Slippery GRP hull

[Sailfree]

Can anyone advise on websites, information sources that cover the resistance of GRP through the water.

Assuming a smooth surface can some coatings (as claimed are used by some olympic teams) improve speed. My understanding is that they disolve over the race and are expensive.

I understand that 3M also produce a film with scored lines i but can't remember whether the lines should go along the water flow or at 90 deg!

More practically does a fine (say 1000grade) wet and dry finish give a better GRP hull speed than a highly polished (water globulating) surface.

Before some scoff at this question remember the Olypics have now banned artificial swim wear material as it had a significant effect on swimmers speed.

 

[awol]

I see a protest under RRS 53 looming!

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.

 

[dt4134]

As Awol says tha racing rules limit what you can do, so I've no experience of the fancier techniques.

I've helped take a hull back to 1200 grit and have gone finer on keel & rudder, it really seemed to have made a difference but the boat had to be dry sailed. On top of the sanding you can waxed and polish the hull, which is allowed.

Flatting racing anti-foul with 400 grit is common too, the keel and rudder give the biggest rewards for the least work.

 

[Boo2]

A boat shall not ... have specially textured surfaces that could improve the character of the flow of water inside the boundary layer.

Doesn't "smooth and shiny" count as "specially textured" then ?

Boo

 

[Sailfree]

As Awol says tha racing rules limit what you can do, so I've no experience of the fancier techniques.

I've helped take a hull back to 1200 grit and have gone finer on keel & rudder, it really seemed to have made a difference but the boat had to be dry sailed. On top of the sanding you can waxed and polish the hull, which is allowed.

Flatting racing anti-foul with 400 grit is common too, the keel and rudder give the biggest rewards for the least work.

Boat will be dry sailed

My first interest is to try and establish is there any difference to speed between a highly polished GRP hull to one say wet and dry to 1000 grade. I have heard some strong opinions stated on both finishes and I am trying to establish the facts.

Lastly one fast boat had a "paint finish skin" on its underside akin to a bubbly roller finish - I would like to establish whether any "types" of finish significantly affect performance. Certainly in Beckworths(? Adviser to the Australian Olympic squad) Speed Sailing book he established that towing a boat without an undercover was seriously detrimental to boat speed just due to minor road grime.

 

[Skysail]

I seem to remember that a fine matt finish delays separation and promotes laminar flow. Many dinghy sailors rubbed down foils and hulls with 1200 grade wet and dry, whether it made any difference or not, it made you feel better!

 

[dt4134]

Boat will be dry sailed

My first interest is to try and establish is there any difference to speed between a highly polished GRP hull to one say wet and dry to 1000 grade. I have heard some strong opinions stated on both finishes and I am trying to establish the facts.

Lastly one fast boat had a "paint finish skin" on its underside akin to a bubbly roller finish - I would like to establish whether any "types" of finish significantly affect performance. Certainly in Beckworths(? Adviser to the Australian Olympic squad) Speed Sailing book he established that towing a boat without an undercover was seriously detrimental to boat speed just due to minor road grime.

I'd say 1200 grit is standard, but some go to 1800 or 2000. You then polish the hull after that. G3 then wax & buff. Repeat wax & buff regularly. So, my experience has always been with wet & dry and polishing being complimentary rather than alternatives.

Certainly for an IRC sized boat, another big advantage is taking a lot of the irregularities out of the hull with long boarding before you get down to the 1200 grit stage.

I'm not sure where a bubbly paint finish stands regarding the RRS, but it does sound like it might be the wrong side of borderline. Maybe class rules allowed it.

What sort of boat is it?

Maybe you could also talk to the regular top three in the same class locally.

 

[Sailfree]

I'd say 1200 grit is standard, but some go to 1800 or 2000. You then polish the hull after that. G3 then wax & buff. Repeat wax & buff regularly. So, my experience has always been with wet & dry and polishing being complimentary rather than alternatives.

