Trim Tabs on a Displacement Hull?

Discussion in 'Powerboats' started by big-boss, Aug 18, 2008.

  1. big-boss
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    big-boss Junior Member

    I think I have a Uniflite 25 ft fish boat w/ flybridge. It has single engine inboard with a keel that runs most of the bottom. I believe it to be late 60's boat.

    It is out of water and I am repainting. I found the reminents of trim tabs on the back-full width of boat. Do they put tabs on displacement hulls? Or do I not have a displacement hull? Thanks.
  2. PAR
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    PAR Yacht Designer/Builder

    Without pictures of your boat it would be very difficult to tell what you have.

    It was once common to use shingles and tabs to correct a bow up condition on some types of displacement hull, but not very common today.

    As a rule, you shouldn't need tabs, unless she's stern heavy, has a particularly narrow stern, is way over powered, etc.
  3. TollyWally
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    TollyWally Senior Member

    I believe a Uniflite is at least a semiplaning hull. They were a well thought of boat in thier day, built up here in Puget Sound country.
  4. fishingaz
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    fishingaz Junior Member

    we owned a roberts longboat fisherman version. full displacement hull and it had a 100 mm wide full width glassed in type of trim that worked very nice at 6 to 8 knots.
  5. DMacPherson
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    DMacPherson Senior Member

  6. heuhen
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    heuhen student in progress

    In norway some people buil trim tabs on ther hull to higher speed. like this Viksund 770 some has a top speed at 15 knoots


    This boat was build by Viksund yacht in 1982, the engine is a 72hp Yanmar
  7. Çemberci
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    Çemberci Senior Member


    NPL model is a good example for Influence of the transom wedge

    oktay çemberci

    Attached Files:

  8. lancingmike
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    lancingmike New Member

    Trim Tab effects

    The force that a trim tab exerts depends on its area, the sine of its angle of attack and the square of the boat speed, so at half the boat speed, the tabs will have only a quarter the power.
    On boats running at the top end of displacement, or into the low range of semi-planing speeds tabs can still help fore-and-aft attitude to a certain extent, and can have considerable influence on roll attitude as hulls of these forms are easier to roll, being more rounded.
    They also tend to stabilise the boat at anchor in a seaway due to the weight of water on top of the tab as that side of the boat rolls upwards and the "stiffness" of the water under the other tab.
  9. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    Is the Viksund 770 pictured three post above on plane with 72hp??? Is that a 7.7 meter boat? That seems like a lot of boat to be on plane with that little of an engine.
  10. rasorinc
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    rasorinc Senior Member

  11. charmc
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    charmc Senior Member

    US Navy refit of the Yorktown class Aegis cruisers currently underway includes retrofit of a stern tab:
    Under the surface, a stern flap has been installed to create a vertical lift force at the transom. This alters the pressure distribution on the aft portion of the hull, reducing the drag and thus modifying wave resistance. Ultimately, less power is required and so fuel consumption is reduced. By maintaining ship speed using less power and a lower shaft speed, the cost of maintenance is reduced and the life of the propulsion plant machinery is extended. Other benefits to stern flaps include reductions in propellor load, cavitation, vibration, and noise.

    As others said, there appear to be significant advantages to adding a stern flap/tab to displacement hulls. But, big-boss, I concur with TollyWolly; a Uniflite 25 would be a semi or full planing hull, and so would benefit even more from adjustable tabs. A Uniflite is a terrific basis for reconditioning; they were known as very strong, well built hulls. In 1965 the US Navy gave Uniflite the exclusive contract to build its 31' River Patrol Boats for Vietnam service. The 31' RPB was the boat featured in Apocalypse Now.

    BTW, Merry Christmas to all!
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Trim Tabs are also a way to correct a poor deisgn, cheaply!
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    FAST FRED Senior Member

    Boats change , engines get swopped , and different owners think different speeds are where they want to cruise ,today..

    Every boat will have an optimum angle of attack, at a selected speed, power and loading.

    Trim tabs are a way to optomize a boat to its current owners needs.


  14. heuhen
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    heuhen student in progress

    The max.. engine you can have one Viksund 770 is 80 hp. that give a top speed at 13 knoots without trim tabs. And it have a 7.70 meter length, beam at 2.50 meter, depth at 0.80 meter and a wight at 2000kg
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