September 25, 2001

Hi All-

While rebuilding my drive train (new ss shaft, cutlass bearing, and PSS shaft seal) Iím considering inserting a flexible coupling. Any of you have any thoughts proís/conís?? I realize Iíll have to shorten the shaft an inch or so and maybe even replace the engine mounts while Iím at it. I definitely donít want to have to go back into the aft bilge area EVER AGAIN!

Bill S. "City Bird" K113

1376 Jan 22, 2002

Bert- (et al), Here is a copy of a response I received from Paul concerning my shaft project.


When one considers the purpose for a flexible coupling (to compensate for any mis-alignment) the importance of proper alignment and the condition of the rubber of the mounts becomes very apparently more critical.

In yachts with a considerably amplified engine mounting angle, rather than the relatively level amplitutude of our boats, and that essentially all of the thrust from that big, three-bladed wheel is transferred to the hull via the mounts and that thrust forces the aft end of the eng/trans upward in forward gear. More importantly, the reversing action draws mightily aft on the coupling and the mounts.

Replacement of motor mounts is a normal (but not frequent) maintenance item for any boat as time, heat, sea water, electrolysis and general deterioration of the rubber is to be expected. Hardened rubber will eventually split but before this, the elasticity is lost and the engine settles somewhat. The immediate result is wear on the cutlass bearing and its inability to cool via increased sea water flow reduction. The shock of shifting is absorbed entirely by the motor mounts when they are in good condition. The isolation of certain noise and vibration is also an important function of the mounts. Once the rubber has hardened or cracked the shock absorption and isolation is transferred to everything else.

Further, the normal consideration of flexible couplings for dampening of unsupported shaft length, causing the "whip" effect, the application in the SW II seems somewhat diminished due to its very short shaft length.

The design and manufacture of flexible couplings demands the insertion of a rubber material that is considerably less capable of axial strains than the castings themselves. To my knowledge, limited to those equipped with Westerbeke power, the original coupling was possibly one of the most perfectly designed flexi-couplers yet as the design limited the amount of rubber mounting. This rubber, in any unit, is subject to wear and deterioration and should likewise be considered normal. However, if you are considering a change, the replacement unit(s) and process has been well documented by Bert deF.

I replaced the motor mounts a few years ago with the Perkins units and found the size and composition to be superior to those offered by Westerbeke (at twice the price). Re-alignment was needed a year later to compensate for the slight and expected compression of all four.

I do suggest to any owner that upgrading the original two stick shift/throttle to the single-lever control will prevent the occasional over-rev shifting that damages the couplings and the motor mounts to a very significant minimum. Morse had been producing a very traditional unit that may still be available with a bit of searching. (*** note The throttle lever supplied must be shortened significantly to compensate for the relatively short "throw" distance of the Westerbekeís fuel lever on the governor). The newer model is just as effective but projects further into the cockpit area and is a bit bulky to handle not to mention very un-traditional in appearance.

I do however suggest that a bullet zinc be installed on the shaft just forward of the shaft seal to prevent the shaft from exiting the boat should any failure of the shaft or coupling occur.

Having read your comments re: removal of the cutlass bearing I offer the following. The shaft/bearing mating surface requires cooling and lubrication by the passage of seawater through it. The relatively short distance between the bearing and the prop is too often filled with the bullet type zinc and restricts this vital flow. First suggestion is to use the collar type zinc as it is shorter. Second, I fabricated a scoop from stainless steel tubing by cutting the one inch tubing section across the bias and welding it upon a simple, slightly curved, stainless base. I bored a hole through the center of the scoop as a pressure relief and another through the deadwood just forward of the bearing and into the shaft tube. The scoop is screwed (sealed with GE 5200) over the cross-bored hole and sea water flows through the scoop, through the cutlass bearing and around the zinc collar. I have experience NO wear in the bearing since.

You certainly had our collective sympathy for the efforts and discomfort you experience while working in the shaft galley but the better you plan for the entire project, certainly the less attention you will pay to it later.