Wetted surface is but one small part of drag.
From a random google search: "the hydrodynamic drag is taken to be the sum of the viscous drag and the wave drag. The viscous drag is the sum of the skin friction (estimated using the 1957 ITTC line) and the form drag (estimated using the empirical formula of Scragg and Nelson (1993)). The steady, upright, calm-water wave resistance is estimated using Michell's (1898) thin ship theory. See also Tuck (1987)."
Think about it another way, when you look at a boat moving through the water there is usually a bow wave and a variety of stern waves. Those waves require lots of power to form and propagate. That energy must come from somewhere, it's drag on the boat hull for which the boat must apply power to overcome. A heavier boat sits in a bigger hole in the water so must move more water to get from point a to b. Therefore bigger waves, thus more power required. If you are interested in the gorey details search around www.boatdesign.net there are tons of pointers to equations and software programs to calculate this stuff.
One more analogy, you seem to be stuck on water friction, think of two identical 18 wheel trucks, one empty and one full of lead. They will both have the same air drag at the same speed because they have the same shape. But the full one will take a whole lot more fuel to move down the road.
Gary Stavrou gstavrou@bigpond.net.au wrote: Regarding Dennis' observations, the constant opposing force that he
refers to would seem to me to be water friction. I cant see why it
shouldn't be the same for a light or for a heavy boat. Surely it's
relative to wetted surface area, not weight.
Gary
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Wetted surface is but one small part of drag.
>From a random google search: "the hydrodynamic drag is taken to be the sum of the viscous drag and the wave drag. The viscous drag is the sum of the skin friction (estimated using the 1957 ITTC line) and the form drag (estimated using the empirical formula of Scragg and Nelson (1993)). The steady, upright, calm-water wave resistance is estimated using Michell's (1898) thin ship theory. See also Tuck (1987)."
Think about it another way, when you look at a boat moving through the water there is usually a bow wave and a variety of stern waves. Those waves require lots of power to form and propagate. That energy must come from somewhere, it's drag on the boat hull for which the boat must apply power to overcome. A heavier boat sits in a bigger hole in the water so must move more water to get from point a to b. Therefore bigger waves, thus more power required. If you are interested in the gorey details search around www.boatdesign.net there are tons of pointers to equations and software programs to calculate this stuff.
One more analogy, you seem to be stuck on water friction, think of two identical 18 wheel trucks, one empty and one full of lead. They will both have the same air drag at the same speed because they have the same shape. But the full one will take a whole lot more fuel to move down the road.
Gary Stavrou <gstavrou@bigpond.net.au> wrote: Regarding Dennis' observations, the constant opposing force that he
refers to would seem to me to be water friction. I cant see why it
shouldn't be the same for a light or for a heavy boat. Surely it's
relative to wetted surface area, not weight.
Gary
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