Soft Bottom - Slow Or Fast

Discussion in 'Sailboats' started by SuperPiper, Apr 10, 2021.

  1. SuperPiper
    Joined: Jan 2003
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    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    My micro-cruiser is in the shed for the winter. I've been completing some epoxy and glass projects. For every hour of glass work, I seem to have 3 or 4 hours of sanding and surface preparation. The boat is cradled on a pair of timbers either side of the retracted keel.

    Depending on direction and frequency, the boat can start to rock and roll while I am hand sanding. Obviously, it's bouncing on its bottom. The hull must be oil-canning while I'm twerking.

    So I'm wondering, if the hull is that soft, is it affecting the boat's speed through the water?
     
  2. hoytedow
    Joined: Sep 2009
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    hoytedow Helmsman

    When in the water it supported along its entire submerged area so any tincanning would be much reduced. The support timbers concentrate the entire weight of the boat along 2 narrow lines. The stress on the hull would be much greater. The greater the surface area contact of the support beams the more the load is spread out, thereby reducing localised stressing of the hull.
    I can only guess. My guess is 'maybe a little'.
    Assuming you have a properly constructed hull.
     
    Last edited: Apr 11, 2021
  3. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Hoytedow, thanks for the reply. I don't usually expect a concise response right away. It's obvious that you completely understood exactly what was happening in the boatshed. Your explanation of the hull being supported completely in the water vs. being point-loaded in the shed makes a lot of sense. You are smarter than I look.
     
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  4. hoytedow
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    hoytedow Helmsman

    You are too kind.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Who knows, if it "oilcans" underway, may even be quicker !
     
  6. tlouth7
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    tlouth7 Senior Member

    Flexure of the hull (or any part of the boat) inevitably leads to energy being converted to heat in the material. This energy must have come from somewhere, and is not available for moving the boat forwards. So on the face of it this flexure is likely to be detrimental, but of course there are situations where deflection can be a good thing. For example flexing of the rig can spill excess wind in gusts, reducing heeling and drag.

    So the question is, can flexing of the hull in waves result in a net thrust? Probably it could if you specifically designed the hull to do that. I suspect that a normal hull will not be the right shape for this, and flexure will actually have a negative effect.

    Anecdotally boats that go soft over time (such as the 420) are known to be less competitive once they go soggy. Whether this is specifically a result of the hull flexing I don't know.
     
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  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    If you look at the extremes; it ought to help.

    An extreme would be a concave hull, for example. Or a hull with ridges (bulkheads showing).

    So, in general, hardly seems helpful. And if it were, we'd never spend hours fairing.

    The real problem becomes if and to what degree. And, of course, whether hull integrity is ultimately at issue from repeated movement. Probably not a big deal.
     

  8. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    The oil-canning you're experiencing is due to point loads on the hull due to the way the hull is being supported. Once the boat is in the water, the hull will experience distributed loads. Convex surfaces resist those kinds of loads very well. It's like cracking an egg with just two fingers rather than your whole hand. With just two fingers, it cracks easy. With your whole hand, it becomes a lot more difficult.
     
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