SM Prop Shaft Seal direction and Bush


Jose Venegas
 

I want to report my experience with the shaft seals and bushing.  
When I bought my SM2000 278,  It had its seal and bushing replaced and the seals oriented as recommended by AMEL.  In less than 2 seasons it was leaking water into the oil and I replaced it again in that spring finding substantial wear in the bushing, as you will see in the pix I have uploaded in face book.  After trying again the same scheme and less than two years later, as we were returning from Cartagena to Florida, just before reaching the windward passage between Cuba and Española, the transmission oil showed again the dreaded white color and I had to sail with no wind around the eastern tip of Cuba.   In Fort Lauderdale, I decided to experiment with the shaft seals direction based on my Mechanical Engineering background:  single lip seals are designed to prevent fluid from crossing the seal in only one direction; Fluid from the side facing the lip apply a pressure on the lip and it that prevents it from crossing to the other side.  Fluid coming from the other side can expand the lip and move across.  This motion of the fluid is helpful as it serves to lubricate the surface of contact between the seal and the shaft.
Based on this idea I realized that having the two inner seals facing in prevented oil leaving and to outer seal facing out prevented water coming in. As a result, none of the seals is lubricated and as the grease is worn out the outer seal will start leaking water in, which is not prevented to get into the transmission by the two inner seals.  The heavy wear and corrosion can be seen in the bushing surface of the outer seal.  So, even if the two inner seals are still viable they will not prevent water entry into the transmission.
My solution was to have the outer seal lip face into the transmission, preventing oil from leaving but allowing water to lubricate it.  Also, the two inner seals were oriented with the lip facing the shaft, preventing water from entering the transmission but allowing oil to move and lubricate them.
After 5 years of use, I noticed a small amount of oil loss ( < 2 cm drop in the tank) which I replaced with a heavier gear oil that completely stopped the oil loss. 
Last week I decided to replace the seals and bushings.  I noticed that the oil was perfectly clean and the two inner seals and corresponding bushing surfaces were intact but, as expected some wear was present in the outer bushing surface.  My guess is that with the heavier oil the outer lip began to work again, this time allowing a small amount of oil lubricating it.

Since after the outer lip function deteriorates the oil loss is very gradual and can be easily replaced with heavier oil, my recommendation is that the two inner seal lips should face the prop and the outer seal face the transmission.  Based on the wear pattern and total lack of wear in the inner seal I estimate that the seal/bushing system would have worked preventing water entrance into the transmission for another two or 3 years.  So my next experiment is to only replace the seals  8 years from now unless I begin to see a drop in oil level which is not fixed with heavier oil which I will report immediately.

Jose Venegas
Ipanema SM2k 278



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