Unless I missed it in Mainesail's post, you do not need a fuse in the positive conductor from the starting battery.
If this "not needed" is defined by the ABYC's "exemption" for cranking batteries then it is pretty foolish to abide by that on small sailboat aux engines that are EASILY over current protected.
Sometimes I agree with the ABYC and in this case I strongly disagree with the E-11 on this. Why? Because it is foolishly & needlessly applied to all boats and is very often misinterpreted as you should not fuse a starting bank
which could not be further from the truth or intent of that exception... There are many of us on the electrical committees who would love to see that wording changed.
So yes the ABYC has an "exception" to this rule for cranking motor batteries. This exception however is more broad based and written to include for large engines which have massive amounts of starting current being drawn. These engines are very expensive to fuse properly hence the "exception".. Think big sport fishing boats, and our engines are about as far away from that as can be... Small diesel or gas AUX engines on sailboats would be best served fused than unprotected.
I quote our good friend Nigel Calder here:
"The net result is that nowadays, electrical shorts are probably the number-one cause of fires on boats.
There is simply no excuse for not protecting all high-current circuits , including the cranking circuit."
Nigel Calder Cruising Handbook
The ABYC also refuses to address the vast majority of boats out there that are fed by a 1/2/BOTH switch where either bank can be house or start with the flip of a switch and NEITHER bank is really considered a "cranking battery". In these circumstances one would be very wise to protect both banks.
Relying on the ABYC exemption in this case is a cop-out and can be flat out unsafe....
Here is a good case study of where this foolishness fails us:
This was a junior sailing program chase boat used at our club. It was a 15' Whaler with a Honda OB motor.
- Because the battery on this boat was not more than 800CCA it was exempted from needing a battery switch. D'oh.. Okay if the ABYC says so.....
This battery was ABYC exempted from over current protection because it is a "cranking battery".. D'oh......
Because of the failings of the ABYC standards in this instance three 8 year olds and a sailing instructor were almost burned up.... Safe..?
A simple $30.00 fuse or battery kill switch could have prevented this yet this boat was ABYC compliant....
Yes a single battery less than 800CCA (exempted from a battery switch) and one that "cranked" a motor (exempted from over current protection at the battery) caused all this....
So yes I often exceed ABYC standards in my wiring because the standards are often quite insufficient or too broadly applied.