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Reverse Locked in

Below is a response to an enquiry from a customer with an older Yanmar 4JH2T with a shaft installation:

With over 65,000  cumulative propeller years of service now going back over 20 years, and say 10 outings per vessel per annum, and say 10 reverse operations per trip - then we are using on very rough numbers some 6.5 million reverse engagements as a basis for developing the response below.

We actually see very very few inquiries dealing with this issue - typically an older installation having difficulty moving the reverse lever to a neutral position after engaging Reverse.

I am assuming this installation is a shaft drive installation, not a Saildrive.

I am not sure if the situation you describe is trying to get the control lever into neutral after sailing without the engine running. In this situation my suggestion would be to round up into the wind, and reduce the boat speed to near zero.

I can see no reason why it would not be easy to now return the control leave it to neutral.

If the unit has cone type clutches - which I described below -  then maybe it is possible that the cones are sticking and have not been maintained correctly as they should be.

If the situation is with the engine running - and not being able to move the control lever to neutral from reverse - then obviously the first objective must be to reduce the torque from the propeller on  the gearbox via the engine.

We have to assume if there was no torque or load, then it would be easy to move the control lever to neutral. Unless the unit has cone type clutches which have stuck together because of maintenance issues.

Some cone type clutches use the torque of the output shaft to assist with keeping the clutches engaged so additional torque will make dis-engagement more difficult. I don't recall a situation where  getting the control lever from forward to neutral has become difficult, it is usually only from reverse. 

One way to do this would be to lower the idle speed of the engine which - if it is too high - as it will be delivering high levels of torque to the propeller and thus to the gearbox.

Stopping the engine while stuck in reverse gear will also immediately reduce the torque load to zero.  It should now be possible to  move the control lever to neutral before starting the engine again - assuming no forward vessel speed.

An obvious solutionwould be to lock the shaft by engaging forward not reverse on his gearbox. If the Kiwiprop unit has been lubricated and in specification there is no reason why you should not leave it in neutral to avoid this problem.

  • So first question - Does the gearbox have cone type clutches ?
  • If answer is  YES - then next question is when were they last maintained - removed and lapped / ground ?

Following is a summary of the situation with the various types of reverse boxes.

The simplest gearbox as used on nearly all outboards consists of a dog clutch which has instant engagement in both forward and reverse. Very cheap to build - simple to operate but very noisy and harsh to engage at other than  idle.
Yanmar have used this type of clutch on all their SD 20, 25, 30, and some early 31 Saildrive models. They are consequently always very easy to engage / disengage  into and out of both forward and reverse.

We are not aware of any difficulty engaging forward, neutral or reverse on this type of Saildrive.

Beginning with Bukh - who designed and built the orogional Saildrive and subsequently Volvo and then others - they chose to use a smoother quieter gearbox with a cone clutch rather than a dog. While they provide smoother engagement - over time the steel cones become glazed and are prone to remain stuck in gear.
The maintenance program is quite clear on the ongoing requirement tp maintain these types of clutches.

Any sticking of the cone clutches will make getting the unit back to neutral difficult.

Both Yanmar on their larger Saildrives SD 40 / 50, Bukh and  Volvo - specify that these cones must be removed and lapped / ground  to remove the polish after something like 200 hours of operation. A failure to do this  maintenance will certainly cause difficulty in getting the gearbox back to neutral.

A YouTube video showing the cones ( @ 10 min ) in the gearbox:  Kanzaki Cone Gearbox Video:

The Kanzaki range fitted to Yanmar engines listed  as having a servo cone mechanical clutch follows.


Note the spiral gears under the cones which provide the servo effort that utilises the torque of the engine to assist and maintain clutch engagement by converting torque to a lateral force that forces the clutrches into the cone. It also makes for more difficult dis-engagement when there is a load from the propeller.

Later model Saildrive units ex Nanni, Yanmar SD 60's and Volvo 150 S units - all OEM'd from ZF - all contain multiple disc clutch packs and consequently do not require these maintenance steps. Neither do ZF or  Twin Disc Saildrive units.

Always check what model gearbox is fitted to the vessel  as with an older model, many different gearbox models were used and it is possible it is not even the original gearbox that came with the engine so many years ago is fitted.

These have changed design by manufacturer over time.

Clutches on Saildrive's have presented many problems to the manufacturers over the years. Today Nanni, Volvo and Yanmar all purchase ZF drives from Germany and sell them under their own name. These units all have separate clutch packs -  for both ahead and astern and are not prone to locking in reverse or ahead.

The more expensive gearboxes  fitted to Shaft drives such as PRM,  Twin Disc have always had multiple clutch packs rather than cone type clutches and thus do not have problems with locking in reverse.

The unit fitted to a Yanmar is likely to be  a Kanzaki unit - normally shipped with Yanmar engines.
Volvo have always shipped with their own design gearbox with shaft installations.

Hurth ZF units - have multiple mechanically operated multi-disc clutches - not cone - and should not lock in reverse.

There are a large number of references on the web to glazed or worn cones on Kanzaki marine gearboxes.

This must be an area to start any investigation of any difficult engagement from reverse to neutral.