Possible to extend LWL by protruding/trailing foil?

Discussion in 'Hydrodynamics and Aerodynamics' started by Niclas Vestman, Jan 29, 2020.

  1. Niclas Vestman
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    Niclas Vestman Senior Member

    About the propulsion part. Here is good link that gives an excelent and short summary of history and significant prototypes.

    WAVE POWERED SHIPS MARINE RENEWABLE ENERGY RESEARCH https://www.bluebird-electric.net/wave_powered_ships_marine_renewable_energy_research.htm

    Maybe unwise to discuss 2 entirely different objectives for the design. Only, if the concept of "artificiall LWL extension by means of trailing semiflexible fin (dolphin tail)" actually would work, there is a high probability that the foil-on-pole flexibility/motions could be made to work for both purposes. Maybe not simultaneously, but changed settings could adapt purpose depending on conditions.
     
  2. Niclas Vestman
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    Niclas Vestman Senior Member

    Question: Except for the obvious advantage of tabs, rudder-foils or interceptors being adjustable to modify trim depending on situational factors.... could a design with foil/tabs mimic a design without f/t but a length longer than that of the boat combined with trim appendages? Meaning. Does the effect of the appendage extend behind the actual tab/foil itself, more or less giving a longer theoretical LWL than the actual physical? Sorry if the anwser is in the linked pdf. Will read it next.
     
  3. Niclas Vestman
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    Niclas Vestman Senior Member

    I know my questions are quite "out there", and most likely compleatly impractical. It's more the general theoretical principle that is interesting to me in this case.
    Even if i associate trim tabs with planing vessels, it is actually a plausible alternative in my line of thinking. Many modern upscale sailboats come with "fold down tailgate style" swimplatforms. One solution could be significantly longer than usual sugar scoops on cats, that can be lifted folded up during transport or storing/morning. Eg making a 8.5m NZ boxrule cat easy transportable with 5' /1,5m fold upp sugar scoops.
     
  4. Niclas Vestman
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    Niclas Vestman Senior Member

    About the PDF, i'm not quite sure if the reduced drag mainly is caused by optimizing trim angle, or if it is a redirection of waterflow, that mimics a longer hull.
     
  5. Abaft
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    Abaft New Member

    Is the speed we are talking about here to do with increased displacement and don't foils reduce displacement?
     
  6. gonzo
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    gonzo Senior Member

    Foils lift the boat, which reduces waterline length. I think what you propose is to use a foil as a pseudo-bulb at the bow. I think that, as a bulb, it will have a very narrow range of efficient speed. Ignoring that, it is possible to modify the flow.
     
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  7. Niclas Vestman
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    Niclas Vestman Senior Member

    Thank you mr G! Love the fact that the knowlegable pros here on BDnet so often come to the rescue. I really appreciate your anwser. You make a very good comparison. I really was thinking along the lines of a "pseudo bulb". And having soaked up som facts and reading suggestions here on BD net on the topic of bulbs, I do get your point on the very narrow range of speed/conditions where a bulb is actually efficient. Thanks for your input
     
  8. Niclas Vestman
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    Niclas Vestman Senior Member

    No, I was thinking about distributing a fixed displacement to extend significantly aft and/or for of the actual hull. Reason 1) increase effective LWL and thereby also hullspeed. 2) increase the slenderness ratio to reduce drag. Sorry for not posting sketches, maybe I should have.
     
  9. alan craig
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    alan craig Senior Member

    I've seen double ended displacement motor boats with a delta shaped foil under the keel at the stern which presumably would lift when the boat squats when it reaches "hull speed". I guess you could also have foils above the waterline near the stern which would do the same job but would not cause drag at lower speeds.
     
  10. tspeer
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    tspeer Senior Member

    The answer is, "Yes." Take a look at this article by Kevin Elway.
     

    Attached Files:

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  11. Niclas Vestman
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    Niclas Vestman Senior Member

    Thank you! Very interesting article, and exactly what I was looking for.
     
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