| "Expensive way to find that the breakers don't always do their job."
Breakers are installed to protect the wiring, not the motor. The breakers have to be sized for the wiring and to handle the starting load. Moderate overloads for extended periods can burn up a motor without bothering the supply wiring, as you discovered.
That is why motor starters and controls have "heaters" before the fuses or breakers, they respond to long term overloads the same way the motor does and provide a trip. Other small motors might have thermal switches to open the circuit when the motor overheats.
Like someone else said, it's corrosion of the contacts that cause the heating on the boatside shorepower connection, not overloading.
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