On Mon, 18 Dec 2006 12:23:32 -0700, "Gilligan"
wrote:


wrote in message
oups.com...
Gilligan wrote:
Frictional resistance varies as the wetted surface area.


Right, but "anonymous" is insisting that the surface area is not
related to the displacement, or the hull shape (or size, presumably?).

A more interesting question would be, if you increase the sueface area
without increasing the cross sectional area, could you approximate the
increase in drag over a given range of speeds? Would changing the
prismatic coefficient be better?

Frictional resistance varies with the square of speed.


Right again.
Didn't I already say that?


The speed will change as of the square root of wetted surface area
change.

But the initial velocity will matter more.

Ask a muddled question, get a muddled answer.

signed- Injun Ear (formerly known as Eagle Eye)


Anon asked:

If the hull
stays exactly the same same size and form yet the wetted surface area is
increased in what proportion does the speed decrease for a fixed power
input?

You answered:

If you want to know what the rate of change will look like, it will
increase geometrically with the initial velocity.


The actual answer is that the speed decreases as the square root of the
wetted surface.

Not for "for a fixed power input"

This is less than a linear increase and certainly not
"geometric" in its common usage. If the wetted surface is 4X the speed
decreases by 1/2. If it were linear the speed would decrease by 1/4, and if
quadratic exponential it would decrease by 1/16.

Interestingly the hull speed formula varies as the square root of the LWL.
Perhaps there is a relationship that disregards displacement or hull shape.