Re: Faster in salt or fresh water?
Roger Long
(some stuff) in which the lack of any mention of wave making resistance and
speed length ratios which change the proportion of resistance due to
friction has me rolling my eyes.
--
Roger Long
Bruce in Bangkok
>On Mon, 17 Mar 2008 12:02:23 -0700, Norman Franklin
><norm...@gmail.com> wrote:
>
>>Under equal conditions are sailboats faster in salt or fresh water? In
>>fresh the LWL is longer, the water is less dense but the wetted
>>surface increases. Salt has less weted surface, shorter LWL and
>>heavier water. I believe salt water has a lower surface tension which
>>may give it the edge. Any inputs? Thanks,
>>
>>Norm
>
>This question has been asked of scientists for which they have no
>definitive answer:
>http://www.newton.dep.anl.gov/askasci/gen06/gen06300.htm
>
>Fortunately for all of you I have the definitive answer and am willing
>to share it with the lesser informed, such as everyone on the group.
>
>The resistance of a hull is proportional to the frictional coefficient
>times the wetted surface area raised to the frictional exponent power:
>
>R~f*S^n
>
>For salt water f=.01055, n=1.825
>
>For fresh water f = .0104, n=1.85
>
>Firstly, let's find the point of equality:
>
>.01055*S^1.825 = .0104*S^1.85
>
>algebriac reduction yields:
>
>1.0144 = S^0.025
>
>raising both sides to the 40th power:
>
>1.77 = S
>
>It is now obvious that a boat with a wetted surface area of 1.77square
>units has the same speed in either fresh or salt water. Hulls with
>smaller wetted surface than 1.77 are faster in fresh water, hulls with
>larger wetted surface areas than 1.77 are faster in salt water.
>
>So the bottom line is that small boats do better in fresh, large ones
>in salt. I could go on and calculate actual hull lengths and
>dimensions but such an analysis is beyond the comprehension of the
>layman populating this group. However, if such an analysis is desired,
>I would be more than pleased to apply my mensa trained intellect to
>such a task and come up with actual numbers for hull sizes and the
>speed difference.
>
>Some of the more astute here will be puzzled by the dimensions of
>square units. Let us say simply the calculations used ratios and not
>absolute dimensions. Chapters 11 through 14 in my new definitive book
>cover all of this in detail and I will gladly provide you an
>autographed copy of the book for the full price plus an additional ten
>dollars. The ten dollars barely covers the cost of maintaining my good
>looks. Five dollars more will get you an 8x10 of me in my black
>speedos.
>
>Nothing pleases me more than knowing how thankful you all are for me
>providing the foundations of true knowledge. Your cups runneth over
>from this one expository, how fortunate you all are.
>
>Jax
And another line in the kill filter file......
Bruce-in-Bangkok
(correct email address for reply)
RichH
Absolute (centipoise) viscosity of fresh is less than salt water;
therefore, the answer is faster in fresh. Applies to and regulates
wave trains and friction factors from a hull. Applies to both energy
solutiona as well as Newtonian solutions. Roll your eyes,
mon. ;-)
Roger Long
"RichH" <rhm...@att.net> wrote
> Absolute (centipoise) viscosity of fresh is less than salt water;
> therefore, the answer is faster in fresh.
I would tend to defer to anyone who can use terms like "centipoise" and
"solutiona" so you are probably right.
It's been 2 - 3 decades since I gave this any though but my memory is that
wavelengths are shorter in fresh water. They certainly appear to be by
observation. Since higher speeds are governed by the length of the vessel
generated waves the possibly wrong explanation I'm carrying around in my
brain is that hull speed will be lower. Of course, the speed / length
relationship of waves could be different.
At this point, I don't claim to know more the average bear on this subject
but I'll be attending sea trials of a boat in fresh water next October and
then going on it's delivery to Maryland so I'll let you know:) There's a
dusty book on the shelf in my office with a chapter on adjusting sea trial
results for water density and fresh water sea trials. If anybody wants
chapter and verse...
Anyway I was just commenting on the OP seeming assumption that it is all
friction.
--
Roger Long
dougk...@yahoo.com
> > Absolute (centipoise) viscosity of fresh is less than salt water;
> > therefore, the answer is faster in fresh.
>
"Roger Long" wrote:
> I would tend to defer to anyone who can use terms like "centipoise" and
> "solutiona" so you are probably right.
>
yeah, "centistokes" is OK but "centipoise" is one to get out the old
text book.
> It's been 2 - 3 decades since I gave this any though but my memory is that
> wavelengths are shorter in fresh water. They certainly appear to be by
> observation. Since higher speeds are governed by the length of the vessel
> generated waves the possibly wrong explanation I'm carrying around in my
> brain is that hull speed will be lower. Of course, the speed / length
> relationship of waves could be different.
>
Wave train speed is also affected by the celerity of the medium.
> At this point, I don't claim to know more the average bear on this subject
> but I'll be attending sea trials of a boat in fresh water next October and
> then going on it's delivery to Maryland so I'll let you know:)
Power boat?
The answer could be different for power vs sail. Both are affected by
friction and wave-making resistance, the power boat's speed will also
be affected by issues of power transfer from the prop to the water.
The sailboat will only be affected by drag (although it's pointing
will be affected very very slightly).
After muddying the water, time for me to git....
Fresh Breezes- Doug King
Roger Long
<dougk...@yahoo.com> wrote .
