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  1. #1
    Old Ski Insanity w/rpm500's Avatar
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    Performance plate mod for the GP?

    We have all been reading of the performance plate offered by JIM's ...
    The posting and development of the plate trueing by FERCHO , from 2FAST4U's knowlage of the GPR's plate warpage .
    The pic's from ABBOTT, of his post on how to trueing 101, great.
    And PHILIP GPR's posting on the plate's angle in relation to the pump... !!!


    Has any one used this information on a GP hull ?
    It sounds logical to true the plate and shim it up possibly to the same angle as the pump nozzel.
    This is always posted as to being one or more stacked washers but never stated as being the same or different from the pump.
    ANY INPUT ON THIS WOULD BE APPRECIATED !!!


  2. #2

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    Don't you have a 0 degree thrust nozzle on that 97 GP?

  3. #3
    ABBOTT's Avatar
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    that is a good question. seems like everything has been aimed at the GPR series these days and some of the GP stuff got left behind.

    Anyone have any good advice with the GP series and the Plates associated with it???

  4. #4
    THE PLATE MAN JIM'S PERFORMANCE's Avatar
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    hello WRPM500, Do you have a factory plate or an after market plate on your ski.

  5. #5
    Old Ski Insanity w/rpm500's Avatar
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    The nozzel o/e in 97' is zero degrees.
    I also purchased a zero AAT (with the new design puck's)from WETWOLF to get that bit more on the bottom end and on the top.
    I have the original plate that came on this ski o/e (long)and a set of
    Riva's billet trim tabs... (large type as this is a 97 hull)
    I also have an new untouched Speed Pro/Mission plate that is a bit longer than the GP's o/e .
    My thought that this plate would give this hull effectively a longer running surface, Make it more stable and enable the hull to run higher out of the water.

    And... Yes, I have read that the Mission plate could crack in very rough conditions, thou I've only read of two ever doing this and they were both surf runners.

  6. #6

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    I used to have the old 97 hull myself (GP1200). Yamaha gave me a 98 hull after I cracked the 97. I wished I knew then what I believe that I know now as I would try what I am about to tell you.

    Consider these points. I believe them to be true. The area of the hull forward of the trim tab area is the effective working part of the hull. It is the "wetted surface" in which the water interacts. It is what the water "sees" at (top) speed. You notice that it is much smaller than the complete hull... less wetted surface, less drag, more speed.

    Without the tabs, I believe that there is not enough wetted surface to support lift at speed. Your boat rises out of the wateras you go faster through it. The dynamic lift (pressure the water is exerting on the hull to push it out of the water)cannot support the weight of the hull and it falls back into the water, creates a lot of drag to slow you down. You then pick up more speed , and your boat begins to lift out of the water again to begin the cycle all over. It is not so much like a porpoise, but like a high frequency buck, and the faster you go, the faster the oscillaton (buck).

    With the trim tabs, you increase the wetted surface AND you move your boat's center of gravity futher back toward the stern. No doubt, your boat rides better, but you still have the problem.... it just shows up at a higher speed. Look at the 98 and 99 hulls. They moved the trim tab area back even more than the 97 with the after market tabs. Yamaha increased the wetted surface (and moved the hull's center of gravity futher back) even more. They did this to compensate (I believe) for increased speeds. For the GP hull design, I believe that the faster you go the longer your boat has to be. There is no boat more prone to porpoising than a short, wide boat.

    The problem as I see it is that the hull is allowed to pivot up and down to much. I would approach the problem as if I were using the ride plate like a wheely bar on a car or motorcycle. A good starting point would be to set the ride plate angle at 2 degrees and increase the angle until you started to porpoise. Use machineist shims, not washers. I believe the sweet spot will be a very narrow window. Washers are so thick, you might go right past the sweet spot. Change the angle in 5 thousandths increments.

    Here's the thing. Make sure that there is no point on the plate that is less than 2 degrees when you set it or that will create a lot of drag. That's where guys like Jim and Carl come in. They are great at eleminating those parts of the plate that create drag.

