Water ingress to gear casing; shaft or seal?

[ribvanw11]

Hi,

Our old Yamaha 3.5AC's gearbox oil goes milky after 10 minutes running in the test tank. but only if the prop is turning (10 minutes in neutral results in no milkiness). We are therefore assuming that the water ingress is via the prop shaft (there is no other way in for the water that is rotation dependent). We have installed a new seal and bearing but milkiness continues. Is the culprit leakage, under vibration, around the outside of the seal or the slight wearing of the prop shaft counterface or something else?

If it is the shaft wear, is a speedi sleeve likely to be successful instead of a new shaft ?

We have pressure tested and all seals retain quite high pressure but then, our problem is ingress of water not egress.

Any advice gratefully received.

Bob

[robert graham]

Maybe bad seal on driveshaft, on top of lower unit? Shifter shaft seal, drain/fill washer/gaskets in good shape? Remove prop and rotate prop shaft to check for any slight bend in shaft from prop strike, a bent prop shaft can cause leaks at seal. Good Luck!

[ribvanw11]

Thanks Robert,

The driveshaft seal (which faces in to the gearbox to seal oil in only) is protected from the water in the leg by a new O-ring around the drive shaft tube. Also, despite the rotating drive shaft and pressure of the exhaust gasses, there is no sign of milkiness when the engine is tested in neutral. The shift boot performed well under the pressure test and does not leak if the motor is tested running in neutral.

We have replaced the drain/fill and detente screw washer/gaskets with new ones.

We haven't tried checking the prop shaft straightness but we did try another one we had in the spares box - no change.

Will give the shaft an eyeballing at next opportunity

Many Thanks
Still intrigued - Bob

[robert graham]

One of my older motors had a groove warn in the prop shaft by the seal, as well as a slight wobble in it, causing water to enter lower unit. I replaced prop shaft from another motor I had and it fixed it. Have you checked carefully for any hairline cracks in lower unit? I've also had those, but had it welded up with success. Good Luck!

[ribvanw11]

Thanks again Robert,

I'm now waiting for a brand new prop shaft to arrive (hopefully this week). I have also identified a stock of Speedi Sleeves locally.

Its now a 'I won't be beaten' challenge rather than a simple repair. Thankfully, the machine in question has a stepsister (branded Mariner) which doesn't have water in the gear oil, so I can use that while I patiently seek to cure the Yamaha.

Regards
Bob

[ribvanw11]

After installing the new prop shaft, we noticed a hairline crack in the casing near the shaft cover. We decided to layer a little epoxy resin adhesive across the surface around the crack.

The good news is that, after 2 15 minute tests, we no longer have water in the oil. The bad news is that we don't know if the cause was the prop shaft or the crack!

It seems odd that the crack did not emit air during the pressure test nor did it let water in if the prop shaft was in neutral.

Equally the wear on the old prop shaft was only 0.05mm (=2 thou")

However, if the problem re-occurs, we have a spare casing and lots of patience.

Signing off

[robert graham]

Maybe get that hairline crack welded up before it gets bigger. I've had mine welded and it's as good as new! Good Luck!