Fore/aft location: widest beam questions

Discussion in 'Boat Design' started by Apple Hill Boater, Mar 3, 2019.

  1. Apple Hill Boater
    Joined: Mar 2019
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    Location: International Fall MN

    Apple Hill Boater Junior Member

    As an exercise in boat design - and maybe building in the future - I am working on a recreational rowing boat. Similar to a wheery, Whitehall, or a Ruth by Gentry.

    My current question at hand regards the fore/aft location of the widest beam.
    • Rules of thumb?
    • Effects of moving it forward or aft?
    • How far from F/A center is ideal, typical?
    • Is fore/aft location dependent on ration of LOA to beam?
    My personal design brief:
    • Stable in open water, sometimes choppy, sometimes breezy weather
    • Able to be rowed relatively fast and track well
    • Able to be easily moved with one rower but able to carry a passenger and some gear for a day on the water.
    • Lightweight (less than 50 lbs)
    • Likely skin-on-frame construction
    Thanks in advance to the boat design brain trust.
     
  2. nzboy
    Joined: Apr 2011
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    nzboy Senior Member

    With a small boat you need to study centre of flotation which will need to take into account where you are sitting in your boat
    this will more than likely dictate where the widest beam will be
     
  3. Apple Hill Boater
    Joined: Mar 2019
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    Apple Hill Boater Junior Member

    Good thought, thanks.
    So if I account for two people, one rowing, and work out from there. Might that be a good approach?
    Should widest beam be where the rower is sitting, or maybe a little aft of the rowing position ? Generally speaking.
    Not quite ready for mathematical modeling at this stage.
     
  4. nzboy
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    nzboy Senior Member

  5. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Selecting the widest point of the boat is not a cookbook kind of decision. The rocker of the bottom will influence the location of the widest point. It is not uncommon with sailing dinghies to have the deepest part of the bottom a bit forward of the mid section and at the same time the widest part of the bottom will be somewhat aft of the mid section.

    The differences have something to do with the entrance angle of the boat, the distribution of the section areas and design displacement of the boat. Human powered boats like canoes and kayaks sometimes have the widest part well aft of the mid section. Not to say that is a recommendation because there are a whole basket of reasons that the designer addresses when he/she does the design work. Do the arithmetic and draw a curve of areas to determine whether the curve is smooth and has no abrupt changes . In my own set of preferences, I do not want my area curve to have reflexive sections. .

    Many catamaran designs have at least two adjacent sections that have the same area and that area is almost always at the largest section area location. Think of the way water is being displaced and then being brought back together as exit point is approached. There must be some kinetic energy in play here. First we are pushing the water aside, and then allowing it to return to its original state. The transition point surely needs to be as gentle as you can make it. My poor old brain is constantly troubled by trying to accommodate Newtons little equation: F=Ma. The little a is factor that denotes acceleration. If we move water or anything else, acceleration is involved. Therefore some Force (F) is involved. M is mass and that is the weight of the water that you are forcing out of the way. That simple equation makes big trouble for all sorts of objects including nice little boats.

    Perhaps some of the other members will have more direct information or comment.
     
  6. Apple Hill Boater
    Joined: Mar 2019
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    Location: International Fall MN

    Apple Hill Boater Junior Member

    That Whitehall sure is a pretty boat and along the lines of what I'm after. As an exercise in learning more about boat design, I'm trying to create my own from scratch. I have no doubt it will end up looking like a master designers, this being a well trod path. But learning how to decide where to put what dimension is part of the experience I am after.

    Good stuff!
     

  7. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Your usage will dictate it to a large degree. You said that it would carry two people, a rower and passenger. With a 14’ boat your seating options will be limited. A lot of small rowing boats will have two rowing positions, one further forward when carrying a passenger aft, and one more centered when rowing alone. If your beam is especially exaggerated in one location, that will have trim implications under different load configurations.
     
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