By Tom Assenmacher
10 January 2009

Posted 1/10/2009
Hit Counter Visits Since 1/10/2009

Following our third transmission failure (ZF 15 MA - 8 Degree Down Angle) in our 1975 MK-II Yawl (#157)  in October, 2008 with a bit over 1150 operating hours, and after a lot of thought, questions asked of technical people, etc., we have decided (sort of) that this recurring problem is more than likely caused by an incorrect transmission damper plate (the connection between the input end of the transmission and the engine flywheel). We re-engined with a Phasor P4-37-05 (Kubota 4 cylinder-37 HP) in 2002 as a result of a catastrophic failure of the original Volvo MD-2B 25 HP engine which suffered a hydraulic lock in the aft cylinder and 'blew' a hole in the cylinder wall.  We did all of the installation ourselves while on the hard at the local Krentz Marina near Kinsale, VA..

The ZF 15 MA transmission is a multiple disc clutch, 8 degree down-angle transmission, and is rated to handle quite a bit more power than the Phasor P4-37-05 output, so we felt quite secure when we chose this engine-transmission combination.  However, we've subsequently had 3 transmission failures in about 1150 hours (engine hours) of operation. 

The first failure occurred about one year (and about 300 operating hours) after the initial engine installation, with the transmission becoming difficult to go into 'gear' in cool weather, and also slipping when up to operating temperature. This transmission was replaced under warrantee. The removal and replacement was done at our dock in Kinsale by ourselves.

The second failure occurred in April 2007, while SHEARWATER was returning from our 2006-2007 Bahama cruise, resulting in another transmission change in a marina near Savannah, GA.  This transmission failure occurred with about 650 operating hours on the unit.  This transmission was not covered under warrantee, so we purchased another new transmission from ZF (it was drop shipped to the marina in less than 24 hours), and installed it ourselves while in the water at the marina.  The replaced transmission was kept by us as a 'rebuildable' spare.

The third failure occurred in late October 2008 near Hampton, VA while we were on our first leg of our second Bahama Cruise.  The transmission began to slip in forward gear, subsequently producing virtually no power in forward gear (reverse seemed to be OK).  This transmission failure occurred with only about 230 operating hours on the unit.  This unit was covered under warrantee and replaced free of charge by a ZF technician at the Old Point Comfort Marina at Ft. Monroe, VA.

Even with a new transmission, we had 0 confidence to continue until we more or less sorted out the real problem with this ZF product (this model transmission appears not to have a high failure rate). The ZF tech indicated that the previous installations were 'first rate' (we did all the previous installs ourselves, which originated with the new engine replacement in 2002), with no problems (alignment, setup, etc. etc.) which we were sort of happy to hear, but that in itself still doesn't inspire the confidence to continue the cruise. 

We then decided to 'scrub' the Bahama cruise, and, since we were still close to home in Kinsale, to return to our dock in Kinsale to sort out the problem. After much discussion, 'pondering', research, etc., we decided to replace the original Hayes damper plate with an R&D damper plate provided by PYI, Inc.  Discussion revealed that the Kubota engine has a fairly light flywheel which requires a more 'flexible' damper plate to dampen the sharp ignition 'torsional impulses', especially at lower operating RPMs. 

Only time will tell if the installation of the new damper plate has cured our transmission problems!!

The following photos document the damper plate installation process.

Transmission Prior To Removal

Bell Housing (With Transmission Attached) Backed Off From Engine
(Note: On SHEARWATER, the bell housing with transmission still attached cannot be removed as a unit as it will not fit between all the plumbing, cockpit drains, etc.  The transmission must be removed from the bell housing in order to remove both components from the engine compartment - not as tough as it appears here!)

ZF 15 MA Transmission and Hayes Bell Housing After Removal

Kubota Flywheel
(Where Damper Plate Attaches)


Front (transmission side) of original Hayes Damper Plate
(The 8 rubber bushings shown are quite stiff, and do not allow for very much dampening)

Back (engine side) of Original Hayes Damper Plate

Front (transmission side)of R&D Damper Plate
Note Elastomeric 'Shock Absorbers'

Rear (engine side) of R&D Damper Plate

The re-installation of the new damper plate, bell housing, transmission, shaft coupling, and shift control cable went smoothly.  Only minor changes in alignment were required, and the entire removal and replace process took approximately 7 hours.  We've run the power plant under load for approximately 1 hour since the installation was completed.  We plan to put a few hours motoring in the local area in the spring prior to venturing far away from home for more extensive local cruising.  We are currently planning on another cruise "Down South" next fall (Fall of 2009).  We probably won't feel comfortable until we have several hundred hours of operating time on this new transmission/damper plate installation - guess TIME WILL TELL!!