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: Sunday, May 23, 1999 6:56 AM
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"


>(SNIP{ about flow)
>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.

I think we are looking at this wrong. I don't think the water is doing
anything but getting out of the way. The waves on the surface aren't causing
any acceleration or deceleration of the "flow" back and forth. The hull is
moving at a relatively constant speed in a given direction and what we have
been calling flow is just the indicators of the direction the boat is moving
as it scrapes through the water. (your surfboard and rudder examples helped
me see this). No water goes down under the surfboard without also displacing
some other water out of the way and since water is not comressible the total
of all these nudges between molecules is upward (making a wave). The
indicators are on the hull so they mostly show which way the boat is moving.
Whether the shape is a rudder blade, surfboard or something in between it
will displace either the water it is moving through (or in the case of the
board at speed it itself will be displaced upwards--planing). If every
molecule of water in the vicinity was tagged and tracked we would see some
draged forward by the hull, others scraped from the surface and flung
forward, much nudged to the side and in so doing nudging others upward to
make room. The net effect is visible at the surface in the form of waves.
No water (other than a few molecules being dragged along) actually dove
under the boat rather the hull bottom in the forebody is contacting water
that hasn't been displaced yet and that still water is holding the little
tufts of yarn in position while the other ends attached to the hull are
pulled forward by it so they pretty much pull straight back at least until
they run into something that causes a major back eddy (separation of flow)
>
(SNIP)

>
>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'm not sure I want to argue with the Admiral but imagine a flat bottomed
ship built like a cow-catcher on a train. It should be clear that most of
the water displaced would be lifted and deposited to the side by the
cow-catcher bow yet if we looked at the little telltales we attached on the
bottom of the hull they would point in the opposite direction of the ships
motion even though no water would have gone downward under the hull at all
to make them do that. Why then are we or the good Admiral assuming some
downward flow based on this "flow" direction evidence?
>
Snip


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