ZF25 Transmission Failure


Dean Gillies
 

Hi All,

After load testing this morning I am 99% sure my ZF25 transmission has failed.  

I know the root cause of the failure, and I will publish separately on that issue later.


I'm told by the ZF representatives that a new transmission will take at least 3 months to supply.  

I don't want to entertain re-conditioning of this box because I know the root cause of the problem.


Our sailing season is over before it started, the admiral is devastated and has lost all confidence in the boat.  We will most likely winterise the boat and go home soon. 

 

However, before I do that I'd like to convert that 99% to 100%, to be absolutely certain that the problem is the gearbox.  I'm finding it difficult to get the required level of professional support here in Kalamata, so I'm hoping for some sage guidance from the group. 


So, the symptoms...


Two weeks ago we started motoring due to no wind. Flat sea, running about 1700-1800rpm.

I started hearing some "modulation" of the engine tone, like wandering RPM. 


I also noticed the disappearance of a "rattle/chatter" sound which has always come from our Morse Control.  It's not a loud rattle, but is clearly transmitted from the gearbox up the gearshift cable and resonates from the hand control unit. (Is that a standard thing - or an early symptom of a gearbox problem?)


I thought we had something stuck on the propeller, so we stopped and I checked it. Nothing.

We started again and the boat would not run up to normal speeds for the RPM we were using.


We stopped again, and I checked the engine room once more. Fluids ok, no leaks, nothing looked out of the ordinary.  So  we continued and I increased the RPM to about 2000-2100, and continued on making only about 4 knots.


After 10 minutes or so, I opened the engine room again to check, and was met with an outburst of smoke, burning smell and everything was very hot.  After stopping and investigating further I found the cause of the overheat was that the shaft brake was stuck closed.  The ZF 25 gearbox was also very hot, and when I opened it to check the fluid again the lower part of the dipstick was melted into an elongated blob of plastic.   

There was no fluid leakage, so I assumed that the fluid  level was still ok. 


I found out how to undo the shaft brake, and wedged a spanner in it to make sure it stayed open. By then everything was cooled down and we proceeded to motor slowly to shore at about 1200rpm. Interestingly the "rattle" mentioned above was back.


We flushed and changed fluids and ran the boat gently for a day or two until we got to a marina.  


On testing the boat, we find that all is well until we exceed 2000rpm. Quite quickly after running at 2100rpm our little rattle stops, then the engine noise starts to waver and sounds like it it reducing in rpm, although the digital readout of rpm does not change from 2100rpm. The boat then starts to slow down from the 8.3 knots it was travelling at with 2000rpm. It drops a couple of knots.  On checking the shaft brake at this point it is still open so that is not the reason for slowing.  If I increase the engine power, we get no more thrust from the propeller.


After this happens, I struggle to generate thrust even at lower rpm.  Leaving everything to cool down completely appears to reset the problem. The rattle comes back, low RPM operation is ok, but when loaded up the gearbox seems to be failing.


The one thing which is niggling in my head is that the engine rpm sounds like it momentarily drops down when this problem occurs, although the rpm readout does not change. However, I can easily increase the engine RPM by pushing the lever, it just does not transfer to the prop, which seems to me like gearbox trouble.


All comments/suggestions appreciated.  We don't really want to give up on our summer cruising, and apart from that its cold back in Sydney!


Thanks all.

Dean

SY Stella 

Amel 54#154





   

 

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