Hull paint

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Michael Brigg
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Re: Hull paint

Post by Michael Brigg » Mon Jul 03, 2017 8:43 pm

As I recall a lot of this started out (I suspect as a marketing ploy) with a paint finish known as "Graphspeed."

Filthy stuff. If you brushed against the hull in your best white ducks you had a stain the size of Russia on them.

We had a guy in the club who put it on his Firefly. Quite cool stuff, and he got to sail a firefly in the style of the style of "Stealth" (the yacht,)
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Re: Hull paint

Post by trebor » Mon Jul 03, 2017 9:21 pm

From what I gathered from the link, skin of hull cannot escape friction from water molecules, therefore reducing surface area of hull in water reduces drag, extremes being foilers, wind surfers etc.
Skill of helm or helm/crew makes more impact on speed.
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Re: Hull paint

Post by jpa_wfsc » Mon Jul 10, 2017 9:31 pm

trebor wrote:Perhaps if the bubbles were allowed to flow through surface and lubricate hull, similar to Dolphins shedding skin cells.
There was an experimental boat built that tried to exploit this - using a set of slots from deck level (in front of the mast) down to the hull. The theory being that the air sucked down would reduce surface drag over the main part of the boats hull. No data as to weather it worked - but I doubt of it did. This boat was parked in our club compound for a good number of years but I never saw it sailing. It was very very heavy and would only have run in displacement mode (when surface finish has little effect anyhow!).

At anything more than a very slow speed, a dinghy's main drag is the form drag: put simply, it has to shove its own weight of water out of the way every boat length forward that you go. This of course results in the waves that make up the boats wake.

At a slow speed, an un-fiared hull because of the dents and bulges, is making more of this work necessary - every dimple followed by a bump is resulting in her having to shove some of her own weight out and back again - adding up to more than her own weight of water moved per boat length of forward movement.

When you plane or use foils, the forgoing no longer applies as the boat is not displacing her own weight anymore. She is instead pushing down with her own weight but the water does not have time to move out of the way, so the boat lifts up instead. Once more, the surface quality of the surface becomes very important, as the boundary layer of water trapped by the slightly not flat surface is more slippery than if it was not there at all.

So - drifting - nothing matters much. Being in a slight puff will have much more effect on your sailing than the surface quality of your paint.

Displacement sailing - the boats shape, and the fairness of her lines will provide the best speed.

Planing or foiling - you need a surface that keeps a boundary layer.

Graphspeed was rather good because it was a thick soft coat, that faired easily, and also had a microscopically designed surface that held a boundary layer. It was thus providing some benefit at all speeds other than in drifting conditions. If anyone has a tin of the stuff, the bluebird project (mentioned elsewhere on this forum!) needs some!

If you really want to go fast, then smear egg white all over your hull just before launching. Peter Scott amongst others used to do this but the effect was probably more psychological then physical. I knew a moth sailor in Salcombe who re-painted her international moth with emulsion paint and buffed it flat between every weekend racing!!

Finally (!) Check Rule 53. Egg White is allowed by the rules!! As are 'eroding' or 'self-polishing' antifouling etc.

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Re: Hull paint

Post by Aquarius » Thu Jul 13, 2017 8:04 pm

In the interests of historical accuracy I record the useless fact that in 1962 I spent hours applying silicone wax polish to the bottom and foils of my father's GP14. I.e. in those days that was reckoned the way to get the last tenth of a knot.

More seriously if you are applying epoxy paint get all (all!) the old paint off before you start and only use epoxy fillers.
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