Generally speaking, torque increases are associated with increased stroke where horsepower increases are associated with increased piston surface area. Running

Generally speaking, torque increases are associated with increased stroke where horsepower increases are associated with increased piston surface area. Running a ship engine with a long stroke (to increase torque) serves to reduce stress on the parts and increase lifespan since these engines have very long life expectancies. Also, you are correct that the lower rpm engine would deliver more efficiency but not necessarily because it is larger/runs slower. It is more efficient because a smaller engine running at a higher rpm would have to work harder to generate the same power to move the ship and its load and would have the undesirable side effect of sacrificing longevity.