Distinguishing Left-Hand From Right-Hand Propellers

Every now and then a boat owner, boat yard or diver mistakenly installs the left propeller on the starboard side of the boat and vice versa. Besides totally changing the handling characteristics of a twin inboard boat, this also reverses shift levers. Pushing levers into forward makes the boat go backward. Plus, it's downright embarrassing.

So how do you tell a left-hand propeller from a right? The first step is to face the propeller from the aft end, where the nut would hold it to the prop shaft. Since propeller shafts are tapered, props have a smaller opening on the aft end and a larger opening on the front, nearest the strut.

Looking at that smaller opening, cover it with the palm of either hand. If you can hide your left thumb behind a blade, it's a left-hand propeller. If you can hide your right thumb, it's a right-hand prop. (Because of the angle of the blades, you won't be able to hide your right thumb behind the blade on a left-hand prop.)