Which is Faster, Smooth Hulls or Rough?
- Thread starter AlastairLC
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Mixing of high and low pressure flow streams at the aft end and bottom of your appendages is a bigger drag producer than skin friction. We did some water tunnel testing of assorted wavy shapes on the aft end of underwater control surfaces back in college under a grant to see if we could reduce the turbulence and thus the "signature" of a submarine. Might be more worthwhile to look into adding what look like corrugations to the aft end of the keel and rudder. Wing keels are also useful to prevent mixing on the bottom of the keel, much like tiplets on an airplane wing.
This is true. A smooth surface allows a layer of laminar attached flow to develop. The smoother the surface the thicker the boundary layer. This creates drag. A slightly rough surface dissipates this boundary layer. This is a real thing and it IS why golf balls have dimples. It is also why you will see small ridges on the nose of an airline jet.
Sorry Jackdaw, I had to Google. I didn't want to just ask without attempting to look into it first on my own.
From the Racing Rules of Sailing:
"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."
That's all it says. How does one, who wishes to sand their bottom, interpret that? 400 grit, 1000 grit, 2000 grit, 8000 grit, 30 grit sand paper? No sanding? Sanding in one direction, sanding only in a circle,... I would wager sailfish skin texture is out, but where is the line on just plain elbow grease and sand paper? Would it be required to be uniform across the bottom? Can you sand with coarse paper near the bootstripe and fine paper on the keel? Is this left to the committee for each race to decide? Everyone sands and many polish or buff. Everyone does it to "improve the character of the flow of water inside the boundary layer."
Is this just a matter of staying within a common norm?
As far as the science goes, no science should EVER be settled. For a thousand years it could have been, and probably was, said that the science of a geocentric planetary system was settled. Newton's LAWs of motion are being reinterpreted. Einstein isn't even beyond question these days. Even centuries old mathematics that seem so irrefutable have been challenged and found in need of amendment. Check out Cantor and his proof of multiple sized infinities and hyper-real numbers.
All we know about science is that there is more we don't know and most of what we do know is only by convention, common agreement, and logical convergence, but our premises are always open for debate. Statistics and proof by authority can take you a long way but it will (almost) never take you to an absolute conclusion.
- Will (Dragonfly)
^^
a. The needs of a hull, keel, and foils are subtly different. The hull needs minimum drag, the keel must balance drag and lift, and the rudder must maximize lift (in general).
b. It's fairly normal to finish the rudder differently, specifically in classes where it lifts. The same is true of dagger boards. (no anti-fouling, high polish).
c. Except for a few one design classes, the bottom can be painted with anything reasonably conventional, left as gelcoat, and even dry sailed.
So I see no reason the keel and rudder on a boat that is coated with antifouling could not use a different paint and finish on the foils and hull if there were perceived benefit. Might it make sense, even for the cruiser, to use a hard paint (yes it would need cleaned, but that is not so much) on the foils and a self-polishing paint for the hull.
For us, of course, this is no more than water cooler conversation, since smoother is always faster within the lumpy limits we will achieve. Are your foils fair? Are the trailing edges thin and square?
a. The needs of a hull, keel, and foils are subtly different. The hull needs minimum drag, the keel must balance drag and lift, and the rudder must maximize lift (in general).
b. It's fairly normal to finish the rudder differently, specifically in classes where it lifts. The same is true of dagger boards. (no anti-fouling, high polish).
c. Except for a few one design classes, the bottom can be painted with anything reasonably conventional, left as gelcoat, and even dry sailed.
So I see no reason the keel and rudder on a boat that is coated with antifouling could not use a different paint and finish on the foils and hull if there were perceived benefit. Might it make sense, even for the cruiser, to use a hard paint (yes it would need cleaned, but that is not so much) on the foils and a self-polishing paint for the hull.
For us, of course, this is no more than water cooler conversation, since smoother is always faster within the lumpy limits we will achieve. Are your foils fair? Are the trailing edges thin and square?
May the earth is flat after all. We shouldn't just let that go.
The point is that while you are correct in some high principle, as a practical matter you are going to be incorrect most of the time. Science starts as a hypothesis that is tested, often adjusted after testing, and overtime the body of evidence grows. Some things get tested enough that they become established fact. 2 +2 = 4. Many others, like Newtons laws, become farther advanced over time, but without invalidating the original work. Although general relativity and even more advanced theories have interesting things to say about motion, Newton is all you will need when working on your boat.
It isn't bad science that is the most common risk. It is conventional wisdom that is not actually based on science, but on repetition and what begins to feel like common sense. Like the notion that dimples in golf balls reduce drag, so clearly there is some application on boats. The problem is the first interpretation is incomplete and the analogy to boats is false. The reason surface modifications were banned from sailing is not because they worked or because they did not work, but because they were impractically expensive in competition. Some classes ban sharp trailing edges on foils, not because they don't work--because they absolutely do--but because they are a pain to maintain.
If you don't like the current science, get involved in testing it. Often it is not so hard. Some amazing things have come out of high school science projects. Remain objective and make sure you have controlled the right variables.
2+2=4 isn’t a fact, in the sense that it has been proven to be true; it’s a tautology, in the sense that it is always true by definition.
It sounds to me like the science around smooth versus rough hulls isn’t advanced enough to be at Newton’s level, let alone Einstein's.
- 2+2=5, for large values of 2.
- Relativity isn't correct, QED is.
when my dad was doing his studies at Princeton, his next door neighbor was Einstein. one of Einstein's passions was sailing. i don't know if this is relevant .....
Word was that he was terrible at it. Not sure that's relevant either!
sailing, skiing, chess, ....... there is an endless quest to do it better. that said. if you are having fun, your doing it right! 
I knew that was coming when I typed it!
The curious thing, applicable here, is that some theories (Newtons laws, and even that the world is flat) work so long as the range of experimentation is limited. The world was flat because unless you asked just the right questions or looked farther than a few miles, it basically worked. And I suspect so it is in this case; unless you are prepared to measure very carefully and change foil designs at the same time, it's going to be pretty hard to improve on what the AC and Olympic guys do.
Consider this. An upgrade that makes sense for certain high speed trimarans, but probably few others. And it only makes sense for those because the rudders were too short. It would make things worse for most boats, adding drag with no upside. In my case, it prevents wipe-outs at high speed. To learn, do the experiments.
on IO's and outboards that called a cavitation plate. on my ascow, where i sail it, it's seaweed that wipes out the rudders, always exciting
It should be called an anti-ventilation plate. But yes, same function. In the case of the rudder, it's helpful when bearing off at over 12-15 knots.
Yeah weed is fun too. I've had those wipe-outs too. I would NOT use a fence in a weed area.
this is called a pitch pole i think
i wonder if this was a weed issue
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bethwaite did allot of experimentation with a fence on his rudders back in the day. thats's where i have read about such things