Exalto oil filled stern tube

[Wilf_williams]

I have a Van Der Stadt 35 with a Yanmar 3GM30F with an Exalto oil filled stern tube.

You can see the photos and diagrams here

https://photos.app.goo.gl/ao5DBM9ihufpCUmi6



After a recent 700 mile trip from Amsterdam to Wales I was very disturbed to find several litres of mayonnaise in the bilge ! This had come out of the Stern tube in-board. I had done a fair amount of motoring due to lack of wind and tide-deadlines.

It was a terrible mess, there was no apparent seawater ingress, I had to leave the boat for a week, and no water had leaked in during those seven days. I felt safe to leave it for a month alone afloat.

I'll be taking her out on 14th July to service the unit, and have ordered the seals and bushes. I wondered if anyone here has had any experience with this type of unit, any heads up would be welcome.

Kind regards

Wilf

[Sailmonkey]

I’ve no experience with that particular prince of equipment, but like all other things mechanical and sealed wear and hardening of the seals will cause leaks. Also of concern may be the alignment of the engine as well as the condition of the mounts. If the mounts have sagged they could be adding wear to one side of the seals and bushes. If a mount has come loose or broken the engine may be whipping the shaft in the tube also causing accelerated wear.

[GordMay]

Greetings and welcome aboard the CF, Wilf.

[RaymondR]

The gunk in the bilge is white because it is oil emulsified with water consequently it may be assumed that water is leaking into the tube somehow.

There is a pipe coming out of the top of the tube. This pipe go up to an oil reservoi, as does another one from the stern end of the tuber. The purpose of this piping and reservoir is twofold. It would thermosyphon cooled oil from the reservoir down into the tube to keep things cool. And, more importantly, keep a positive head on the seals. Oil seals are generally only designed to retain the oil from one direction. If the hydrostatic pressure being generated by the head of oil in the reservoir and the feed tube to the stern tube falls below the hydrostatic head generated by the sea water, sea water will find it's way into the tube and mix with the oil. You will note from the diagram that the height of the oil in the reservoir needs to be at least 1.5 time the depth of the aft end of the stern tube. This is so to ensure the generation of the requisite seal activating pressure.

The high shearing effects of the shaft spinning in the tube and the bearings will then break up the oil and disperse it into the water forming an emulsion.

The bearings are what are called "plain bearings" ie. they are bushings. There may be wear in them allowing the shaft to vibrate and this will cause the seals to leak.

I would clean up the mess, flush through the tank and piping and stern tube with diesel and replace the oil and see if anything leaks. If you were motoring with a largish following sea the surging pressure may have caused the leakage.

You might also check the lateral clearances with a dial gauge to check if the bushes need replacing.

[Boatwright]

I worked on a N.F. Herreschoff schooner (Brigadoon sister) this year with a similar arrangement.

The seal at the aft stern tube end is a standard [U]double[U] lip seal. That is two sealing lips designed to keep water out and oil in. There is another seal inboard to keep the oil in the tube. The shaft is supported by bronze bushings. The bilge was also a mess.

I think it likely you will find worn bushings and bad seals at both ends. Repair requires hauling the boat and removing the shaft. You will also need proper tools to pull the bushings.

By the way, Coast Guard regs now require the use of food grade oil for these setups. We are trying canola (rapeseed) oil because of its resistance to gumming. The oil tank is located well above the waterline designed to maintain positive hydro-static pressure of oil against water.