Auto rotation can be initiated by many things  from fouling of the blades - barnacles etc to picking up a piece of seaweed on a blade.

It is for that reason we recommend leaving the unit in forward once feathered to prevent any further rotation.

Reverse should always be avoided because if the unit - or any type of prop - fixed / feathering / auto prop were to engage and slowly rotate the motor over as compression was lost in each cylinder in turn it is possible for the engine to suck back seawater from the exhaust lock …   enough said.

BACKGROUND:

Yanmars SD20's and early SD30's have dog clutches. Simple cheap robust but a noisy crash engagement.

Volvo's started with cone clutches in their 110S & 120S  legs and gearboxes and have hold untold trouble to the extent they have produced the 130S & 150S - which contain normal multi plate clutch packs

Yanmars SD31, SD40 SD50 all contain cone clutches and again have been a nightmare for them warranty wise and have just in the past few weeks introduce the SD60 with multi plate clutch packs.

Just google " Yanmar or " Volvo  Saildrive clutch " to get an extent of the problem.

Nanni who used to use Volvo legs now use Twin Disc who offer the SP60 with clutch packs.

ZF offer a Saildrive - again with clutch packs.

Bukh switched from cone clutches to multiplate.

The net effect is that cone type clutches in the marine environment do not work well over long periods of time. The salt in the air and seeping past the seals is sufficient to destroy the metal surfaces of these types of clutches. In use operation can also lead to glazing / wear / polishing - all sufficient to cause problems.

Now think of the propeller types out there:

Fixed                          Will rotate when sailing unless locked in gear

Folding                      Will not rotate once folded

Autoprop                   Will rotate when sailing unless locked in gear

Max-prop                  Will rotate when sailing unless locked

Kiwiprop                   Will not normally rotate  - can under exceptional conditions

So couple these two issue together and we conclude:

If you have a Yanmar SD20 leg - it is impossible to hurt the clutch because it effectively doesn't have one. So leave it in forward to prevent any rotation.  Who wants the noise of constant rotation of the drive when sailing..

If you have any of the cone type clutch units - leaving it in neutral will allow rotation of the driven vs driving clutch faces of the cones and this will over time glaze  / polish / wear the metal surfaces and cause clutch failure if you have a fixed / feathering type clutch.

Again - can be solved by leaving in Ahead.

On this issue we feel sure there is a lack of understanding of the options out there and how detrimental constant rotation is to a cone clutch. It was not envisaged to be so when the units were announced for low powered applications.

So in all cases - including new multipack units engaging forward will stop wear on these clutches and prevent a lot of the many serious and ongoing problems both Volvo and Yanmar have had - sufficient to replace their cone type units.

So it is of the above reasons we maintain our recommendation - leave it in Ahead when sailing after the unit has feathered. This is particularly relevant to high usage sailors on long passages of course.

Each of the manufacturers will have their own recommendations - most say leave in neutral and you will need to form your own opinion on this but we would argue they have not thought it through correctly to accommodate all propeller types.

We have spent many long hours on the issue of leaving the unit in Ahead when sailing to lock the shaft and prevent wear while understanding the issues that the different propeller types have on saildrives and on shaft installations.

In this case perhaps  the manufacturers don't know best as they simply did not have the information or reflect upon real world operating conditions and the multitude of options not available some 30 years ago.

Unless there is some fouling or weed caught up - Kiwiprops will remain feathered in neutral any way - so putting them in Ahead ( If that locks the gearbox ) is simply extra  insurance.

This situation is somewhat complex and there is no simple straightforward answer to all scenarios  - but we have to deal with the real world where there are no yes / no answers to these situations where there are so many variables.