Rule of thumb for wetted surface area
 

"Goofball_star_dot_etal" wrote in message
...
On Mon, 18 Dec 2006 11:41:53 -0700, "Gilligan"
wrote:

Frictional resistance varies as the wetted surface area.
ok

Frictional resistance varies with the square of speed.

ok (force)

Power to overcome skin friction (speed x force) varies with the cube
of speed.


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


Not "for a fixed power input"

V=(P/k * A)^1/3
V speed, P power k a constant

For small increases in area, the decrease in speed will be a third. 3%
increase in area will give 3/3 = 1% decrease in speed.

k is a drag coefficient and A is wetted surface area?

If so then and P is constant:

dV/dA = .333*P/k*A^(-2/3)

Simplifying:

dV/dA ~ 1/(A^0.6)

If A is very large then dV/dA ~ 0

If A = 1 then dV/dA = A

If A1 then dV/dA ~ large

Since in real units (ft^2, meter^2) A is not near unity, lets throw out that
case. Also toss A=1 for same reason. In reality then A1. If so then:

dV/dA = 1/(A^0.6) * some constant. The curve approaches zero for A large.

The speed will change as the 2/3 power of wetted surface area rather than
the square root (1/2 power).

Rule of thumb for wetted surface area
 

"Ellen MacArthur" wrote in message
reenews.net...

"Anonymous" wrote
Does anyone know the rule of thumb........

Somebody told me how that rule of thumb saying got started. They said
back in old England
men were allowed to beat their wives with a stick but it couldn't be any
bigger around then their
thumbs. Isn't that awful? :-(((

Cheers,
Ellen

Come on Ellen, you saw that in 'boondock saints', the reply to which was
'maybe it should have been rule of wrist' and im not sure if it was a true
story or not :P

Shaun

Rule of thumb for wetted surface area
 

"Shaun Van Poecke" wrote
Come on Ellen, you saw that in 'boondock saints', the reply to which was 'maybe it should have been rule of wrist' and
im not sure if it was a true story or not :P

Never heard of it.... But, here's a google search find:
http://tafkac.org/language/etymology/rule_of_thumb.html

Judge Thumb... http://www.worldwidewords.org/qa/qa-rul1.htm

Cheers,
Ellen