[Rhodes22-list] Re: Pointing

[Peter Thorn]

Slim,

As I understand it, the lift from the underwater fins of a sailboat derives
from the cockeyed angle that most sailboats actually present to their true
windward course.  Sailboats do not point in the exact direction they are
moving, since leeway causes them to "crab off" a little to leeward.  If you
have Dennis Conner's 1994 book "Learn to Sail", there is a sketch on p 104
that shows this effect.  As a result, the bow points a tiny bit to windward
of the course the boat is really moving.  This angle will result in the
trailing (aft) windward edge of the foil developing turbulence and a
slightly longer path for the waterflow, and thus lift.

Although the fins are symmetrical (so they can work on either tack) and not
as efficient as asymmetrical airplane wings are, they can still provide a
small amount of lift.  A slight amount of weather helm (have you heard it
said to tune your boat to have a slight amount of weather helm?) will
generate some rudder lift, since the rudder is held a tiny bit off
centerline to compensate for the w/h.

Perhaps this could be inspiration for Stan's next R22 development
breakthrough:  asymmetrical twin centerboards!  Raising and lowering boards
every time one tacks may not be so popular, but my guess would be that if
any interested prospective R22 buyers have the money, Stan could find the
time.

Slim, I am not an engineer but a general contractor with a life-long passion
for sailing-- so I read a lot about it.  Perhaps Roger or Brad could offer a
more scientific explanation.

Fair winds,

PT

----- Original Message ----- 
From: "Steve Alm" <salm at mn.rr.com>
To: "Rhodes" <rhodes22-list at rhodes22.org>
Sent: Thursday, September 23, 2004 3:26 AM
Subject: [Rhodes22-list] Re: Pointing


> Peter,
>
> Hold on, thar!  "Lift" from the keel, CB and rudder?  The underwater
> appendages are symmetrical with the hull and cannot provide any lift.
They
> only serve to prevent lee way, or to provide lateral resistance.  That
part
> I agree with.  Brad might have a better description, but lift happens when
> air (or presumably water) has to travel farther around one side than the
> other, creating a difference in pressure between the two sides.  Lift is
> created by the curved shape of the sail or airplane wing and will stall if
> not going fast enough.  The keel, CB and rudder do not have that kind of
> shape.  I'm with you on the rest as far as pinching vs. pointing goes, but
> it's the sails that stall out, not the keel, CB or rudder.
>
> Slim
>
> On 9/22/04 7:58 PM, "Peter Thorn" <pthorn at nc.rr.com> wrote:
>
> > Hello Ed,
> >
> > If you verify that you're able to point your R22 35 degrees off the true
> > wind, I certainly would like to visit Lake Hartwell to see that.
Perhaps
> > it's the apparent wind, the combination of the boat's velocity across
the
> > bottom combined with the true wind direction, that's making you think
you're
> > pointing so close. On a reasonably fast boat like R22, the apparent wind
> > angle can move quite a bit forward.  In an extreme example such as
iceboats
> > (that travel many times the true windspeed) the wind indicator points
almost
> > straight forward.
> >
> > Are your headsail sheets led to tracks at the foot of the cabinhouse
roof?
> > That, I think, would certainly improve pointing.
> >
> > It's good to be aware of the difference between pointing and pinching.
> > Sailing too close to the wind can cause the underwater foils to slow
down
> > then stall.  That's pinching.  When the keel, cb and rudder stop
producing
> > lift, the boat will start to produce a lot of leeway, or sideways drift.
It
> > is very difficult to detect leeway when aboard the boat that's making
all
> > the leeway.  The bow points higher, so the skipper might think he's
pointing
> > pretty high because the sideslip is so hard to feel.  To avoid this
> > condition, foot off and keep the boat moving.  After regaining speed,
head
> > up a little.
> >
> > If you have a GPS you can verify your pointing angle by measuring your
> > heading (not the direction the bow is pointing), tack to the other tack,
> > measure heading again and divide the angle difference by 2.  I think
someone
> > mentioned this not too long ago on the list.
> >
> > I too have wondered about the diamond board, and would guess Phil Rhodes
> > original cb is pretty hard to improve on.   A while back Roger wrote a
very
> > scientific sounding comparison, do you recall that?
> >
> > Perhaps you Lake Hartwell guys should conduct on-the-water pointing
trials
> > and settle the issue.
> >
> > PT
>
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