DSK wrote:
Nav wrote:

Froude found that the wave making resistance rises as:

L^3 aW(v^2/Lg)


Thank you, that was what I was thinking of but couldn't remember the
formula.


where a is the form coefficient, w weight which suggests that the wave
making resistance rises with the square of speed. A factor that
increases the steepness somewhat is the squat of the vesses as speed
rises changing a. The skin friction directly rises with v. So that's
resistance, but the power needed then rises with the cube of v.


Overall resistance rises as a higher power because it is the sum of
friction, which rises somewhat less that the square of velocity, and
wave making resisitance, which rises as somewhat more than the square of
resistance.

Yes, but usually the skin friction is low compared to wave making until
you start to plane...


The usual hull speed formulas make no use of anything as
complex as the Froud number, just waterline length and an approximate
speed length ratio.


Yes, and there's a lot of design in controlling wave making scale
factors (until you get to a very long thin hull).


Now let's see you explain prismatic coefficient and all it's ramifications!


The prismatic coefficient (Cp) is the ratio of the immersed volume to to
the area of the midsection times LWL. A big Cp means full ends and a
small Cp fine ends. The coefficient should exclude appendages, bulges
and deep keels. The ramifications would require a book!

Cheers