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Thread: Length Beam Ratio

  1. #1
    Join Date
    Jun 2020
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    Hawley, Pennsylvainia, United States
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    Default Length Beam Ratio

    Hi, I have been doing research on Downeast style hulls, in an effort to design one myself. A good friend of mine is assisting me, and told me to look into length beam ratio, and decided I would ask the forum. Is there any formula for finding this?

    I would like my hull to be 25ft LOA, with the capabilities of a cruiser and workboat. I would like the beam so that it doesn't compromise stability.
    May the winds be forever in your favor,

    Ty

  2. #2
    Join Date
    Nov 2004
    Location
    Port Townsend WA
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    14,462

    Default Re: Length Beam Ratio

    Depending on the depth and displacement of the hull, if the boat is to have a fixed ballest keel and is not designed to plane
    six to seven feet of beam is an accepted criteria for a twenty five foot sail boat. Make a half model and see how you like the shape. Trust your instinct!
    Jay

  3. #3
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    Jun 2020
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    Hawley, Pennsylvainia, United States
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    15

    Default Re: Length Beam Ratio

    Would that be the same for a downeast lobster boat?
    May the winds be forever in your favor,

    Ty

  4. #4
    Join Date
    Jun 2020
    Location
    Hawley, Pennsylvainia, United States
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    15

    Default Re: Length Beam Ratio

    Would that be the same for a downeast lobster boat, or would the shape of the hull affect that?
    May the winds be forever in your favor,

    Ty

  5. #5
    Join Date
    Nov 2006
    Posts
    9,136

    Default Re: Length Beam Ratio

    I have an old book written by Royal Lowell, "Boatbuilding Down East", wherein he describes the building of a thirty-six foot lobster boat. The beam of this design is eleven foot six, so three-to-one. Lobsterboats are being made much wider these days but that's for commercial reasons and probably requires an inordinate amount of power.

    Jim

  6. #6
    Join Date
    Jun 2000
    Location
    Pleasant Valley NS Canada
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    21,467

    Default Re: Length Beam Ratio

    Length-to-beam ratio is usually the length and beam on the waterline, but Downeast boats and others of the type usually have plumb bows and sterns, or nearly so, so that LOA and BOA can be used without undue numerical error. Going fast usually entails a high L/B and load-carrying capacity usually means a low L/B. (This is a bit simplistic, as prismatic coefficient, block coefficient, and angle of buttocks also enters into the picture.) But to keep it simple, fast Downeast and Northumberland Strait type boats usually have an L/B ratio of 3.75 - 4.35:1. Fast Cape Island lobsterboats are usually a little chubbier, with an L/B of 3.5 - 3.75:1. A couple of decades ago, 'normal' ratio was between 3.00 / 3.25:1, but (at least here at the southern tip of Nova Scotia) boats have become ever fatter, so the new normal is 2.60 - 1.95:1, and some are going as big as 1.5:1, which I find mildly ridiculous.

    Stability is also a factor. Simply put, the beamier the boat the more stable it is. But beam affects speed, so too narrow (to go fast) makes the boat pretty tender at rest. Also, if this is a boat to be occasionally trailered, if you exceed your state's maximum road width, you have to pay for a wide-load permit and maybe even hire escort vehicles.

    So, before I get too deep into the subject for anybody to bear, my advice: For a 25-ft LOA boat that is legally trailerable, beam will be 8.25 ft. This works out to be a L/B ratio of 3.03:1, which puts you right in the 'sweet spot' for the best compromise between internal volume/stability and legal towing size. With the buttocks kept long and flat, you should see a comfortable cruising speed of around twelve to fifteen knots with relatively low horsepower.
    Hope for the best, but plan for the worst.

  7. #7
    Join Date
    Jan 2003
    Location
    New Zealand's Far North
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    9,764

    Default Re: Length Beam Ratio

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    I would rather have doubt than be certain and wrong.
    Richard Feynman.

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