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

(SNIP)


>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).

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

(SNIP)

>
>Do you think a Hobiecat's narrow hull is inefficient? My understanding is
>that it is so fast because rather than having to climb over the bow wave
it
>simply cuts through it . Being so narrow and moving water only a little to
>the side and hardly downward it wouldn't suffer much from the Bernoulli
>effect either.

Actually the Hobie cat hull has very poor efficiency at kayaking speeds.
Used to kick their butts all the time in my Thistle. Of course, when the
speed got up to the point where I had to plane and they didn't, then they
kicked my butt. Boats with high L/B ratios don't experience the same
problems that boats with low L/B ratios experience. To assume, however,
that there would be no difference would lead one to erroneous conclusions.
One cannot assume that the flow regimes for a scow will be identical to
than of a SWATH ship. The principles remain the same but their degree
varies. In this way, the flow along the bottom of a wide shallow boat
varies from the flow along the narrow bottom of a deep narrow one in
degree. We cannot assume that, because a deep narrow boat has so little
flow along its narrow or nonexistent bottom that there would be no flow on
any other hull form. In the same way we cannot assume that, the flow on the
bottom of a wide boat has to be duplicated on a narrow one. What we do is
study the boats we deal with. Thus we are careful not to apply the design
principles of a submarine to a sea kayak and in so doing avoid cowcatcher
bows.

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 ................


>I think our differences here are because you are looking at what the
>apparent flow over the hull is as it moves through the water and I am
trying
>to understand what is actually happening to the water as the hull goes by.
>You are riding on the boat and I am swimming in the water. I realize that
>skin friction and separation drag is going to be determined by how the
water
>appears to flow past the hull. I also realize it would make little or no
>difference in the physical effects if the water flowed by a teathered hull
>or the hull moved through still water.  I'm just trying to point out that
>the telltales are going to mostly point in the direction of travel and if
>the hull is in the way they will point as closely as they can to the
>direction of travel.

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. I cannot debate the
philosophical aspects of sitting or swimming. They would not let me swim in
the test tank possibly because they saw how much coffee I drink. :-)


>
>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.

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.

>
>>The flow indicates where the water goes. To assume that it doesn't go
where
>>it is going kind of boggles.
>
>Again you are an observer riding the boat.

?????

>
>The flow lines over the hull are the resultant of the motion of the hull
and
>the direction the water is actually moving.

Yes.

>If the water moves 2" down while
>the boat moves 200" forward the flow would appear to be at a slight
downward
>angle but the water itself would have actually have moved 2" downward and
>slightly forward (due to the friction with the hull).

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.

>So the water is mostly
>moving up or down with the waves and being pulled along at some speed
>between as fast as the boat (or even faster for spray flung forward at the
>bow) and not forward at all some distance from the boat (which gets wider
as
>the boat moves past resulting in the wake behind the boat.

The waves are continuously created by the energy expended by the boat.
These
waves get left behind as can be seen in the wave train where the energy
dissipates.

>I don't know why the flow lines around a moving hull are as they are (and
>I'm not disputing the direction of the lines just maybe the expanations
>given--that I don't see clearly)
>I'm trying to understand why they are that way in fairly simple terms and
>basic principles that even I can understand.

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.

>
>It is getting into our busy season and I am spending way too much time in
>front of this computer so I'm going to try to taper off my responses, but
if
>I can't control myself more I'll have to quit Paddlewise cold turkey. It
has
>been fun.


Making a living always comes first. Will miss your valuable input.

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


***************************************************************************
PaddleWise Paddling Mailing List
Submissions:     paddlewise_at_lists.intelenet.net
Subscriptions:   paddlewise-request_at_lists.intelenet.net
Website:         http://www.gasp-seakayak.net/paddlewise/
***************************************************************************