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Thread: Sponsons

  1. #1

    Sponsons

    I watched some close up video of my ski running across the water mostly paying attention to lift,attitude and looking for drag areas (abnormal outward spray).
    The two main places were sponsons and trim tabs, Sponsons being the worst so I will work on those first.

    They are currently stepped, but heres a few things I was considering and why.

    1.sharpening the bottom- a sword cuts through water better than a 2x4.
    2.trimming the bottom-the shallower they cut the less drag there should be.
    3.Flatening the boottom-making them work more like skis or hydrofoils to create lift and give more of a sliding action over cutting the water.
    4.Angle the last several inches of the sponson to the same angle as the ride plate- this would keep the plate and sponsons from fighting against each other and work together instead.

    Im sure most if not all of these things have been tried, if you have any feedback on any of these or a combo of these I would love to hear it. It would keep me from having to buy 4 sets of sponsons and weeks of testing.

    Thanks, kevin


  2. #2
    spud's Avatar
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    Bill and I have talked a lot about sponsons. There are many things to try and the stock sponsons are cheap. I do think they are important for stability. For me the stock sponsons with the little holes filled flush in the stepped position are fastest. Up would be faster but my handling goes to heck.

  3. #3
    MR. SCOVILLE UNIT SUCKMYWAKE's Avatar
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    the sponsons are out of the water at speed. that is why there are skegs on the ride plates

    let me regroup. the sponsons are barely in the water
    Last edited by SUCKMYWAKE; 12-29-2006 at 09:03 AM.

  4. #4
    All hail the Chief! fullboogie's Avatar
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    Quote Originally Posted by SUCKMYWAKE View Post
    the sponsons are out of the water at speed. that is why there are skegs on the ride plates
    Then why do we step them to prevent (or cure) porpoising?

    Pork, don't assume that sharpening anything will help. We've already proven that with the intake grate. Knife-edging the leading edges of the grate does absolutely nothing for speed. Don't know why, but you may find the same result on the sponsons. Interested in your findings, though.

  5. #5
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by fullboogie View Post
    Knife-edging the leading edges of the grate does absolutely nothing for speed. Don't know why...
    I think that some of that may be efficiency. It's tough to wrap the mind around, but at different angles of attack, the knife edge on a grate will actually "break" the water, as opposed to allowing it to "flow" smoothly.

    This is a portion of an article written by Kevin Cameron...



    As the boat gets under way, another source of intake pressure develops: the speed with which the water approaches the grate. This dynamic pressure is why cavitation is much less likely at higher boat speeds.
    Because the grate does not squarely face the water, it has to be equipped with one or more vanes whose purpose it is to scoop water up into the intake tunnel. These turning vanes are in effect little wings. Each of them has a high-pressure side (the side facing the incoming water) and a low-pressure side (the opposite). And like wings, these vanes can stall. The flow over the high-pressure side has no choice but to follow the vane surface, but the flow trying to follow around the low-pressure side may "feel" so much centrifugal force as it makes the turn that it can no longer remain attached to the surface. If this happens, the grate vane itself generates a sheet of cavitated flow, which can then cavitate the pump itself. Impeller and pump specialists point out that leading edges of things like the anti-swirl vanes behind the impeller are rounded for a good reason. The angle at which the water approaches them changes with impeller and boat speed, and round edges are easier for the waterflow to curve around. Knife-edging seems like it ought to be an improvement, but when the water approaches such an edge at an angle, the flow will cavitate behind it on the low-pressure side, reducing efficiency.

  6. #6
    Good info guys. At full speed most of the sponson is out of the water but the last few maybe 5 inches are in the water and the spray was being sent outward from that. That may be from that last bolt hole that you mentioned spud. My ski is like a bronco unless the sponsons are stepped, its like night and day.

    Has anyone tried leaving them stepped but making the last few inches the same angle as the ride plate? I think mine was 4.85 degrees.

  7. #7
    spud's Avatar
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    There was an old picture of a Fountain ski that had the blades cut off with the mounting body of the sponson left intact. I couldn't tell you if its stepped or not.

    That really isn't the kind of speed I'm after. I want to be able to turn if I need too.

    My skis ride like crap if I don't leave the sponsons stepped.

  8. #8
    Good point spud.

  9. #9
    One day at a time..... N8R's Avatar
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    i have a set of sponsons that i had made and will test after i hit 80. i will post the results later.

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