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!
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 you're
overpowered on a small sail. (The latter problem is from loss of control
as the vertical component of lift from your fin wants to make your board
fly.)
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.
Paul Billings <pab@maui.com> describes a more agressive technique:
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 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."
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, too. Will Estes <westes@usc.com> suggests the fix I
use: 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.
...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).