That last 10% of efficiency.

Discussion in 'Boat Design' started by river runner, Nov 3, 2011.

  1. river runner
    Joined: Jul 2011
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    river runner baker

    This should probably go under a previous post.
    I've been told a couple times now that I shouldn't worry too much about some design element, such as LCB forward or aft of middle, because it only makes a small difference in efficiency.

    My resposnse is this: imagine you are on a canoe trip. You've been paddling all day. It is getting late. You are tired, hungry and can't seem to find a good campsite. Do you think I don't care about that last 10% of efficiency?

    I've been in that spot on more than one occasion. Every percent of efficiency counts when you are the motor.
     
  2. DCockey
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    DCockey Senior Member

    Numbers in my comment may have been incorrect so I deleted it.
     
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    You make it sound like a military exercise. Where's the enjoyment in fretting over a minor difference in your progress ? Stop and smell the water lilies. :D
     
  4. SamSam
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    SamSam Senior Member

    Fine entry and exits, the sharper the better. Long and skinny is faster. Round bottom is the least wetted surface. Asymmetrical shape is fastest, I read ages ago that reverse asymmetrical, with the fatter part forward, was faster than the opposite and was even not allowed in class racing.

    You might look outside the box, as in other than the shape of the canoe. A smooth (no scratches, gouges) bottom freshly waxed can be a few percent. A lightweight skin that flexes is inefficient, as you lose an efficient shape. Better paddles can add 10% or more efficiency. Carrying less weight and stowing for best trim can also help.

    Try only going on free flowing rivers and forget the paddling. ;)
     
  5. fcfc
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    fcfc Senior Member

    It would be very very much harder to find the last % of efficiency within the constraints you have decided for yourself than to gain efficiency by changing a bit your self decided constaints.

    For your example, if you want an efficient canoe, instead of finding the optimal LCB for the loading and power the padller will have (NB you can find an optimal LCB, but it will depend on speed and loading) , just add length to the canoe, or try to lighten it.
     
  6. jehardiman
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    jehardiman Senior Member

    There are too many variables that you have no, or limited, control over to ever say that you have the absolute "correct" LCB. As fcfc pointed out, the "optimum" depends on displacement, length, and speed. So lets say you want that last percent; that implies that you can't change displacement. That means that you carry the weight of your loadout food and water with you ALL THE TIME. Now I know you would use the standard ship method and put in aux ballast tanks, but ziplock bags work just as well (the Navy uses them all the time...they are called "piddle packs"). So to get the "optimum" for a week long trip you will need about 1 cf of water ballast. But why carry that water when you don't need too? Also, you can't let it rain, as that will change the displacement, altering the optimum LCB. And taking a trip where the wind changes direction and velocity is totally out, as well as any lake or river where there are waves. Really, because there are so many unknowns that you must encounter, wringing that last percent out of resistance and propulsion really isn't mission effective. As was pointed out in the other thread, the change in non-optimal LCB is small, and there are other, larger, issues in a HPV to worry that you have the exact one. Place the LCB 4-5% aft of WL midships and call it good.
     

  7. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    jehardiman and other have provided good advice good advice.

    (A re-write of my earlier, now deleted, post.)

    Be aware that at typical canoeing speeds (not sprint racing) viscous drag accounts for 80% or more of the total drag. A change of a couple of percent in LCB location will change the wave drag by a couple of percent or less, and have virtually no direct effect on viscous drag. The net effect on total drag will be less than a percent. (That's assuming LCB was in a reasonable location to begin with, and the hull form is not extreme, and no "tricks" were used to change the measure LCB location.)

    On the other hand a change of wetted surface area translate an equal change in viscous drag, which at canoeing speeds will be seen as an almost equivalent change in overall drag. So worry about wetted surface area first.

    Also, in use the LCB will be located where the CG of the boat, occupants and other contents is. That is simple hydrostatics. Even a symmetric hull can have LCB forward or aft of the center of the boat depending on where the occupant(s) is/are located and how the boat is loaded.

    Back to optimizing LCB location for minimum drag. The "optimum" location for minimum drag depends on the speed. So do you want to optimize for when you are trying to get maximum speed for a short time (further aft) or for slower speed, long distance cruising (not so far aft)?
     
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