Fin vs Full Keel Revisted

Discussion in 'Boat Design' started by MichaelinSM, May 9, 2011.

  1. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    You start with a statement of requirements, so far you have the above. You don't give a draft that is significant as is the expected performance. Nor do we know of the expected crew, ease of rig handling, expected performance under sail and power.

    For sailing the underwater lateral plane area really is a function of sail area. The planform is then a function of draft. The lateral area needs to be sufficient to counter leeway for windward sailing.

    Longer keels often with attached rudders or double hung balanced rudders are simply sensible options for larger cruising boats.

    So you have 100 feet on deck, medium to medium heavy displacement seaworthiness is already inherent providing you keep the beam down and design to a reasonable GZ curve.

    I'd design to a 9 or 10 knot optimum and shape the Prismatic coefficient around that keep the rig smaller rather than larger. But pick the lowest wind speed you want to do that 10 knots in and you'll have an idea of the area of light air sails you want to fly. If you stay in the trade winds you only need half the rig you need if you want to sail around in idle breezes. All that comes into the statement of requirements.

    A 100 footer can sail in 20 knots with a draft limited (long keel) and still stay under 10 degrees of heel and have a limit of positive stability of 0ver 140 degrees. It'll sail to windward at 45 degrees well enough.

    A ULDB like an Open 60 or 70 is not a comfortable vessel but it can outrun weather only with advanced weather routing and 15 knots sustained speed. You cant outrun a gale if you are caught in it, that's not possible. The Open Series type racers are not just not suitable as cruisers and it's not helpful to compare them in a discussion about cruising boats.

    Some people, notably Claughton at Wolfson and others sucha s Renilson at the AMC have further refined Marchaj's work and observations and introduced further work on the safety of yachts in rough unpredictable seas.
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