With this post, the Boats Blog welcomes a new author, Ed Sherman, lifelong student of boats and their systems. In addition to writing for numerous magazines and blogging at EdsBoatTips, Ed works for the American Boat & Yacht Council on a variety of projects, including developing its certifications curriculum. Click on his byline, above, to read more about Ed. If you have a follow-up question for him on this topic, leave a comment; otherwise, drop me a note c/o firstname.lastname@example.org and I'll pass it on to him.
—John Burnham, Editor
Earlier this summer a friend of mine, who I consider a very knowledgeable boater, came to me with this rather interesting experience and question. What happened to his boat could happen on yours, and the problem is really easy to correct:
Recently a strange thing happened on my boat. We were in our slip having dinner on board and could smell a strange odor, sort of like plastic melting. Finally, we saw some smoke coming up from around the perimeter of a lift out cabin sole panel. I lifted up the panel and saw that my bilge pump was burning! I had a fire extinguisher right near the galley stove and put the fire out quickly with no real damage to anything but the bilge pump itself. My question is, if the bilge pump had some kind of electrical short circuit in it, how come a fuse didn’t blow?
In this case we are dealing with what the American Boat and Yacht Council refers to as a “locked rotor” condition on the bilge pump motor. With any motor circuit, the locked rotor condition creates an unusual electrical response that boils down to one of the electrical fundamentals all ABYC-certified electrical technicians are taught.
A locked rotor can be caused by a piece of debris getting lodged between the pump housing and its impeller, or maybe a motor armature bearing that simply wears out and seizes so the motor can’t spin. In this case I’m sure the problem was caused by some piece of debris that had been drifting around in the bilge for who knows how long.
What happens electrically is that the motor will try to turn over but since it’s jammed, it can’t and it converts the energy being supplied to into the only thing it can, heat. Initially however, there will be a spike in current draw as the motor tries to turn. This will blow a properly rated fuse, but may not blow a fuse with a higher rating due to engineered trip time delay curves that are designed into all fuses and circuit breakers. Heat adds resistance to an electrical circuit in most cases. So, if resistance increases, amperage flow decreases. Amperage is what blows fuses and trips circuit breakers. If the fuse installed is not passing enough amperage, it will never blow and current will just keep flowing. Eventually, things like bilge pump motors can actually catch on fire as in this case.
The simple solution is to be absolutely certain the fuse in this circuit is not rated at any more than the motor manufacturer recommends. Most all of the small DC motors I’ve checked out in recent years have a maximum fuse size printed on a label on the motor. I would add that the vast majority of boats I check have motor circuit fuses that have too high an amperage rating, and they will not blow in a locked rotor situation.