Hull finish

Paint it orange!!! that will be really fast!!

I will go out on a limb on this one.

Based on the test data I have seen (which is limited) the finish should be absolutely clean and fair, finished to 1600 grit or better.

The flow around catamaran hulls is laminar. The water touching the hulls does not move. Wax will not will not hurt anything. If the wax is thick enough to fair imperfections and strong enough to stay on it will help a little.

In simple terms

Touchup any scratches
Fair the hull, get rid of any bumps or hollows
Polish it until it's real shiny
Keep it CLEAN

Carl,

are you sure flow around a catamaran hull is laminar? Perhaps in perfectly flat water, but in any kind of waves?
I asked around about finish on foils some time ago, and had a very credible source claim a 600 grit finish was good enough for the speed range between 10-14 knots. This is the range cats mostly sail in around here, so that is what we used. My source had done a tank test and 600 grit was the conclusion. I dont see why finish on the hulls should demand more than the foils?

Well the sanding of the hulls first of all needs to conform with the flow of the water. From bow to stern. Will it make that much difference between 600 and 1600 grit sanding. No it will not, but it may make the sailor faster. Kind of like wet sanding a ACC boat with 1200 every morning before sailing, if they still do that. Psychologically it does wonders. If you think that the boat is faster, it makes you feel faster and therefore you tend to be faster. Awlgrip is the finish on my boats. 600, 800, 1000 then sprayed the top coat. You do not want an absolutey mirror or glass finish on the boat. There needs to be some roughness in the finish. 600-1200 grit doesb this well enough.

Tom,

do you happen to have read the

Talent, or training and skill

thread <img src=

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so, you sanded the primer to 1000 gt then shot top coat? any problems with the paint sticking? i have never gone above 240 before painting. there is no need.

How well do you sail? This is not a rhetorical question as my point is, make sure your boat is in good shape and learn to sail extremly well first. Once you have exhausted all you need to learn on making the boat go fast then work on the .005 knots you will increase with the finish on the boat.
A fast boat does not make you a fast sailor!

JMO... <img src=

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Clayton

you shouldnt sand past 200grit for a primer coat.

The paint needs some

teeth

to adhere to.

I find that the clear poly I use sticks better and lasts longer when I sand with 60grit only. It gives the finish somethign to grab onto. This varies by paint type, but I never sand past 200grit before applying top coats for any paint type.

Once the top coat is on, then you can sand that with 600, 1000, etc to get a mirror finish.

Bill

My boat is painted with Awlgrip and I remember reading in their application guide that they DO NOT recommend wet sanding the final top coat or applying any rubbing/polishing compount finesse-it, etc...

The paint is so thin, I think this makes sense. they don't even recommend waxing of any sort. only product recommended is Awl-care which supposedly is some type of liquid polymer. anything else they say could damage the finish.

anyone try the new Holmenkol stuff??

I know guys that put a clear coat on after the top coat and then wet sand the clear coat, they stop if they see any color in the sanding slurry.

I thought this was a good idea with the high tech paints, they are very thin.

Bill

Rolf

In terms of classical fluid dynamics, it's laminar. In CFD, it's laminar or transition to laminar.

The test data I have seen, has always been about the same.

Special surface finish makes a small difference (bumps grooves, etc)

Fair and smooth makes a difference.

Drag continues to decreases with polish. An English test of foils showed drag continued to decrease up to 3200 grit (test limit). Also the higher the polish, the better flow stayed attached. I have never seen enough data to say where the area of diminishing returns is. I used to say 400 grit, now I polish my boat the best I can.

The biggest factor is clean. Everything else is trivial compared to this. It does not take many grains of 40 grit beach sand stuck to your hull to offset everything else.

Carl

hmmm... interesting, i've heard of guys wetsanding the awlgrip top coat, then shooting a layer of clear Imron over it. didn't hear anything about wet sanding that again... I think for every

expert

there is another way of doing it... Either way, it's good to hear different opinions.

Very true but..... As others have said the psychology of

I think and therefore I am

does play some part. If you are sailing a boat that you perceive as being inferior then your own performance will be at least some part effected.

If you are sailing what you believe is the fastest boat in the fleet this will have the effect of better feeling of wellbeing in your own mind and you will sail somewhat better.

