Clean Bottom, Fast Bottom

Proper prep and effective paint will boost boat speed.

According to Interlux’s technical group, there is a formula that links speed to surface profile (measured in microns, peak-to-trough height) as shown in this chart (Y axis = speed in knots; X axis = roughness in microns). It shows that a drop from 254 microns (10 mils) to 75 microns (3 mils) gives a speed increase of about .225 knots. Simply translated: As surface roughness decreases, you’ll see marginal increases in speed.

I often read that hard antifouling paints are preferred for racers. Does anybody know how much faster my boat would go if I use a hard paint versus an ablative one?

Ed Lentz
Jeanneau Sun Odyssey 44
Antigua, West Indies

Determining how much faster a boat can sail with one paint versus another would be difficult as the prep and application of both paints would have to be identical, as would test conditions, in order to have a fair trial. None of the antifouling manufacturers we spoke with are aware of any documented head-to-head speed tests of paints.

The key to maximizing boat speed actually has more to do with choosing an effective paint, properly prepping the hull, and keeping the bottom clean. A smooth, clean hull will be the fastest.

Some feel that ablative antifoulings are better suited for race boats because the surface is constantly wearing itself away to create a smooth finish, and moisture in the coatings actually reduces turbulent flow/friction. However, most paint manufacturers and boat owners claim hard paints are the best for racers because they can be wet-sanded and burnished to a smoother finish, and many contain Teflon, which adds to the slickness. The downside is that hard paints release biocide more slowly, are more prone to slime buildup, and eventually get hard-shell fouling. A very hard racing paint like Interlux VC Offshore is easily burnished, but it provides only medium fouling protection, making a pre-race bottom wipedown a must.

Fouling is what affects boat speed the most. The small bumps of a thick, sloppily applied ablative will slow a boat down, but not as much as growth on a bottom that isn’t cleaned regularly or is coated with an ineffective paint.

So your first step toward a turbo-fast hull is choosing a quality paint. Talk to other racers in your area to see what works for them and read our recent antifouling test reports (March and October 2011). Our recommended hard paints included Epaint HP-ZO and EP2000, Pettit Trinidad and Trinidad SR, Interlux VC Offshore, Blue Water Copper Pro SCX Hard, and Sea Hawk Tropikote.

The next step is proper paint preparation. Be sure to sand the hull to a fair finish and fill in any gouges or scratches. (On smaller boats, using a template to get a fair shape may be an option.) To get the smoothest bottom, you’ll want to have the paint sprayed on, but you could also use a short-nap roller. Once the paint is dry and hard, burnish the finish.

Most boat owners are content to just wet-sand the hull using 400-600 grit sandpaper, but others take it a step further and follow up sanding with 1000- to 1200-grit paper and/or smoothing the leading edges with fine bronze wool. We’ve even heard of racers using a polishing compound and electric buffer to get a high-gloss bottom, but this is overkill, in our opinion. While burnishing to a mirror finish certainly increases boat speed, the gain is in the 10ths of a knot—of course, even that narrow margin can change places at the finish line.

After you splash the boat, clean the bottom regularly. Even a thin coat of slime can create enough drag to keep you off the podium. Check out the chart below to get an idea of how a bumpy surface can affect your speed.

Look for our latest long-term bottom paint test results in the next issue.

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