delta pad stability effects

Discussion in 'Stability' started by rappitysnap, May 1, 2012.

  1. rappitysnap
    Joined: May 2012
    Posts: 2
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    Location: florida

    rappitysnap New Member

    my boat is a 29 c-hawk pilothouse 15 deg. deadrise parker style hull. light displacement 5700# without power dry. currently the boat is powered by a 300hp fourstroke on a bracket. Boat seems a little stern heavy with this set-up. my plan is repower with a 6bt cummins diesel. The boat as it is has a fast snappy roll on a beam drift. So i am wondering how much if any difference will the stability increase with the weight of the diesel compared to the outboard. Also I was considering cutting out a delta pad for the running gear to minimize draft and ease for trailering. What kind of effects does the average pad for this purpose have on stability and bouyancy. Will the pad raise or lower the waterline at the stern?
  2. MLC
    Joined: Dec 2011
    Posts: 14
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    Location: Port Townsend WA/ New Oleans

    MLC Junior Member

    It sounds like you will be adding around 500-800lbs, (roughly estimated), depending on the various modifications you undertake.

    Adding weight will decrease the BM, (Metaceteric Radius), Increase the VCB (Vertical Center of Buoyancy), and switching from an outboard to an inboard diesel will, in all likelihood, lower the center of gravity. The decrease in BM will have a negative effect on stability, the increase in VCB and the decrease in VCG will have positive effects on stability.

    Exact predictions can't be made without the lines plans, but my guess is that you will not have any reduction of stability directly due to the re-power.

    As for the quick "snappy roll," more knowledge of the hull would be required. The increased weight would act to slow the roll, but the lower VCG would likely act to speed it up. I can't guess more likely result in this case.

    If the snappy roll was the cause for your concern for the stability, you can relax. A quick roll period is usually a sign a of good stability. The quick roll is generally caused by large righting arm, which causes the vessel to react quickly to the motions of the seaway.

    Regarding the delta pad, I am not 100% sure on what it is. I am picturing it as a flattened portion of the bottom near the stern. If that is what you are talking about, you will reduce the draft, since the water plane at the delta pad is significantly smaller than the water plane at the designed waterline. Depending on how much you wan't to reduce the draft, a tunnel might be an option.

    The effects on stability and buoyancy should be negligible under normal circumstances. If trim at planning speeds is a concern, it is simple to adapt design to account for this.

    Before I get ahead of myself, some important questions to ask would be: how fast do you plan to go, how fast did it go before, how much power does the b-series you are looking at have.
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