dovekie
Well-Known Member
Any advice appreciated on this. We have a Lewmar 185TT 12V bowthruster, factory fitted (Southerly Yachts).
1. The unit has (massive) 12V cables to and from the domestic batteries, and a small (4 wire) connector from the control unit at the helm.
2. It has it’s own anode (see pics) and no bonding to the main hull anode which is 7m aft at the stern of the boat,
3. All good for 10 years, except surprisingly, annual inspection showed no pitting or corrosion on the bowthruster anode at all.
4. Last summer, the gearbox simply broke off in the tunnel, leaving just a stub. We removed the stub, it had corroded through rather than sheared we think (no shiny metal).
5. We assumed (perhaps wrongly) that the anode had poor electrical connection to the gearbox, which had ultimately corroded away.
6. Refitted a new gearbox / prop / anode assembly, carefully checking that there was electrical continuity (<0.2 ohms as I recall) between the anode and the gearbox.
7. Launched 23 days ago, and the anode, the locknut, the bolts holding the gearbox face and the gearbox housing itself have virtually dissolved away (pictures).
8. The motor works fine. The boat is generally on a mooring but in that 23 days had 2d on a pontoon with mains hook up. It is fitted with a galvanic isolator (Sterling)
9. The main hull anode is not dissolving away. The boats propellor (an Autoprop) has its own anode which has dissolved perhaps 10% if it’s mass. The internal bonding to the galvanic isolator and hull anode all looks OK, but I need to dismantle more of the boat to thoroughly check that.
The engineers at our home marina suspect that the may be a stray current leak developed in the bowthruster motor, perhaps the actual cause of last summers failure, and then the rapid failure this year. I am inclined to agree, but any thoughts gratefully appreciated. The motor is now removed but on board (6 hours away).
1. The unit has (massive) 12V cables to and from the domestic batteries, and a small (4 wire) connector from the control unit at the helm.
2. It has it’s own anode (see pics) and no bonding to the main hull anode which is 7m aft at the stern of the boat,
3. All good for 10 years, except surprisingly, annual inspection showed no pitting or corrosion on the bowthruster anode at all.
4. Last summer, the gearbox simply broke off in the tunnel, leaving just a stub. We removed the stub, it had corroded through rather than sheared we think (no shiny metal).
5. We assumed (perhaps wrongly) that the anode had poor electrical connection to the gearbox, which had ultimately corroded away.
6. Refitted a new gearbox / prop / anode assembly, carefully checking that there was electrical continuity (<0.2 ohms as I recall) between the anode and the gearbox.
7. Launched 23 days ago, and the anode, the locknut, the bolts holding the gearbox face and the gearbox housing itself have virtually dissolved away (pictures).
8. The motor works fine. The boat is generally on a mooring but in that 23 days had 2d on a pontoon with mains hook up. It is fitted with a galvanic isolator (Sterling)
9. The main hull anode is not dissolving away. The boats propellor (an Autoprop) has its own anode which has dissolved perhaps 10% if it’s mass. The internal bonding to the galvanic isolator and hull anode all looks OK, but I need to dismantle more of the boat to thoroughly check that.
The engineers at our home marina suspect that the may be a stray current leak developed in the bowthruster motor, perhaps the actual cause of last summers failure, and then the rapid failure this year. I am inclined to agree, but any thoughts gratefully appreciated. The motor is now removed but on board (6 hours away).
