Bow Thruster Corrosion

Over the last decade more and more boaters have either bought or retro-fitted bow thrusters. In some cases people have had to buy several........... Why? You may ask. DC stray current corrosion to the extreme has been the cause for more than a few bow ...

3rd June 2010.
By Ed Sherman

Over the last decade more and more boaters have either bought or retro-fitted bow thrusters. In some cases people have had to buy several……….. Why? You may ask. DC stray current corrosion to the extreme has been the cause for more than a few bow thruster drives literally disappearing from boats. A prime example of what happens is shown in the photo below, which I took on a boat that was less than two weeks old.

As you can see the drive propeller is completely missing and the gear housing looks like a giant sea monster took a bite out of it. DC stray current is the fault here and it could have been prevented quite easily with a little knowledge and care. You don’t want this to happen to your boat so follow the tips here and you’ll be able to miss out on this rather expensive adventure. The picture below, of the same boat gives some clues as to what happened here:

In the photo above you can see the bow thruster motor in the center. What you can’t see is that the boat’s air-copnditioning ductwork has been moved out of the way to get the photo. This whole installation was located in a very crowded compartment under a berth with a simple lift up cover that was just inviting the owner to throw some extra gear in the space available. This particular boat lost it’s thruster drive because in the installation process someone short-circuited the DC positive feed wire (shown disconnected) to the case of the motor. Now on this particular thruster motor, the case of the motor was not electrically isolated from the drive leg outside the hull. The net result of this was that once the drive anode depleted, which probably took about 24-48 hours, the drive housing was the next bit of metal in the corrosion food chain.

So, considering that the vast majority of bow thrusters are located similarly to the one above i.e. under a berth, often with lots of convenient storage space around it, you need to be extremely careful not to bump into the wiring that supplies the thruster motor. I recommend building a cover of thin plywood to fit over the motor and wiring assembly, or in some way building an isolation barrier between potentially useful storage space and the thruster motor. This problem is all too common, and really expensive to fix. It may or may not be covered by insurance and typically won’t be covered by any warranty.

The bottom line here is simple, keep those bow thruster motors protected so that nothing can bump into the power cables supplying them. 


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Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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