Does anyone know the rule of thumb for wetted surface area? 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?

TIA

"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

"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

"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

Anonymous wrote:
Does anyone know the rule of thumb for wetted surface area? 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?


There is no simple "rule of thumb" for this. How do you increase
surface area and keep hull size & form the same? Is displacement held
constant? Have you already factored out wave-making drag?

(signed) Injun Ear (formerly known as Eagle Eye)

wrote:
Anonymous wrote:
Does anyone know the rule of thumb for wetted surface area? 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?


There is no simple "rule of thumb" for this. How do you increase
surface area and keep hull size & form the same? Is displacement held
constant? Have you already factored out wave-making drag?

(signed) Injun Ear (formerly known as Eagle Eye)


I'm asking about rate of change, not absolute numbers so the assumption
of not changing the surface area and hull form is valid. It's an element
of calculus if you have studied mathematics. To give a realistic
example, change the hull from smooth to one of those lapstrake types.
Leave out drag and reread the first three words of the title: "Rule of
Thumb".

Anonymous wrote:
I'm asking about rate of change, not absolute numbers so the assumption
of not changing the surface area and hull form is valid.

It may be valid, but is it physically possible?


.... It's an element
of calculus if you have studied mathematics.

Even if you haven't studied calculus, it's still calculus


To give a realistic
example, change the hull from smooth to one of those lapstrake types.
Leave out drag and reread the first three words of the title: "Rule of
Thumb".

Leave out drag? Wasn't the question about drag in the first place? So,
what is the new question?

If you want to know what the rate of change will look like, it will
increase geometrically with the initial velocity. Does that answer any
of your questions?

signed- Injun Ear (formerly known as Eagle Eye)

This may help:

http://oneoceankayaks.com/kayakpro/kayakgrid.htm

Frictional resistance varies as the wetted surface area.

Frictional resistance varies with the square of speed.


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

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)