No, I don't think they require you to put a fuse there if it is the standard alternator charging
the starter battery
(but they do when you charge house batteries). Then again, I am no expert on ABYC. In fact, I think they impose no requirements at all, just suggestions.
I have 3 alternators on my engine
: the standard one which charges the starter batteries
(and uses the same cable as for starting) which has no fuse, and two extra ones that charge the house batteries, which do have a fuse each. And the boat
has the ABS approval so I guess it's okay.
On a down to logic level: the wiring
between starter battery (both positive and negative) and engine should be rated to easily carry the starter current
and also easily carry the alternator output current
. The alternator isn't going to output twice as much suddenly, but if you change to a high output alternator you might need bigger wires (which is forgotten sometimes).
Your 40A alternator doesn't sound like it could fry the cable that also starts the engine so you should be just fine without the fuse.