Re: [Paddlewise] Pro's and Con's of the "Swede Form"

-----Original Message-----
From: Matt Broze <mkayaks_at_oz.net>
To: 'Paddlewise' <paddlewise_at_lists.intelenet.net>
Date: May 29, 1999 1:54 AM
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"


>
>Does this increased velocity (over the velocity of the boat is moving
>already I presume) of flow and the resulting decrease in pressure come
about
>due to the longer distance that water must travel around or under the hull
>(Bernoulli)?

Whew, lots of Questions.  Yes.

>Or is it the result of the pressure differential between the
>front and back of the moving hull (inertial, form or wake drag--pick a
>name)? Both?

Must be some interaction although I can't recall reading how much or if it
is. My answer, Not sure.


>Both plus wave drag? Or are waves just the result? Or are these
>all just different ways of describing the same phenomenom?

The waves result from energy expended they do not cause resistance.

>If the waves are only the result of drag forces rather than a contributor
as
>well how come we don't get the same drag in a uniform fluid (such as
>underwater). And why does the imposition of another interface between
>different densities of water (such as fresh water overlaying salt water)
>create wave at this interface and add more drag to a hull?

The wave making ressitance is the net fore and aft force acting on the hull
due to fluid pressures acting normally on all parts of the hull. When a
body travels at or near the surface the variation in pressure causes waves
that alter the pressure distribution and the resultant manifests itself in
wave making drag as seen in the waves created.

>
>John wrote:
>>Once the speed reaches a point where the dynamic forces can
>>offset Bernoulli's effect the boat begins to climb. This can be seen
quite
>>nicely in the tank. As the good admiral said, the change in trim at high
>>displacement speeds is a symptom of speed.
>
>Can you please explain the reason for the change in trim of the boat using
>physical principles rather than using vague terms like "symptom of speed"?

I will try. As the pressures vary along the hull waves are created. These
waves alter the immersed volume. Since, in displacement mode, the boat must
displace a constant volume of water the trim must adjust to suit those
changes in pressure.  The boats settles into the trough to adjust its
displacement.

>
>Me, still floating around in the water getting hypothermic, wonders if you
>are still riding on the boat. As I watch your fast boat move from a
>displacement to a planing condition it moves a whole lot horizontally and
>very little vertically the resulting motion of the CG looks to me to be
>"climbing" at a shallow angle until levelling off once fully planing (I'm
>talking direction of CG here not trim angle). Now if I was riding on the
>boat and looking only at a very sensitive altimeter I might be seeing this
>as a vertical climb.  I suspect you mean something else but I can't fathom
>what that might be.  Does an airplane climb only vertically by your
>definition of climb? Is vertical the only way to climb by definition?

If you were in the water and looking through a surveryor's level at a fixed
point on the hull located at the CG you would see the fixed point lower
first and then begin to rise vertically as the boaty lifted onto a plane.



>>
>John asked:
>>How far away?
>
>20 or 30 feet should do it  ;-) probably a lot less.

OK, this explains the problem. We aren't talking about the same thing.


>Here is what a text ("The Science of Yachts, Wind and Water" by H.F. Kay
>page 124) I just reread says:
>"As the boatspeed approaches the surface speed of its fundamental wave,
the
>drag increases considerably because the boat is trying to go faster than
>this wave, therefore it is trying to climb uphill out of its own
fundamental
>wave trough in a situation analogous to the high speed aircraft at the
sound
>barrier trying to go faster than the speed of sound. Furthermore the boat
>tends to 'squat' low down in the water because the buoyancy is operating
>more at the less effective fore and aft regions than at the bulbous centre
>region where the water is forced lower."

Yes, I read similar comments and still have trouble with the "climb"
business. On a purely logical basis how you can climb something you create
continuously for, as observation of a boat getting up on a plane show
reveals the bow wave gets smaller as the boat rises. if you climb it it
should remain until you get to the top (how can you climb something that
just keeps getting smaller and if so are you climbing or is the bow wave
just getting smaller?)  I can understand Taylor, Lewis, Savistky and others
more easily since what they say I can confirm in the tank and elsewhere.

>This is obviously one of the places I got my (mis?)education. I sure wish
I
>could understand this subject starting from the basic principles of
physics.
>Waves are a result of drag as Nick says. Is it also possible that they
then
>interact with the hull and each other in ways that add to or decrease the
>drag on the hull rather than be just a forgotten burden illustrating the
>drag but not contributing to it? The above "Kay" explanation though not
>getting down to basic principles is attractive in that it explains the
trim
>change when pushing above hull speed. It also seems to explain why in
>shallow water with its reduction of wave speed this squatting happens at a
>lower speed. This explanation may be dead wrong but I have yet to see
>another explanation for the trim change that takes place. Anyone care to
>try?


I see your point. Does the wave cause the resistance or does the resistance
cause the wave?

In the first part of this post I provided a general concept of wavemaking
that I borrowed from "Principles of Naval Architecture" by SNAME. This
explanation says that the waves result from the pressure differentials
along the hull and the net pressure. If we can change the pressure pattern
we can eliminate the waves and this is what happens with some of the
exotically shaped hulls of Inui and others. By modifying the flow (that
nasty word again) one can modify the pressures. What we see when we look at
waves at a distance from the hull is the net result of resistance and the
result of our manipulations of the flow.

I hope this helps.

If readers think this discussion seems overly complex or even inconclusive
they should read Dr. Takao Inui's paper "Wave-Making resistance of Ships"
and the comments at the end of the paper.  Clearly a full understanding of
what happens does not exist at this point. What I have provided reflects my
understanding of the texts and papers I have read (a lot and maybe even too
many).

I try to caution people who use my prediction program about its
shortcomings and I know of no other developer of a program who would even
hint that their program tells the full story.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/





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