Smooth hull for racing?

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moj779

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Hi all,

The underwater bit of my boat has been treated with Epoxy sealer, possibly 6 or more coats judging by the thickness (1.5 to 2mm). The finish resembles orange peel. I was told by a surveyor while I was inspecting the boat for the first time that sanding this smooth would not make her any faster. Can this be true? I would like to give her every chance in competition but it looks like a lot of work for minimal rewards. I would appreciate your views.
 
Having done it on a number of boats I would say without doubt that it will make the boat faster. It is also incredibly hard work. Get your crew involved, working together in adversity (or at least a freezing cold boatyard) will help build up both team spirit and their shoulder muscles for next season.

You are not just trying to smooth the surface but also fair the hull so that those bumps and dimples that are more likely to cause turbulent flow are removed.

You need to pay particular attention to the keel & rudder and especially their leading edges.

Power tools are little help as they are likely to create new bumps & depressions of their own. You need to use long boards working at 45 degrees and moving over the surface to avoid wearing one part paticularly hard. Keep an eye on the crew to make sure they don't over do any one patch. Start with something like 120 grit sandpaper and work up to a finer grade.

If you're planning to go without anti-foul (i.e. dry sail the boat) go right down to 1200 grit sandpaper and polish the hull afterwards.

If you're planning to use anti-foul there's no need to go all the way to 1200 IMHO. Buy good racing antifoul and sand it lightly afterwards to make sure its surface is as smooth as you can get it.
 
G'day Corinthian, and welcome to the YBW forums,

Smoothing will help, but, are you saying you have epoxy resin showing, or an epoxy coating of some kind? Because if it's epoxy resin it will need covering or UV light will more than halve it's life.

If it's an epoxy based coating, chances are it will be very hard sanding, but not as hard as epoxy resin is to sand.

You will need long boards more often called torture boards to remove a thin even layer, not fast and not easy work.

You may find a number of areas that require filling rather than sanding as well.

If you do use and epoxy resin and 'closed cell' balloons or spheres, much easier to sand but will need covering as above to avoid UV damage.

A simple way to check for low spots is to get hold of a flexible steel strip and hold it at 45 degrees and slide along the curve of the hull looking for gaps.

Smoothing will reduce resistance, but so will a small amount of weight, for a lot less effort.

Hope this helps.

Avgoodweekend......
 
Re: Smooth hull for racing? no!

sanding this smooth would not make her any faster. Can this be true?

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One possibility is the 'golf ball effect', where a non-smooth surface traps air, creating lower friction & hence more speed!
/forums/images/graemlins/wink.gif
 
Re: Smooth hull for racing? no!

I seem to remember it was fashionable in the late 60s early 70s to coat dinghy bottoms with "Graph speed" which looked like and probably was very expensive black board paint, it left a finish that was slightly rough.

All sorts of claims involving laminar flow were invoked, then it seemed to die the death and we all went back to sanding and polishing.
 
Apparently its not simply a smooth surface thats required for speed and there is some technical support for the dimpled golf ball finish. Try it and see.

My hull has that finish - comes from rollering on the epoxy when there's no one available to brush it smooth. Won the race today but it has to be said that the result was totally down to the skill of the skipper and nothing to do with the boat performance /forums/images/graemlins/wink.gif
 
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Yes it will - just a bit But are YOU really that fast. Around the cans, screw up one tack & you'll negate and difference you just made.

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What a bizare comment. Sounds like you've given up before you even start.

If you don't do everything within your power (and budget) to make your boat as fast as possible you always have an excuse. And when you have an excuse you have a reason not to perform as you could and start to settle for middle of the fleet.
 
Welcome to the forum!

I kind of agree with Flaming about doing all you can to get everything right for racing but would temper that with keeping it in proportion to the type of racing you do.
Serious racing = do everything
Club racing = maybe not quite so diligent to avoid making it too much like hard work rather than fun

I have a programme written for me by the Wolfson Unit (serious tank testers) that allows me to calculate speed drop on a given hull based on average surface roughness against a perfectly smooth surface.
As an example a 12m LWL boat that can do 8kn with a perfectly smooth hull will lose 0.61kn with a surface roughness of 500 microns (0.5mm). With a surface roughness of 75 microns, equivalent to 2 coats of AF, the same boat would lose 0.18kn.

