replacing the rear seal in the transmission/reduction...

If the rear oil seal in the transmission fails, oil will be spattered 360°around the transmission coupling (the coupling to the propeller shaft) and the oil level of the transmission will decrease to zero. This needs a solution before the engine is used again.

My 2001 Tartan 3500, #131, has a 3GM30F Yanmar engine (the ‘F’ means fresh water cooled with a heat exchanger using sea water as the exchange medium). This has a KM2P, 1:2.62, in forward, ratio, Kanzaki transmission, also made by Yanmar, as standard. The area we need to operate in is very easy to access under the berth in the aft cabin.

Replacing the rear seal in the transmission/reduction gear. Aligning the Tartan 3500 engine with the propeller shaft.

This seal fails because of poor alignment of the propeller shaft with the transmission or overheating (no oil left). My situation was unknown, but I suspected poor alignment causing a leaking (original) stuffing box which I had sought to remedy with a PSS dripless seal. The old stuffing box had indicated previous chronic leaking (that’s sea-water in) to me, and maybe the transmission seal had already been damaged?

I recognized the oil seal damage when I lost oil and the transmission did not immediately engage with the engine running. It turned out that my engine was nowhere even close to being in alignment (1-1/2° too close to level!) (see below). The Tartan yard that originally sold the boat is responsible for this as it should have been part of her commissioning check out.

It is not a particularly difficult seal to replace and inexpensive ($5 from Yanmar and $1 for the “O” ring). The limitation on easy, in-boat replacement

carbon graphite flange

rotor

output shaft

stern tube

propeller shaftbellows

coupling

inside of hull

lock nut

transmission housing

oil seal

hose clamps

O ring

PSS seal


is having enough room in the space between the output shaft of the transmission and the propeller shaft coupling (rear) to slide the propeller shaft back and remove the output shaft coupling (forward bit). You could also worry about the torque wrench you should get into that space with a 30 mm socket on it to accurately torque the lock nut (the manual says 61.5 to 83.2 ft/lbs (10 +/- 1.5 kg-m)) on the output shaft, if you are so inclined. I wasn’t.

You should think about seawater coming into the boat whichever type of propeller shaft seal you have. The seal is below water level. Do not walk away from the boat for a week or so looking for parts or daydreaming, as is my wont, and leave this seal susceptible to leaking after the propeller is nudged by your diver or an adventurous harbor seal. You will get some water in when you are messing with it and this will be retained by the non-draining well under the engine. We pumped this well out with our manual engine-oil-draining pump.

The first step is, as above, to ensure you have enough room to at least remove the forward part of the coupling which measures 1-1/2”, 38 mm, long. On my PSS seal (see diagram) I could move the stainless steel rotor forward, along the propeller shaft, 1-3/4”, 44.5 mm, which indicated I could then slide the shaft back the same distance whilst maintaining the integrity of the seal. In fact the rubber bellows around the stern tube could also be loosened and slid back about 3/4”, 19 mm, so giving me a total of 2-1/2”, 63.5 mm. The rotor had two set screws (which need replacing once you loosen them) and the rotor is hard to slide on the shaft - mine needed to be tapped with a hammer initially, before it became a bit easier to slide.


•I marked the couplings together (with a Sharpie) so I could put them back in the same orientation (I can’t think why this matters, I just read it in some book).•I undid the four bolts that hold them together, marked the shaft next to the rotor and the rotor itself for re-alignment later (same caveat).•I moved the rotor forward on the shaft and slid the shaft back, bit by bit, until I gained the clearance I needed to get the forward part of the coupling out.•I increased this clearance by loosening the seal clamps on the stern tube and sliding the bellows and shaft back before retightening the clamps.

The forward coupling is held onto the splined output shaft by a specific 30mm reverse-thread locknut, that, on Catcher, was barely finger tight(!). This nut has a thin lip facing aft that tapers to an edge. This edge is supposed to be bashed into the threads to lock the nut once you’ve torqued it down. On the forward side the nut has a circular recess next to the shaft, as if to hold an “O” ring, but it doesn’t. However there is an “O” ring forward of this nut, it just sits around the output shaft. Replace it too.

•The oil seal sits in the transmission housing and has to be pried out with a screwdriver. Be careful not to scratch the surface the seal sits in on the


housing, as this could cause any new seal to leak. Its a bit sticky, but with patience (and the destruction of the old seal itself) it comes out.

•Clean the surfaces with a rag and tap the new seal in (flat closed surface of the seal faces aft) with a wood block and a hammer until it is flush (screwdriver as punch through the bolt holes on the coupling); see the picture on the right:

•Put a new “O” ring on the output shaft, put the coupling back in place and the reverse-thread nut and tighten it to about 70 ft/lbs (95 Nm). I used a plain wrench and tugged hard on it because that is all that would fit.

for'd part of coupling,removed

propeller shaft, moved aft

bellows, squinched aft

PSS sealaft part ofcoupling

rotor slid for'd on shaft

stern tube

output shaft

inside of hull

oil seal

transmission housing

hose clampsloosened


•Using a rectangular metal punch/chisel and hammer, knock the locking lip into the threads in just one place.•Undo your rotor and slide it aft as you slide the shaft forward, same with the bellows, until all is in the original position.•Replace the rotor set screws, tighten well and tighten the bellows clamps.•With the four coupling bolts back in place, but not at all tight, check that the two couplings have an even gap between them all around. Use anything metal that is of an even-thickness to evaluate this (a feeler gage if you have it). If they do not have a completely even gap all around, your engine is not in alignment with your propeller shaft and should be corrected using the adjustable height fore or aft mounting bolts.

As mentioned above, my engine was way out of alignment. I undid the nuts above the aft engine mounting plates until the lock washers beneath the nuts were free, then I tightened down the supporting nuts under these plates to allow the engine to drop. I measured the engine plate height from the engine mount with a 6” (150 mm) steel ruler before I started and reduced this height by my calculated 0.34”, 8.6 mm, before I re-checked the coupling. After some back and forth I got the space between the two coupling parts measuring the same all around. I then tightened down the top nuts onto their lock washers as tight as I could. See picture below...


I refilled my transmission with 0.3 liters of 30 SAE (that’s ‘weight’) Castrol oil (the oil level is checked with the dipstick sitting on top of the threads, not screwed in) and ran the engine, engaging forward and reverse gears a few times. Then I re-checked the tightness of the coupling bolts and the engine mounting plate top nuts and checked for any leaks in the dripless seal or the transmission seal. Thank goodness - there were none. The next day I checked the oil level again after the oil had settled and took a little bit out with a plastic glue syringe with a bit of tubing on the end, to make it correct.

©Philip Roberts, 2013, Palo Alto, California, USA.


replacing the rear seal in the transmission/reduction...

Documents