Fluctuating LWL and wave patterns.

Discussion in 'Boat Design' started by Omeron, Mar 6, 2008.

  1. Omeron
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    Omeron Senior Member

    This has been wondering me for a long time.
    We all know the relationship between LWL, Hull speed, and the wave pattern which dictates this relationship for displacement hulls in still water.
    What i think is lesser known is what happens when a displacement hull with long overhangs moves through an existing wave pattern and buries her long bow overhang in such a way that LWL increases significantly with the period of the waves.
    How does this change the hull speed as we know of?
    When the bow hits the oncoming wave and buries into it, LWL at that
    instant is increased. But moments later and possibly before the wave exits from the stern, bow raises and LWL is shortened again.
    How does this irregular LWL effect the formation of waves?
    Is it beneficial to the hull speed in general or does it generate an abnormal wave pattern which would slow the boat before it reaches its normal hull speed?
    Think of a sailboat with long overhangs like the J class and imagine left over waves with a long period.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry to disappoint you, but the notion of hull speed is mostly used by people who know nothing about hydrodynamics.
     
  3. Omeron
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    Omeron Senior Member

    Hi Leo,
    Your comment perplexed me.
    To me it could mean three things.
    A- it is a dumb question to ask...
    B- if you do not have enough knowledge on hydrodynamics you should
    not ask such superfulous questions...
    C- nobody knows it anyway, just do not worry your pretty head with these
    things...
     
  4. sighmoon
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    sighmoon Junior Member

    Reassured that my lack of knowledge allows me to have a stab at this:

    The 'hull speed' limit is the point at which the boat has to pass its own bow wave to get any faster. However, as the wave builds (in flat water), it would be higher bow and stern than in the middle, so would be longer than the static waterline. The heeled waterline is also longer than the measured waterline. These factors account for the shape of the J's - as a rule cheat.

    If you're going into waves, they slow you down. If they're coming form behind you, you can surf a little and go faster. However, I don't believe the waterline length is a factor.
     
  5. jehardiman
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    jehardiman Senior Member

    Could be any of those things depending on the extent of your knowledge of Hydrodynamics. ;)

    The simple answer is that the seaway always decreases speed because it always increases required power. If you really want to know how much it changes and have the money, the power, and the coding ability to pull down a few weeks of supercomputer time, I would look to the recent works of Dr. Robert Beck from the UofM on the effect of waves on powering.

    As many of us have tried to point out, a fixed, constant, "hull speed" dependent only on LWL is an improper concept foisted upon the world years ago by ignorant, but well published, authors.
     
  6. Omeron
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    Omeron Senior Member

    Ok. lets not get bogged down with the hull speed.
    To me what is interesting is the dynamics of the wave pattern.
    In still water, and in equilibrium, the hull digs itself a nice cradle when the speed of the hull equals the period of the wave. or something like that.
    What intriques me is what happens when the nicely formed wave suddenly
    receives a message that the hull which created that wave is no longer the same hull.
    In that instance you would expect a longer wave to be formed by the longer LWL. if that longer LWL was sustained long enough.
    But probably it does not, because the already formed wave does not care
    how the parameters have changed after it has formed.
    Or does it? Since water is incompressible, probably everything that happens in an instant modifies the form of the wave which was about to exit the stern unaware of what was happening at the bow.
    This was what i was after.
    Does the wave get itself modified by the changing LWL, or is it history after the main wave is once formed?
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No wonder you're perplexed. What I wrote reads far more harsh and brash than I intended.

    Others here have answered your question. You should also do a search for the many previous discussions of the concept on this board.

    You might also like to search more generally for "added resistance" which is (very roughly) the additional drag due to a hull's inability to ride up and down waves without ploughing in. Be prepared for some pretty tough mathematics!

    All the best,
    Leo.

    Positive: Wrong at the top of one's voice.
    Ambrose Bierce.
     

  8. jehardiman
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    jehardiman Senior Member

    Actually, nothing like that. What happens is the interaction between the forward pressure disturbance is addative with the aft quarter pressure distrubance causing an increase in power due to decrased pressure on the after skin plating. It did not "dig" a hole in the water that it has to climb out of.

    It is almost always recieving that message for a hull of finite length. It is a function of the rate of curvature of the skin plating.

    No, the length between crests of the pressure wave train is a function of the speed of the pressure disturbence, not the length of the hull. For an infinite hull, there is only a bow wave, which oscilates due to gravity effects across the surface of the fluid and therefor down the length of the hull.

    Each change of curvature in the hull surface and change in the fluid free surface causes a differential change in the pressure disturbance. These pressure changes provide the energy to generate an implusive wave and becuse of infinite fluid and gravity effects, they also generate a periodic wave to distribute that energy throughout the fluid. So what you have is an infinite number of infinitely small impluse waves and thier associated periodic waves that interact to to form the wave train that is associated with the hull moving through the water. Now given that the celerity of the impluse wave is a function of speed and the periodic wave period is only a function of the density to kenimatic viscosity, there are a given speeds at which the effects are additive or subtractive...i.e. the humps and hollows in the residuial powering curve. The change in skin friction due to changes in the wave pattern are also linked to the viscous pressure effects, but are usually handled as independent.
     
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