Sport Today

can someone give me the basic physics of why a surfer floats?
Is that really a case of floating, that is, upward force on the
surfboard canceling the gravity on the person, or does the
forward movement have something to do with it?

All responses, posted or mailed, appreciated.

Victor.
--
Victor Eijkhout ................................ `There are also a few bugs,
Department of Computer Science .......... though not as many as I've come to
University of Tennessee ............... expect in new [MS]Windows products.'
Knoxville TN 37919 ................................ (from a software review)

   O
   I\   Brian Kiviat (301)227-1833

  /     David Taylor Model Basin
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                              /         side view
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     \           /                       cross section
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Anyway, if you think about it, you'll see that by moving forwards, the craft
forces water downwards.  This, in turn, forces the craft upwards, so less of
the craft is in the water leading to less friction with the water and allowing
it to move faster.  That, basically, is the principle of hydroplaning and it
has everything to do with the forward motion of the craft and the shape of
its hull.  On a surfboard, the forward motion is supplied both by gravity and
the forward motion of the wave.  The shape seen by the water depends on
what the surfer is doing, while t***, at any rate, the surfboard's shape
allows hydroplaning.

- leo

   O
   I\   Brian Kiviat (301)227-1833

  /     David Taylor Model Basin
   \___________________________
                             \\

   >The water passing by the fin give the board lift, thus counteracting the
   >gravitational pull on the windsurfer/surfer (weight).  The faster you go,
   >the more lift created by the fin -- that's why a smaller fin is better
   >for faster speeds (too much lift causes instability).

   I don't think a fin with the airfoil section perpendicular to the board
   can create a force in the upward direction.  The force is always in
   the perpendicular direction to the airfoil section.

The board provides lift when it's moving, and if the leeward rail is
tilted down a bit (good style!) you get lift from the fin and can ride
on it alone.  

--

IBM T.J. Watson Research Laboratory
(914) 945-2573

As for surf here: not much to talk about, but waves for the
desperate.

Jim

When people refer to 'lift' generated by a fin, they are talking about the sideways
force generated by the fin which helps the board go upwind. A fin which generates
no lift would only let yuou sail downwind. One would assume
that when people say 'lift' that they are talking about an upward force, but
not in the case of fins. The reason the word 'lift' is used is because it
is analogous to the lift generated by a horizontal airfoil. Because a fin is
vertical, the lift is horizontal.

The instability you mention which occurs at high speeds with big fins is
because the horizontal force generated is strong enough to try and
roll the board over.

Jim Munro

Sinkers of all kinds(surfers, windsurfers, skipping stones) stay above water
because their upward force equals or exceeds their weight. The only upward
force a sinker can have is caused by velocity and the appropriate angle of
the bottom with respect to the surface of the water. This causes
"planing" (for windsurfers), part of what's called "dropping in" (for surfers),
and skipping (for flat stones). Like the skipping stones, once any of these
lose sufficient velocity, they fall off their plane (and sink).

Windsurfers are powered by the wind, stones by your arm, but it's more
complicated for surfers. Surfers are using at least 3 different methods of
powering their vehicle to get and stay on a plane(surfing). The first and
obvious one is paddling with their arms. Ouch. This is strenous and slow: this
method alone will never achieve a fast enough speed to get onto a plane.

The other, only slightly less obvious power surfers use is the forward
motion of the wave itself. While waves are relatively slow (by windsurfing
standards, at least), they do have *almost* enough speed to allow a plane.

The third, least obvious power surfers use is the "undertow" effect
(sorry, I don't know the real term for this), which is the effect of the
wave "sucking up" the water just ahead of the actual wave itself.
Assuming (in our time frame of a few seconds) that the volume of the ocean is
fixed, waves don't just appear: the water has to come from somewhere, and part
of where the water comes is immediately in front of the wave. As the
(not yet breaking) wave moves forward, it causes the water immediately in front
of the wave to rush backward toward the crest.

For example, if you watch surfers trying to catch a wave, you'll notice that
they paddle to get some forward motion going, but just before the wave
overtakes them, while they are paddling the hardest, they will suddenly slow
down and stop (or even go backwards slightly). This is caused by the water
rushing toward the crest.

Why is this important ? Relative water velocity. The total of paddling in a
backward-moving stream of water(paddling + "undertow" effect) and the
forward rush of the wave combine to exceed the "planing" velocity of the
surfboard in question. When that is exceeded, the surfboard planes
(or "surfs"). As long as the wave exists, and the surfboard stays between
the crest and the forward extent of the "undertow" effect, the surfboard
will plane.

Paddling is really only used to initiate the plane and to prevent being
sucked back over the crest, and missing the wave. Another technique used
for smaller boards, which need more speed (and skill) to plane, is to use
the force of gravity ("dropping in") to initiate the plane. The surfer
paddles, but stalls the board by preventing the proper planing angle until
he is relatively high on the wave, near the crest, then he/she pushes down
on the front of the board to get a planing angle and presto, surfing begins.

Surfers can also get faster velocity for tricks and manuevers by slowing or
steering the board until it is high on the wave, then rushing down the wave
at warp speed using the force of gravity. Which is way fun :-)

Well, that's *MY* version of physics of surfing. On the other hand, thinking
about it as an ability given to a few exceptional individuals directly
from God is probably just as valid.

Other issues like what is a good shape for the board belongs in a never-ending
discussion about the perfect board for which condition, and the subject of
what skills and how to perfect those skills is also in a different discussion.

How about some of you aerospace people in the Pacific Northwest giving us
more specifics ? I know you are out there somewhere. What is the best board
angle relative to the water for planing ? I've heard 4 degrees,
but I'd like to learn more. (Just curiosity until I can stop thinking
about sailing, and start doing it !!!)

Jaime "surf's when no wind" Cordera

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