#19 - Bottom speed-- the bottom finish-sanded (nose-to-tail) with 400 grit wet-n-dry provides optimum surface texture. Too rough or too water repellant (slick) creates turbulance between laminar flow layers beneath hull. NO wax of any kind! The edges above waterline also need this treatment.

Some point to the fact that water beads on a waxed surface while exposed to the air and that this is due (correctly) to a phenomena called “surface tension”. They drown in the shallow end of the logic pool by thinking (incorrectly) that this “tension” must “pull” on the submerged hull, slowing the boat down. I must pause to laugh. If this was true, wouldn’t it mean that if you gave that same tenacious water thousands of tiny scratches to hold onto by wetsanding, it would pull just that much harder?...

Water does have a surface tension property, but it’s dependent on the interface... Luckily for us, what the air/water interface is doing has nothing to do with how it reacts submerged at the hull/water interface... Water and air boundaries are completely different--different densities, different viscosities, just different. The fact that water is beading in the air has no relevance to our discussion...

The difference in drag along a 16-20 foot hull between a wetsanded hull and a waxed hull has never been experimentally confirmed, to my knowledge. That's how minute the difference is. Until someone drags a 600 grit sanded hull through a tank of water with transducers to measure drag, and then drags the same hull after waxing and proves that there is a significant drag increase, I must insist that the lower drag will be attained with the waxed (smooth) hull.

Frank Bethwaite, on page 263 of his brilliant book “HIGH PERFORMANCE SAILING”, states: "At practical yacht or dinghy speeds, only the bow area of the hull can hope to run with a laminar boundary layer. Under this area the surface should certainly be highly polished. But beyond this zone the flow will become turbulent, and under turbulent flow a highly polished surface will not be any faster than some rougher surface, provided always that the roughness is less than some small fraction of the boundary layer thickness." While Mr. Bethwaite does not concretely recommend waxing/polishing the entire hull, he doesn’t preclude its success and distinctly promotes leading edge treatment. The key is to have your hull as aerodynamically smooth as possible to keep the flow attached for as long as possible, keeping the transition from laminar to turbulent flow as far downstream from the leading edge as possible.

Conclusions?

An individual who has sailed with and around Dennis Conner (of America’s Cup fame) related a quote to me in which Mr. Conner was asked why he wetsanded his cup boats. He replied that he had absolutely no idea, but that if he didn’t, he was sure the other teams were, and by God he was going to do it too, if for no other reason than to level the playing field...

I believe he should wetsand, then follow up with a silicone-based polish. It is interesting to note that when Dennis lost the Cup to New Zealand and subsequently took the Catamaran “Stars and Stripes” to get it back, he was not only wetsanding but using a controversial coating (polish) that I believe was called “Shark Skin”. It’s amazing to watch how in their desperation to go 0.001 knot faster, the best sailors allow themselves to get sucked into trying every bottle of snake oil on the globe. The fact is. any possible difference that “Shark Skin” could have made, as compared to wetsanding or wax or silicone polish, is so miniscule that it can’t be measured from the noise.

Go ahead and wax your hull. It will protect it from UV damage, keep it looking shiny and, thanks to Billy Crystal, we all know it’s better to look good than to feel good... If someone beats you and they wetsand instead of waxing/polishing, they were a better sailor, not a better boat prepper. Even if it was Dennis Connor. Even the best can be scientifically misled. We won the spring A-fleet series on Cayuga Lake (NY) in a heavily waxed J33 this year. I have race-prepped boats from Sunfish® to Cats for people, including waxing and silicone-based polishes, and in regattas they have never finished worse than before, and sometimes much better.

I admit there are arguments in opposition to mine that can sound pretty convincing. The only “fact” in this debate, though, is that it is a debate. Most aerodynamicists will admit that this is still as much art as science, and the more we learn and understand, the more we realize we don't understand and have to learn.

