Most efficient bow and bottom shape for shallow water?

Discussion in 'Boat Design' started by river runner, Sep 26, 2011.

  1. river runner
    Joined: Jul 2011
    Posts: 172
    Likes: 6, Points: 0, Legacy Rep: 91
    Location: Colorado

    river runner baker

    Let's forget current for now. The subject is complicated enough without throwing that into the mix.
    If you look at a racing canoe on top of a car, three things that should be apparent are:
    1- It's Swede form
    2- The bow is pointed
    3- The bottom is round

    I think we've established that Swede form is still more efficient in shallow water (I guess we did.) What about the other two? As the water gets shallower, does a punt shaped bow ever become more efficient than pointed, at canoe speeds? Assuming you don't drag bottom with either one, does a flat bottom ever become more efficient than round, at canoe speeds?
     
  2. jehardiman
    Joined: Aug 2004
    Posts: 2,957
    Likes: 481, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    No, you should have not drawn that conclusion. A "swede" form (or more correctly aft LCB form, ...I mean who wants a boat shaped like a turnip anyway) is not more efficient in shallow water than a fish form (that is forward LCB form). It is better to say that a LCB aft form is more efficient for the identical displacement, wetted area, and LWL than a LCB forward shape for Froude numbers above the skin friction vs pressure drag transition point. This is a very explicit limiting statement and is based soley upon the entry of a LCB aft form having to be geometrically finer than a LCB forward form which reduces pressure drag and does not consider directional stability. Once you leave the limits of identical displacement, wetted area, LWL, and fixed directional stability all bets are off and there is no "best" shape in an equally weighted trade-off space.

    As far as shallow water goes, it is not so much the bottom shape as it is prismatic coefficient, waterplane area, and the blockage factor. A wider, shallower, flat bottom or a narrow deep-V can have less drag depending on the viability of the necessary draft to volume to stability (assuming fixed length) trade-off. As I have said many times before, there is never a "best" hull form, only ones that better meet the mutualy exclusive requirements of the design.
     
    Last edited: Sep 26, 2011
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