Originally Posted by colemj
Locking the gear in forward (while going forward) is like parking your car facing down a hill and leaving it in first gear. The turning force will act on the whole gear train and try to turn the engine over. Putting it in reverse locks the gears against the turning force (unless you are sailing in reverse!).
I am pretty sure the gear ratio is not the same in forward/reverse on the Yanmar SD20. I know they are the same on the Volvo 120SD.
Volvo recommends putting the saildrive transmission
in reverse when not using the engines.
I agree with the scenario of parking your car. But I disagree that is analogous to the SD20 saildrive. The SD20 is a different design than a car transmission. I pulled the service
manual/parts list and studied the SD20.
1) The SD20 is the same gear ratio forward and reverse, as Lucky pointed out.
2) The direction is determined by the dog clutch (#44) meshing with either the top gear assembly. (#36) or the lower gear assembly. (#36).
3) The main drive gear is the same part as the upper and lower gear assembly. (#36).
4) All the lower shafts/gears will see no difference in stress whether one locks the prop in forward or reverse since the direction selection is made at the top of the unit.
5) Since all (3) of the upper gear assembly are identical, there is no more or no less stress placed on any of the upper components when locking the prop in forward compared to reverse.
6) The end result is the locked prop will either try to turn the engine in it's normal rotation or reverse rotation. Since the locked prop doesn't ever have a chance of actually turning the engine (just not enough power there) in either direction, it doesn't matter if you lock the prop in forward or reverse.
After examining this, I do agree that locking the prop has no significant wear on the SD20 (in contrast to what the Yanmar dist. told me). The wearing of the ears on the dog clutch due to the extra shifting in/out of gear is so insignificant over the life of the clutch, I'm not going to worry about it.
I like forward as it keeps the shift levers out of the way so I can play with sheets