Now, why do engines all turn in the same direction?

[Bernard Littau]

Quite a large number of private yachts are twin engined.  Many of these have
engines that rotate opposite to each other, in order to get the best
propeller action with a minimum of loss.  You don't want to add an extra
gearbox or gear to make the props counter-rotate, so you build the whole
system with one side the mirror of the other.