Re: Prop is spinning freely

Danny and Yvonne SIMMS

A great reply Bill. Best explanation Ive seen. However I am going to drop a curly one here. I have a Volvo and a ZF 25M gearbox and an auto prop.For reasons I have not discovered it will sometimes lock in reverse, sometimes in forward. I have to observe the shaft spinning and engage one or the other. The correct selection will exhibit a satisfying clunk and the shaft stops dead. The wrong results in a soggy almost stop, in which case obviously I change to the other. It is random, so I just take care and always check.

Kind regards and wishing you all the best for the new year.


SM 299

Ocean Pearl

On 11 January 2021 at 14:18 "Karen Smith via" <karenharmonie@...> wrote:


I think you are mixing up the ZF25M transmission and the ZF25. The document you reference is for a ZF25M, not the ZF25.  This has been a common source of confusion on the forum, since both of these are the common transmissions used in many Amel boats. The similar model numbers for two very different machines has always been an issue.

This is my understanding of this un-necessarily complicated issue:

The ZF25M is a smaller, lower horsepower rated, transmission that shifts mechanically. I believe none of these transmissions were ever fitted with a shaft brake, because none was needed.  The ZF25, even though it has a very similar part number, is a very different beast.  I doubt the ZF25M and the ZF25 have any parts in common.  The ZF25 is a higher horsepower rated, hydraulically shifted, transmission.  As far as I know (and this might not be correct) the Volvo TMD22 engines all had ZF25M and the Yanmars all had ZF25.  Note that these model numbers changed when the original Hurth brand was bought out by ZF.

As the document you quoted points out, the ZF25M can free-wheel without damage, other than routine wear on bearings and seals. With a normal fixed prop or a MaxProp, putting it in reverse will lock the rotation.  The AutoProp is a different story.  It rotates in the opposite direction when free-wheeling, and needs to be put in FORWARD to lock it.  The rule for operation should be “Do not allow the transmission to free wheel while in gear—ever.”  If the prop rotates while in reverse, put it in forward.  And the other way round.

The ZF25 relies on the hydraulic pressure generated by the rotating input shaft (from the engine) to shift.  If the engine is not turning, it will always be in neutral, and neither forward nor reverse can be engaged no matter what the position of the shift lever.  Therefore, the prop will always spin when the engine is not running.  Most hydraulic transmissions require the engine input shaft to be turning to pump the fluid internally to lubricate the moving parts. The ZF25, however, is allowed to rotate with the engine off.  It will be noisy, and add routine wear to the bearings and seals. There is no way to stop the rotation without a shaft brake.

Here are the rules:

If you have a Volvo with a ZF25M transmission:
  • Sailing in neutral is acceptable with any propeller. The prop WILL spin with a fixed prop or an AutoProp, and MIGHT spin with a MaxProp. While spinning, the prop is noisy and adds routine wear and tear to the bearings and seals. 
  • With a MaxProp or a fixed prop, you can sail in reverse and the prop will not turn. This is acceptable.  
  • With a fixed prop, if you sail in forward, the propeller will spin, and the transmission will be damaged quite quickly.
  • With a MaxProp, if you sail in forward, the propeller MIGHT spin, and if it does the transmission will be damaged quite quickly. 
  • With an AutoProp, you can sail in forward and the prop will not turn. This is acceptable.
  • With an AutoProp, if you sail in reverse, the propeller will spin, and the transmission will be damaged quite quickly.
If you have a Yanmar with a ZF25 transmission,
  • With any propeller, the shaft can spin when sailing. This is acceptable, again, other than the noise and additional routine wear on bearings and seals.
  • The only way to stop this rotation is to use a shaft brake.

Bill Kinney
SM160,  Harmonie
The boat is in Charleston, we are in Antigua finishing up a delivery.

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