Probably isn't going to take you from last to first but it might possibly win you a few places that you otherwise may not have got.

Tiger MIke

Oh and all scientifically based tests I have seen conclude that smooth polished boats will have less resistance thru the water than it's un-polished counterpart.

Clayton has it right.

Spend more time greasing the nut on the tiller.

Carl,

I dont know enough fluid dynamics to say who is right regarding laminar flow around hulls. It does seem a bit counterintuitive that a hull moving trough even small waves should have laminar flow. I know Tom Speer once posted that he was uncertain how much laminar flow even our foils see. Perhaps another reason why laminar sections dont always deliver to their theorethical promise?

The guy I referred to with the 600grit finish knows his stuff tough. Among other interesting stuff, he has buildt banana foils for ORMA60 tris. Better finish will give less drag, that is accepted. But how much work is it to put on that 2000grit finish vs the returns? Would be fun to see theorethical and practical data for drag from the same foil or hull at 600, 800, 100, 1200 and 2000grit in both flat water and different waves.
For foils, I would think adherence to the theorethical sectional shape to be much more important. A small building error could make a big difference.

What do you guys think of a rough surface to reduce friction such as orange peel texture or even a fine gritty surface like those neet-o new swimsuits from the olympics.

-Rob V.

a lot of research is going into the skins of fast swimming fish such as Mako sharks. They have finly roughed skin which desturbs the attraction of water and speeds their movement markedly.
Personally I feel time on water is better than time on beach polishing.

You guys ever read the book, High Performance Sailing by Frank Bethwaite? He has a whole chapter on smoothness. The short answer, as stated above, cleaner and smoother has less drag. You don't see airplane racers or glider pilots out there with 600 grit sandpaper. They rub and polish until it looks like glass, then they will smack you if you touch it... <img src=

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But during the America's Cup some years ago, Dennis Conner tried some kind of ribbed stick on stuff on his boat to somehow reduce drag by keeping the boundry layer attached or something like that. I think he eventually ditched the idea. <img src=

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I like to polish my boat with Teflon until my hand just slips right off, boards too. Makes it tough to climb back on after a crash though! <img src=

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3m had/has the product that dc used in the cup. it had/has been banned by all of the major sailing federations.

BUT...was it banned because it was faster or because they weren't sure if it was or not, so to be safe, they banned it?

T.O.B. is a myth. Old, sun-baked chalky gel coat has been found to be identical in texture to Mako skin. Furthermore, by sloughing off, it acts as a lubricant and is technically illegal. This explains why old sods on old boats go so well.

Nice Try, I had a old Hobie with chalky gel coat. I couldn't win a race. Sanded it down, Awlgrip paint (sprayed) and started winning. Traded it in on a new model and won even more. I now have a H-18 that needs a

bottom Job

. Hulls & sails make a big difference, right behind the

nut

on the tiller.

Rolf

You are getting into area where I wish I had the time, money and smarts to setup an exhaustive test program to find out.

I think the guys at Boeing, NASA and NASCAR have already done all the math. I don't see them doing anything with sandpaper, just lots of polish and elbow grease.

How many sailing catamarans have Boeing, NASA and the NASCAR guys buildt? <img src=

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Not to be rude, but there are differences. As it is, I dont know anyone besides Frank Bethwaite who has done practical tests on this. I am pretty sure there is a point of diminishing returns for going to finer and finer surfaces. But where this point is, is still pretty much an unanswered question it seems.

For what it is worth, I have sanded a boat and it seemed faster. Painted a few, and they seemed slower. I know theoretically it should be impossible to notice a difference. I also cannot prove you didn't hide speed stripes under your topcoat, but I can prove you had nominally more time-on-boat after the paint job than before .

Rolf,

Maybe you can convince the US Naval Academy Hydromechanics lab this is a worthwhile research project?

Rolf, I don't know about actually building cats but I do know there are a whole lot of NASA guys and Boeing guys racing cats here in the USA. Can any of you types tell us which is faster and why?

As for the NASCAR guys? They bring the beer... <img src=

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How about my where I spent my college days:

Probably want to rent out the Low-turbulence Free Surface Re-circulating Water Tunnel.