Mailport April 2012 Issue

Mailport: April 2012

Fast Bottom

In regards to a reader’s query on how much faster his boat would go with hard bottom paint than ablative antifouling (PS Advisor, February 2011): As a world-respected boat yard for racing sailboats (and a “speed shop”), that question is one of Waterline Systems’ most commonly asked.

A fast bottom needs to be fair, and it needs to be smooth. How smooth your bottom should be relates to the thickness of the “laminar sub-layer,” the layer of water that lies closest to your hull. This sub-layer is really thin and varies with speed and distance to the leading edge. The faster you go, the thinner the layer; and the closer to the bow (leading edge of a foil), the thinner the layer. As long as your wetted surface is smooth enough that nothing pokes through the sub-layer, the hull is hydro-dynamically smooth. Anything that pokes through the sub-layer creates a big drag penalty.

The basic formula for determining admissible roughness is: Ka= 8/V (knots) in mils, where Ka= admissible roughness height, 8=kinetic viscosity of water, and V=Velocity in knots. When we do the math, we can relate the required smoothness to sandpaper grits. Bear in mind though, that this only relates to a properly sanded surface. For example: Where admissible roughness at 2 knots (velocity) is 4 mils, it’s similar to 220-grit sandpaper; an admissible roughness of 1.1429 mils at 7 knots is like 400-grit sandpaper; and an admissible roughness of 0.2857 mils at 28 knots is like 1200-grit sandpaper. Remember that the sub-layer is thinner near the front of your hull and foils, so if you are feeling ambitious and want to sand more, concentrate there.

The primary advantage of hard racing paints is the ability to sand them to a consistent finish. Ablative paints are simply too soft to be able to accurately sand to an acceptable finish. Once you have achieved the desired sanded finish, the water just doesn’t care. Buffing and burnishing looks good, but doesn’t make you any faster.

In our experience, the best hard paints are not always the best from an antifouling standpoint. If you keep your boat in the water and want peak performance, find a good diver and get real friendly with him. Don’t let your diver use anything abrasive on the bottom. Frequent cleanings with a sponge of soft cloth are the only way to go.

How much faster will his boat go? That’s a bit hard to pin down to an accurate number. It will depend on the design of the boat, how bad the current bottom is, and how good the new bottom will be. I suppose one could run it through a bunch of software and get a pretty good theoretical number.

Will Harris

Waterline Systems

Next: Free Charting

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