Reading through some experiments with kiteboards, a perspex bottom to the board seemed to make a big difference to the top speed. This appeared to be because of the super smooth finish that perspex has.

Looking at the bottom of my boards, they don't appear to be that shiny -- has anyone tried applying a slippery film to the bottom of their board, or perhaps polishing it? Did it make a difference?

Glider pilots spend hours and hours polishing every surface of their wings, they seem to think it makes a difference. Certainly from my R/C plane days, the way you covered a wing used to be "Solarfilm", a heat-shrinking adhesive film that was as smooth as a baby's bum...

And yet there are those damn golf balls, with dimples that are supposed to make it go further.

I guess it's all to do with the boundary layer, isn't it? And complicating this is the way that we sail over chop so we're really riding on a cushion of air and water. I'm seriously thinking about buying a couple of rolls of solarfilm and giving it a go [}:)]

Way back in the days, I remember hearing that Dennis Connor attached a thin film with riblets to the bottom of his America's cup 12 meter and proclaimed that they thought the boat went faster. It was only later that they realised that they'd put it on at 90 degrees to the diresction it was supposed to go on. Sounds like a placebo to me.

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Originally posted by Gestalt
sailkote can also be applied. the US windsurfing team has done lots of testing with sailkote.

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Soooo...any results or theories from this testing? or is it classified!

There was a tread on GPS Speed about this and if I remember the gist of it went.

A glossy finish picked up speed easier, but as the water sticks to the gloss at high speeds it shears the water and offers resistance.

A rougher finish at low speeds does not accelerate as well but at high speed the roughness traps air (like the swim suits) as air has a lower shear factor it offers less resistance.

Fins on the other hand want the water to grip so are better suited being pollished. If the air sticks to the fin then you are on your way to cavitation. So it's always best to clean up your standard fins, Techtonics and other of that ilk are already done.

Please note..My memory may be wildly inaccurate on this, The SP Crew and CL would have more accurate info.

Wow, definitely some food for thought there.

Perhaps the trapped air might be part of the answer, but I'd love to see some data.

Or just polish the part of the board that's out of the water when planing... that should get me from slogging up to planing speed quicker. Maybe?

After Stars and Stripes won the Americas Cup back from the Aussies in 1987, it was revealed they had been using riblets on their boat to reduce drag. NASA pioneered the research with 3M and contacted us to run some tests on speedboards to see if there was any benefit. Initially the boards were faster but the material had a very short life, when you span out it was a very big moment rather than progressive when the film was not fitted. For more insight into riblets have a look at these articles www.nasa.gov/centers/langley/news/factsheets/Riblets.html and www.aerodyn.org/Drag/riblets.html Sanding the board in s straight line bow to stern can achieve the same type of effect.

Board finish is a bit speculative...

Fin finish is a bit more scientific.

A polished fin will be more likely to produce low drag laminar flow. But it is also more prone to stall (then ventilate => spin out) due to laminar separation, especially at lower speeds. If the foil section is designed to avoid laminar separation over a reasonable operating range, then its OK.

Most fins work better sanded. This is because they suffer from laminar separation when polished. Its a combination of the foil shape, and machining tolerance that makes it susceptible.

By wet sanding the fin, you increase the turbulence growth rate in the boundary layer, so that it transitions earlier into turbulent flow. Turbulent flow has enough momentum in the direction towards the foil (the fluid tumbles so there is an inward velocity component though the boundary profile), to keep the flow attached to the fin. So flow separation is delayed or reduced, the maximum lift is increased, and the drag due to the size of the wake is reduced. If the transition to turbulent flow occurs before the pressure recovery region begins, then laminar separation is avoided.

This is the reason why golf balls are dimpled- to avoid ,assively unstable and draggy flow separation by inducing turbulent flow in the boundary layer so it stays attached.

All this means is that your fin can be pushed harder before it lets go, and is less prone to spinout at high speed... There is an optimum grit that will give you a bit of laminar flow, and a stable transition. This can be between 240 and 1200 depending on the fin. 400-600 seems to be standard.

But if the fin is designed and manufactured well, polished finish gives the potential for lower drag (in the right conditions) by allowing more laminar flow.

So the question "should I polish my fin or not?", is answered with "depends".

Polished or not polished is great, until you put your board down on the sand while adjusting your harness or putting your helmet on, or something like that.

What I am trying to say is, that for most of us, if we polished the board or fin, it would only last about 5 minutes until we scratched some part of the surface and then the theory is shot out the window.

For the average Joe Blow, I dont think it would make enough difference to matter, one way or the other.

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Originally posted by Gestalt
everyone i spoke to said it was the thing to do.

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Exactly my point... everyone says it, the message is spread and believed... But why

Maybe slowboat should experiment with a bunch of different bottom finishes on a number of different boards in a single size (eg SP50 with polish, sanded and golf-ball like bottom) from his range and work out which makes him go fast, faster and fastest! Then he can share the knowledge with us mere mortals, and we'll get the finish on the boards he prefers (but still be 10 knots slower ).

BTW, I wonder whether longboard racing would be different (different water/air flow under board) given that longboards glide (are quicker in non-planing conditions) and when planing are not travelling at anywhere near the top end speeds of modern speed boards...

Sounds interesting Goo Screw...

Is that SP in the background?

Also, where do you buy spray putty? Is it polyester based?

So you're not worried about the polyester eating into the foam? Not really a problem with a new board, but for one that's a bit dinged up would you still use the spray on stuff?

If that was the case I would glass it,fill it,bog it first.
Spray putty is only good for about a 2mm fill at best so I would not load heaps into the holes.
I was not aware polyester had an adverse affect on foam.

I found out when I made a fin, forget the sequence now, but I think I put a polyester filler coat on an epoxy layup, never went off!!! When I inquired, was told why.

I think Goo Screw definitely has the right idea about flat being better than wavy on the board hull for sure. Use a good straight edge on your board and what you often see will make you weep.

I have noticed the Phenomena that Slowboat mentions about spin out on a highly polished fin. Some fins I have polished went to hell in a handbasket. Spin out everywhere. BUT, the really accuratly foiled fins that Mal Wright has been making lately seem to definitely benefit from mirror polish and don't have spin out problems. Same with a very nice Lockwood fin I have.

Any reasonable fin will benefit from careful sanding and filling with soft putty spray to get the waves and bumps out of it. I have recently revived some '90's slalom fins this way and they are as good as, or in some cases, better than new. The soft spray putty is very easy to work with but also sratches up again very easily so care is needed when launching and landing, something I have particular trouble with!

Just for future reference, spray putty is acrylic (at least the can I bought is!)
I think that means that it uses air or maybe the moisture in air to set, rather than having two different parts that you mix.

Interesting what people are saying about a flat bottom being better... I take it the real speed freaks aren't big fans of concaves or cutouts?