First off, Yanmar
engines are required to be put in neutral while sailing or raised out of the water
. It's in the manual.
is famous for glazing over. However, the transmission
fluid should be checked first and you have to understand that the 'check' is to unscrew the dipstick, and have it rest on the threads ... then pull it and read the level.
controls should be checked next by disconnecting the shift cable at the transmission
and shifting the transmission by hand. If it slips then, it's probably the cone clutch
, if it goes into gear
it could be just a worn shift cable or a cable adjustment or someone put the shift cable on the wrong connecting hole on the transmission shift lever.
Lastly, if the fluid level is good and no one put transmission fluid in it (It takes 30 weight oil) the fix is to dismantle the unit and rough the cone up with sandpaper.
Unless you have the service
manual for the transmission and know how to break and reseal the case, you might need professional help at that point.