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Troubleshooting
Bow thruster barnacles: why it fouls, how to keep it clean
A thruster tunnel grows barnacles faster than the rest of your hull, and the growth slowly costs you thrust. This page explains why it happens, which paint to keep off the thruster, and what to check at every haul-out.
At the stern. A stern tunnel thruster is the same kind of unit as a bow one, and 61 of the 140 tunnel models this site lists are sold for either end. So it is covered by the tunnel rules below. The units made only for the stern are external, from Max Power, Sideshift, Sleipner Side-Power, Yacht Thruster and ezDrive, and the sections below on the other kinds cover the Sideshift and DockStar ones.
The short answer
Unless the thruster is running, the water in it barely moves. Still water in a sheltered spot is exactly what weed, slime and barnacles want.
The zinc anode fights corrosion. Antifouling paint fights growth. Neither one can stand in for the other.
VETUS says not to use copper-based antifouling on or near it. Copper sets up corrosion that the anode then has to fight.
It happens so gradually that the first sign is usually a change in feel. On a retractable, growth can stop the unit coming down at all.
It is lifted clear of the water on its track when you are underway, so it does not foul.
Your haul-out routine in four steps
- Clean the tunnel and propeller. Clear whatever has grown since the last haul-out. The section Why a tunnel grows worse explains why there is more in there than on the rest of the hull.
- Check the zinc anode. Replace it if it is significantly eaten away. It protects against corrosion, and corrosion and growth are two separate problems.
- Paint with the right antifouling. Use the copper-free type if your thruster's manual calls for it. The section The antifouling that can do more harm than good explains what copper does beside a zinc.
- On a retractable, check the hatch. Make sure nothing has grown into the path the unit comes down through. The section What it is costing you explains why a jammed hatch is the worst case.
The same list, with a link to every maker's own schedule, is in what to check at every haul-out.
Why a tunnel grows worse than the rest of the hull
You Lifting the boat out of the water on a travel lift or a trailer so work can be done below the waterline. It is billed separately from the job itself. and the tunnel is caked. The propeller looks like it is wearing a coat. If you noticed anything before the haul-out, the thruster felt a little weaker than it used to.
None of that is a defect in the thruster. It is what happens to any surface that sits in the water with nothing moving across it. A thruster tunnel is one of the worst spots on the boat for it.
Marine growth, what a yard calls Weed, slime and barnacles growing on anything that sits in the water. Inside a thruster tunnel it quietly costs thrust, and on a retractable it can stop the unit coming down., needs sheltered still water and a surface to grip. Most of your hull gets some relief from that when you are underway, because the boat is moving through the water.
A thruster A tube bored straight through the hull below the waterline, with a propeller in the middle of it. It is the conventional thruster. Fitting one is mostly a labor job rather than a hardware one. gets no such relief. Unless the thruster itself is running, the water in it is as still as water gets on a moving boat. It is dead water, and dead water is what weed, slime and barnacles want.
Every thruster that sits in the water is affected. A fixed external unit stays in the water full time. It fouls on the housing and the propeller like any other underwater fitting, and Sideshift’s own rule is to scrub it as needed.
A tunnel thruster is worse. The tunnel stays dead water even while the boat is underway and the rest of the hull is being flushed clean.
Corrosion and growth are two separate problems
Both get dealt with at the same Lifting the boat out of the water on a travel lift or a trailer so work can be done below the waterline. It is billed separately from the job itself., which is probably why they get mixed up so often. They are separate systems, and one does not do the other's job.
Corrosion protection is a zinc A block of zinc or aluminum bolted to the thruster that corrodes on purpose, so the seawater eats it instead of the gearbox. It is replaced when it is about half gone, usually once a year.. Underwater metal parts on a boat are always trying to corrode. A sacrificial zinc anode gives that process something cheaper than your thruster housing to eat first. VETUS ships a zinc anode with its bow thrusters for this. It has nothing to do with marine growth.
Growth protection is antifouling paint. Either a biocide (a poison to the growth) or a slick coating it cannot grip keeps barnacles, weed and slime from getting hold in the first place. It has nothing to do with corrosion.
You need both. Skip the zinc and you get corrosion, however good your antifouling is. Skip the antifouling, or use the wrong kind, and you get barnacles, however healthy your anode is.
The antifouling that can do more harm than good
The instinct at haul-out is to treat the tunnel like the rest of the hull and use whatever antifouling is already in the bucket.
VETUS's own manual says not to. Do not use copper-based antifouling on or near the bow thruster.
