Despite extensive reading on this subject my knowledge is little and often contradictory, but as I understood it it is unusual to overprop to the extent that the engine achieves less than 80% of max rpm.
Reasons for overpropping include matching the power curve, better fuel consumption, lower rpm for a given speed (and hence quieter engine)
Reasons for not overpropping include not being able to get full power from the engine (though many narrowboats are overengined anyway) and being less likely to overload the engine in trying to get to max rpm.
I am shopping for a propeller at the moment and would expect the supplier to discuss with me the ramifications if deliberately over propping.
I take it the Op's propeller has been on from new and has never done more than 2000rpm?