Hull speed

Discussion in 'Boat Design' started by HJS, May 9, 2010.

  1. HJS
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    HJS Member

    Back to my original question in post 1.
    Is there a better measure than the waterline length to refer to when we compare different hull shapes. For example, can the longship's metacenter radius be a better measure?
    JS
     
  2. TANSL
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    TANSL Senior Member

    Okay, back to your original question in post 1.
    "compare different hull shapes" is too generic. You can compare boats in relation to a certain parameter depending on which one hull will be more appropriate than another to achieve a certain objective. In relation to what do you want to make this comparison?
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    I have yet to see any evidence presented to support this statement.
     
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  4. gonzo
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    gonzo Senior Member

    If we were to take two hulls of identical shape but different metacentric radius, the stability would be different. However, I can't see how that would affect the hull resistance which determines the speed for a particular power. For example, one hull can be built with a thick planking and the other with a lighter planking and added ballast to get the same weight.
     
  5. TANSL
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    TANSL Senior Member

    I don't know how two identical shaped boats could have different metacentric radii.
    Nobody builds a boat "with a thick planking and the other with a lighter planking". That is totally stupid. A boat is built with the lightest possible planking, with the lightest possible structure, and, in the area in which we are talking, putting ballast on a boat, which might not carry it, is only conceived as the solution for a very serious design mistake.
     
  6. jehardiman
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    jehardiman Senior Member

    Ok, lets approach this systematically:
    Yes, there is, it is the stern buttocks because they provide the lift and the sinkage to any given hull form. The bow forms could be similar, but it is the stern forms that define the "lift angles"... or lack of them...
    No, because that is a STATIC measurement. Once the vessel is MOVING, the "normal" measurement of LMR is all ... "confused"...! This goes back to... "what is buoyancy"?... and.... "what is stability"? ... and "what is suction"? ... and... "what is thrust deduction"? There are "WAY TOO MANY (tm)" nooks and crannies to go down in a "simple"; broad-brush; addressing of the question.
     
    Last edited: Aug 23, 2021
  7. Will Gilmore
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    Will Gilmore Senior Member

    Thinking about this in terms of bow and stern volume as well as fore and aft cg balance, a broader shallower bow would push up more as it gained speed, floating the bow higher than a narrow Vee bow. This would have the effect of moving the bow wake farther aft along the hull. In such a case, the result is an effectively shortened waterline.

    Once the second wake peak reached the transom, any overhang would be gone and the flotation of the transom balanced with the fore and aft cg would determine where the best place on the forward face of the stern wave the transom should fall. Are all shapes traveling up hill once the peak of the stern wave moves aft of the transom or is there a way to trim the boat such that she is better balanced with the transom down the forward face of the stern wave?

    Can you move weight aft to re-trim the hull lines so that a squatted stern is more level (or 'natural' may be a better term)? If such a condition were achievable, then the "Hull Speed" would actually be some point where the wave length of the wake was longer than the waterline.

    -Will
     
  8. jehardiman
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    jehardiman Senior Member

    How much fantasy do you want to invest in? Can two ~100 kg people in a 3m, 200 kg, RIB do this? Yes. Can you do this effectively in a 20m, 10 tonne, PB or "all ocean capable" MV....not so much. So... "horses for courses"... and the answer is "it all depends..."
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    In "theory" yes one 'could' argue this, but piratically no. The constantly changing waterline (as you see it) owing to the passage of waves acts both as +ve or -ve effect (as waves pass) and = no net change.

    You're missing the point of "design".
    Either the vessel under investigation will either:
    i) run at this speed, ergo creating the squat you cite
    or
    ii) shall run through this hump region creating said aid squat and in which it is not an issue. Other than at the ephemeral level.

    Because if the 'design' condition is that of i)...you modify the lines and LCG/LCB to ensure this is NOT an issue. Since why run with a large attitude if the objective is zero or near zero attitude and you can 'design'; it to be so?
    Simple!
     
  10. RampantMule
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    RampantMule Junior Member

    If i was some computer/software guy, i would certainly try to do my own tests.
    Example : I would build two identical hulls, but with different transoms and see how they behave. After that i could compare the results with my theory.
    Personally i would so much like to see your results if you proceed to do something like that :)
     

  11. HJS
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    HJS Member

    We've done it. Two boats were built with the same dimensions, weight, length and width. The difference was mainly in the shape of the transom above the water surface. The sailing characteristics differed markedly. The new boat could carry more sail, had less wavemaking and was therefore faster.
     

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    Last edited: Nov 13, 2021
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