Nautica rib jet 10. Plowing, undriveable

Discussion in 'Boat Design' started by bbar, Aug 3, 2022.

  1. bbar
    Joined: Jun 2009
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    Location: new jersey

    bbar Junior Member

    Hi, I have a "tender"; a 2001 Nautica rib jet 10 (ft). Yamaha jet ski driveline. I got it as a basket case and have put it back together but have a major ride problem and hope and I can get some help here.
    When the nose comes down onto plane, it just keeps going too far and begins to plow; causing it to catch, pitch, and skid, as it tries to throw me off. It has very long steering response and is basically undriveable over 24 knots and about 3000 rpm. I have changed jet nozzles from 0 degree to 5 degree up angle (which should have helped keep the nose up) with no difference. No hook in the hull. I have deflated the tubes and put 2 cinder blocks on the back swim platform (maybe???a slight improvement). The 11 gallon gas tank is almost all the way up front (and is permanent). I have run with different amounts of gas (down to 1/4 tank) with no difference. The motor is ahead of center. I don't know if this is just a bad design or what??? I'm at the point where I think I have to consider doing something to the underside of the hull (chine? sponsons?) I will attach pictures of it including the bottom a little later tonight. Appreciate any help.
     
  2. bbar
    Joined: Jun 2009
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    Location: new jersey

    bbar Junior Member

    here are pictures to go with separate post concerning plowing and undriveable condition.
     

    Attached Files:

  3. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    The forward shape ("buttocks") of this hull is showing convex surfaces to the incoming flow, which causes strong local reduction of static pressure; the round shape is "sucked down", and the spray shooting upwards follows the collar, cutting off the only possible ventilation path. Together with the waterjet's high trust line you get a very strong down-pitching force. The center of the lateral area moves forward, while the center of mass "stays behind"; it is like a dart, thrown backwards.

    The required medicine is a set of spray rails (with SHARP edges!) from about midships all the way to the stem, probably two each side. If you can produce a pic from the side, it might be possible to indicate the recommended positions. I don't see anything peculiar with the jet installation, the small "step" aft of the inlet base is ok.

    Edit: Oops saw that there is a sideways image already; I'll see if I can add a few lines to it later.
     
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  4. bbar
    Joined: Jun 2009
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    Location: new jersey

    bbar Junior Member

    Thank you for info. Here are 2 more pics of side. Look forward to recommendations of placement and where I can buy. IMG_20190802_162349_786.jpg IMG_20190910_130844_558.jpg
     
  5. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    I have indicated the approximate positioning of the spray-rails on the pdf pic included. The upper should be a smooth continuation of the existing one, but with a width of about 60 mm, tapering to zero at the stem. The lower rail about half way between upper rail and keel, stopping aft at about the front of the trailer side support. This rail may be slightly wider, say 70 to 80 mm aft; the forward tapering starting ~500 mm from the stem. Outside edges to be sharp (which is a challenge for a grp job).

    I'm not aware of any off-the-shelf "add-ons", so you have to arrange the pieces either by making up loose bits from alu or wood and screwing to hull, or go for a build up of glass and epoxy.
     

    Attached Files:

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  6. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Those things used to fly.. couple herring fisherman had them back in the herring heyday as they could go over the nets without snags and test roe quality. Used a fairly generic yamaha power plant used in the stand up, waveblaster and and some early waveriders. I wonder if somehow it was "repaired" over the years with parts that changed its performance. I know the intake grate design on the super jets was very finicky for getting correct ride attitude on that short water line with different weights.
     
  7. bbar
    Joined: Jun 2009
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    Location: new jersey

    bbar Junior Member

    Sorry for delay in responding.
    Comfisherman: It was missing part of the pump when I got it, but I worked with a yamaha junk yard and they matched it to the era of stuff you spoke of and the pump they sent me bolted right up. I see no evidence that any of the driveline was altered.

    Baeckmo: thank you very much for the thoughts and specs for spray rails. I have been looking and trying to learn and when I googled "spray rails" I see several pre-made "bolt on" rails One is "smart rail" from intergrity marine #m1000. another is from Taco., another from hamilton marine. I wonder if any of these would work for what you want to accomplish on my hull (and give a sharp edge like you wanted) If you have a minute to look at these and let me know your thoughts, I would appreciate it.
     

  8. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    That interchangeability is what I'm getting at. In those years Yamaha had three different jet skis ranging from the super jet that 7 ft long all the way up to a 11 and 1/2 foot multi seat ocean rig. With the Japanese penchant for interchangeability all could swap jets, grates, motors and impellers between them. Problem is each is setup to trim rather different from each other. This is a good thing for tuning to rider weight and machine design, my sister is 5'4" and 125 lbds and I'm a foot taller and twice the weight. Needless to say I had a lot harder time learning as the ski seemed to hate me. Ended up having someone recommend a grate design that had a different fin setup, when I bolted it on the difference was immediate.

    I'm guessing your grate is off a superjet or waveblaster as they by design pull the bow down to offset a rider on the extreme stern of a short waterline length. I'm 2700 miles away from where the family stack of skis ate to take picture, but I'd guess a specialty ski company could give guidance.

    What cc engine does it have? I'd be curious what generation system it had. Grate looks to be off a gp series that's bigger than the fx I'm referring to. They have their own ecosystem of parts as well.
     
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