38 COACHING FUNDAMENTALS
But the question is how can an angle of attack be achieved
with a fin that cannot pivot, as it is literally bolted onto the
centreline of the board? Water, in our minds, is flowing equally
over a symmetrical fin and equal flow cannot create lift. Towing
a rig-less board in a straight line behind a powerboat, for
example, would not create any lift on the fin regardless of how
fast it went. Furthermore, we know that lift from fins is created
in such a way as to pull it towards the windward side of the
board. We know this as, when planing, we place our feet in the
footstraps on the windward side of the board to force the fin
back down which squeezes the board forwards. If we kept our
feet on the centreline of the board as we started planing then
the lift from the fin would tip us off the board to leeward.
So how does the fin create lift and why is the lift to windward?
Dave Gollick (keen UK windsurfer and founder of
windsurfingfins.co.uk), Dietrich ‘Rick’ Hanke (founder, fin
designer and CEO of Maui Ultra Fins and German Aerospace
Engineer), sail coaching buddy Matt Wright and windsurfing
luminary Sam Ross all found time to chat to me about it and I
am very grateful to them for their input.
When you are planing along you might be forgiven for thinking
that your board is pointing in the direction of the course you
are sailing, i.e. thinking that, if your board is sailing across the
wind, the centreline of your board (the axis running nose to tail
through the mastfoot) is also pointing across the wind. Well,
perhaps surprisingly, that is not the case. The centreline of the
board is not identical to the sailed course so there is a very
small offset angle between the fin and the direction sailed -
typically 2-5 degrees (or higher when planing upwind when the
board is slow and the side force from the sail is high). This
offset represents the ‘side-slip’ or ‘angle of attack’ or ‘leeway’
that we need to create lift. It is naturally created by body weight
pushing through the feet on the board’s tail as a result of
hanging out from the rig to balance the lift from the sail.
The reason that the fin creates lift towards the windward side
of the board is that, being offset from the direction of travel,
the fin has its windward side tilted very slightly away from the
incoming water flow and therefore the water flow has further
to travel over the windward side. This creates high pressure
on the leeward side, low pressure on the windward side
and – hence – lift to windward.
If we push too hard against the tail of the board then the
angle between fin and direction of travel becomes too great
(at about 8-12 degrees) and flow separation occurs (the fin
breaks free from the flow of water and spinout occurs where
the board skids downwind). Skill, instinct and experience
allow the windsurfer to make small dynamic adjustments to
prevent or correct spinout in much the same way as a bird
limits stalling by natural talent.
THE PHYSICS OF HOW A WINDSURF FIN
CREATES LIFT TO WINDWARD
TRUE WIND
Apparent
wind
Fin
Centreline
of board
Direction of
travel
Very small offset
(2-5º)
Flow of water
(To balance the
direction of travel)
Lift of fin to
windward
Low pr
essu
re
Fin
High pres ure
s
uk
WIND
SURFING
The natural offset of the centreline (and fin) to the
oncoming water creates lift on the fin in the same
way as oncoming air creates lift on the wing
of an aeroplane