Date: October 2011
At anchor off Muara, Brunei.
PROBLEM: Clutch slip......forward gear
engages, but increasing the engine revolutions results in the propeller
shaft slowing down.
My boat is fitted with a Mercedes
OM636 engine, coupled to a Hurth
hbw 125 gearbox. The initials HBW stand for Hurth Borg
Warner. Hurth gearboxes (German manufacture) are a common fitting
on boats and have now merged (perhaps they always were the same company)
with ZF Marine (Zeppelin Foundation) and the HBW
125 has been replaced by the ZF 12M - essentially the same gearbox but
with optional gear oil cooling.
Description: The engine is coupled to the
transmission's input shaft (36), on which two gears are keyed, one at either
end. A second shaft, the output shaft (66), has two more gears riding
on it; one of these engages one of the input gears directly (44 - forward
gear), and the other engages the second input gear via an intermediate
gear which reverses the direction of the output shaft, and thereby gives
reverse gear (65). These two output gears are mounted on bearings and
rotate freely around the output shaft. The drive gears impart
continuous forward and reverse rotation to the output gears. Each
output gear has its own clutch, and between the two clutches is an
engaging mechanism (60). Moving the engaging mechanism one way locks
one gear to the output shaft, giving forward rotation; moving the
mechanism the other way locks the other gear to the shaft, giving
In a manually activated two-shaft transmission
such as this, when the clutch-engaging mechanism is first moved to
either forward or reverse, it gently presses on the relevant clutch.
This initial friction spins a disc carrier (57) which holds some steel balls
in tapered grooves. The rotation drives the balls up the grooves.
Because of the taper in the grooves, the balls exert an increasing
pressure on the clutch, completing the engagement. Only minimal
pressure is needed to set things in motion, and thereafter, this clever
design supplies the requisite pressure to make the clutch work, but
without the need for oil pumps and circuits as in a hydraulic
My forward gear problem would have to
be caused by either the balls in tapered grooves jamming, and therefore
not fully engaging the clutch, or by the clutch plates (48 to 51)
slipping. Reverse gear worked fine which sort of eliminated the
ball and groove diagnosis and the clutch plates became the main
suspects. It is possible to replace the plates, and a kit is
commercially available. However, because of time constraints and being at
anchor in what is known locally as 'lightning alley,' I really wanted to
get the problem resolved as quickly as possible and a complete gearbox
strip down, inspection, ordering of new parts, delivery and finally
fitting them would take too long. I decided to just replace the
entire gearbox with new and examine the old box at a later date.
There was also a clattering noise coming from the box as the engine was
switched off which indicated that the damper plate fitted between the
engine flywheel and the gearbox input shaft was kaput, and this also
would have to be renewed.
I've mentioned several times in my log
that the only access to my engine is through the cockpit floor - there
is no access at the front, rear, or either side. Furthermore, the
steering pedestal protrudes into the under floor space reducing access
even further. The picture on the right, looking down on the rear
of the engine and the gearbox, may give some idea of the problem.
After first removing the gear change
linkage, the fuel return pipe and the throttle linkage, I then had to undo the eight bolts
securing the flexible shaft coupling and push the propeller shaft
backwards (after ensuring the rudder was central and locked) to allow
enough room to undo the six bolts holding the gearbox and mounting plate
to the flywheel housing. This part of the job didn't actually take very long as I
had renewed the shaft coupling only a few months ago.
The prop. shaft flexible coupling
The gearbox and mounting plate removed.
The new gearbox was delivered from
the UK (my special thanks to my friend Rob for this!) and was in 'bare'
condition. So while that and the mounting plate were being paint sprayed
ashore, I turned my attention to the damper plate. A mirror inspection
revealed that only a half of each of the six securing bolts were
visible, preventing the use of a socket. Ring spanners could not
be used either because of the protruding 'spline' centre section.
In any case, measurement of the new plate showed that the aperture in
the flywheel housing was too small to accept the plate, and that the
flywheel housing would also have to be removed, along with the exhaust
pipe. As the housing
also incorporates the rear engine mountings, I had to borrow a chain hoist
from the Royal Brunei Yacht Club to take the weight of the engine while
I removed the housing. Once this was off, it was a straightforward
job to remove the old plate which had a completely shattered
polyurethane central section, and replace it with the new plate.
The old damper plate
inside the flywheel housing and with it's broken
polyurethane shock absorber.
The new damper plate................fitted to the flywheel (complete
with oily finger marks!)
Once the damper plate had been renewed, the flywheel
housing could be refitted and the engine lowered back onto it's
mountings. The new gearbox and mounting plate (now painted) could
be fitted next.
Old and new gearboxes.
New gearbox paint job!
Once the gearbox was in place, it was a case of aligning
the output flange with the propeller shaft flange, bolting the flexible
shaft coupling back in place and reconnecting the Morse controls and
exhaust pipe. Finally, the alignment needs to be checked again
with feeler gauges and adjusted accordingly.
On my boat because of the access problems, engine
alignment is a very difficult part of the job and one which I will not
describe on this page.
So......gearbox changed while at anchor!
My grateful thanks to Rob in Yorkshire for locating a new
gearbox (cheaper than I could find on our limited internet), and for
sending it out from the UK to Brunei.