But I also know a lot of sailors who do the same thing and have never heard of one who had a transmission problem as a result.
I know of many who have had to rebuild Kanzaki gear boxes but don't know the causes in each case.
My main concern with a freewheeling prop while sailing is unnecessary wear on the entire drive train.
If the gear box is splash lubed then it is fine to let it spin and Yanmar stands behind this.
Cutlass bearings I have seen go 6000 engine hours before needing replacement and as much as 40,000 nm boat travel distance.. Alignment issues are usually the culprit in cutlass wear as they do not allow a water/film layer between the shaft and rubber.
While Yanmar may recommend leaving the transmission in neutral when sailing,
(1) I would like to read or hear their views of the additional wear this causes, and
I have spoken with them at length about this and also seen the cones of a gear box that needed re-building due to locking the gear. The guys who report they can't get it "back into neutral" "without starting the engine" are specifically why this MSA was created.
If it locks up you should never "force" it back to neutral and should always start the motor in reverse then switch to neutral. Yanmar does not make these MSA's on a whim. Reverse is used so limited that wearing out the cone is rare from locking but the prop does not simply load up the cone and always bury it into the reverse gear especially at low sailing speeds. It can cause the cone to chatter in the gear which can cause undue wear.
The cone is an aluminum bronze alloy and the reverse gear is iron. The cone gives not the reverse gear. After a while of this loaded chattering the cone can wear to a point where you may have problems. When you put it in reverse the motor can tend to feel like it is "hopping" because the cone is slipping until it has been driven deep enough into the gear to lock up and stop slipping. This MSA is more about the problems surrounding the ability to not get it out of gear than anything and the resulting damage that can ensue if trying to force it back into neutral.
(2) I'd like to read or hear what the makers of the transmission have to say about this issue as Yanmar doesn't make transmission, I believe.
Unless you have the rare Yanmar that has a Huth/ZF box, which you can lock in reverse, or a hydraulic Kanzaki box which must have an external shaft locking device, then the Kanzaki box is a Yanmar product and they are the manufacturer.
Finally, if any one out there has a credible story about how his/her transmission was specifically damaged by locking it in reverse while sailing, I would hope they would post it here.
This was a cone clutch from a full keel boat. They had a fixed two blade they locked in reverse. The motor began bouncing violently when put in reverse as the cone was apparently slipping and not fully engaging. The cone was worn according to the re-builder on the reverse side of the cone. They usually see the forward side worn. Motor had 1400 hours.