Brian Whatcott wrote in message
...
On Thu, 1 Apr 2004 12:24:46 +0100, "JimB"
wrote:

An experimental rig for visualizing fluid flow over
rudders etc., is easy to make and provably representative of
2-D flow.
snip
At the top of the incline, permanganate crystals trail stream
lines
down the incline.

Nice little touch!

This is an easy way to show the "molecules give lift by hitting
the
proximal surface" enthusiasts how fluid dynamics really works.
(about two thirds of the side force from the distal surface,
and one
third from the proximal surface.) You can work it out from the
streamline spacing over both surfaces.

OK. This is straightforward foil in a free flow. It confirms the
point (among others) that pressure drop and speed change are
linked.

However, our steering rudder in reverse is a foil in (lets call
it) convergent flow, where, if the pivot was actually at the prop
origin, the flow lines would always be along the rudder with no
deflection. As the rudder moves away, then stream deflections
occur, but the speeds (and forces) drop right off, and the flo is
funny too, showing a strong s bend.

And on top of all of that, my fundamental momentum theory sais
that all this input water is starting at zero velocity relative
to the boat, but exiting the prop with a new velocity. So up
stream action (rudder angle) would only have an effect if it
changed the downstream velocity.

This is quite feasible, since output velocity is not constrained
(as from a hosepipe - Ugh - Feyneman again) and if there's a
lateral component at the input end, I'm thinking it would be
present at the output end. An extreme model is looking at an
elliptical duct on the input side canted at an angle to the prop.
So I'll go away and get my brain around that idea to see where it
takes me. It does remove the need to think about all the various
forces on rudder, prop, hull etc and their interactions and
connections in a complex pressure field.

JimB