Well, a 3-1 ratio (or any ration) is just gearing At any given RPM
3 engine turns to 1 prop turn would give more torque than 1-1 but not more horspower, likewise 3 prop turns to 1 engine turn gives less torque but not less horsepower. the power produced is your constant the torque is the variable. Yes, there is some lost
horsepower in the transfer but not significant I believe for our applications.
There are two main reasons for the bigger engines, 1. Heavy boats regardless of displacement
require gobs of torque which is more easily produced from lots of cubic inches at low rpm's than from a smaller engine at low rpm's (think of cubic inches in terms of displacement) so a big engine running de-tuned can economically produce lots of torque
2. It is more economical to produce engines of any kind in quantity, so by detuning the same engine that would plane out a 45' Sports Fish
to power your 36' displaement trawler
makes sense to the builder
. the same engine can be used with/without turbos, intercoolers etc. and serve both purposes. also the great majority of diesel
engines come from a modification or slight redesign of an existing power plant and in the Western world that more often than not, means bigger from the start.
Just my $.02