How to remove a stern tube & Why would you want to!

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When I brought St Paddy she had her original Petter Mini Twin. A wonderful engine, for its day. It had a few problems, the spreading of oil around the bilge was easily solved, but the lack of power was not.

I had to half dismantle the Mini Twin to get it out, it then took an angle grinder to remove the bed. This I chucked over the side at 3 am. The noise of it landing on concrete was very satisfying. To cut a long story short I glassed one of Jeremy's new 2GM beds and fitted a new Yanmar.

 When St Paddy was launched she motored much quieter, and with her bigger 3 bladed prop, faster too. I'm sure thou, that the transmission noise was always there, drowned out by the dear old Petter. So the project for the next winter was to fit a Halyard Aqua Drive. This helped reduce the noise but the Tufnell stern tube bearing would still not last. Each year I'd made a new one and the last was a precision fit.

The problem here, I think was two fold. The side load caused by the 3-blade prop, and, some miss alignment. The Tufnell bearing in a past life had spun in the stern tube and intern wearing it oversize.

 So here is what I want to pass on. I wanted a sailing boat that could motor well and as quietly as a modern boat. The Yanmar is quiet and the engine compartment totally soundproofed. The problem was propeller noise. Most of it transmitted through the hull by the tuffnell stern tube bearing. By fitting a cutless bearing I hoped that the rubber component would reduce the noise transmission to the hull. The problem here is, these must be water lubricated. With the present set up this would not be possible.

 I needed to remove the original stern tube. Some said drill it out, this might be possible but high-speed steel would not drill GRP for very long, secondly I wanted to stay inline with the hull lamination and engine bed. The only way I could think of achieving this would be to line bore. Knowing that I would not find such a machine already in use I set about making one for myself.

The finished machine used a Tungsten cutting tool that removed the GRP tube from inside and cut with an inboard direction. The size of the cut would then be increased and another cut taken. With about five passes the cut broke out into the inside of the skeg. (If I had been using a drill it would have dived, That would have given me an oval hole and I probably would not have been able to continue) The last 'finish' pass, produced a parallel hole, accurate in diameter.

One surprise, which I didn't expect, was problems with the lay-up of tube. It was difficult to see what was happening while I was machining but there was definitely woven cloth coming out that had either lost it's resin or the resin failed to impregnate it properly. It is worth remembering that if the tube fails then water could enter the boat, once pass the bearing it would run into the skeg cavity. From there it would be seen trickling from under the stuffing box on its way to the bilge. The flow would depend on the clearance on the bearing. Easily handled with the bilge pump, but difficult to stop.

 Now that I had a round, parallel hole all that was needed was to do was machine a bronze replacement. This I designed to hold a full size cutless bearing and clamped between shoulders to secure it from rotating. On the inboard end, I fitted a pressure feed from the engine cooling system. This has it dangers but with water intake enlarged and all the sharp turns removed I hoped to increase the flow rate enough to make up for the loss.

 When St Paddy was launched the difference was immediately noticeable. The rudder could then be heard fluttering against the skeg support, steel on steel. You guessed it. I then had to silence that. It now has nylon inserts and the whole assembly is bolt together. (I changed inserts in less than an hour and didn't need epoxy anywhere)

 Conclusion. We sailed off two years ago, heading south. We spent Last winter in Greece and will be in Venice this summer. The Engine has run well over a thousand hours since I fitted the new stern tube. Even with some of the cooling water feeding the cutless bearing and the sea temperature rising to over 20 degrees C and at times 25 degrees C we still haven't had any cooling problems. The last test was motoring forty miles against a steep sea and with twenty knots of headwind. Why was I not sailing, That’s another story, I'll tell you if you want me to.

I have included some photos of me operating the machine. The motor in the picture was not powerful enough. I expected this and that is why it's only jury-rigged. I had to buy nice new blue one with a reduction box.

If any other owner is following in my footsteps and would like to know more details, then contact me st.paddy@o2.co.uk I will be happy to help where I can.

Robin.