Hull speed

Discussion in 'Wiki Archive' started by Guest625101138, Apr 20, 2007.

  1. Guest625101138

    Guest625101138 Previous Member

    My first boat book predated metric conversion in Australia and the 1.34 has just stuck in my mind. I regularly do meter to feet conversion so the 3.28 also comes easily to mind.

    Rick W.
     
  2. Kerry Thomas
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    Kerry Thomas Junior Member

    The Froud number was for the average L/B ratio of merchant ships in his time. Very wide or narrow hulls have a different constant.

    Also we can get higher power these days with the same displacement.
    A 50m tug hull with 80 ton bollard pull would have been impossible then.

    Many displacement vessels now have enough power to exceed hull speed, usually with the stern wave above the funnel:) I hate to think how much fuel they are wasting to get that extra few knots.
     
  3. Guest625101138

    Guest625101138 Previous Member

    Kerry
    "Hull Speed" is a defined quantity that is only related to the LWL and the gravitational constant. The value shown in the equation is NOT A CONSTANT related to L/B but a physical property of gravitation waves.

    All hulls have a "Hull Speed" by definition. It is the speed where the boat speed matches the phase velocity of a wave having the same wavelength as the hull waterline length. The phase velocity for deep water is:
    v = (g * Wavelength / 2 / pi) ^ 0.5

    In applying this formula you have to use consistent units so for metric, wavelength is in metres and g is 9.8. Velocity will be given in m/s. For imperial it is feet, 31.8 and speed is ft/s. If you want to get speed in knots you need to know 6080ft/nm or 1852m/nm and 3600s/hr.

    I simply find it easier to remember 1.34.

    Rick W.
     
  4. Kerry Thomas
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    Kerry Thomas Junior Member

    Exactly.


    Quote; I can't find the right references, but I know that the hull speed formula for extremely narrow hulls (very high L/B ratio) is very different. The problem is I can't remember the point at which the 1.34 constant gets tossed. :confused: This reference gets at part of the subject: "There is a sense in which multihulls are always superior to monohulls from the point of view of wave reduction, and in particular wave resistance reduction. After all, since wave resistance varies as beam squared, the total wave resistance of two separate half-beam hulls is half of that of one fullbeam hull." (OPTIMUM HULL SPACING OF A FAMILY OF MULTIHULLS, Tuck & Lazauskas, University of Adelaide, 1998)

    The point is that the hull speed of very narrow hulls is much higher than for hulls with L/B of 3 or less. "Narrow" and "high ratio", of course are subjective terms. They are defined, I just can't find the reference now.[/QUOTE]
     
  5. Guest625101138

    Guest625101138 Previous Member


    For any given LWL the hull speed in deep water DOES NOT CHANGE irrespective of the hull shape. Hull speed is not a speed limit. Hull speed is simply the phase velocity of a wave having wavelength the same as the LWL of the hull. Absolutely NOTHING to do with L/B ratio.

    Rick W.
     
  6. masalai
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    masalai masalai

    Ok, now I am being thoroughly dumb/ignorant/confused. (Not unusual as I come to grips with failing memory & precise definitions)... Is there a way to evaluate the potential velocity of a long skinny hull (half a catamaran) at various Kw of power delivered through a reasonably effective choice of 2 different screws (optimised to work fully submerged - one without, and the other, with designed "cavitating blades", supported by exhaust injection)

    I have moved my request to here, I feel this form is accidently the best I have produced? http://www.boatdesign.net/forums/showthread.php?t=20633&page=6 at post 81 :D Is it worthy of further modelling to make and put to a 1/10 test? What suggestions?
     
  7. Kerry Thomas
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    Kerry Thomas Junior Member

    I do not recall saying hull speed is a speed limit. Quite the opposite.

    I am trying to track down the paper to cite.
    This was a study on why multihulls and very narrow vessels appear to dis regard the Froud number.
    The conclusion was, hull speed, defined as the point where wave making resistance is such that power required to increase speed slightly increases exponentially. This is what we want to know in the real world!
    Not only beam, but angle of entry and displacement also should be factored in. Giving us a different constant and factors.
     
  8. Guest625101138

    Guest625101138 Previous Member

    It is not reasonable to redefine Froude's work. Hull speed, as he coined the term, is determined by LWL and the gravitational constant. It is based solely on the phase velocity of gravitational waves in deep water - end of story. NOTHING to do with L/B of a hull.

    Hull speed, as Froude observed it, has more relevance for wide hulls than for slender hulls. The velocity corresponding with hull speed results in rapidly rising wave drag with wide hulls.

    One observation I have made with very light displacement hulls is that the hull speed corresponds with the design speed when the hull is designed for minimum drag. As the displacement increases, the hull speed is increasingly higher than the design speed for the minium drag hull.

    I have attached drag curves for two hulls with the same length therefore the same hull speed. One displaces 10t while the other displaces 300kg and is optimised for 8.1kts - the hull speed. You can see the heavy displacement boat drag is dominated by the wave drag throughout the speed range while the light displacement boat drag is dominated by viscous drag. Even so the optimum length for the light displacement boat results in the design speed and hull speed being almost the same. The hull speed is just where the wave drag is starting to kick in despite this component of drag remaining very small.

    Point is that hull speed still has significance for very light displacement hulls as it provides the length that results in the minium overall drag for that speed. This is significant for craft like rowing sculls.

    The curve for the light displacement boat also shows that there is no point where the wave drag is increasing exponentially. Your redifined term has no meaning for slender hulls. Wave drag remains a tiny component of the total drag.

    What I want to know is the total drag on the hull. To determine this I just load the hull coordinates into Michlet and it gives me the answer over my selected speed range. Nothing too complicated there. No need to be concerned with exponentially rising wave drag. It may or may not be the case.

    Rick W.
     

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  9. charmc
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    charmc Senior Member

    "Thin hulls can readily exceed their hull speed. Wider hulls require much more power to exceed their hull speed." Rick W

    True. Tugs are an example. This neat video of the annual tugboat race in NY harbor shows the concept: massive power = massive wakes and a (very)few more knots.

    http://www.splashvision.com/Video/10964_2007-nyc-tugboat-races-II.html
     
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