Is My Starter Fuse Going To Work?

Although not required by the Coast Guard, some boatbuilders install fuses in the starter circuit -- and sometimes those fuses can't handle the high ampere draw of the starter motor.

24th January 2014.
By Ed Sherman

Question: I bought a new boat at the Annapolis Sailboat Show this year. I’m thrilled with the boat but I sure do have a question about something I just noticed as I was emptying her out for the winter. The photo shows the dedicated battery for the auxiliary engine starter motor. I just noticed the fuse and holder mounted on the bulkhead behind the battery.

Chances are, this fuse blew when overloaded by the starter motor, and the crew simply put both cables on the same stud to complete the circuit and bypass the blown fuse.

Chances are, this fuse blew when overloaded by the starter motor, and the crew simply put both cables on the same stud to complete the circuit and bypass the blown fuse.

It looks to me like the fuse would never blow because the two cables are on the same stud and the electrical current wouldn’t actually flow through the fuse. Am I missing something here?

Answer: No, you’re not missing a thing. If in fact that fuse is for the starter circuit — and I agree it looks like it, based on its location — it would never blow. Looking at the photo at a larger size, it looks like the fuse is rated at 100 amperes. I’ll bet it blew at some point during the boat’s delivery and this was the solution — to bypass the fuse entirely.

Standards do not require a fuse for starter motor circuits because it is a bit of a problem with some starter motor circuits, in that they draw very high current, sometimes over 1,000 amps. The cost of circuit breakers in that rating range is extremely high. For years the USCG has felt that if the wiring is installed properly, the likelihood of a short circuit or over-current situation with starter circuits is quite remote, and that statistical data just doesn’t support the need. Good enough.

That said, some boatbuilders have opted to install either a fuse or circuit breaker in their starter circuits for an added assurance if the worst happens. I suspect that the 100-amp fuse might be just a bit too low of a rating for the starter in your boat and that something like a150- or even 200-amp fuse would be more in order. It looks like the wire gauge used is adequate to support that rating.  Most of the smaller four-cylinder diesel starter motors I’ve tested in the last 5 or 10 years draw up to about 185 amps of current if all is well.

You should get this confirmed by a good engine mechanic before you make the final decision on whether you want to use a fuse or not. Remember, it is not required.

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About the author:

Ed Sherman

Ed Sherman is a regular contributor to, 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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