How smooth is smooth enough?

Discussion in 'Boat Design' started by RHough, Oct 19, 2006.

  1. RHough
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    RHough Retro Dude

    Please forgive me if I've missed a good answer to this. I've done several searches and have found some comments, but no answers that are very satisfying.



    I am about to make a decision on how smooth the bottom finish on my sailboat will be.

    I have a chance to to try a new ceramic surface coating.

    How smooth is smooth enough? 300 grit? Polished?

    I've heard people say that a polished hull and foils are no faster than 600 grit, I've even heard claims that a polished surface is slower.

    How smooth is too smooth?

    Cerakote(tm) is available in both a satin and gloss finish. It is very slick (too slick for marine growth to stick on it), I am advised that if the topcoat is abraded (sanding or cleaning with scotchbrite) the coating looses some effectiveness in keeping growth off.

    Is it possible that having a very smooth, hard, surface on the hull would be slower than regular bottom paint?

    I won't be upset if the anti-fouling properties are not as good as bottom paint, but I'll be sideways if the hull is slower with a gloss bottom at $400/gallon.

    Thanksfor any help ... If I've missed the information, just point me to the right thread ... :)
  2. jehardiman
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    jehardiman Senior Member

    The answer is, as usual, it all depends. Big question is if the surface is hydrophobic or hydrophillic? Hydrophopic surfaces tend to perfer smoother finishes, hydrophillic surfaces are more tolerant of surface roughness. Next in importance is Renyolds regime...are you long and slow or short and fast? And where is your seperation point? All these matter if you want to say "bottom coat A is better than bottom coat B".
    Last edited: Oct 19, 2006
  3. RHough
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    RHough Retro Dude

    I'm a 11,000# sailboat with a 25 foot waterline (D/L ~ 300). I have a water line beam of about 9.5 feet ... so I guess I'm short and fat? :(

    I don't think I'm hydrophobic ... I'm a boat and I've been sailing for 31 years ... :D
  4. PAR
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    PAR Yacht Designer/Builder

    With your high D/L you're not climbing over the bow wave, so any grit will suffice, if you're pleased with the finish. Frictional resistance reductions will amount to very little potential in performance on a 300 D/L craft, for the most part. It may be a measurable amount, but likely in 100ths of a knot. 2,000 grit would be polished, 220 grit would be more then adequate for your boat.

    Put a good paint job on your boat and she'll be fine, probably no faster, nor slower, just better protected. Leave the polished bottoms to the folks with double digit D/L yachts.

    From a technical stand point, Jehardiman is correct, you could see, very small gains (that measurable thingie again) if you had a golf ball type of surface on your bottom. Maybe a thick nap roller without tipping it off.
  5. jehardiman
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    jehardiman Senior Member

    PAR is correct, with a D/L of 300 220 grit will be more than sufficient. Just make sure it is smooth and step free. Surface finish gains here are measured in 1/100th of a knot.... unless you have a Gas Turbine for propulsion...;)
  6. yokebutt
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    yokebutt Boatbuilder

    Make it shiny, 800-1000 grit wetsand and then rubbing compound. The reason is simply that it's a hell of a lot easier to keep it clean.

  7. RHough
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    RHough Retro Dude

    Here is an interesting article from Sailing World, June 1996


    "As for what is "hydrodynamically smooth" aft of the transition point, when sailing at 2 knots, it's any scratch smaller than 4 mils (thousandths of an inch). At 12 knots, the "admissible roughness" reduces to under 1 mil. A human hair is approximately 2 to 3 mils in diameter, and a bottom finished with 400-grit sandpaper should have a hydrodynamically smooth finish aft of the transition point for speeds up to 7 knots. So, for most keelboats, a bottom which is finished with 400-grit sandpaper in the aft sections is adequate. For planing dinghies, which sail faster, the aft sections of the
    bottom need to be smoother."