    I wished I could give you the reader's digest version, but this stuff is complicated. That is probably why you have a 9 year old boat, and the problem still has not been worked out!

    Hope this at least gives you a starting point.

  7. #7
    Old Ski Insanity w/rpm500's Avatar
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    Thanks Salty for the obsevations on this!!
    I have thought thru some of this before, and appreciate the new insights.

    I dont have an issue with ride.... so far anyways , mines more of a question of setting up with the plate milled/surfaced/angle , using new information recently gained from tuner's working the GPR hull, to improve handling and speed of GP hull.

  8. #8
    ....back from the dead.... J-ME's Avatar
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    Great thoughts on the GP Salty. My beliefs concour with yours.

    One thing for sure most ride plates are a mess when it comes trueness, flatness etc.

    I spent 2 days hand filing the bottom of my Riva Limited plate and 1 day filing the mounting bosses just to get them true.

    I haven't even begun to mess with the angles yet.

    The GP is VERY sensitive to weight balance. Mine changes attitudes according to the quantity of fuel in the tank.

    Most that ride in rougher water have the plate shimmed down to keep the nose down to eliminate porpoising and stay hooked up....at the cost of several miles an hour.
    I may try ballasting the nose and using fewer ride plate shims to see how it goes. I am sure others did this in the day, but I want to see for myself.

    I know mech. trim tabs worked excellent for the old GP on off shore conditions but scrubbed alot of speed in calmer waters even when up.

    One thing for sure, the sweet spot is VERY small. One day I found my old GP's sweet spot by coincidence with just the right wind and water conditions and it went faster than it never did before. Didn't have a GPS but SMW and I guestimated about 66 MPH by the way it "dusted" his GPR.

    Does anyone know if Ocean Pro ever made a decent ride plate for the GP?
    Their plate seemed to work the best on my Raider in rougher water without scrubbing that much speed.

    More to come as soon as I get on the water.

    Great thread W/RPM, you must of read my mind.

  9. #9

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    One thing that I believe you have as an advantage over the GPR is that the nozzle on the 97 is 0 degrees. This means that all of the engines thrust is directed straight forward through the hull rather than a 5 degree angle downward. It is by far more efficient.

    What I am saying is that the only lift that is created is dynamic lift of movement (and shifting your body weight toward the stern ) Obviously, you get plenty of lift without the 5 degree nozzle.

    A 0 degree nozzle does not interfere with the angle at which the boat travels through the water. The 5 degree does... i it wants to put the boat at a 5 degree angle of attack. True enough, stockers with the 5 degree nozzles routinely get 2-3 mph better speeds. What I believe though is that if you had a 0 zegree nozzle and could balance the hull at 2 degrees, then the hull would behave as if it were longer and thinner. A long, thin hull is always faster than a short and fat one. You would get your 2-3 mph back and them some! Not only that, a boat traveling at 60 mph with a 2 degree angle of attack is a much better ride than one traveling the same speed at 5 degrees. A long trued plate would come in very handy here.

    The old GP does not have as steep a deadrise angle, as the GPR. One the whole, it cannot lift out of the water as much as the GPR can without going through that porpoising thing. There is not a lot of wiggle room. A 2 degree set up won't allow the bow to lift to high. The whole hull would have to lift at 2 degrees, which is exactly what you want.

    I think that this is another reason why a longer plate is so advantageous. You have a longer surface area to plane. If you could set the plate at 2 degrees, that nose would automaticly be planted there at 2 degrees. The longer plate set at 2 degrees sets your center of lift more forward and the center of gravity futher aft. That set up minimizes porpoising and increases speed.


    A 98-99 GP with a 5 degree nozzle has a much greater tendency to slide compared to a 97 with a 0 degree nozzle set up the same way. That is probably the biggest reason we had to use those huge sponsons to grab the water in the turns. Not enough dead rise. It's like riding a flat bottom boat compared to a GPR! Maybe that's a little exaggeration

  10. #10
    just GOT TO DO IT! great lakes gpr 1200's Avatar
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    salty I just pm'ed you

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