Hull paint

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Slingbat
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Hull paint

Post by Slingbat »

I'm just about to start stripping the paint off the bottom of my miracle, as far as I can tell it's some cheap white gloss over a few layers of other paint, plan is to go at it gently with a heat gun and scraper and sand and really stubourn bits.

Now for repainting, I was given a 5l kit of Jotun pengaurd epoxy paint that I have painted the internal floors with and it gives a satin type finish, is there any reason I couldn't paint the bottom with this as well? Or is it best to have a glossy as possible surface finish, or is it likely that real world I will notice little difference

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Tom
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TheGuvnah
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Re: Hull paint

Post by TheGuvnah »

Ooh good question!

Funnily enough I've just been reading through a few old issues of the Enterprise Assoc quarterly and in the Spring 1980 diary there is an exchange of letters on this very subject. Instinct says that a mirror smooth gloss finish will offer the least resistance to adjacent water molecules and thus render the best boat speed but apparently this is not necessarily the case. The thinking goes like this...
For minimal frictional resistance one needs a hull surface which causes no turbulence whatsoever but to which a thin water film can remain attached by means of a microscopic roughness. This thin layer ensures that friction only acts through adjacent water layers and is thus at a minimum.

A beautifully painted or sprayed hull with a high gloss will repel water and this causes friction between paint and water which is higher than the friction resulting between adjacent water layers.
It is the same with a glossy fiber-glass hull where there is similar friction between the polyester gel coat and water.

The best practicable surface can easily be obtained by treating the hull, both below and above the water line, with 400 grade waterproof abrasive paper. You should never use car wax or silicone wax or other water repellant finishes on a fast hull.
It is a well known fact that a dolphin is a fast mover and its skin has a surface layer of slime which holds a thin layer of water. Here we see the same effect where friction is only between adjacent layers and hence is low.

Furthermore it is of the utmost importance that the hull surface is absolutely grease free and you can keep it this way by rubbing very lightly with the same fine waterproof abrasive paper or you can clean the hull with an ordinary clothes washing detergent.

You can check the efficiency of the surface very easily. Throw some water onto the surface and if drops remain then it is water repellant. If the water spreads evenly in a microscopically thin film then it is a good surface for speed.
So it's all about laminar flow and the idea being that the microscopic granulations retain a layer of water molecules that are then carried along with the hull and the only friction results from the shearing of the bonds between water molecules which is very low and certainly less than that between paint and water.
the ideal surface for small boats has about 0.005mm granular roughness and this can be obtained by rubbing down with 400 grade abrasive paper.
the contributor also makes the point that a varnished hull will not necessarily be the best finish because owners will not be using any fairing or filling compounds to take out any of the minor (but still drag inducing) dinks, dents or irregularities.
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Slingbat
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Re: Hull paint

Post by Slingbat »

That certainly makes sense, so use my free paint and apply to a good smooth surface and then go and get her back in the water or would the true benefit be in giving her that final loving rubdown with some 400 w'n'd. Alternatively should I accept that a miracle with sail no. 708 is never going to be a speed demon and have done with it and get sailing
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TheGuvnah
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Re: Hull paint

Post by TheGuvnah »

Slingbat wrote:Alternatively should I accept that a miracle with sail no. 708 is never going to be a speed demon and have done with it and get sailing
Yeah probably.

I've no idea of the actual degree of enhancement such treatment would get you, what do we think? a tenth of a knot if that? Hull form and sail trim would far outweigh any performance gained by doing it.

I suppose the satin finish would to some extent fulfill that role of capturing a layer of slick static water against the hull. I'm thinking that a different set of priorities would apply to a boat kept on a mooring where the roughness would attract more algal accretions and staining than a gloss finish. To my mind a glossy gleaming finish just looks better. 8) If I was actively and seriously racing then that'd be a different matter. It'd make an interesting 'tank test' though to see the actual difference it made.
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SoggyBadger
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Re: Hull paint

Post by SoggyBadger »

You're probably talking about tiny differences in performance between gloss and matt finishes. I think it's more important to get a really smooth finish on the foils.
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Pat
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Re: Hull paint

Post by Pat »

You're probably talking about tiny differences in performance between gloss and matt finishes. I think it's more important to get a really smooth finish on the foils.
Or yet more important, for the crew to get more practice time in the boat :)
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JimC
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Re: Hull paint

Post by JimC »

Frankly I think the stuff about repelling water is a load of nonsense. The only person I know of who did real testing and measuring of drag was Bethwaite, and his opinion was the smoother the better.
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TheGuvnah
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Re: Hull paint

Post by TheGuvnah »

Found a fairly broad but concise discussion of the subject here by someone called Paul Grimes; was a Collegiate All-American sailor at Brown University, an engineer for Tillotson-Pearson, and is currently owner of Fusion Composites in Middletown, R.I.

http://www.sail.ie/SW/Bottom%20Finishes.htm
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Rupert
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Re: Hull paint

Post by Rupert »

Fascinating! So my bumpy, dirt covered hull isn't quick?
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trebor
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Re: Hull paint

Post by trebor »

Perhaps a layer of air trapped in a micro fibre finish.
The question should be do we want to dramatically increase the speed of normal sailing dinghies ?
Foiling boats, Wind surfers and Kite boards provide increased speed for those that want it.
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JimC
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Re: Hull paint

Post by JimC »

trebor wrote:Perhaps a layer of air trapped in a micro fibre finish.
The question should be do we want to dramatically increase the speed of normal sailing dinghies ?
Foiling boats, Wind surfers and Kite boards provide increased speed for those that want it.
Ah well, this is where you have to study another aspect - drag at different speeds. Improved smoothness of the hull will make its biggest difference at low speeds where skin friction is dominant. So if you halve skin friction drag with mega surface preparation you might go 10% faster in 2 knots of breeze, but only 2% faster in 10 knots of breeze (these are made up numbers).

Not sure anyone will find an issue with improved light airs performance.
Last edited by JimC on Sun Jul 02, 2017 7:36 am, edited 1 time in total.
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trebor
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Re: Hull paint

Post by trebor »

Perhaps if the bubbles were allowed to flow through surface and lubricate hull, similar to Dolphins shedding skin cells.
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TheGuvnah
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Re: Hull paint

Post by TheGuvnah »

trebor wrote:The question should be do we want to...Wind surfers and Kite boards provide increased speed for those that want it.
Best reply so far!
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Re: Hull paint

Post by Rupert »

I'm not sure me wanting to go 0.01% faster than my competitor in order to get off the line quicker and therefore get my nose in front is going to make the Firefly as fast as a kite. It is all about getting a racing edge. Mostly, though, it helps in my head.
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trebor
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Re: Hull paint

Post by trebor »

Class racing, they would all have this new finish, so nothing gained.
The end result will be the same people winning who were winning last week.
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