new flat hull shapes

Discussion in 'Sailboats' started by bob the builder, Aug 4, 2009.

  1. bob the builder
    Joined: Jul 2009
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    Location: mooloolaba

    bob the builder novice

    are flat, fat and planing

    the question is, how much more drag do they have? twice?

    (they've surely got more than twice the SA of a minimum drag jobbie)

    Van Gorkom


    open 60
  2. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    A real drag

    When you talk about drag, you need to specify which kind.

    In sailboats, there are three kinds of drag.

    1.) aerodynamic, caused by freeboard, cabin structures, and rigging,
    2.) friction drag, caused by water molecules trying to slide past the hull surface, and
    3.) wave making drag, caused by the hull making waves as it goes through the water.

    A general purpose sailboat, with limited Sail Area (SA), the goals are to limit drag by reducing, first, frictional drag, then wave making drag, then aerodynamic drag.

    This is where the round bottom and deep fin keel come in.

    Most modern sailboats have flattish bottoms, in that whatever curves they have save little surface area over a dead flat bottom.

    This is for two reasons:

    1.) The flatter bottom provides greater initial stability allowing more SA to be carried. This SA increase, by the way, is quite out of proportion to the added hull area. This has been well known since Viking times

    2.) The flatter hull tends to make less waves when going through the water at the higher speeds that the increased SA allows, furthering the advantage. Such hulls can surf when running before large waves and, if extreme enough, can even plane like a powerboat.

    Naturally, if you are going to go to that extent, aerodynamic drag must be dealt with ruthlessly, hence the low, streamlined houses and the very limited rigging.

    As you can see, there are few simple answers in boat design.

    In most circumstances, a sailboat's drag is about half friction and half wave making. So even if a flattish hull had twice the surface area of a deeper rounder hull, which, by the way it usually doesn't, it would have only 33% more drag. And that is only when it is going at a relatively slow pace of 1.0 * Water Line Length^0.5. The faster the boat goes, the lower the percentage of the increased frictional drag.

    Wave making drag increases with the square of the increased speed (twice as much speed means four times as much drag) where frictional drag increases in direct proportion to the increase in speed (twice as much speed means twice as much drag).
    Last edited: Aug 4, 2009
  3. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    generally for the same size sailboat, wide beam flat bottom hulls are designed to plane. When the hull reaches planning speed, it lifts out of the water and drag drops way off, down to perhaps 10-20 percent of a non-planning hull at the same speed. But if you are not on plane, the displacement hull, designed narrow and with more rounded surfaces (to minimize wetted area) will have less drag than a wide flat planning hull at the same displacement speed. Narrow rounded hulls do not plane easy, and wide, flat, sharper corner of a planning hull will have higher drag below the planning speed.

    So it will cost you at one of the speed range or the other.
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