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1- Yes, fuse the starter circuit. It is an ABYC "exception" to the rule not saying to not do it. With small sailboat aux engines there is no reason not to fuse these circuits. Just last week a customer called me while on a cruise. His starter button stuck and was keeping the solenoid engaged. He is very aware and shut down within seconds. If he was not so aware an engaged starter could have caused massive issues and the potential for a fire his starting battery is fused..
This customer was very happy I talked him into a starting circuit fuse:
This was what happened to one of our Yacht Clubs junior racing program boats. Just seconds before the battery cable erupted in flames there were four kids under 8 years old and a 17 year old instructor on-board. It almost burned our entire fleet of Opti's, our 420's and our docks. These boats all have fusing now.... Even the simplest boats can suffer dead shorts....
2- With boats and battery wiring my simple rule is the bigger the wire the better. I happened to be at the Landing School one day when Roger H., the systems instructor, was demonstrating the differences between battery cables on motor starting, voltage drop etc. etc.. It was an amazing demonstration of the differences between small and large battery cabling.
The smallest wire I will use on an inboard diesel is 1GA. The engines just start better and with less voltage drop to the starter. The "circuit" is the full length of all the battery cabling not just the + wires and some of these starters are loading the wire with 200+ amps of current... I have to remove far more starters for re-build on boats using 4GA battery cable than I do on boats using larger wire. I can't recall the last time I had a Sabre starter re-built (they used 1GA wire or larger) but I have done three Catalina's (4GA wire) and two O'day's (also 4GA wire) in the last three months. Catalina began using 1/0 wire back in the late 90's.. Cape Dory was also known for "dinky" wire and I have to get many of those starters re-built too. This is pretty typical. I find that boats wired with small wire chew through starters faster than the boats I work on with larger wire. It is enough so that I don't find it to be just happenstance. It also leaves you little room for adding inverters, windlasses or other high draw items when you have small battery cabling..
A recent CD-36 was tripping the windlass motor. The windlass wiring was the proper size but the rest of it was 4GA and of a pretty decent circuit length..... Re-wired the bank and battery wiring with 2/0 wire and the motor literally jumped to life and the windlass never again tripped. Owner actually asked if I had put a new starter on the Perkins 4-108 because the difference was that dramatic..... Most engine manufacturers want to see less than 3% voltage drop on the starting circuit because the batteries also "dip" when hit with the in-rush and starter loads. 4GA wire, even on a very short circuit, can be tough to hit 3% with. Even just a 12' circuit, a pretty common circuit length range on many boats, will yield a 5+% VD on 4GA wire. That same circuit with 1GA wire is 2.5% VD...
4GA can work but the circuit would need to be really short and you'd be limited in what you can add to it. Small wiring also makes fusing a bank for starting very tough without exceeding the wires max ampacity rating and accommodating starter loads at the same time. Larger wire has more benefits than just voltage drop...
