Traditional hull high Cp designs sailing downwind

Discussion in 'Sailboats' started by Vineet, Sep 26, 2022.

  1. Vineet
    Joined: Apr 2021
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    Location: usa

    Vineet Junior Member

    Hello all,
    Can any good folks offer an explanation of the downwind rolling effect of heavy displacement, high Cp boats and also explain why they are heavy on the helm sailing downwind in a bigger seastate? Why and how does this occur?

    Thank you
  2. Skyak
    Joined: Jul 2012
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    Location: United States

    Skyak Senior Member

    I am not sure I understand the question, but I know and have experience with three large contributors to role sailing downwind.
    1) The large area sails at high angles of incidence that you need for low apparent wind speed are naturally unstable aerodynamically. As they build and shed vortices the force will be cyclical.
    2) The "heavy displacement" boats you are asking about are likely to be experiencing very low frequency wave motion sailing downwind driving rolling motion.
    3) Sailboats have a low intrinsic natural frequency and high moment of inertia (by design in the heavy boats you are asking about) so they are susceptible to the driving forces from 1 and or 2 when they hit the boats natural frequency.
  3. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Is this even true?

    I would think the low cP vessels would have this problem more than the high cP ones.

  4. Skyak
    Joined: Jul 2012
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    Location: United States

    Skyak Senior Member

    The three contributors I listed are present in all vessels to varying degrees in different conditions. You are likely just thinking that a light sailboat with lots of form stability and a tall mast would be more violent bobbing around in steep waves compared to the relative comfort of a heavy displacement cruiser. The OP didn't ask about a comparison and likely would never want to be on a light sailboat.

    To answer the second part of the OP,s question, heavy full keel sailboats have keel hung rudders that are intrinsically unbalanced and limited in span. If you don't want to extend below the bottom of the keel (NOBODY wants to extend the rudder below the keel) there is no alternative to add power other than extending cord further from the pivot center which increases the moment exponentially. Down wind and down waves the heavy sailboat hits maximum speed so the force on the rudder is maximum (force is proportional to velocity squared) and when the boat is heeled rudder effectiveness declines so the force on the helm is at its maximum.
    Vineet likes this.
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