How do I free a bow thruster propeller rubbing against excess antifouling?
My bow thruster isn't spinning freely because there is a bit of extra antifouling paint in the way inside the tunnel. I painted it quite thickly this year, and now the blades are rubbing against the tunnel wall. How do I sort this out without damaging the propeller or the leg?
Answer
To free a bow thruster propeller binding on excess paint, drop the unit off the splined shaft and physically sand back the tunnel wall to restore the factory clearance. Never rely on the thruster motor to wear away the paint, as friction will overload the electrical circuit, blow the main ANL fuse or burn out the motor relays.
Preparation and safety
Before reaching into the tunnel, completely isolate the thruster by turning off its dedicated battery switch and pulling the control fuse. If someone accidentally touches the helm joystick while your hands are in the tube, the torque will cause severe injury.
Trying to power through the paint is a common boatyard mistake. A jammed prop pulls massive amperage and immediately blows the main thruster fuse (usually 250A to 400A), costing £30 to £50 to replace. The sudden resistance can also shear the stainless steel drive pin inside the gear leg.
Removing the propeller
To clear the paint properly, you need the propeller out of the way. Most popular thrusters from brands like Side-Power (Sleipner), Vetus, Lewmar and Quick share a similar assembly sequence.
- Remove the anode: The central propeller nut hides behind a conical zinc or aluminium anode. Undo the retaining screw (typically a 4mm or 5mm Allen key) and pull the anode off.
- Undo the prop nut: You will reveal a central nyloc nut, usually 13mm or 17mm. Wedge a block of softwood between a propeller blade and the tunnel wall to stop the prop turning, then use a socket wrench to undo the nut.
- Pull the prop: Slide the plastic propeller straight off the splines.
- Save the drive pin: Watch out for the stainless steel drive pin located on the shaft behind or inside the prop hub. Remove it safely so you do not lose it in the gravel under the boat.
While the propeller is off, inspect the shaft lip seal for wrapped fishing line. This debris is a notorious cause of gear leg water ingress and subsequent gearbox failure.
Clearing the excess paint
Thruster clearances are deliberately tight to maximise water flow and thrust, leaving a gap of just 2mm to 4mm between the blade tip and the tunnel wall. Standard ablative (self-polishing) hull antifoul is thick and builds up over the seasons to rapidly eat up this gap.