V-bottom or flat bottom?

Discussion in 'Boat Design' started by HJS, Oct 14, 2020.

  1. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    The notion that a certain shape "cuts" through the waves is a false impression. Water is displaced in some manner or other and "cutting" is not a suitable description for a displacement situation. Water must move aside if in a moving displacement regime. It is not at all unusual for a flat entry to be superior to a sharper entry if the hydrodynamics are well planned. The deal is that the physics obey the basic idea of F= Ma. How severe is the acceleration of the mass of water in whatever case......both at the front and the rear of the immersed body.

    Vee bottoms are definitely not more stable than flat bottoms. That is particularly true when initial stability is in question. Here I am referring to a section that has equivalent chine width dimension. Flat bottoms are not more likely to cause water to enter the boat. To be sure the flat bottom will be more likely to pound if presented entirely upright and facing the chop. A slight heeling attitude mitigates the slamming rather nicely.
    In terms of power required to propel the boat, a flat bottom is clearly more economical in power demand at speed. . All this depends largely on the configuration of the quarter beam run.

    There is more.................A vee bottom has somewhat less wetted surface than a flattie of equal mid section area. That is at least a marginal advantage for slow speed operation, as in trolling speed. A vee bottom is not the most efficient in terms of wetted surface per displaced area. The flattie is the worst of them and the Trapeze bottom is very close to the best of section shapes. As is the realities of boat design and other world realities, there are no best designs for all circumstances.
    Dejay, HJS and bajansailor like this.

  2. HJS
    Joined: Nov 2008
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    Three planing boats

    The boats compared here have the same interior space, load capacity and speed.
    The planing bottom surface has different length-width ratios and different deadrises.

    The drag is related to the boats' total weight including payload. The reported horsepower at 25 knots assumes 65% efficiency.
    Only the V20 has a marked hump and a large trim angle.

    On the flatter models, the adjustable aftship interceptor reduces the power requirement by an additional 25% by keeping the entire aftship bottom dry and optimizing the trim angle.

    The vertical acceleration is reported with g-numbers. It is a measure of how hard the boat goes in the oncoming waves.
    In the above example, the significant wave height, h/3, is 0.25 m, which is one third of the maximum wave height.

    A paying guest should not be exposed to more than 0.6g.
    Normally you can handle 1.5g for one to two hours.
    To be just as comfortable, the V20 has to run nine knots slower than the narrower V12.

    Theoretical calculations according to D. Savitsky, J. Koelbel, J.A. De Groot and others.
    The calculations are confirmed with model tests and full-scale tests.

    Attached Files:

    Last edited: Mar 20, 2021
    Dejay likes this.
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