Rule of thumb for wetted surface area
 

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

Rule of thumb for wetted surface area
 

"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

Rule of thumb for wetted surface area
 

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)

Rule of thumb for wetted surface area
 

Rule of thumb for wetted surface area
 

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)

Rule of thumb for wetted surface area
 

This may help:

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

Rule of thumb for wetted surface area
 

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.

Rule of thumb for wetted surface area
 

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)

Rule of thumb for wetted surface area
 

wrote in message
ups.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. 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.

Rule of thumb for wetted surface area
 

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.


Gilligan wrote:
The actual answer is that the speed decreases as the square root of the
wetted surface. This is less than a linear increase and certainly not
"geometric" in its common usage.

Sorry, that wasn't too clear. The -rate of change- will vary
geometrically, not the decrease in speed.



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

Perhaps, but perhaps not. The hull speed formula is rather basic and
does not give precise results. Or would you seriously suggest that hull
shape, displacement, cross sectional area, etc etc, don't matter in the
slightest and that LWL is the only determinant for speed/drag ratios?

signed- Injun Ear (formerly known as Eagle Eye)