Ask SAIL: Best Anti-fouling for your Prop

I have a 2006 Najad 440 center-cockpit sloop with a 75hp Volvo saildrive. Last summer I kept the boat at a marina in Harpswell, Maine, and as the season progressed, I noticed it was slower under power. When I hauled her in the fall, the bottom was clean as a whistle (thanks to Sea Hawk AF-33 paint), but the bronze propeller blades and hub were completely covered with barnacles. I would like to put antifouling paint on the propeller. I’ve researched the topic, but have found no definitive answers as to whether it’s a good idea.
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Nick Orem of Newbury, Massachusetts asks:

 I have a 2006 Najad 440 center-cockpit sloop with a 75hp Volvo saildrive. Last summer I kept the boat at a marina in Harpswell, Maine, and as the season progressed, I noticed it was slower under power. When I hauled her in the fall, the bottom was clean as a whistle (thanks to Sea Hawk AF-33 paint), but the bronze propeller blades and hub were completely covered with barnacles. I would like to put antifouling paint on the propeller. I’ve researched the topic, but have found no definitive answers as to whether it’s a good idea.

Don Casey replies:

If I had the definitive answer on how to keep propellers clean in seawater, I would be cruising in the south of France while my prop-coating company made fat deposits into my personal account.

You can paint a bronze prop, but not with paint that contains copper. You also have an aluminum saildrive leg, and I wonder how you keep that clean. Normally the same antifouling should be used for both the leg and prop, but if your leg has been barrier-coated, then painted with a copper paint (high-risk behavior, in my opinion), you’ll have to use a different paint on the prop. If your saildrive has a copper-free coating, you can also apply that to your bronze prop, but only after priming the bronze with an epoxy-based barrier coat.

Another popular option is to use a zinc-rich aerosol paint, such as Pettit’s Barnacle Barrier, or the similar but cheaper “cold galvanizing” sprays available at home-supply stores. This has the appeal of being simple, inexpensive and risk-free.

Instead of painting you could also try a silicone-based coating, such as Propspeed, a lanolin coating like Propshield, or a heavy wax, such as Sex Wax surfboard wax. Propspeed is highly praised, but is relatively expensive and must be carefully applied. It also depends on motion to keep props clean, so it may be less effective on a sailboat that sits idle or rarely runs under power. Propshield and similar products have a relatively short life span, not more than six months. A wax coating will last an even shorter period, but some can be reapplied underwater.

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