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

(SNIP{ about flow)
>
>Why do you think it appears to do that? Are these hulls pushing hull speed
>and the trough that follows the bow wave is actually what is "diving
under".
>It sure seems to me that the net effect is that the (incompressible) water
>had to be displaced to the side and upward first and then is falling back
to
>below grade as the hull is passing through.

I suspect that to most people it  "appears" to be pushed to the side mostly
because one can so easily see the divergent wave system and assumes that
the water gets pushed to the side. Actually the bow wave doesn't go
anywhere, it remains at the bow. The transverse system is created
continually and the flow continues as the trough (and then later to more
crests and troughs depending on speed).

If we watch the crests and troughs along the hull we have to explain why
the water appears to flow aft instead of back and forth. Tough to do if the
water flows in and out rather than just accelerating and decelerating fore
and aft.

The flow under the hull exist regardless of speed. Another interesting
group of diagrams in Taylor shows the net sinkage of the hull due to the
increased flow velocity under the boat (Bernoulli's theorem at work). Many
people make the same assumption that the water is pushed to the side and
flows back at the stern and some of it does. The flow, however is three
dimensional not two dimensional.

The amount of flow in any direction depends upon the hull shape and, more
importantly, the ratio of beam to length. At one extreme, a flat plate like
a rudder,  the flow goes around, at the other, a surfboard, most of the
flow goes under. Sea kayaks, being relatively wide and short , have
considerable flow under the boat. Fine ends don't seem to have any effect
in
that eventually the flow has to get under the boat.

(SNIP)
>
>I don't have a copy but confirmed this with some other texts. Were any of
>the tests pictured done at very low speeds? I have trouble believing that
>the flow along the hulls surface will follow the buttock lines at low
>speeds.

Not sure about very low speeds but Taylor tested as low as S/L 0.6 which is
getting down there where wavemaking doesn't have much effect. I would guess
that, at speeds below wavemaking the water still travels along the hull
longitudinally rather than axially since if it didn't our use of friction
formulas would not apply since the "length" of the surface would across the
hull surface not along the hull surface. This problem of flow bothered me
when developing the hull length correction I.E. whether to use just the
waterline shape or to include the change in volume at the ends. I ended up
using both (the ultimate compromise :-))

To quote the Admiral, " Perhaps their most notable feature (flow lines) is
the strong tendency of water to to dive under the forebody, as it were."


>
>I forgot to mention that one. I'll have to remember to add it to my list.
>I'd like to learn more about this before I make that claim though. Do you
>have a source that explains why there is less energy loss. Is it just the
>loss due to pitching more or is it also the greater pressure exerted on
the
>blunter ends by the waves?

We can see the visual evidence in the spray created. It takes energy to
splatter water all over the place. Also, one can easily imagine the energy
lost when a boat slams. The best treatment of this I have found is in Kents
"Ships in Rough Water". In simple terms, the boat creates waves that
radiate away from the boat as it plunges into the waves. The larger the
waves, the more energy used to create them. I like to use diving as an
example of what happens. A good clean dive makes small waves and the diver
plunges deeply into the water. Do the cannonball and you make huge waves
and don't go very deep.

Havelock used this formula to obtain the mean wave pressure on the hull:

R= 1/2gp^2Bsin^2a

where
g = acceleration of gravity
p = density of water
B = beam
a =equals waterline the half angle forward

Obviously much too simple :-) but it hints at  the importance of hull
fineness forward.


(SNIP)
>
>Most people aren't looking at the waterline at all but at the seam line
(top
>view) when deciding if a boat is Swede form  or fishform and that can be
even
>more misleading than just looking at the waterline. For instance, our
Escape
>looked way more Swedeform than it actually was because there was
>considerable flair behind the cockpit (to get more secondary stability and
>storage room) but at the same time the flare was reduced markedly to near
>vertical sides just in front of the cockpit (in order to make it narrow in
>the paddling area). It then flared out again further forward into a ship
>like bow. It looked radically Swedeform but it was only moderately so.

Good point. I get so absorbed with what I see under the water that I
completely ignored that part of it.

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




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