Saturday, July 15, 2006

How Slippery Is Your Bottom?

We all know that we need to keep the hulls of our racing boats clean and smooth. We are sure that if the hull is covered in scratches or grime or barnacles it will slow us down. Smooth is fast. But how smooth? Is it sufficient to wet-sand the bottom of the hull with 400-grit sandpaper or do we need to sand it with progressively finer and finer grades, and polish it until it shines, and apply special slippery finishes (if our class rules allow them)? If you are a racing sailor you can really obsess over this stuff. How slippery should your bottom be?

You could ask the scientists and engineers and they will confuse you with talk of boundary layers, and laminar and turbulent flow, and Reynolds numbers and drag coefficients. Just remember folks, these are the guys that designed the Titanic, not to mention racing yachts whose hulls snap in half and canting keels that leak. It should be obvious that despite all the high-faluting talk of hydrodynamical and aerodynamical theories, nobody really understands everything there is to know about how to make a yacht go fast. How boring it would be if they did. The rest of us would have nothing to argue about.

All the theoretical spouting about hull smoothness seems to assume that water at some point is flowing smoothly in a layer close to the hull. Hello people. Come sail a Laser some time. It's bouncing around in the waves and the chop. I find it hard to imagine any water molecules flowing smoothly like they do in all the fancy hydrodynamics textbooks.

So how do you know if your hull is smooth enough?

Feel the surface with your fingers? Who says that water "feels" the hull the same way your fingers do? Admire the shiny polished surface that you can see your reflection in? Why should how the water flows over the hull have anything to do with its optical qualities? (And by the way, that reflection of you is even uglier than the real thing.) Oh, so you applied Teflon polish to your hull and the water "beads up" on it? So, you're telling me that your hull repels water and you think that's a good thing? Hmmm - don't know about that.

And what about riblets? Sharks seem to do pretty well with a rough skin. Perhaps tiny little grooves or even dimples (think golf balls) would be faster than a perfectly smooth finish. Dennis Conner seemed to think so with his riblets on Stars and Stripes in 1987.

Or is it all psychological? Will you be fast if you think your hull is fast? If you spend hours sanding and polishing your hull will it give you the feeling that your preparation is as perfect as it could be and that you have left yourself with no excuse to lose. Will your brain magically make you superfast? Maybe.

What do you think?


Shopping City Chaplaincy said...

Personally I think as long as it is reasonably smooth it should be good enough.

Before I paint I clean and sand the hull with 400 Wet&Dry then fill and sand any digs, scratches big enough to fill, small scratches I let the paint fill.
I sand down between coats with 400 to 600 grade Wet&dry.
I paint using a roler and may tip off with a brush.
I don't polish.
This gets a nice finish that is as much about having a boat that looks good as having a racing finish.

Do I think it makes that much difference? No not really. It is more important to point the thing in the right direction and trim the sails correctly. I guess it might make a difference to the really top sailors but I think that even at that level mental attidude has more to do with it and it takes more than a nice finish to give you that edge.

Anonymous said...

I really don't know how much a smooth bottom on a Laser helps in most conditions. Right now, I'm about to haul the Pretty Gee out as her waterline is too low and I'm getting nasty barnacles on the gelcoat where it is below the waterline. UGH.

The theories on what makes a boat faster is all nice, but I think that alot of the theory tends to fail in Real World conditions.

Carol Anne said...

Tim, I like the coinage "attidude."

If you da Man, you don' need no fancy bottom job.

Shopping City Chaplaincy said...

Ah! I normally check my spolling but I must have missed that one! Quite a neat mis-spelling though!

Pat said...

Let's see... 2000 fine sandpaper might be overdoing it, but barnacles definitely aren't fast!

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