For what it's worth you have a lot more than 6 coats of epoxy if it's 1.5 - 2mm thick. Typically you would struggle to get 100 microns (0.1mm) from 1 coat with a brush or roller without it sagging and running badly.
 
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Riblets

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From the ISF Rules of Racing:

53 SKIN FRICTION
A boat shall not eject or release a substance, such as a polymer, or
have specially textured surfaces that could improve the character of
the flow of water inside the boundary layer.
 
I was of course hoping everyone would agree and say " No, it wont make any difference, save yourself the bother". However it looks as though we will be kept busy this winter. Thanks for your comments
 
The boundary layer is the name given to the tiny turbulent bubbles on the surface that can actually lubricate the major water flow. So some roughness can give less drag.

However you must realise that this is only a consideration at low speeds. As soon as your hull starts approaching hull speed ie above 5 knots then the wave making resistance becomes the significant drag.

Anyway bottom line. I would not spend a huge amount of effort smoothing the hull. Any large lumps or variations can be gound down to make you feel better. But far more significant to race results are good sails, clean bottom (as in no weed) a skilled crew and mostly a skilled helmsman (especially on the wind)

By the way we finally got my little boat cranked up yesterday for a first and fastest in club race. Not so much success so far this season.

good luck for your racing olewill
 
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If you're planning to go without anti-foul (i.e. dry sail the boat) go right down to 1200 grit sandpaper and polish the hull afterwards.



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One world champion dinghy sailor told me never Polish a hull. Get it smooth but leave it with a 1200 finish so that you get a wetted surface,

Never knew whether the advice was good or the reason he was world champion and I was not.

Finally decided it was because I was a crap sailor but always wondered about the advice!
 
Pasarell said:
I have a programme written for me by the Wolfson Unit (serious tank testers) that allows me to calculate speed drop on a given hull based on average surface roughness against a perfectly smooth surface.
As an example a 12m LWL boat that can do 8kn with a perfectly smooth hull will lose 0.61kn with a surface roughness of 500 microns (0.5mm). With a surface roughness of 75 microns, equivalent to 2 coats of AF, the same boat would lose 0.18kn.
Click to expand...

At what point is a hull for practical purposes "perfectly smooth"? So, maybe <0.01kn slower than a perfectly smooth hull. Does a new, well applied gel coat get you there?

Also interested to know the result of your program for a 28ft (Cavalier 28) LWL boat using AF vs one using Gel Coat.

Also, interested in your comments about what proportion of the hull is worth paying attention to. At some distance from the bow, the flow over the hull becomes turbulent at which point the surface becomes much less critical.

Thanks for your contribution.
 
Having spent many miserable winter days burnishing hard racing af. for one design racing I do not envy you. But why attack the epoxy coat, surely it would be better to apply a high solids primer like Primocon over it and burnish that, if rubbed each time with just 2 or three coats she will be smooth, but if you are that good, you then must use hard racing on top and burnish that too. You must dry sail her now or scrub at least once a fortnight.
When we got fed up and started using just soft antifouling put on with a roller and laid off with a soft tipped brush our results were no worse than when we laboured on it.
If you are not always the first boat across the start line then do not bother.
 
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Last season I brought her bottom back to the original hull which gave a much smoother finish. I couldn't believe the difference it made to performance. Anything that reduces drag is for sure going to make your boat faster. But the results will differ from boat to boat and perhaps not worth the hassle on some. But for me, well worth it.
 
JayEm said:
Also, interested in your comments about what proportion of the hull is worth paying attention to. At some distance from the bow, the flow over the hull becomes turbulent at which point the surface becomes much less critical.
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That is one of the factors. In prep'ing a hull for racing you're trying to delay the onset of turbulence.

The keel and rudder are especially important because it is not just increased drag but reduced lift that results from turbulence.

Of course you should do everything if you're serious about racing, but if you want to limit time and effort spent on it focus on the appendages, leading edges especially and the forepart of the boat.
 
Pasarell said:
As an example a 12m LWL boat that can do 8kn with a perfectly smooth hull will lose 0.61kn with a surface roughness of 500 microns (0.5mm). With a surface roughness of 75 microns, equivalent to 2 coats of AF, the same boat would lose 0.18kn.
Click to expand...

few things stop me in my tracks but that did. Are you sure you have those figures right? Does people's experience confirm such a large change? Thats more than 2%
 
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