So it looks like boats sanded with 100 grit with the flow would be the fastest.

I always knew my 18 was fastest with the 100 grit finish.

Everyone can be happy all the time...
Due to flow types The ideal is a polished hull for the forward third, sanded for the rest, and a waxed stern to repel water..

Mike,

One of the articles indicated that this advantage was lost if the angle of attack (direction of water vs. direction of riblets) increased over 15%. I would guess that the bobbing up and down of the boat would cause large deviations of the AOA, which would reduce the efficiency.

Also noted was the increase in total wetted surface caused by the riblets, which if memory serves, increases drag... Do the benefits outweigh the drawbacks? You be the judge.

Interesting discussion nonetheless....

There was a paper in Nature, v388, 21 Aug 97, pp 753-755 with a summary on pp 713-714 that described a test using random protuberances. The random did better than the regularly placed roughness, presumably because the regularly placed roughness produced a coherent vortex structure, albeit smaller than the smooth surface structure. Has the added benefit of not needing to be aligned with the flow, though the test setup in the the Nature paper was flow in a channel, not past an object.

The proper way to test something like this would be to use the smooth surface area of your object as the reference area. That way, any effect of the larger gross surface area are factored into the difference in drag coefficient. However, a vendor presenting the data might disagree with what is proper.

gelcoat shmelcoat, buy a boat with a high quality paint job. Super slick yet not too slick to screw up attachment, no wax or slime needed to restore surface, just detergent.... low maint. Easier to touch up and repair scratches and dings.

Jay,
Yeah I read that and the depth of them need to tuned the expected speed. However they seemed to work down under where the boats were sailing in big seas.

For what it's worth I like a scratch free smooth surface polished with a non-silicone polish. The water sheets off the hulls, doesn't bead.

Eric are you saying the ARC's are painted? Who's twisted idea was that?

I was at Robbie D's this eve and asked him what was on his Tornado....It wasn't gel coat.

Then...What do those guys know anyway.

I think it's more about surface tension, adhesion and low/high surface enrgy interfaces. A displacement hull has to push aside an amount of water equivalent to that it displaces at the speed of motion. The water 'wets' the surface to some degree and adheres viscously impeding the hull's motion. The water moves out of the way as fast as the hull goes, up to a point, at which the hull begins to plane. At that point the water is no longer 'wetting' the hull surface. So planing happens when the hull moves swiftly enough to exceed the capacity of water to move out of the way. Note that this is a pretty good analogy to subsonic and supersonic flight.

But there are other ways to reduce surface wetting, certain surface coatings for instance. Low surface energy coatings cause water droplets to bead up rather than spread out into a thin film, implying poor wetting and thus adhesion.

Jimbo

To further stir up the issue, the most applicable test data I know of, is rudder testing by some British grad students (Looks to me like they had too much free time). If I remember correctly, wax made no difference. The lowest drag and best flow attachment was with a 3200 grit finish. A real 3200 grit finish would be like a brand new camera lens.

So get to work. Try a high speed buffer, white plastic rouge, and continous water mist. Oh and be sure to clean the bugs off your hull after you trailer to the water. And the sand or grass off when you get it in the water. And Oh, maybe the scum, slime, and bits of stuff while its in the water.

Or you could just wax it to make it pretty and wash it before you race.

(For those of you whose native language is not English or have a hard time with humor, the last half of this was humor. In the real world, a 3200 grit finish on a catamaran might last until the dust settled on it. I would be hard pressed to maintain an average 600 grit finish at a regatta)

Carl

Carl, do you have a reference for that rudder testing?

Part of the study was on madforsailing.com or the Daily Sail with a link to the university library for the complete article. I read when it was a free website and did not think to copy it. Maybe someone with a subscription to The Daily Sail could find it for you.

I was told that someone found the article by going into the Online University Library search system. I can't confirm that.

There are actually lots of articles available on this subject however most are hard to for the majority of people to read. For example.

The Relationship Between Frictional Resistance
and Roughness for Surfaces Smoothed by Sanding
Researcher: Assistant Professor Michael P. Schultz
An experimental investigation has been carried out in the 380-foot towing tank at the U.S. Naval Academy Hydromechanics
Lab to document and relate the frictional resistance and roughness texture of painted surfaces smoothed by sanding.
Hydrodynamic tests were carried out in a towing tank using a flat plate test fixture towed at a Reynolds number (ReL) range of
2.8´106 – 5.5´106 based on the plate length and freestream velocity. Results indicate an increase in frictional resistance coefficient
(CF) of up to 7.3% for an unsanded, as-sprayed paint surface compared to a sanded, polished surface. Significant increases
in CF were also noted on surfaces sanded with sandpaper as fine as 600-grit as compared to the polished surface. The results
show that, for the present surfaces, the centerline average height (Ra) is sufficient to explain a large majority of the variance in the
roughness function ( ∆U) in this Reynolds number range. The results of this study have been published in the Journal of
Fluids Engineering


If you have access to an online search and retrival like a University Library. You will find many articles on the subject.