Certainly for an IRC sized boat, another big advantage is taking a lot of the irregularities out of the hull with long boarding before you get down to the 1200 grit stage.

I'm not sure where a bubbly paint finish stands regarding the RRS, but it does sound like it might be the wrong side of borderline. Maybe class rules allowed it.

What sort of boat is it?

Maybe you could also talk to the regular top three in the same class locally.

Cant understand your first para Except to get smooth, Wet & Dry gives a surface that retains water and its argued gives a separation in boundary flow and is faster. Polishing (even with G3) causes water to globulate and some argue causes more drag.

One of the very top sailers had the bumpy finish!!

 

[dt4134]

I see what you mean, just stick at 1200 grit to get the rough finish.

In that case we've always polished. The reason for working up to 1200 grit and beyond is that long boarding was necessary to take irregularities out of the hull, which are going to be present in pretty much all GRP boats as built.

Polishing makes more sense to me, as friction predominates at slow speed in light airs so I feel trying to prolong laminar flow is better, but I'm sure there's not a concensus on it.

Maybe one to post on a racing forum, like Sailing Anarchy. Best of luck. I suspect you'll be likely to get lots of opinions, but I don't know that there's a received correct answer.

 

[jwilson]

Can anyone advise on websites, information sources that cover the resistance of GRP through the water.

Assuming a smooth surface can some coatings (as claimed are used by some olympic teams) improve speed. My understanding is that they disolve over the race and are expensive.

I understand that 3M also produce a film with scored lines i but can't remember whether the lines should go along the water flow or at 90 deg!

More practically does a fine (say 1000grade) wet and dry finish give a better GRP hull speed than a highly polished (water globulating) surface.

Before some scoff at this question remember the Olypics have now banned artificial swim wear material as it had a significant effect on swimmers speed.

Used to be gospel that a very fine flat mat finish was fastest, but that was generally in boats that spent most time at displacement speeds. These days 49er and other skiff sailors go for ultra-polished.

I think it was one of the Aussie Americas Cup boats that used a textured film, and I think it was essentially ultra-fine grooves up and down, not along the hull.

 

[Skysail]

If you rub your finger tip along a polished wet surface, there is appreciably more drag than on a fine matt wet surface.

 

[dt4134]

If you rub your finger tip along a polished wet surface, there is appreciably more drag than on a fine matt wet surface.

Leave the boat in the water for a couple of months without anti-foul and you'll have a nice slick green surface that your finger will just glide across.

 

[jwilson]

Leave the boat in the water for a couple of months without anti-foul and you'll have a nice slick green surface that your finger will just glide across.

Scrubbed a boat once that had just a trace of slime, speed increased about 5%. Serious race boats get scrubbed shortly before every race if too big to dry sail.

 

[dt4134]

Scrubbed a boat once that had just a trace of slime, speed increased about 5%. Serious race boats get scrubbed shortly before every race if too big to dry sail.

Been there, done that, quite a few times. The most notable improvement is how much higher you can point with a clean hull.

 

[grumpy_o_g]

Don't know if it's applicable but in aviation it's an accepted fact that a very smooth surface with a very narrow boundary will tend to separate suddenly and completely, whereas a rougher surface will have a thicker boundary layer with a more gradual transition between the laminar and non-laminar flow. This is for lifting surfaces so will be relevant for the keel/board and rudder only on a boat presumably.

Absolutely sharp trailing edges are another thing that can cause problems I was told once as a dinghy sailor (didn't make any difference but that's cause I was a **** helmsman). I later learnt that for slower airflows and more dense fluids (e.g. water) a flat trailing edge provides an small but contained area of cavitation or turbulence where the different pressures and flow speeds can sort themselves - or something like that.

 

[fireball]

Back in the dim distant past - I raced against a guy who polished his shrouds on his new 420 ... pity he was a **** helm (then) and didn't feature in the race whilst I was in contention in my 30yo 420 - with unpolished hull or sails!!