    "The worthy opponent of these positive effects is the state of the water. Turbulence at the surface from waves, microorganisms, and contaminants can all be disruptive. Yacht designer David Pedrick, who has dealt with this question during several America's Cup efforts, feels that the imperfect sea state usually wins out. "We've used electronic sensors and microphones to test for laminar flow," he says. "You can get some, but not much." The best chance for laminar flow is on the keel and rudder, both because of their convex shape and because they are immersed below much of the disturbance. Aerodynamicist and dinghy designer Frank Bethwaite questions "whether any surface is 'smooth enough' for a racing dinghy," when it comes to foils."
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    BOATMIK Deeply flawed human being

    Frank's testing on hull surface smoothness was flawed if it continued to reflect the material published in Australian Sailing magazine a few years ago.

    Which is unusual for Frank. Rather than swear on a stack of bibles I would be happy to swear on a stack of his Frankness's articles. Massive contribution - usually ahead of the game and usually right.

    I don't have a copy of the article on me - it might have made its way into his book.

    He published a graph with drag figures for a smooth hull and a roughened hull.

    Speed was plotted against drag for each on the one graph.

    If you drew a line through the graph at a drag figure and worked backwards there was a good 2knots difference between speeds between the sanded and polished hulls at that given drag figure. This is probably at least 20 or thirty - maybe 100 times the differences we see on the race course.

    Unless I've got something wrong.

    I find this doubtful.

    The boundary layer (the thin layer of water that more or less sticks to the boat) gets thicker at the water travels down the length of the boat. So any surface roughness is more and more easily able to hide inside the boundary layer where there is much less relative movement between the hull bottom and the water.

    Perhaps it is a reference to the bow rising out of the water at high speed where the aft part of the bottom might be the first part to make contact with the water. I suspect that the water would be so disturbed and aerated that any meaningful reduction in drag through surface smoothness would be be unlikely. Or at least I find it so.

    Just one final thing.

    The Psychology of it all.

    I usually use a 400grit finish if I can be bothered to do anything at all to the hull. Usually I will take great care with the centreboard and rudder - particularly the leading edges and makes sure they are pretty good with the 400 grit and a long based flat sanding block (not too hard or too soft please).

    My feeling is that foil section shape and a clean leading edge is much more important that the most perfect of finishes.

    If I do this I feel I don't need to worry about the bottom of the boat and so can concentrate on my racing - at which I have been reasonably successful. Not the America's cup mind you, but enough club and class trophies to keep me happy enough.

    If that is not smooth enough for some other people then - sure - they should work to finer levels of smoothness. Whatever it takes so that they can just forget the bottom of the boat when they are out there and focus on the racing.

    If someone reckons a freshly painted bottom is best - then that's what they should sail with. And that's what they will do best with. It's not too bad hydrodynamically and wonderful for their psychology. They know in their water that their boat is the best one out there - so they can forget about it.

    Thats the real purpose of a smooth bottom - smooth enough that you don't feel you have to worry at all when you are sailing.


  9. Karsten
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    Karsten Senior Member

    First there is smoothness and fairness. A hull is not fair if there are larger iregularities in the hull like a 1mm deep depression over quite a large area. The water basically has to change direction to flow into the dent which slows the boat down. That's why some race boat owners pay alot of money to custom fair the hull even on a female moulded production boat.

    Hydrodynamically smooth is if the little blobs and grains on the surface do not extend past the boundary layer. The boundary layer is the water very close to the hull that basically gets dragged along. So any imperfections within this area are not going to slow you down much.

    So the question is how thick is the boundary layer? It depends on the local Reynolds number. In the laminar area the boundary layer is very thinn. So you need a good surface finish. On a "normal" boat you can probably forget about the laminar area because it is so small and due to turbulence in the waves it might not be there at all. The thickness of the boundary layer also depends how far back of the boat you are. At the front it is very thinn and by the time the water reaches the transom it has dragged along more water further away from the hull. So the boundary layer is thicker at the back and therefore a good surface finish is less important. On an oil tanker you might find that football sized barnicles have little effect on the drag at the back of the hull. Also next time you fly have a look at the rivets in the fuselage. In the front of the plane they are flush but at the back they are often dome head rivets because the rivet heads are hidden in the boundary layer and don't have much effect on the drag. So if you can't be bothered to sand your whole boat start at the front.

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