Join Date: Apr 2006
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Some transmissions are designed to be cooled by the engine raw water while the engine is running, and they will overheat and be damaged if the transmission is spinning without the coolant flow. So, if the prop is auto-rotating while you are sailing with the engine off---it turns the transmission and damages it. (I think the actual problem is that internal parts are "glazed" from the heat damage, and the fluid itself overheats and is damaged.)
For these transmissions, it is important to put the shift to reverse, so you lock the prop and don't turn the transmission while sailing.
With a folding prop, locking the shaft this way also ensures the prop will stay folded and not flop down to create extra drag, regardless of whether the transmission requires it. (You'd normally mark the prop shaft so you can visually align it to "blades closed, facing sideways" before locking the shaft or putting it into reverse to lock it.)