News f2s wrote:
"Gary" wrote in message
newssEtf.107527$2k.93334@pd7tw1no...

Actually, you quoted, and your source expressed himself very
poorly, using some technical sounding words incorrectly.


A fin keel boat separates the water flow across the keel
creating
differences in hydrodynamic pressure resulting in better
performance and
efficiency to windward.


As you can see in the attachment, the boat is sailing to
windward on a
close reach or close hauled. Notice how the boat does not sail
perfectly
straight, but rather at a slight angle (yaw)(angular component).


I'm afraid the guy who wrote that lot confuses the issue by using
many terms differently from those in common use.
'Straight' usually means traveling in a straight line. 'Yaw' or
'angular component' usually means a rate of turn. So his first
proposal seems to mean that the boat is turning - which I'm sure
is not his intention in this case. What would be very clear to
most people would be to say 'The boat does not go where it points;
it travels through the water slightly to leeward of its heading'.
A step further would be to define this difference as the angle of
leeway. Which is Roger's point exactly.


The
water hits the downwind side of the keel creates a pushing force
called
drive. The water bending around the windward side of the keel is
forced
to follow more of a curved path. The curving of the flow of
water across
the windward side creates an area of less pressure (hydrodynamic
lift),
and has a tendency to pull the boat up closer to the wind. This
same
effect also occurs at the rudder resulting in a lifting force
off of the
rudder and better efficiency to windward.


Well, I've never heard of this 'pushing force called drive' in
relation to keel behaviour, but I'm not very widely read. I'd have
called it 'a lateral force'.

And I would have said 'any symmetrical hydrofoil held at an angle
(in this case leeway) to the water flow will generate a lateral
force and some drag'.

For further explanation one might say 'It does this by deflecting
the water flow towards the chord line of the foil' and 'for this
to occur there is necessarily low pressure on one side of the
foil, and high pressure on the other'.

One might then go on to explain that much of the drag element
caused by the keel is due to water flowing around the lower tip of
the keel from high to low pressure, creating a vortex. This would
lead nicely to an explanation of why long keels suffer more
leeway, instead of your sources incredibly garbled account of why
long keels don't allow an angle (which, fair enough, you didn't
quote).

I think you've got yourself a very bad textbook here.

JimB



The discussion is over. John and I have agreed on the point. Sorry.