prop suddenly turning


Dear Amelians,
I am heading for Bermuda and then Marinique..  I just noticed that the propeller shaft was turning when I was sailing and the throttle handle was flat forward as Amel said to dol.  I have no shaft break and an AutoProp propeller.  My boat is SM 216.  I have owned it from new.  There is a placard attached to the boat in front of the shift lever.  The placard says, quite strongly, to always have the shift  in forward when sailing and I have always done this.  Just now, I noticed that the shaft was rotating slowly with the gear in forward and the boat going about 5 knots. It was slow and I could stop it with my foot.  Does this mean that the clutches in the transmission were slipping and wearing?   I don't understand how the shaft can turn when it is in gear and the engine is stopped.
I tried putting it in reverse and the shaft stopped.  Doing this means passing through neutral to start the motor.  In neutral, the prop spins fast in reverse and putting the transmission in forward causes a big jerk on the transmission clutches. 
I don't know what to do and I would be grateful for advice.
S/Y Ladybug, sm216  350 miles NW of Bermuda.

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