Deep and narrow?

Discussion in 'Boat Design' started by JohnBobson, Dec 7, 2020.

  1. JohnBobson
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    JohnBobson New Member

    I'm sure there is a reason, but how come there aren't many deep and narrow boats?

    Basically instead of a 30 foot boat with a 10 foot beam and 5 foot draft.

    Why not a 30 foot boat, with the buoyancy part of the hull 2 and a half foot wide all the way down to the same 5 foot draft? Then build a cabin on top above the waterline?

    It seems when heeling (or dealing with a side wave) it wouldn't shift the center of buoyancy as much as wide and relatively shallow displacement hulls have. And if you put more of the weight right at the bottom of the draft, then you could try and shove the center of gravity down more than normal keels?

    It seems like the much more narrow hull might move ok through the water? A catamaran is faster than a mono, but if you take both amas and their displacement under the water, then it's actually pretty deep combined (so seems similar in amount of water displaced with the thinner hulls).

    The only thing I can think would be a big issue is the lower center of buoyancy. A normal sailboat has one heck of a high COB, which makes it easier to get the center of gravity under that for stability. So seems like maybe if you made it heavier and tried to get that keel weight as low as possible it could compensate for the lower center of buoyancy?

    Anyhow, just armchair theory. I'm sure there is something obvious I'm missing.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    The most obvious thing is that a boat with 5 feet draft and 10 feet waterline beam, as opposed to one with 5 feet draft and 2.5 feet waterline, but widening quickly above the waterline, will have a lot more displacement. So the latter boat will need to be quite light. It sounds like you are talking about a box keel thing, like the Bolger boats.
    Bolger.jpg
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Welcome to the forum JohnBobson,

    2 things basically:-

    1. increase in wetted surface area = more friction = more drag
    2. .stability!

    No.2 being the dominant one!
     
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  4. The Q
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    The Q Senior Member

    An old design style of boat was nicknamed a "lead mine" they were long, thin, had a very low coach roof, if any, but gave full head room because you were effectively standing in the keel.
    They tended to look beautiful, not looking like a bulbus caravan on water. But they were their nick name.. Lots of lead in the bottom of the keel to keep the boat up the right way..
     
  5. latestarter
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    latestarter Senior Member

    Physics.
    Initial stability is based on the water plane, it is proportional to the length x the cube of the beam. So a 2.5' wide boat will be 64 times more tender than a 10' wide one.
    Load carrying. For each extra lb added the thin boat will sink 4 times more the wide one.
    Comfort, living on a heeled boat is not popular.

    This assumes that the designer of the wide boat was not trying to optimise his keel.

    If you make it heavier with the same length and beam the draft must increase.
    Maybe you could replace the lead with Osmium or Iridium ;)
     
  6. JohnBobson
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    JohnBobson New Member

    Been trying to find out more about that after you posted it, but I can't find much information on them. It seemed like they were power boats though?

    I was able to find some examples of the box keels too. It seemed like a lot were pretty shallow (and more of a feature to reduce draft?).

    I figured it might be more stable in waves though? A catamaran is pretty dang stable because of the form, but then when they hit a steep wave that stability seems to be a drawback and the boat matches the wave slope.
     
  7. Waterwitch
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    Waterwitch Senior Member

    What you are suggesting has been tried but not as extreme as 2.5 ft beam. for a 30 footer. it is a very tender hull form prone to flooding even with heavy ballast is the reports of people recreating old english designs. https://goodoldboat.com/images/articles/6beam.gif
     
  8. sharpii2
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    sharpii2 Senior Member

    A major problem is that it doesn't develop much righting moment until it heels about 30 degrees.

    Imagine standing in a telephone booth that is tilted 30 degrees.

    If it has sufficient freeboard, it will be quite seaworthy indeed. Maybe even more so than a more conventional hull.

    But seaworthiness is not the only design criteria of a boat. Usefulness for its intended purpose counts almost nearly as much.

    In fact, once an acceptable level of seaworthiness is reached, other design criteria quickly take over.

    I once drew a section drawing of a plank-on-edge type of hull.

    It's underwater section was deeper than it was wide.

    The center of buoyancy ended up to windward of the center line. But the center of gravity ended up even more so.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously something 30 feet long and two feet wide is pretty useless, I surmised the OP was alluding to boats with box keels that suddenly widen dramatically at the waterline, which I am sure has its own problems, notably slamming.
     
  10. JohnBobson
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    JohnBobson New Member

    That's pretty interesting. Any chance you have links to find out more? Or ships that I could try googling?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    That one waterwitch posted, when the tide goes out.....Houston, we have a problem.
     
  12. bruceb
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    bruceb Senior Member

    Look at a J-90 for a fairly modern one done properly. They sail quite well but really lack on interior space. Nice boats though.
    B
     
  13. Waterwitch
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    Waterwitch Senior Member

    The genre of narrow Victorian era English boats are described as plank on edge English cutters, or six beam English cutters. Even a 30 ft six beam cutter would have a 5 ft beam.
     
  14. tlouth7
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    tlouth7 Senior Member

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  15. JohnBobson
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    JohnBobson New Member

    Thanks for the link, it was interesting to read.

    I don't know too much how to estimate the weight of the boat (minus the keel), but it seems like it would be pretty heavy? 1500 pounds maybe? Maybe more? Then add 3 people, of 180 pounds average, and that's another 540 pounds up top. Along with the 200 pound engine.

    It makes me think the 2000 lbs of keel weight wasn't enough to significantly lower the center of gravity, all considered.

    That seems like it would be the biggest problem, especially since the center of buoyancy is so low.

    I'm guessing since it wouldn't get much / any stability from the form, it would probably have to have an extreme weighted keel to make up for it. If you even can.
     
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