Water is pretty sticky: dip a
paddle into the ocean and it comes up with much water attached. Move a
board through the ocean – be it surf, windsurf, stand-up, or kitesurf -,
and it will drag a substantial amount of water along, which, as it
eventually cannot hold on to the bottom any more, emerges behind in the
form of a wake. The energy that you generated with your sail, kite or
paddle, went into accelerating not just you and your board, but this
rather substantial mass of water as well.
To go faster, then, we need to
decrease the amount of water “sticking” to your board – “reducing the
drag” in techno-lingo, since the best wake is no wake!
In extensive tank testing for
world-class racing boats, it was established 40-some years ago that a
glossy surface has substantially more drag than a matte one. In tests I
was involved in for an America’s Cup boat, we found that simply sanding
a glossy bottom with 600 grit paper, reduced the surface friction by
about 5% at ½ hull speed, i.e. at about 4.5 knots. Instead of sticking
to the glossy bottom, the water molecules would be “tripped up” by the
minute ridges left by the sandpaper. This slight turbulence would reduce
the thickness of the film of water being moved along with the vessel
(the “boundary layer”), and thereby reduce the overall drag.
Surfboards are traditionally
finished super-glossy-shiny. Windsurfers, on the other hand, have long
since followed the lead of sailboat racers and taken the gloss off the
bottoms of their craft.
If you wonder if your bottom is
too shiny, throw a cup of water at it: if the water beads, like it will
on a freshly waxed car, then the surface tension is high, and therefore
its resistance going through the water is high.
To change it to a low-drag
surface, sand it in a circular motion with 600 grit wet & dry paper,
until water thrown at it runs off in sheets – WITHOUT ANY BEADS forming.
Instead of wet & dry sandpaper,
you can also use a “Purple Pad”, a Scotch brand synthetic wool pad
designed to burnish metal and take off rust. Again, a circular motion is
To keep this fast surface fast,
wash it now and then with soapy water; and lightly apply the Purple-Pad
when the bottom appears yellow.
How big a difference will this
make? After a bottom repair a while back, I decided to Purple-Pad the
entire bottom of a windsurfing board. The owner noticed with a raised
eyebrow, but did not comment on it when he picked up the board. However,
he was back 4 hrs later, demanding to know what exactly I had done to
his board: a friend, who had always easily passed him before, was no
longer able to even keep up with him.
Or this recently from Shawn C:
“I purchased an old Hifly 265 poly board that I used a couple of times
and really enjoyed. I noticed that the board had a few small gouges on
the bottom so I thought that I would sand them down using wet/dry 600
grit as you suggested. I then mistakenly waxed the bottom and had
horrible performance on the water.”