A lot of mid range engines run 20A fast blow. The following is speculation based on my other engines and a few other things.
The key switch Is fed 12v from that fuse in the engine to operate all circuits that require to be off when the key switch is OFF to prevent battery drain. Exceptions are trim and running lights which are powered directly, not associated with the switch.
In the ON or Start Position the 12v fed to the switch is distributed to key contacts that control certain things, like choke power or starter solenoid power for starting.
The choke or in later engines, Fuel Enrichment, both use an electrical solenoid to move a plunger and effect work. On the choke it sucks up the mechanical choke arm and on fuel enrichment it opens a fuel valve to feed fuel directly to the carbs for starting.
Measuring the 12v wire to these solenoids to engine ground (battery -) will yield resistances of 25-100 ohms range (battery disconnected for safety). If you have zero or a very low ohmic value expect a short somewhere blowing your fuse.....could be within the component, could be in the wiring harness to the component where chafing has exposed the power wire to a grounded surface, metal boat, exposed metal shield on another wire, rubbing on exposed metallic control linkages for steering, throttle, or shifting.
12v/20 amps is less than an ohm for a circuit resistance value that is a fuse blower for sure. If you measure the wiring from the switch terminal to the solenoid and get good resistance, then the switch could be shorted internally which happens over years of usage, and water intrusion. You might also move things around....that get moved when underway, and see if you develop a short on the meter.
On most engines the engines are stopped by shorting out the triggers. The ignition switch in the OFF position is connected to engine ground. Again a source for problems with worn out switches.