Copper-based paint on an underwater metal fitting sets up Metal eaten away because two different metals sit in seawater connected to each other, or because stray current leaks into the water. It is the damage an anode exists to absorb. Most warranties exclude it. between two different metals. That is the very problem the zinc anode a few inches away is there to fight.
Pair the wrong paint with the zinc and you can eat the anode faster than normal, or corrode the housing anyway. The anode then gets blamed for a job the paint made harder.
Check compatibility before you paint the tunnel. The tunnel's own material changes which paints suit it. It can be steel, fiberglass (GRP) or aluminum, and it varies by model.
A copper-free antifouling made for underwater metal fittings is the usual default. If you are not sure what is already on the tunnel, or what the manufacturer allows, ask before the next coat goes on.
What it is costing you
Growth inside a tunnel narrows the tube and roughens the surface the water has to move through. A propeller working through weed, or a caked housing, is pushing against more than it was designed for.
Nobody publishes a figure for how many pounds of thrust a season of growth costs. The direction is not in question, though. It costs thrust, and so gradually that you notice a change in feel long before you think to check the tunnel.
On a A thruster that lives inside the hull and drops down through a hatch when you press the button, then pulls back up. It leaves no permanent hole in the water flow. thruster it can go further than a slow loss. The unit drops down through a hatch in the hull when you need it. Growth around that hatch, or on the mechanism itself, can stop it coming down at all.
A fixed tunnel gets weaker. A fouled retractable can stop working.
Why a unit that lifts out of the water skips all of this
An external unit that lifts clear, like DockStar, rides on a track at the bow or on the The flat surface across the very back of the boat., and it stays on that track almost all the time. You lower it into the water to dock or leave the dock. Once you are underway, you lift it clear of the water on its track with a rope and pulley, so it does not foul.
If barnacles and haul-out costs are pushing you toward one style over another, that counts in its favor. It has trade-offs elsewhere. You raise and lower it by hand, and each one is custom made for its boat.
The thruster finder, our free tool that matches a thruster to your boat, lays those trade-offs out against every other style side by side.
External units stay in the water: the Sideshift rules
An external unit has no tunnel, but it sits in the water all season with its propellers exposed, and it grows the same beard. Sideshift’s manual gives four rules:
Clean it with a scrub brush as needed. Never pressure-wash near the joystick or gauges.
Antifouling is optional, recommended where growth is a problem, applied to the paint maker’s instructions, and never over the sacrificial anodes.
Look at it every few weeks while the boat is in use, for paint failure or corrosion on the motor and propellers. Anodes going fast, or corrosion appearing, mean stray current, and the manual says act at once.
Replace the anodes at least once a year. If the motor ever comes off its mount, fit a new strap kit so two different metals do not touch.
The motor itself is sealed and needs nothing. DockStar, which lifts clear of the water, asks for less still: a freshwater rinse and a wipe with a silicone-dampened cloth, weed and line cleared off the propellers, and a silicone spray on the track. Its user manual is linked from the DockStar review.
Retractables: the leg, the hatch and the yearly strip-down
A retractable tucks its propeller into the hull between uses, so it fouls less than a tunnel. Even so, the makers ask for more work, because the leg and hatch add moving parts that growth can jam.
Lewmar, in its retractable manual: check the anode after three or four months on a new install to learn its rate; then at the annual service clear debris from tunnel, propeller and hub, replace the anode, replace a damaged or heavily fouled propeller, grease the exposed seal and shaft, and fit a new tunnel gasket if the hub has been off.
Quick, in its BTR manual: dismantle once a year, clean propellers, tunnel and gear leg, replace the anodes, grease the CV joint, check for water inside, and never paint the anodes, the seals or the shafts the leg pivots on.
What to check at every haul-out
The zinc anode. Check its condition and replace it if it is significantly eaten away. It is corrosion protection, and it belongs on this list because it is the same trip up the ladder.
The antifouling coat. Confirm it is still intact. If your thruster's manual calls for copper-free paint, confirm that is what went on, rather than whatever was left over from the The sides of the hull above the water..
The tunnel and propeller. Clear whatever has grown since the last Lifting the boat out of the water on a travel lift or a trailer so work can be done below the waterline. It is billed separately from the job itself.. Otherwise it keeps costing thrust, and you only notice once the loss is significant.
On a retractable, the hatch and mechanism. Confirm nothing has grown into the path the unit deploys through.
None of this is unique to thrusters. It is the same care you give zincs anywhere else on the boat. The thruster is simply the underwater fitting that is easiest to forget, because it sits inside the hull, out of sight.
The maintenance page has every maker’s own schedule, and the anode page covers the anode on its own.