garvellachs
Well-Known Member
A post for information rather than a question. All last year our newish rubber 30mm prop shaft seal leaked, bought from ASAP as a replacement for the old Volvo lip seal. The old Volvo one didn't leak - I replaced it because it was old and likely to be well-worn. The shaft was replaced at the same time.
It leaked only when the engine was in gear on tickover speed, forward or reverse. At idle out of gear it was OK; at speed it was OK. But a steady drip emerged when in gear at tickover speed.
I wracked my brain all summer wondering why. In the end my best guess was that the 30 year old engine mounts had allowed the engine to slump down slightly and that the new seal was less tolerant of this than the old. It certainly felt as if it was made of stiffer rubber - it was definitely harder to burp for instance. So over the winter I splashed the cash and bought new mounts (Yanmar 3HM), fitted them and aligned everything carefully, and I went back to a Volvo seal. Result: this season no leak. And when I measured where the shaft was in the log with the seal removed but still with the old mounts, it was definitely lower than centre (but there was no knocking).
A bit too much change to say definitively what the cause of the leak was, but for interest I took the ASAP seal home and put it on an old but clean and serviceable 30mm shaft, vertically, with the lip seals at the bottom. I poured water into the seal from above to see if it leaked. No leak.
Then I applied finger pressure to the other end of the seal where the clip goes to simulate it being out of alignment/centre with the prop shaft log. Instant leaking.
This is despite the seal having a long molded-in cutless bearing between the clip and the lip seals to hold it in alignment.
I still don't really understand why the seal only leaked in gear at tickover speed? I suppose it's possible that the engine rises slightly when power is transmitted to the prop, improving the lack of alignment?
My conclusion is that these rubber lip seals really do need the shaft to be well-centred in the log, and I wonder too whether the Volvo ones are a little more tolerant of this than the newer makes. Plus I'm not convinced they are made of the same rubber. All just my opinion of course.
It leaked only when the engine was in gear on tickover speed, forward or reverse. At idle out of gear it was OK; at speed it was OK. But a steady drip emerged when in gear at tickover speed.
I wracked my brain all summer wondering why. In the end my best guess was that the 30 year old engine mounts had allowed the engine to slump down slightly and that the new seal was less tolerant of this than the old. It certainly felt as if it was made of stiffer rubber - it was definitely harder to burp for instance. So over the winter I splashed the cash and bought new mounts (Yanmar 3HM), fitted them and aligned everything carefully, and I went back to a Volvo seal. Result: this season no leak. And when I measured where the shaft was in the log with the seal removed but still with the old mounts, it was definitely lower than centre (but there was no knocking).
A bit too much change to say definitively what the cause of the leak was, but for interest I took the ASAP seal home and put it on an old but clean and serviceable 30mm shaft, vertically, with the lip seals at the bottom. I poured water into the seal from above to see if it leaked. No leak.
Then I applied finger pressure to the other end of the seal where the clip goes to simulate it being out of alignment/centre with the prop shaft log. Instant leaking.
This is despite the seal having a long molded-in cutless bearing between the clip and the lip seals to hold it in alignment.
I still don't really understand why the seal only leaked in gear at tickover speed? I suppose it's possible that the engine rises slightly when power is transmitted to the prop, improving the lack of alignment?
My conclusion is that these rubber lip seals really do need the shaft to be well-centred in the log, and I wonder too whether the Volvo ones are a little more tolerant of this than the newer makes. Plus I'm not convinced they are made of the same rubber. All just my opinion of course.
