a '74 Apollo 18' skiboat with a stock Berkeley waterjet. The boat does OK in straight but in a tight turn it handles poorly and also cavitates the impeller. This (in my opinion) is because the hull slides sideways and the intake for the jet has two tall fins on each side of the intake that serve as nice dams against the intake water in a hard turn. I am about to do some significant changes "just for fun." I am going to move the engine about 2' forward and add some hull fins below the CG just as with most modern prop ski boats and wakeboard boats.

    1st question: Has anyone ever done something similar with a waterjet and does the presence of the hull fins 4' in front of the water intake mess up the flow to the impeller?

    2nd question: Has anyone ever removed/reduced the intake fins on the Berkeley jet? If so, does it help or hurt?
  2. nevd
    Joined: Nov 2003
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    Location: Australia

    nevd Junior Member

    Water jet trim angle


    Moving the motor forwards will decrease the trim angle unless other weight is moved back to compensate. This decreased trim angle will decrease efficiency if the boat is already running at too low a trim angle and will probably make the jet unit more likely to cavitate.

    Low trim angle will also make any tendency for handling problems in following seas more likely.


  3. Black Swan
    Joined: Jul 2004
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    Location: Mission, B.C. Canada

    Black Swan Junior Member

    The turbulence from a "skeg" may be dissapated by the time the water stream reaches the jet intake, but it will depend on speed. The skeg used on tournament ski boats is intended to assist in straighter tracking while the skier alterantively is pulling the boat "off course" from both sides. Moving the engine forward may increase "wetted surface" of the hull, thusly increasing hull resistance and decreasing top speed. Similar problems occur when fuel or other tankage is added forward - the planing aspect of the hull is altered and performance suffers.

    There may be an "aftermarket" rudder available for your jet which should dramatically improve your cornering performance. There may also be special impellers available to better resist ventilation, or at least have a better recovery.

    If you're going to try adding a skeg, (one or more), be sure that it is installed "structurally", there is a whole world of stress on a skeg. I've seen improperly mounted ones rip the bottom right out of a boat.

    Good Luck!

  4. orvill
    Joined: Aug 2004
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    Location: Abbotsford bc

    orvill New Member

    I built a jet boat out of a vdrive hull one thing i proved if you set the pump back it dosn't cavitate.the stern of a boat normaly stays in the water.As your pump wears it will cavitate easier.the finns are to stop the hull sliding on turns.
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