Design modification when the designer suggests it is possible?

Discussion in 'Boat Design' started by Murky Deep, Feb 11, 2021.

  1. Murky Deep
    Joined: Jun 2020
    Posts: 26
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    Location: Massachusetts

    Murky Deep Junior Member

    I am putting together this boat from one of Gardner's books for fishing and general use (first image). In the current build, I am not changing anything from the design other than using marine plywood for the bottom instead of cross plank. Cross plank demands more skill at carpentry than I possess.

    Yesterday I reread the chapter searching for some piece of information and noticed the second photo, where Gardner states the boat redrawn with "considerably more" rocker would make a good sailboat for sheltered or semi-sheltered waters. In the future, this is a very tempting proposition, as I like tinkering with designs. The question is how much more? From the many hundreds of hours I have spent staring at boat plans, It seems that the stern transom would also need to be pinched in. The boat has 3" inches of aft rocker, in the sketches I have made, it looks like a sailboat with 6-7" of aft rocker.

    Any input or suggestions?

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  2. Olav
    Joined: Dec 2003
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    Location: Filia pulchra Lubecæ

    Olav naval architect

    Murky Deep,

    to be honest I would then consider a new design from scratch if you intend to build a sailboat.

    Sure, just about anything will sail in some way if you put a rig on it, but I doubt the boat from that book would be a good choice for sailing. The wide stern would cause terrible drag when immersed and with the almost wedge-like shape the bow would sink in significantly when the boat heels.

    A little sharpie hull like this is drawn very quickly and could then be designed for this particular purpose (i.e. sailing instead of motoring). Shoot me a message if I can be of any help; sounds like a nice fun winter weekend project to make some drawings and crunch some numbers. :)
    BlueBell and bajansailor like this.
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    It says "boats of this type" meaning flat bottom boats with a square stern; not that boat in particular.
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Can you explain the physics of that for me ?

  5. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    When a sailboat experiences wind forcing the boat forward, the wind force also generates a turning moment that seriously forces the bow down into the water. Sailboat hulls generally have hull forms that address this force better than hull forms that are not designed for sail. We are talking about the force driving the boat forward. Of course the wind force also usually generates sideways (athwartship) forces causing the boat to list, but for now just consider a boat running directly with the wind, without significant side forces.

    Consider a hull with propeller power (or paddle/oars), the force on the propeller shaft, driving the boat forward, is just below the waterline, and does not generally push the bow downward. A sailboat gets its driving force from the mast, roughly applied 1/3 the way up the mast, significantly giving a turning moment that can only be balanced with the bow going deeper, and the stern rising slightly. If the hull form does not gain considerable "Bow buoyancy" when pushed into the water, it will pitch down further than what might be considered prudent.
    rxcomposite and fallguy like this.
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