NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/rec/windsurf/4171

Text File | 1992-11-16 | 2.8 KB | 55 lines

Newsgroups: rec.windsurfing
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From: [e-mail] (David Abraham)
Subject: Why do blade fins work better?
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Date: Mon, 16 Nov 1992 19:44:30 GMT
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So- the title says it.  Seems like for (at least) straight line speed
blades are dominating.  I like the one I have pretty well, although to
tell you the truth, aside from feeling looser I can't swear it really
is faster since I haven't done side-by-side comparison.  Anyway, let's
assume it is faster.  The question is, why?  The differences I see are
two- the obvious one is the aspect ratio, and the other is the thin
foil that is possible with the stiffer G10 material.  As far as aspect
ratio goes, I know that most low drag shapes tend this direction, such
as sailplane wings and so on.  For high lift at low speeds airplane
wings tend to be stubbier with thick airfoils.  Not so efficient, but
if you have plenty of power they are more forgiving.

With a thin foil you would get less lift for a given planform and a
given speed.  Also, you have less drag.  I don't know how this
balances out- why is it that it's more efficient to have a thinner
foil but pay by having a greater fin area to compensate for this
effect?

Now, as far as aspect ratio goes, the statement is that equal areas in
different shapes give different efficiencies.  I am thinking that this
is because turbulence becomes more severe with the low aspect fin,
adding drag.  I did a very (!) crude calculation, and for a fin of 10
cm. width moving at 10 m/s (about 20 mph), width a kinematic viscosity
of water of 0.01 cm**2/s we get a Reynolds number of a million.  This
is to my understanding well into the turbulent regime, and I am hard
pressed to see why a factor of a few in the width is going to be
important.  (Laminar flow happens up to Reynolds numbers of thousands,
not millions).  The other cute thing that comes out of this is, if
these factors of a few are important then you could see a similar
effect when sailing in really hot or cold water.  The difference
between water at 0 degrees and say 35 C is about 2.6 (in viscosity),
an equivalent change to going to a blade from a old-style fin.  The
sense of the change is such that colder water would be lower drag.

Just rambling on.  Any people out there REALLY know what's going on?

--
David Abraham            [e-mail]
IBM T.J. Watson Research Laboratory
[phone]