Rule of thumb for wetted surface area
 

wrote in message
oups.com...

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?


The hull speed formula is a rule of thumb. If anything were to change on the
formula it wouldprobably be the coefficient and not the order of the
dependence on LWL. The hull speed formula determines hull speed, not drag.
Do you think the hull speed formula is valid for wave piercing hulls where
the bow rides in the wave rather than on top of it?

Rule of thumb for wetted surface area
 

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.

Rule of thumb for wetted surface area
 

"Anonymous" wrote in message
...
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'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".

Rule of Thumb does not normally call for a knowledge of calculus...

Rule of thumb for wetted surface area
 

"Edgar" wrote in message
...


Rule of Thumb does not normally call for a knowledge of calculus...



Here are some rules of thumb for the applications of integrals in calculus:

http://www.math.hawaii.edu/~lee/calculus/Integrals.html

Is this abnormal?

Rule of thumb for wetted surface area
 

"Gilligan" wrote in message
. ..

"Edgar" wrote in message
...


Rule of Thumb does not normally call for a knowledge of calculus...



Here are some rules of thumb for the applications of integrals in
calculus:

http://www.math.hawaii.edu/~lee/calculus/Integrals.html

Is this abnormal?

No, I would say it is normal. His 'rules of thumb' are directed at people
who have not yet, or do not need, to learn the theories in their entirety,
and he admits that they do contain inaccuracies.
That is exactly what a 'rule of thumb' is- a shortcut for everyday use for
those who do not wish, or are unable, to get too deeply involved in the
basic theorems.

Rule of thumb for wetted surface area
 

"Edgar" wrote in message
...

"Gilligan" wrote in message
. ..

"Edgar" wrote in message
...


Rule of Thumb does not normally call for a knowledge of calculus...



Here are some rules of thumb for the applications of integrals in
calculus:

http://www.math.hawaii.edu/~lee/calculus/Integrals.html

Is this abnormal?

No, I would say it is normal. His 'rules of thumb' are directed at people
who have not yet, or do not need, to learn the theories in their entirety,
and he admits that they do contain inaccuracies.
That is exactly what a 'rule of thumb' is- a shortcut for everyday use for
those who do not wish, or are unable, to get too deeply involved in the
basic theorems.

Rules of thumb might even be useful for those highly skilled in the art who
wish to reach a quick, back of the envelope solution.

Rule of thumb for wetted surface area
 

"Gilligan" wrote in message
. ..

"Edgar" wrote in message
...

"Gilligan" wrote in message
. ..

"Edgar" wrote in message
...


Rule of Thumb does not normally call for a knowledge of calculus...



Here are some rules of thumb for the applications of integrals in
calculus:

http://www.math.hawaii.edu/~lee/calculus/Integrals.html

Is this abnormal?

No, I would say it is normal. His 'rules of thumb' are directed at
people
who have not yet, or do not need, to learn the theories in their
entirety,
and he admits that they do contain inaccuracies.
That is exactly what a 'rule of thumb' is- a shortcut for everyday use
for
those who do not wish, or are unable, to get too deeply involved in the
basic theorems.

Rules of thumb might even be useful for those highly skilled in the art
who
wish to reach a quick, back of the envelope solution.

Yes, indeed. It does not make sense to calculate something to many places of
decimals when the input data is not in itself all that accurate. If you do
that you are just deluding yourself.
The best rule of thumb I can think of off the top of my head is the rule of
12ths for calculating tidal heights. Works pretty well on the whole for
practical purposes but ignores barometric pressure effects and is not valid
for ports such as the Solent in Uk that have special tidal effects.
I used it to good effect once in the port of Treguier in France. There is up
to 30 feet tidal range there. The tide was starting to fall and several of
us were anchored lower down the river eyeing the last good place to anchor
nearer the town. It was clear all were wondering if they would find
themselves aground at low water. I used the 12ths rule and decided to go for
it and it worked for me as at low water I had 1 foot below my keel. Everyone
else had a long trip in the dinghy!

