Spinout FAQ - it's done!

Spinout FAQ - it's done!

Post by Tom von Alt » Sun, 02 Oct 1994 09:22:46

Ok, that's it.  Gotta go to Maui, so if you have any complaints, tell them
to the wind.  I forget if I'm supposed to "turn this in" somewhere, or just
post it....  this final version is only very slightly modified from the 2nd
draft I posted.  (It includes a little detail about my references.)

                        Hewlett-Packard Disk Memory Division

~~~~~~~~~~~~~~~~~~~~~~~~~ schnitt hier ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

     rec.windsurfing FAQ - SPINOUT


There you are, sailing along, powered up in some conditions a little beyond
where you've been before, you hit a little chop and then

You're still sort of on course, but the board is pointing about 45 degrees
closer to the wind, and blasting along sideways.  You're still on a plane,
but you're not sure you should be.

Whoa.  Now that you've stopped and jumped in the water and turned your board
over, you can see that the fin is still there, and just fine.  (Or not, in
which case you need to look for the "busted fin" FAQ.)  What happened?

You've just experienced SPINOUT.

That nice, smooth flow of water on either side of the fin that was providing
the (sideways) lift to keep you on course, and heading a little upwind is
not so nice and smooth when this happens.  Your fin has "stalled," and at the
new angle of attack, 45 degrees or so, there's hardly any lift, but plenty of
drag.  What's more, it's a long way back to that smooth flow!

There are three causes of spinout, and even more ways to cure it.  The actor
in the scene described above has come to be known as "ventilation" and is the
way most of us first encounter spinout; after going airborne off a wave, we
lose it on re-entry.  If air can get at the root of the fin, it can be drawn
into the low-pressure flow on the windward side and lead to a stall.  The
turbulence resulting from chop and the landing helps this happen, as does
putting the fin back in the water at the wrong angle - it needs to be pretty
close to "on course."

The other term that gets tossed around is "cavitation," which is low pressure
boiling.  It's a known and well-studied problem for propellors, and for
hydrofoils at speeds above 40-45 knots.  You probably weren't going that
fast, were you?  Whatever the threshold, any nicks, dings and other surface
imperfections will bring it closer, and increase speed-robbing drag as well.

The third cause is a fin that doesn't match the conditions.  Too small a fin
for that big sail will do you in, as will too big a fin when the wind is
blowing "small sails."  (The latter problem is from loss of control as the
vertical component of lift from your fin wants to make your board fly.)

How do you keep it from happening?  Keeping those dings tuned out of your fin
is certainly important, and better fairing (smoothing of the transition)
between the fin and the board can help.  Technique has a lot to do with it,

    1) As soon as you spin out, try to remove all weight from the
    back foot.  I find that in most cases this alone will allow the
    board to correct on its own.  Usually the spin-out is initiated
    by too much weight on the back foot, and once the spin-out
    begins, weight on the back foot keeps it going.

    As you get better, you will find yourself becoming very familiar
    with the sensation of the fin just as it is about to begin to
    spin out.  There is a moment before the spin out when you can
    feel the fin lose force against the water.  If you act quickly to
    remove weight from the fin just at the moment when the force of
    the fin is lost, the spinout will correct before it even happens.

    2) Learn to grab the board with the back foot and literally yank
    it toward you (to windward).  The idea is to force the fin back
    to the direction you're going and re-establish smooth flow around it.

The side force that you apply to your board and fin is what "drives" the fin.
If you drive it too hard, or when it's not completely in the water (or if
you don't have enough fin to start with) it stalls.  In the worst case,
you have to get out of the straps and move your weight closer to the
mast to get it off the fin.

    ...You must absorb the bumps with your legs.  When going over the top of
    the chop, let the sideways pressure off a bit (don't push so hard with
    your back leg).

    If it happens, pull HARD with your back leg and push with the
    front.  Actually it's more of a jerk than a pull.

    And another approach is to just do a little chop hop to get the
    fin out of the water and situated properly while in the air.  This
    assumes you can land without spinout, however.  :-)

There are plenty of experiments in fin design going on, and you can certainly
join in that fun.  Hydrofoil designers have been working around cavitation
for more than 20 years, and with the sailing speed record topping 50 knots,
it's certainly a problem for windsurfing's "leading edge."

If your designer hasn't made any gross mistakes (or found the Holy Grail),
fin SIZE is the key parameter you need to pay attention to.  If you can
control your sail in the conditions you're in, but you spinout easily, or
can't point as high as you want to, you need MORE fin.  If the board is
getting squirrely, trying to fly on its own, leaving you overpowered and out
of control, you need LESS fin (and maybe less board, too).

_Fundamentals of Fluid Mechanics_, 2d ed., Munson, Young, Okiishi, 1994
        A recent, and excellent, engineering textbook
_Surf_ magazine (German), May 1992, and August 1993
        Watertunnel testing of fins; good photos, but not much test data
_Life in Moving Fluids: They Physical Biology of Flow_, Vogel, 1981
        An excellent introduction to fluid dynamics principles, without the
        mathematical emphasis in an engineering text.  Interesting
        biological examples, too.
_Hovercraft and Hydrofoils_, McLeavy, 1976, p. 143-145
        These were the rage back then.  High speed foil overview
_The 40-knot sailboat_, Bernard Smith, 1964 (!)
        Interesting comparison with the Yellow Pages Endeavor

Thanks to the many rec.windsurfers who helped me improve this FAQ!
               -- Tom von Alten