Undesirable inclination

Discussion in 'Powerboats' started by vander, Jul 20, 2005.

  1. vander
    Joined: Nov 2003
    Posts: 8
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    Location: Spain

    vander Junior Member


    Some days ago I launched a small fishing boat, 7,45 Loa and 2,40 Bwl.
    She has a deep V shape in bow and almost flat aft, with full keel and conventinal rudderstock.
    Engine is a 250 Yanmar, and 2800 Kg of displacemnt.
    When I am near 18 knots, the boat starts an inclination to portside.
    The weights onboard seem to be very equaled, and I have to put down the trim tab to correct this. At this speed the boat has a nice trim and good planning
    I don't find the reason of this, could you give me some help?

    Best regards
  2. mmd
    Joined: Mar 2002
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    Location: Bridgewater NS Canada

    mmd Senior Member

    Vander, this can be a complex problem (it is called roll instability), and is usually related to hull form or appendages. Donald Blount and Dean Schleicher published a good article on the subject in Professional Boatbuilder magazine (Edition No. 84) in August 2003. Professional Boatbuilder also ran an article on planing hull stability in Editions No.31. It may be worth your while to order these back issues from http://www.proboat.com/

    The causes for roll instability listed by Mr. Blount and Mr. Schleicher are :

    • Boat too heavy for its size
    • Longitudinal centre of gravity too far forward relative to longitudinal distribution of chine beam
    • Hook in hull near transom
    • In profile, the buttock curvature is too extreme near the bow quarter length or extends too far aft
    • Rudderpost ventilation
    • Rudder ventilating near hull
    • Rudder toe-in or toe-out not properly set
    • Trailing edge of rudder too close to or extends aft of transom
  3. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    Did You check initial stability of You boat at zero speed? How much is GM?

  4. vander
    Joined: Nov 2003
    Posts: 8
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    Location: Spain

    vander Junior Member

    I attacht hullform while I find out some data from the hull

    Attached Files:

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