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

-----Original Message-----
From: John Winters <735769_at_ican.net>
To: 'Paddlewise' <paddlewise_at_lists.intelenet.net>
Date: Wednesday, May 26, 1999 5:29 AM
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"


>(Lots and lots of SNIPS)
>
>I said:
>>It seems to me that any speed increase in the flow over the boat speed
>could
>>be due to the orbital motion of the wave trough and any extra sinkage of
>the
>>hull (beyond that caused by its own speed) reflects that the boat is in
>the
>>trough. The boat speed itself should cause some sinkage  (the fact there
>is
>>flow and that it is under and to the sides of the hull (but not on top)
>>would cause the Bernoulli effect to be balanced to each side but the
>>unbalance effect below the hull would make the hull ride lower. I don't
>see
>>how increasing the speed of the flow implies the need for a greater volume
>>of water from anywhere but above (dropping down to make a wave trough).


John said:
>The flow diagrams do not show flow from the surface to the bottom. The
>"source" of the flow comes from ahead not the side.

My point of view is from the water watching the boat move by. You seem to be
observing from the boat. The source of the apparent flow is because the boat
is moving. It comes from the front only if you are riding on the boat. Jump
off for awhile.
>
>(SNIP)
John said:
>Boats do not climb their bow waves nor do they cut through them. Boats
>create their bow wave. Regardless of where it lies on the boat the first
>wave created (called a pressure disturbance) is the bow wave. I know I
>sound like a broken record on this but ................


Of course the boat creates the wave but that wave sure looks to me like a
hill to climb that is bigger than the hill that would have to be climbed to
reach planing speed if one started from a position where the wave hadn't
already been formed. I am well aware that true planing is beyond human power
except with the use of hydrofoils (at least at the present time and into the
forseeable future).
The powerboat sure looks to me to be climbing out of the hole of the first
trough behind the bow wave when it is moving from displacement to planing
speeds. A waterskier starting from a dock finds it much easier to plane than
one being dragged through the first water does (or to eliminate some
variables --has slowed to the point of sinking before power is again
applied).
Since planing is using an angled plane and speed to maintain a higher
position against the force gravity, it sure seems to me that if you start
from a lower position you need to climb up to the higher plane. If you have
pushed up a wave in front of you you will have to climb even higher to get
it more behind you.
>
John said:
>I don't get the point. The flow markers (whatever they are) will point in
>the direction of flow and that isn't  always in the direction of travel.
>The flow is what actually happens to the water.

No, the flow is what happens to the water/moving boat combination (or
boat/moving water combination). The water itself mostly moves mostly up and
down in an orbital motion as the boat passes. If you would jump in the lake
it might be easier to see it my way.;-)
>
>>I said:
>>I was merely proposing a shape that would show a flow sraight back on the
>>bottom that clearly did not require a downward flow yet exhibited the same
>>behavior of the telltales as was being claimed as evidence of a downward
>>flow of water.
>
John said:
>I recognize what you were doing. Imagine a ski gliding across the surface
>of the snow. Clearly the ski glides across the surface of the snow. Suppose
>we turn the tip down instead of up. Then the ski would plow under the snow.
>Should we conclude from this that skis do not glide over the snow because
>we can create a ski that does not? One should not apply the design
>principles of a snowplow to a Ferrari.

I don't think I was. Sometimes it is easier to see things when you take them
to extremes. If evidence is being used to show something to be true but if
that same evidence can also be shown to be present due to a different cause
it would cast some doubt on the real meaning of that evidence.

John said:
The water moves with the hull in the boundary layer not in the free stream.
>This discussion has to do with the free stream flow.
>
Most telltales are inside the boundary layer. Earlier you mentioned some
that were not. Hold them far enough away from the boat and they all will
point only in the direction of motion.

John said:
>Well, I never said I was a teacher. I think the best thing would be to read
>some of the better texts. Since Taylor isn't easily found you might try
>"Principles of Naval Architecture" by the Society of Naval Architects and
>Marine Engineers. Sometimes heavy going but gets you there.

I have read the relavant parts of "Principles" but it has been awhile. I do
have Aero-Hydrodynamics of Sailing by Marchaj in front of me right now
though. If you have a copy check out the picture on page 77. That picture
(of a sailboat pushing hull speed) looks like it might expain why the flow
lines dive under the boat and follow the buttock lines (at least when one is
pushing hull speed). I still doubt we will see this at very slow speeds
though.

As long as you have the book open check out the flow lines on the hull of
the boat on page 279. They appear to be diving all along the hull and only
following the buttock lines in the forebody. In fact it looks to me that
they may be moving to fill in the low pressure area behind the hull like I
proposed might be happening earlier. Of course that huge keel is probably
confounding things.

Matt Broze
www.marinerkayaks.com
Thoeries are nets:
Only he who casts will catch.
---NOVALIS
>

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