Rule of thumb for wetted surface area
 

"Edgar" wrote in message
...

"Gilligan" wrote in message
. ..

"Edgar" wrote in message
...

"Gilligan" wrote in message
. ..

"Edgar" wrote in message
...


Rule of Thumb does not normally call for a knowledge of calculus...



Here are some rules of thumb for the applications of integrals in
calculus:

http://www.math.hawaii.edu/~lee/calculus/Integrals.html

Is this abnormal?

No, I would say it is normal. His 'rules of thumb' are directed at
people
who have not yet, or do not need, to learn the theories in their
entirety,
and he admits that they do contain inaccuracies.
That is exactly what a 'rule of thumb' is- a shortcut for everyday use
for
those who do not wish, or are unable, to get too deeply involved in
the
basic theorems.

Rules of thumb might even be useful for those highly skilled in the art
who
wish to reach a quick, back of the envelope solution.

Yes, indeed. It does not make sense to calculate something to many places
of
decimals when the input data is not in itself all that accurate. If you do
that you are just deluding yourself.
The best rule of thumb I can think of off the top of my head is the rule
of
12ths for calculating tidal heights. Works pretty well on the whole for
practical purposes but ignores barometric pressure effects and is not
valid
for ports such as the Solent in Uk that have special tidal effects.
I used it to good effect once in the port of Treguier in France. There is
up
to 30 feet tidal range there. The tide was starting to fall and several of
us were anchored lower down the river eyeing the last good place to anchor
nearer the town. It was clear all were wondering if they would find
themselves aground at low water. I used the 12ths rule and decided to go
for
it and it worked for me as at low water I had 1 foot below my keel.
Everyone
else had a long trip in the dinghy!


Good job!

I have my own rules of thumb for bidding contracts. Once, with a group of
"rocket scientist" we were to bid on a project. They all jumped in
calculating the minuteau of the project - 5 of them spending days. I told
them they were crazy and did my estimation in 45 minutes. We differed in
total by about 1% and categorically were within 5% of each other. 5 man days
vs 45 minutes and got the same numbers. I was never invited to do proposals
again.

Rule of thumb for wetted surface area
 

"Edgar" wrote
Yes, indeed. It does not make sense to calculate something to many places
of
decimals when the input data is not in itself all that accurate. If you do
that you are just deluding yourself.

Exactly. And one of the first things to do is figure out whether more
accurate data is available, and whether or not it is worth whatever
investment is necessary to get it.
OTOH if you have enough data, the decision makes itself.


The best rule of thumb I can think of off the top of my head is the rule
of
12ths for calculating tidal heights. Works pretty well on the whole for
practical purposes but ignores barometric pressure effects and is not
valid
for ports such as the Solent in Uk that have special tidal effects.
I used it to good effect once in the port of Treguier in France. There is
up
to 30 feet tidal range there. The tide was starting to fall and several of
us were anchored lower down the river eyeing the last good place to anchor
nearer the town. It was clear all were wondering if they would find
themselves aground at low water. I used the 12ths rule and decided to go
for
it and it worked for me as at low water I had 1 foot below my keel.
Everyone
else had a long trip in the dinghy!


I guess it was too late to go back & buy a shallower draft boat...
maybe a centerboarder....

Gilligan wrote:
I have my own rules of thumb for bidding contracts. Once, with a group of
"rocket scientist" we were to bid on a project. They all jumped in
calculating the minuteau of the project - 5 of them spending days. I told
them they were crazy and did my estimation in 45 minutes. We differed in
total by about 1% and categorically were within 5% of each other. 5 man days
vs 45 minutes and got the same numbers. I was never invited to do proposals
again.

A mistake. They should have made you the chief estimator. I adopted
that title informally and thought it was a joke until a few years back
I had to conference with a guy at one of those big-name int'l firms
whose offficial title was "Chief Estimator." How!

signed- Injun Ear (formerly known as Eagle Eye)

Rule of thumb for wetted surface area
 

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.