mercruiser (351 Ford) inboard shut down while running, now blows 15 amp start fuse! help!!!!!

I have a 1975 Bertram 28 with twin 351 cid Ford V-8's, Mercruiser original set-up. Engine has old style ignition system (distributer with points, condensor), mechanical fuel pump, 4 barrel carberator etc. The port engine shut down while running this past summer, just stopped...no noises. I found the 15 amp fuse from the ignition switch blown. When I try to restart, fuse blows everytime. Checked starter by jumping it and engine cranks so not seized-up, on-off start toggle switch at helm, both seem fine and also replaced solienoid. Haven't checked coil, alternator or main 30 amp resetable circiut breaker, etc. Not sure where to start with this problem. Don't think it's a wiring problem or short, just my hunch. Was thinking of pulling parts from the starboard engine to isolate problem, with the exception of the starter because of reverse rotation. Can anyone give me some direction? HELP!!!!!

I found the 15 amp fuse from the ignition switch blown.

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Ayuh,... What's covered by this fuse,..??

I can't think of such a fuse in the starting circuit, or the ignition circuit...

The fuse is located in the main 12 volt electrical panel in the vee berth. The bertram wiring diagrams shows that 15 A. fuse line goes to the "on-stop" switch at the helm. From the the PORT shunt a #8 red wire goes to that fuse/switch. The shunt connects to the load side of the master "on-off" switch. Everything then comes off the master switch...solenoid, starter coil, etc. The shunt (load side) #8 red wire connects to the alternator.
Don't know why when I turn on "on-stop" switch fuse blows immediately. I don't even get a chance to push the momentary start button! This all started with the Port engine shutting off while underway, just died blowing the same 15 amp. fuse. I thought short first, but find no evidence of that. I thought faulty "on-stop" switch (no power, no spark). I thought coil may have shorted out. Not sure how alternator plays into this whole thing. I thought maybe main on-off switch is bad.
You may have determined I'm not good with 12 volt electrical systems and have been unable to get a mechanic involved because of the current season. Any advice would be much appreciated!