What are These?

Discussion in 'Hydrodynamics and Aerodynamics' started by fritzdfk, Feb 26, 2015.

  1. fritzdfk
    Joined: Jul 2004
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    fritzdfk Junior Member

    These photos show the stern of one hull on a small power cat. It is hard to tell but the hulls have a horizontal flat (still V) section on each transom acting as built in trim tabs. The hulls have rocker up to the start of the "trim tab". The photos show added on vertical pieces on either side of the "trim tab". The vertical pieces are not from the original designer but added by a builder. He says they contain the flow past the trim tab section and also contribute to tracking of the boat. I can readily understand the they would contribute to tracking but what do you think about "containing the flow" over the trim tab?

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Does the boat run OK, or is there an issue you think these appendages are causing ? I think it likely those fins would increast the effect of that "hook", depending on what speeds we are talking.
     
  3. BMcF
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    BMcF Senior Member

    "End plating" transom trim tabs or interceptors (and even fins and foils) does result in some increase in total lift, but those skegs are so large that I would assume their contribution to directional stability is their main purpose.
     
  4. NavalSArtichoke
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    NavalSArtichoke Senior Member

    I agree. These appendages would function like skegs on a barge, to increase directional stability
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Perhaps only serve to support the boat when beached, while protecting the outboard.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It is hard to see in the pictures, but there appears to be an abrupt change in the bottom aft, as well as those skegs. Of course they will have an effect on directional stability, but I agree with what the OP says about containing the flow over the "tab", especially where there is a decent angle of attack, they will act as a fence to some degree.
     
  7. fritzdfk
    Joined: Jul 2004
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    fritzdfk Junior Member

    Thanks for the replies. I suppose only experimentation would show how big they need to be to 1. add needed directional stability and 2. increase lift from the "trim Tabs". I have built the same hull and I do not have the foils. The boat is somewhat annoying to steer requiring constant attention and there is squatting of the sterns under power. It is only a moderate speed boat and cruises around 10-12 knots. I am considering adding the foils to my boat but am wondering if they have to be so deep. Improving steering would be great and limiting squatting would help as the single outboard is centrally mounted and configuring the outboard mounting height and outboard fairing design would be simpler and maybe be could be kept higher out of the water.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    What you pick up in lift, you will pay for in drag, probably. Is this a sail-boat conversion ?
     
  9. fritzdfk
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    fritzdfk Junior Member

    The design was originally a sailboat but was modified by the designer for power. The hull profile was changed, I don't know exactly how.

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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, I had a mental image similar to that. You could probably improve the skegs by tapering the trailing edges, to lessen drag slightly.
     
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