PBO reader Andy Peach has a question about the right type of propeller antifoul to use. Our paint and antifoul expert Vyv Cox has this advice…
Andy Peach writes: “What’s the best preparation and antifouling paint to use on a propeller and P-bracket?
“I’d heard that lanolin was worth a try but when I lifted the boat mid-season I found most of it had worn away from my bronze prop and fouling was starting to rear its ugly head.”
PBO paint and antifoul expert Vyv Cox replies: “Antifouling a boat propeller, and to a lesser extent a P-bracket, poses some very specific problems.
“The first of these is that the speed of the propeller through the water generates considerable erosion, requiring specific propeller antifoul paints that show high resistance to this mechanism.
“A further problem is that some of the best antifouling paints work better in some places than others, while some that work well in one part of the country, or indeed the world, do not work at all well in another.
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“The manganese bronze selected for many propellers and P-brackets is in fact a modified brass, although many after-market propellers are genuine bronzes, mostly with tin additions but some with silicon.
“In all cases these alloys are more noble than copper, which means that if plated with copper metal or coated with copper-based antifouling it will be the coating that corrodes, not the item itself. (My article on copper plating a propeller was printed in PBO in 2019).
“The next problem is getting the antifouling paint to adhere to the copper alloy. Serious abrasion with something like 80-grit paper before applying a conventional primer can be successful but an etch primer is far more effective. I’ve had excellent experience with Hammerite Special Metals primer, as the photographs above show.
“I sail in the Mediterranean, where Velox antifouling paint combats tubeworm quite well but other specific antifouling paints may be more effective in other waters.
I first antifouled my propeller in 2012 using this combination of paints, applying two coats of primer and four of Velox.
“At the end of the season (see photo top left) it was evident that the primer was in excellent shape but that the antifouling had lost some adhesion and had chipped in the typical cavitation areas.
“Subsequently I have abraded the primer using 180-grit and applied only two coats of Velox. I did not paint the P-bracket, using the same methodology, until 2013.
“Amazingly, I have never renewed the original primer. The second image (top right) shows a later season. Both photos were taken after a six month season in the water and before being pressure washed.”
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