(1975 MK-II Yawl, # 157)

Receives A  New Transmission

By Tom Assenmacher

 Hit Counter Visits Since 10/24/2010

After 5 failures of our Hurth/ZF 15MA transmission in about 1750 hours of operation (that's engine hours, and includes time charging batteries, etc.), we decided to switch to a different brand of transmission. 


A bit of background for those who may not have followed the 'saga' of transmission failures:

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 5 transmission failures in about 1750 hours (engine hours) of operation. 

The 1st failure occurred about one year (and about 300 operating hours) after the initial engine installation in 2002, 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 2nd 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, and 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 3rd 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.  Since we had 'lost confidence' in the Hurth/ZF transmission, we subsequently 'scrubbed' the Bahama 'mission' and returned to Kinsale in early November, 2008 (we subsequently got our Bahama 'Fix' by renting a cottage on Man O' War Cay, Abaco for February and March 2009).

Following the third failure, we decided that perhaps the problem lay in the damper plate, and subsequently after quite a bit of research, etc., installed a more 'flexible' damper plate.  This installation in  is covered in the article INSTALLING A NEW TRANSMISSION DAMPER PLATE  IN SHEARWATER

With renewed confidence brought about by the installation of the different damper plate, we subsequently cruised to the Abacos again in the fall of 2009.  We had an uneventful cruise until we reached Green Turtle Cay, where we experienced the 4th  transmission failure.  As we had a re-built transmission aboard, we installed it in Green Turtle and spent the rest of the cruise in the Abacos.  We rebuilt the recently failed transmission while in Man O' War Cay, Abacos.


The 5th transmission failure occurred on our return to Kinsale in early May, 2010 while sailing off-shore from Ponce Inlet, FL to Southport, SC,  While motor sailing in nearly calm winds about 60 NM south of Charleston, SC, the trans failed again (same problem - forward clutches worn out!).  We subsequently made port in Charleston, and had SHEARWATER trucked back to Kinsale (she goes to windward very well at about 60 kts, with no masts or sails up).  Our friends Kip and Linda Newbould of nearby Coles Point, VA (Boat Lovers Transport), the owners of MISCHIEF, a 1970 MK-I Yawl, picked us up in Charleston with their massive Volvo tractor/trailer. 


With SHEARWATER on the hard at Port Kinsale Marina, we set about figuring out the next step in curing SHEARWATER's propulsion problems.


After much research (on the Internet, phone calls, etc., etc.), we finally decided on a TWIN DISC transmission, Model M60A, a 7 degree down angle transmission.  This transmission uses  a 'Cone Clutch' vice a 'multiple disc clutch' used in the Hurth/ZF.  The Twin Disc transmission is basically the same physical size at the Hurth/ZF 15MA which it replaced.  The existing damper plate, adapter plate, and bell housing all fit the Twin Disc perfectly, so no modifications were required in those components.  The Twin Disc being a 7 deg. down angle vice an 8 deg. down angle, required a slight adjustment of the engine mounts which was easily accomplished.   Since the Twin Disc trans was about 1.5" longer than the Hurth/ZF, we were able to use the 'old' prop shaft by shortening the shaft a bit, and machining a new keyway on the transmission end of the shaft.  No modification was required on the prop end of the shaft.


Since the new Twin Disc transmission had a different reduction gear ratio than the Hurth/ZF (2.4 - 1 vice 1.8 - 1, we needed a different prop.  Using the  prop formulas, we quickly determined the new prop dimensions (14" x 16" RH vice the old 14" x 11" RH prop).  Fortunately, our friend Bill Pittman of Reedville Propeller in nearby Reedville, VA, was able to find us a used prop which he refurbished (polished/balanced/re-pitched) and took our old 14' x 11" prop in trade.  He charged $120 for the 'new' prop which was a REAL good deal!


One area which required 're-engineering was the fact that the transmission shift lever was on the port side of the transmission vice the starboard side as on the Hurth/ZF.  We had to design/build a new shift cable bracket assembly to accommodate this physical difference.  A day of 'pondering' the problem, and a few hours in the 'shop' we had a new cable attachment assembly.


We finished the installation of the new transmission in July, 2010, and as of October, 2010, we have about 30 operating hours on the transmission.  We are quite happy (so far) with the Twin Disc transmission.  It seems we have less vibration and less noise with the Twin Disc.  We also appear to be 'propped about right', as we can just obtain max operating rpm at full throttle, and achieve hull speed at the top of the engine torque curve (rpm wise).  Also, the rpm at hull speed is also near the area where the torque curve intersects the optimal fuel consumption curve (i.e., 'Max Range' for the boat).


If all works out, we fully intend to 'head south' to the Bahamas next fall (October, 2011).


The following series of photos provide (more or less) the design and installation process.  We did the entire installation ourselves, along with 4 of the 5 failures. Of the 5 Hurth/ZF failures, only one transmission - the 3rd failure - was replaced by an 'outside mechanic' - a Hurth/ZF technician who could find nothing wrong with the installation.......  We are quite good at Removing and Replacing (R & R) transmissions!!!






Removing The Prop


Old Prop


Removing The Hurth/ZF Transmission

Twin Disc With Adapter Plate Attached


Prop Shaft (Transmission End) - Required Shortening And New Keyway




Prop Shaft (Prop End) - Required No Modification



Trial Fit Of Twin Disc (It Fit OK!!)



Old Shift Cable Bracket (From The Hurth/ZF Transmission)



"Re-Engineering" The New Shift Cable Bracket (Photo #1)



"Re-Engineering" The New Shift Cable Bracket (Photo #2)



"Re-Engineering" The New Shift Cable Bracket (Photo #3)




"Re-Engineering" The New Shift Cable Bracket (Photo #4)



"Re-Engineering" The New Shift Cable Bracket (Photo #5)


"Re-Engineering" The New Shift Cable Bracket (Photo #6)



"Re-Engineering" The New Shift Cable Bracket (Photo #7)




Final Installation