When I was little, my dad had a yacht which had been a northern European coastguard patrol vessel. He bought it as surplus military and converted it into a yacht.
The engine, a Stork Werkspoor diesel, well that was quite a story. It was massive, almost 6' tall, with a gigantic flywheel.
Each cylinder had a shelf in the casting, on which you placed a blow torch. They had to be lit about half an hour before you wanted to start the engine. When the individual cylinder heads were red hot my dad stood on a handle and pushed himself down from the roof, taking real good care the flywheel was positioned in just the right place, or it would kick back (I tried, it almost launched me through the roof).
That engine drove a massive 5 blade prop with variable pitch, so no gearbox, you adjusted speed just like an aircraft, by changing the pitch of the prop.