The bottom line is: as long is there is argument and the differences are microscopic anyway, I’m willing to err on the side of making my boat prettier and easier to maintain and, at the same time, spend more time on the water practicing skills.

For those who may be curious at this point, here is what I do to my boat:

(1) I wetsand by hand all the way up thru 500, 600, 800, 1000, and on to 1200 or 1500 grit 3M papers. I rub each grit of sandpaper in one direction only. (Obviously you don’t want to sand through the gelcoat, so prudence is essential here.) As I switch to each higher number paper, I rub at a 90 deg. angle to the previous. In this way, I can easily see when the tiny scratches from the previous paper have been removed. I keep alternating as I go up.

(2) I apply 3M Fine-Cut rubbing compound with a 7” orbital buffer. This removes the rest of the sanding swirl marks.

(3) I proceed with a good quality polishing compound (3M, or Turtle Wax).

(4) I finish with a silicone-based polish (Starbrite Boat Polish, McGuiar’s #53/#53 Boat Polish, or Eagle Poly-1; whatever I have in the shop).

Yes, it’s possible I have too much time on my hands, and yes, after all this I can still get my backside handed to me on the race course, but it’s not the hull prep of my boat that’s to blame.

Now, if you’ll excuse me, I’m going to put in some water time. See you there.

Cheers,
Christopher H. VanEpps
Aeronautical Systems Engineering
Lockheed Martin

For decades, modern designers and engineers concerned with movement efficiency focused on the coarse shape and smoothness of an object. Howard Hughes’ H-1 Racer, for example, an aircraft which broke numerous speed records in the 1930s, sported revolutionary design features such as retractable landing gear and flush rivets. More recently, armed with greater tools for observation (such as scanning electron microscopes) and manufacturing, designers and engineers are developing an appreciation for the impact of finer-scale surface interaction dynamics.

For example, while a shark’s coarse shape is famously hydrodynamic, shark skin is anything but smooth. The very small individual scales of shark skin, called dermal denticles (“little skin teeth”), are ribbed with longitudinal grooves which result in water moving more efficiently over their surface than it would were shark scales completely featureless.



Over smooth surfaces, fast-moving water begins to break up into turbulent vortices, or eddies, in part because the water flowing at the surface of an object moves slower than water flowing further away from the object. This difference in water speed causes the faster water to get “tripped up” by the adjacent layer of slower water flowing around an object, just as upstream swirls form along riverbanks.

The grooves in a shark’s scales simultaneously reduce eddy formation in a surprising number of ways: (1) the grooves reinforce the direction of flow by channeling it, (2) they speed up the slower water at the shark’s surface (as the same volume of water going through a narrower channel increases in speed), reducing the difference in speed of this surface flow and the water just beyond the shark’s surface, (3) conversely, they pull faster water towards the shark’s surface so that it mixes with the slower water, reducing this speed differential, and finally, (4) they divide up the sheet of water flowing over the shark’s surface so that any turbulence created results in smaller, rather than larger, vortices.

Why It Matters

New surface coatings for boats which emulate shark skin texture and fine-scale movement have been shown to reduce fouling by 67% over conventional surfaces, and at 4-5 knots be completely self-cleaning. Due to their clean surfaces, boat hulls treated with these new shark-inspired surfaces are subsequently much more energy efficient.

The applications for synthetic shark skin surfaces extend beyond boats, however, and range widely, from better-performing medical implants to faster swimsuits. The swimsuit company Speedo, for example, has incorporated shark-inspired textures into their swimsuits. The 3% improvement in swimming speed due to the original “shark-skin” suit likely contributed to the fact that 80% of the swimming medals won in the 2000 Olympics were won by athletes wearing Speedo’s Fastskin suits; swimmers wearing the suit also broke 13 of 15 world records. Speedo has made further modifications to their Fastskin suit based on continued research of shark skin and increased the swimming speed of its wearers further, generating further anticipation over the suit’s performance in the upcoming 2008 Olympics.