Effect of Anti-Fouling Bottom Paint on Boat Speed
The effect on boat speed of the surface condition of the portion of a boat’s hull that is in the water is not known with any particular precision. I don’t khnow of any solid data about this. You can find anecdotal remarks putting forth the notion that a planing hull with a surface of anti-fouling paint will experience a speed reduction compared to the same hull with a smooth, clean, polished, and waxed gel coat surface; even if that were true—and I don’t believe there is any actual measured speed data to support it—the effect is likely to be minimal.
In the realm of ultra-competitive sailboat racing where a difference in boat speed of one-tenth of a knot is considered a huge advantage, it was found that a speed INCREASE could be achieved over a smooth and hard hull surface by creating a particular texture to the hull surface. As I recall, those modifications were latter outlawed by the competition committees as being unfair due their extreme expense.
The adverse effect on boat speed for a moderate planing speed boat from a bottom coat of anti-fouling paint that prevents marine growth on the hull is likely to be of lower magnitude than the effect of the marine growth if allowed to accumulate unchecked.
Any effect on speed notwithstanding, a fiberglass boat hull left continually in the water needs a protective epoxy barrier coat to prevent osmosis of water through the fiberglass.
The only option to avoid a barrier coat would be to haul the boat out of the water periodically, say every two weeks, and let the hull dry thoroughly in open air.
Optimizing the performance of Boston Whaler boats
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I had old and flaking epoxy-based bottom paint on my 1997 Outrage 20. I sanded it off and surprising found no measurable difference in speed or economy. It sure looks better without the old paint though.