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?
The theory is that shiny is slower than matt. It's not very intuitive, shiny repels water and it will "bead" on a horizontal surface. A matt surface is wettable and doesn't bead. If you tilt both surfaces, the water will start flowing on the matt surface before it does on the shiny surface. The reason as I understand it, is that on a beading surface all the water has water to surface friction, but on a non beading surface, most of the water has water to water friction. There is only a layer 1 molecule thick that has water to water friction.
The recommended finish is 400/600 wet and dry. That's why most boards have matt bottoms these days. It would be interesting to do the experiment though, and see what the difference is.
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 [}:)]
interesting question and 1 that will probably gather a few responses.
to be honest i haven't got a bent clue as to what the fastest finish is, how ever i can tell you that carbon art speed boards have a polished finish-at least mine has anyway and i know its good for at least 48knots.
also the new tmv fins produced by Mal Wright, are also highly polished and these too have been clocked in the mid forty knot range as well.
this is a good question for the techno heads to answer and debate on about boundary layers and the like.
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.
for windsurfers it is better to have a mat finish.
the mat finish allows a thin film of water to adhere to the board. the thin film of water then has better slip against the water your sailing through.
sailkote can also be applied. the US windsurfing team has done lots of testing with sailkote.
quote:
Originally posted by Gestalt
sailkote can also be applied. the US windsurfing team has done lots of testing with sailkote.
quote:
Originally posted by Gestalt
for windsurfers it is better to have a mat finish.
the mat finish allows a thin film of water to adhere to the board. the thin film of water then has better slip against the water your sailing through.
sailkote can also be applied. the US windsurfing team has done lots of testing with sailkote.
Do you have any supporting links
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.
After awhile the bottoms of my boards get a little crappy looking so I give them a light sand with 1200 wet and dry. From nose to tail in a straight line. Dont know whether it makes them faster but it gets rid of all the dirty hand marks, grass marks etc.
they look better.
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?
I have two boards, a 2006 Naish Wave and an RRD FSW. The Naish has a matt finish, RRD gloss. Both look great (lots of nice colourful graphics) and both feel exactly the same - other than the difference in shape/size. i have new board coming - Kinetic, and have no idea what the finish will be!
I always remember some of the best Lazer sailors in the nationals back in the UK used to get new hulls quite regularly. The better guys would always take some 1200 wet n' dry and sand the whole bottom section and reomove the sheen/gloss - they thought it was quicker but ultimately went upwind better.
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.
Didn't the Aussie olympic swim team develope the "shark skin" swim suit that holds air bubbles on the surface to aid them to go faster?
I would imagine this would also hold true for the hull of your board.
DavMen
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".
Seems like there is not a lot of evidence either way... just theory...
What is needed is testing! How about a bunch of boards which are the same (give or take manufacturing tolerances) with the same riders, same sails (no change of trim), same fins and under the same wind conditions on the same day... Then the speeds on each board by each rider can be compared...
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.
quote:
Originally posted by Pugwash
Seems like there is not a lot of evidence either way... just theory...
What is needed is testing! How about a bunch of boards which are the same (give or take manufacturing tolerances) with the same riders, same sails (no change of trim), same fins and under the same wind conditions on the same day... Then the speeds on each board by each rider can be compared...
quote:
Originally posted by Gestalt
everyone i spoke to said it was the thing to do.
).
I am a compulsive sander.I cant help it,every time there is no speedsailing for a few weeks I start sanding
I sand untill I can see carbon,then I paint with spray putty and then I sand again.Even my new board,a Tabou Manta only lasted a few weeks before I started sanding through the pretty pin stripes.It will probably get a dose of spray putty next week.
The bottom of your board looks shocking after a sand with a good straight edge(300-600mm long)there are spots and different colors that start to pop up everywhere which isn't good for board re-sale,but from what I have seen there is no production board that still has a flat bottom a month after production.
My latest routine is nose to tail first with 600 straight edge and 600 wet&dry and then directionally from the nose to the tail with 320 a few times with a 6" block.
This might not be your cup of tea and a sanding block in the wrong hands could probably wreck a good board in seconds but my angle is a flat board is faster than a lumpy one.
Sounds interesting Goo Screw...
Is that SP in the background?
Also, where do you buy spray putty? Is it polyester based?
Yes thats Sandy Point and it must be going off because I am holding onto my 4.6.
I have been getting pressure packs of Motorspray or K&h from the local autopaint joint for a fair while and it sticks well whith a harsher sand and a good clean(has never flaked off).
From car pannel repairs I do know polyester dries harder and would probably be better if you have a spray gun.
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?
You'd only put spray putty on after it was all glassed and sealed, so core foam shouldn't be a problem.
But if you've just glassed with epoxy, the problem is that epoxy and polyester interfere with each other's setting. You have to make sure the epoxy is well and truly cured before putting polyester anywhere near it. (about 2 weeks at 20C or 8hrs at 50C)
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.
Thats probably a good blend to avoid by the sounds of things or it could all stay pretty sticky I'm guessing.
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.
quote:
Originally posted by Goo Screw
I was not aware polyester had an adverse affect on foam.
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! 
I have two slalom boards of the same model but one is a 80L and the other 90L. They were 2nd hand acquisitions and the 80 came with a polished bottom, care of the previous owner.
Not sure if it's just the feel of the phsically smaller board... but the smaller polished one seems to unstick a lot easier (using the same sail). It does feel more slippery and I seem to get an extra knot or so without hard work. I haven't had that much time on the water with the smaller board but if it continues to feel as slippery as it has been I'll definitely consider polishing the bigger bigger one.
Des
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?
