box keel runabout and safe speed

Discussion in 'Boat Design' started by srimes, Sep 17, 2008.

  1. srimes
    Joined: Sep 2008
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    srimes Senior Member

    I'm considering building a small (12'-16') flat-bottom w/ box keel, low-power runabout. The inspirations for this boat include the Boldger Sneakeasy and the displacement glider.

    It seems to me that the boldger step sharpies aren't quite what I would call "box keels." The center sections look quite wide and shallow, with significant keel depth only at the bow. I'm thinking of having the box keel be more narrow so that it runs in high-speed displacement mode, with a l/b ratio somewhere between 12 and 20 (I know, big range).

    My main worry is safe handling. I don't want the boat to "trip" over the keel. I'm not designing a fast boat, the expected max speed is in the 15-25 range.

    I'm also curious about picking the box keel l/b ratio.
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    FAST FRED Senior Member

    Most of the boats I have found with box keels do not attempt to operate at SL over 3 or so.

    At that speed the boat is basically PLAINING and although there might be some dynamic lift from the bottom of the keel (like a water ski) and a true plaining hull design is called for.

    At a SL over 1.3 and under 3 the box keel does seem to lower resistance , and wake making.

  3. srimes
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    srimes Senior Member

    I understand that their effeciency vs. simply planing drops off at higher speeds, but are they safe? What happens when the bow pierces a wave during a turn?

  4. messabout
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    messabout Senior Member

    Use Bolgers ratios. The old boy has "been there, done that". If you make the box long and skinny, it will contribute lift in proportion to its width while in planing regime. That implies that you would need a bunch of power to get the boat up on the box which is now a ski. Boats like Sneakeasy operate nicely with very modest power. Part of that is due to the considerable length of the boat. The shorter box keel tugboat (forgot it's name) takes more power than Sneakeasy to operate at equivalent speeds. That may be the result of differences in weight and windage.

    Box keels have the advantage of enhanced longitudinal bottom stiffness while having the disadvantage of lesser bottom stiffness athwartships. You have to fix the sideways disadvantage with plenty of frames.

    Keep us posted about your progress.
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