>
>> At this point, I don't claim to know more the average bear on this
>> subject
>> but I'll be attending sea trials of a boat in fresh water next October
>> and
>> then going on it's delivery to Maryland so I'll let you know:)
>
> Power boat?
This one:
--
Roger Long
RichH
RichH
tsm...@gmail.com
> "RichH" <rhmp...@att.net> wrote
>
> > Absolute (centipoise) viscosity of fresh is less than salt water;
> > therefore, the answer is faster in fresh.
>
> I would tend to defer to anyone who can use terms like "centipoise" and
How about adiabatic or kinematic viscosity? No, I will not pretend to
know the answer, but I read that wave speed is dependent on both
density and bulk modulus (eg. the speed of sound in a liquid is ~
sqrt(bulk modulus/density)). That's what the wiki says, and I'm
sticking to it! :) In practice, the speed of sound in sea water is
higher than in fresh so I think that must mean that wave speed is
higher in sea water. No? But, I'm doubtful that such effects will be
large enough to overcome differences in flotation, the relationship
between skin and wave drag and other complications that vary form boat
to boat. I've raced the very same boats on Lake Ontario and on the
Atlantic and any differences in speed were below the threshold of knot-
meter error.
-- Tom.
RichH
deg. f.)
So, with the same energy applied a boat in saltwater will be 9%
slower, than in fresh water due to the frictional effects
(viscosity) .... but uncorrected for the slight deeper immersion of
the boat in freshwater (buoyancy).
The density of fresh water is 62.3 lb./cu.ft (70°F); saltwater 64.0
(70°F) .... so that the boat's center of mass will be 3% lower/deeper
in the water.
and that will have to be factored into the above when correcting for
'projected cross secional area
The 'trivial' answer would be 9%-3% = 6% slower in salt water than
fresh.
:-)
jeff
...
> The density of fresh water is 62.3 lb./cu.ft (70°F); saltwater 64.0
> (70°F) .... so that the boat's center of mass will be 3% lower/deeper
> in the water.
> and that will have to be factored into the above when correcting for
> 'projected cross secional area
>
> The 'trivial' answer would be 9%-3% = 6% slower in salt water than
> fresh.
>
So while we're doing "trivial" answers, I always thought of wave making
resistance in terms of the energy required to push aside an amount of
water equal to the displacement of the boat. Since the displacement
doesn't change, why should this affect the speed?
Goofball_star_dot_etal
Complete BS in this thread noted..
Justin C
>>if such an analysis is desired,
>>I would be more than pleased to apply my mensa trained intellect to
>>such a task
Mensa trained? So they knew you were a genius before you were able to show it
and so "taught you to be smart"?
AFAIK mensa don't teach you to be smart, you have to be that in the first
place. Something I think you've just failed to demonstrate.
Justin.
--
Justin C, by the sea.
Jere Lull
>> On Mon, 17 Mar 2008 14:04:00 -0700, JaxAshby <jaxa...@ao1.com> wrote:
>>> if such an analysis is desired, I would be more than pleased to apply
>>> my mensa trained intellect to such a task
>
> Mensa trained? So they knew you were a genius before you were able to
> show it and so "taught you to be smart"?
No, they probably taught him his social skills. ;-)
> AFAIK mensa don't teach you to be smart, you have to be that in the
> first place. Something I think you've just failed to demonstrate.
Mensans aren't necessarily smarter by anyone's estimation, just better
at taking tests. Often, they're less able to focus on the task at hand,
more likely to confuse similarity with identity, mistake knowledge in
one arena with wisdom in any.
--
Jere Lull
(Currently, I'm a local Mensa editor, the Admiral is our local Prez,
and we're fairly well known nationally. I have observed that those who
feel the need to assert their "Mensa-ness" usually aren't very
accomplished.)
RichH
> resistance in terms of the energy required to push aside an amount of
> water equal to the displacement of the boat. Since the displacement
> doesn't change, why should this affect the speed?
No, the relative immersion DOES change (thus skin friction and cross
sectional impact/friction changes), with the change in water density,
salt water being more dense than fresh.
You can ignore wave making, etc. as in physics or engineering your can
approach an answer from either the 'action side' OR the 'reaction
(wave making) side' and still arrive at the exact same answer.
action=reaction
Just as a pump requires more energy to 'push' a thicker more viscous
fluid than a thinner less viscous fluid ... same with a boat when you
look at the 'energy' needed instead of the F=ma stuff. Viscosity is
the 'friction' of fluids, the higher the viscosity the more work/
energy it takes to do the same job.
richforman
>
> So the bottom line is that small boats do better in fresh, large ones
> in salt.
That's the result I've always gotten with my pwc's, faster top speeds
on the gps in fresh water, and since they are indeed small boats, well
anyway that conclusion does jibe with my experience.
> Some of the more astute here will be puzzled by the dimensions of
> square units.
I was indeed wondering about that (guess that puts me among the
astuter). Still don't know what it means (well, you didn't really
explain it), but again since my boats have all been small I'll just go
with the "small boats" part of your theory and agree.
richforman
Ryk
<e3019b4a-0c85-4413...@e39g2000hsf.googlegroups.com>
RichH <rhm...@att.net> wrote:
It is important to note that the viscosity of fresh water varies by
almost a factor of two over the range of water temperatures we sail in
and that viscosity is only a big factor in light winds. When sailing
is actually fun, it is almost all inertial rather than skin friction.
Likewise the speed of sound in water has little to do with the speed
of the surface waves that limit classical hull speed.
The whole thing is considerably more complicated than simple ratios of
properties.
Ryk