The ratcheting is the overrun clutch in the lower unit and involves the prop shaft, clutch dog around the shaft, and the F and R gears. When decelerating, any time the engine rpms are lower than the corresponding rpms required to turn the prop at it's current rate, the overrun clutch allows the prop to free wheel. Were it not for that, when you cut the throttle, your prop revs would cut an equal amount and the effect would put your teeth into your windshield if you had one. Some engines have an overrun clutch in R gear. Mine does not and as such, in R, the prop will turn the crankshaft in either direction. Never figured out why some do and some don't have one in R. If it's necessary, which I doubt as you don't go all that fast in R, then why don't all have it. If not, then why do some have it.
Back in the early days of farm tractors with transmission driven Power Take Offs, if you didn't have an overrun clutch on your mower, when you cut the throttle, or pushed in on the clutch, the energy stored in the turning blade and drive mechanisms associated with the mower would drive your tractor and it would wind up in a fence, in the side of a barn, in the pond, or who knows where. With the overrun clutch, as soon as you cut the throttle on the tractor it is isolated from the turning blade of the mower allowing you to stop, shift, or whatever manuever you chose.
Mark
