Question: I decided to leave my boat in the water over the winter this year and just had it hauled so we could check the anodes and reapply anti-fouling paint. I am now in total shock! The photo you see here is what is left of my bow thruster after sitting in the water all winter. What is going on?
Answer: Oh my, you had an expensive winter indeed.
What you see here is the net result of electrolytic corrosion, otherwise known as DC stray current corrosion. This is quite unfortunate because it is so easily prevented. The DC positive cable that goes to your bow thruster motor has somehow short-circuited to the motor case, and I’ll bet it completely discharged the battery supplying the bow thruster over the winter. This is one of many reasons why I always recommend a battery master switch shut off for all battery installations, regardless of the power output of the battery. This gives you a way to shut electrical power down completely when you are away from the boat, whether for a day, or all winter.
Bow thrusters are almost always mounted under forward V-berth areas of the boat. This area typically makes a great “catch-all” storage area for things you don’t need to access too often. I’ll bet that in your case something bumped into the positive battery lug where it connects to your thruster motor and bent it into the motor housing. This causes a low level short circuit that unfortunately didn’t provide a good enough electrical connection to blow a fuse or trip a circuit breaker. But it did allow enough electrical current to leak out through the drive leg and electrically eat away the bow thruster. This is a really expensive way to learn the following lesson: keep that locker empty!
The good news is that drive makers these days are starting to electrically isolate the motors from the in the water sections of their drives so that this can’t happen. This is definitely worth looking into, since you are going to have to replace this gear anyhow.
Read more about Storage around Bow Thrusters