The previous engine was a Mercedes 36HP - Marinized by Solè in Spain
The new engine is a Volvo
The propeller is three blade, the same propeller fitted before, checked by a specialist, not worked-upon, only re-fitted.
discharges from the same above waterline outlet as before.
at which the engine run when experiencing the unprecedented propeller noise
was in the same range (1500-2200) as with the previous configuration.
I must add that the Volvo, in a newly soundproofed engine room, is remarkably silent.