The downside is - he's now quite good ... authored (or co) videos & books and would appear to be enjoying life earning a living at the sharp end of sailboat racing ... bitter? me? no!

 

[Deleted member 36384]

..... Flatting racing anti-foul with 400 grit is common too, the keel and rudder give the biggest rewards for the least work.

I cant vouch for the 400 grit exactly but that grit I am sure produces the roughness that gives the best surface for the water molecules to do what they do with least resistance. It was either that the molecules rolled along the surface without sticking to the hull or that they stuck firmly and the boundary layer sheared with less force between the stuck to the hull molecules and the free molecules, or maybe a combination of both.

I too would be interested in any facts from those in the know. I understand that slight flats on trailing edges with sharp breaks is best for reducing drag on the back of keels / dagger boards and rudders.

 

[dt4134]

I cant vouch for the 400 grit exactly but that grit I am sure produces the roughness that gives the best surface for the water molecules to do what they do with least resistance. It was either that the molecules rolled along the surface without sticking to the hull or that they stuck firmly and the boundary layer sheared with less force between the stuck to the hull molecules and the free molecules, or maybe a combination of both.

I too would be interested in any facts from those in the know. I understand that slight flats on trailing edges with sharp breaks is best for reducing drag on the back of keels / dagger boards and rudders.

I'd always assumed that the flatting of the anti-foul was just to get rid of the surface roughness and irregularities after anti-fouling. My understanding was that 400 grit gave most of the benefits and there was nothing to be gained from finer sandpaper.

I'd always believed it was to try to maintain laminar flow as long as possible, but that was just my belief.

I do the same on trailing edges, having the flats at a slight angle.

 

[Sailfree]

Appreciate the comments regarding antifoul but I am trying to establish the ultimate "legal" finish for fastest GRP hull.

So dry sailed , no antifoul - now do you polish the hull with say MER or get a polished finish with say Farcela G3 or leave it say with a soaped 1200 wet and dry which leaves a very fine matt finish.

For the inexperienced when I did car resprays you could get a polished surface using a rubbing compound like G3 or G7. The surface looked polished but was spotlessly clean with no contaminants. Using a polish add silicones etc to the surface but both finishes will cause water to globulate in the same way. More friction?

When rubbing down with Wet and Dry if you were trying to do it very finely you first rubbed a bar of soap across the wet & dry paper to partially block the courseness of the paper. It made the rubbing down action much finer and lubricated it at the same time but left a very smooth finish but matt and with very fine scratches.

What suprises me is that from one article I read it appears that the fastest (now illegal) hull will have a 3M film with minute ribs in it so perhaps a more course wet and dry with the sratches in the "right" direction might best.

I am surprised at the lack of published info on this but possibly it all makes so little difference that its only Olympic dinghy sailors that bother to go to these lengths and they want to keep their knowledge secret.

 

[Quandary]

Appreciate the comments regarding antifoul but I am trying to establish the ultimate "legal" finish for fastest GRP hull.

I am surprised at the lack of published info on this but possibly it all makes so little difference that its only Olympic dinghy sailors that bother to go to these lengths and they want to keep their knowledge secret.

This discussion has been going on for donkeys years with very little progress, even in one design racing you need to be very very good before it makes as much difference as a perfect start or one perfectly executed gybe. The advantage is so small that it is negligible in most normal forms of yacht racing. For what its worth at one stage (quite long ago) we used to antifoul our one design with hard racing, applied in thin coats with a roller and laid off lightly at 90 degrees to the water flow with a very good quality brush. (it was then scrubbed with a soft floor brush at least monthly). We soon found that a 2 sec advantage on the start line was worth a lot more, as was continuous monitoring of correct placement of crew weight. If you are dry sailing all you need to look for is a smooth clean bottom unless you are so really really good that you would not be asking the question here. How many races have you lost by less than ten seconds?