Would full extension swim platform add speed?

Discussion in 'Hydrodynamics and Aerodynamics' started by Danviletim, May 18, 2014.

  1. Danviletim
    Joined: Nov 2013
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    Danviletim New Member

    We have a 62 striker and need / want a swimplatform. We plan on having an integrated aluminum platform fabricated and welded on. The question is whether this is benefit to go a bit further and extend the hull by 3 '.

    The boat has had a trim plane / after plane added that is 10" and supposedly added 3knots. Boat has 3412 cats that wot at 29kts.

  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If it really added that much speed, the boat must be stern heavy, I'd experiment with a temporary set-up before going to a final decision. You could support in with struts off the upper transom, and attach to the existing extension as well
  3. Yellowjacket
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    Yellowjacket Senior Member

    Whether or not you would gain speed with an increase in planing area is dependent on where you are now with the planing flaps, or to put it another way, if the planing angle is too high at max speed now, then adding area and reducing the planing angle will gain speed.

    If the planing angle is low, then adding area won't help. Typically planing angle should be around 3 degrees at best efficiency.

    So the real question is, what is the planing angle now when running at WOT.

    Also remember that if you make a change to reduce the drag, you may need to change props to take advantage of it.

    Lastly, increasing planing area will likely improve the efficiency at cruise speed even if you don't get any more speed at the high end.
  4. tom28571
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    tom28571 Senior Member

    The effect of adding a transom hull extension is, or can be, a fairly complicated design question. Its effect on performance will be different across the speed range and depends very much of what the boat performance was before making the addition. As yellowjacket says, the trim angle of the boat will be changed (reduced) by adding the extra planing area aft. Highest efficiency, or lowest resistance, is where the sum of wavemaking and frictional resistance is minimum and that varies with the deadrise of the boat with deep V hulls needing higher trim angle (as much as 6 1/2 degrees) than low deadrise hulls which usually take about 4 1/2 degrees as optimum.

    Now, optimum trim angle depends on the use the boat is intended for. High trim angle may give higher top speed but is rougher riding, makes for poorer steering and is, generally less satisfying to the passengers. Low trim angle allows the sharper bow to penetrate waves rather than the flatter bottom further aft so less slamming results. Low trim angle also means that more waterplane is contacting the surface and that leads to quicker planing at lower speed.

    Racing boats go for minimum resistance and cruising boats accept lower WOT speed for easier low speed planing and a more comfortable ride. A designer will, or should, choose a desirable trim angle and design the boat for that rather than draw the boat and have the trim angle be a surprise. I design cruising boats and choose a trim angle of 2 ½ to 3 degrees for quick low speed planing, smoother ride in chop and better view from the helm over the bow. This means lots of waterplane (low bottom loading) at the cost of a knot or so of top speed which I consider a good tradeoff in a cruising boat. Other features like trim tabs and a hook or rocker in the aft bottom can be used to affect or correct a poor trim angle also.

  5. tom28571
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    tom28571 Senior Member

    I notice that after all the explanations, the OP's question was not really answered.. The most likely effect of adding a transom extension is that the hull speed will be increased and the WOT speed may likely be decreased. Whether this is good or bad is up to the user of the boat.
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