Antifouling and anodes for thruster props and tunnels

A thruster spends most of its life sitting in the dark in a tube full of seawater, which is exactly what barnacles like. When a thruster that used to spin the bow briskly starts to feel tired, the first suspects are the battery and the cables. The next is growth on the props, the gear leg and the inside of the tunnel. Sleipner lists marine growth as a common cause of lost thrust, and barnacles on the prop are among the worst offenders.

The rules before the products

Sleipner's manuals give a short list that applies to most tunnel thrusters:

• Antifoul the gear leg and propellers before every season.
• Never paint the anode, the rubber seals or the propeller shaft.
• Do not fill the tracks in the gear housing where the propeller hub runs.
• Inside the tunnel, use a thin primer and two coats of antifouling only. No gelcoat or thick build in the bore, because the clearance between tube and prop is tight.

Sleipner also points out that legal rules for antifouling vary by region, and suggests asking locally which products are allowed and work in your water.

Propspeed

Propspeed is a foul-release system rather than a biocide paint: an etching primer followed by a slick clear coat that growth struggles to hold onto. The process is strict. Strip old coating, sand, clean with Propclean, apply Propprep, two coats of etching primer, then the clear coat. Propspeed wants at least 8 hours of cure before launch in warm weather and longer when it is cool; it does not recommend application below 50°F.

Standard Propspeed is aimed at underwater metals. That makes it a good choice for bronze or metal thruster gear legs and metal props, and a poor match for the composite props many thrusters use. Sleipner's SE80 and SE100 IP manual, for example, lists fiberglass-reinforced composite props.

Foulfree

Foulfree is Propspeed's biocide-free foul-release coating for non-metal underwater parts. Propspeed lists thrusters, specifically nylon and composite parts, among its target surfaces, along with transducers and stabilizer fins. A Multi-Surface version extends it to plastics, composites and painted aluminum. For composite thruster props and plastic tunnel parts, this is the coating that was designed for the job.

Interlux Trilux 33 aerosol

Trilux 33 is a copper thiocyanate antifouling with Interlux's Biolux slime booster, formulated to be safe on aluminum. Retailers list it for aluminum outboards and outdrives and all underwater metals, at roughly 21 square feet per 12 oz can. The aerosol is handy for small parts like a thruster gear leg. Mask the anode and seals first.

Pettit Barnacle Barrier 1792

Barnacle Barrier is a high-zinc coating for bare underwater metals: aluminum, steel, stainless and bronze among them. It is aimed at running gear rather than plastic, so it fits metal thruster legs better than composite props.

Anodes

Most tunnel thrusters carry a small sacrificial anode, often on the prop nut or tail cone. Sleipner's guidance is to replace it before every season or when about half has eroded, and to use thread glue on the holding screw so it cannot back off. In some waters an extra anode may be needed to last between haul-outs.

Material matters:

Sleipner sells zinc and aluminum versions for many models, and Vetus lists specific anode sets such as SET0148, SET0149 and SET0151 for different BOW thrusters. Aftermarket makers like Martyr and Tecnoseal produce pattern anodes for Sleipner, Vetus and Lewmar. Pattern parts are fine if the fit and alloy are right; the main thing is to replace them on schedule.

What to use where

• Composite props and nylon parts: Foulfree, or your regular bottom paint if it is compatible with the substrate and the thruster maker allows it.
• Metal gear leg or metal props: Propspeed for long-lasting foul release, or an aluminum-safe antifouling like Trilux 33 if the leg is aluminum.
• Tunnel bore: Thin primer and two coats of the hull antifouling, per Sleipner.
• Anodes, seals, shafts: Nothing. Ever.

Haul-out sequence that works

1. Pressure-wash the tunnel, leg and props as soon as the boat comes out, before growth dries hard.
2. Remove the anode and inspect the prop, drive pin and hub.
3. Scrape and sand the tunnel bore lightly, keeping the coating thin.
4. Mask the seals, prop shaft and anode seat, and keep paint out of the hub tracks.
5. Apply the coating system to the leg and props, following its cure time before launch. Propspeed, for example, lists 8 hours at 80°F and up, 12 hours in the 70s, and 24 to 36 hours in the 50s.
6. Fit a new anode last, with thread glue on the screw.

Weak spots

Thruster props and legs are small, awkward parts, so it is easy to rush them at the end of a bottom job. Every option here needs the thruster out of the water and the parts clean and dry, and foul-release coatings live or die on prep. Copper paints on aluminum invite corrosion. And no coating fixes a missing anode.

Verdict

For most tunnel thrusters, a season-start routine of fresh antifouling on the leg, props and tunnel plus a new anode handles growth and corrosion. Use Foulfree on composite parts if you want a coating that lasts, Propspeed on metal, and keep paint off anodes and seals. Our maintenance checklist puts this in order, and our repair service handles the props and seals when growth has already done damage.

Questions people ask