Selva saildrive diaphragm replacement

[Mark L]

Hello,
I have a selva saildrive unit fitted to a beta 28 which has started letting water into the boat, particularly when the engine is running. The water is seeping in between the seal and the inner clamping ring (i.e. saildrive side rather than GRP moulding side). Not enough to be dangerous, but enough that we couldn't leave the boat on a mooring unattended and obviously concerning. We were a good distance from anywhere to lift out when I noticed, but I managed to reduce the rate by slackening off the nuts on the inner ring slightly (tightening them made it worse). The boat is now out of the water for a more detailed inspection. I have had the boat for a year and reportedly, the saildrive diaphragm was professionally replaced 3 years ago. A slightly suspicious sign is that there seems to be a large amount of some sort of caulking/ gasketing material between the inner ring and the diaphragm which may indicate a previous problem (my understanding was that the diaphragm seals on most saildrives are supposed to be dry fitted). I will lift the saildrive out to have a better look and might as well replace the seal at the same time anyway.

I have a couple of questions: 1. Does anybody have any suggestions as to why a relatively new seal might start leaking and is there anything particular I should be looking at when inspecting it?
2. Does anyone have any detailed instructions for replacing the seals on the selva units? The manual is pretty light on detail (torque settings etc) and beta were not able to supply any more detailed instructions with the part.

Any other thoughts or opinions welcome.

 

[RichardS]

There was a thread a week or two ago where, for some reason, the top of the saildrive where the diaphragm is situated, was seriously corroded and will need to be replaced. I suspect that getting a good seal against a corroded area like that would require a lot of gasket sealant.

I think that the previous OP suspects that galvanic/electrolytic corrosion of some kind is probably the culprit.

Richard

 

[Mark L]

There was a thread a week or two ago where, for some reason, the top of the saildrive where the diaphragm is situated, was seriously corroded and will need to be replaced. I suspect that getting a good seal against a corroded area like that would require a lot of gasket sealant.

I think that the previous OP suspects that galvanic/electrolytic corrosion of some kind is probably the culprit.

Richard

Yes, I fear that it may be something like that; I imagine that the stainless studs would be a likely location.... Oh well, only one way to find out I suppose. I presume that any serious corrosion on that surface of the saildrive would be pretty terminal ?

 

[RichardS]

Yes, I fear that it may be something like that; I imagine that the stainless studs would be a likely location.... Oh well, only one way to find out I suppose. I presume that any serious corrosion on that surface of the saildrive would be pretty terminal ?

It's not really my area but it will totally depend on what you find. With light corrosion it might be possible to abrade it out but still leave enough circumference to fit the band. If this isn't possible I think that replacement parts are the only solution as I don't think using sealant on a corroded area is a good idea on something as important as the diaphragm.

If it is badly corroded the question is why? It's something that probably needs to be investigated as it might just repeat itself unless something is modified .... anodes, galvanic isolator, saildrive isolation etc depending upon what is needed.

Richard

 

[richardbrennan]

I was that OP and it was a Volvo Saildrive, but I suspect the basics are the same.

I started off asking my engineer to replace the seal, but when he took the leg out, it was clear that there was significant corrosion around the join between the upper and lower legs and the intermediate plate that fits between the two. After splitting the leg, the extent of the corrosion was apparent, the engineer said he had never seen anything like it before. The picture is of the upper leg, but the other components were equally badly corroded, and the seal sits in the channel around the outside. The engineer doubted it would seal properly, and the yard asked for a written indemnity absolving them for any subsequent failure should I decide to have it cleaned up and put back, so it was time for a new leg.

The cause of what was almost certainly electrolysis remains unclear. It could have been due to faulty initial installation, stray currents in the marina, or the anode failing to work properly. I suspect the latter as it did not change much over the past couple of years in spite of there being a good amount of zinc remaining.

Good luck, if you need more info do PM me, and if you need a new drive, try Marine Enterprises, I am not sure if they deal in Silva drives, but he supplied mine for about half of what Volvo wanted.

Rick

 

[Mark L]

Thanks both, that sounds like sensible advice and lots of things to look for. I guess that I will just have to take a look and see... Thanks for the offer Rick, I may well be in touch depending on what I find.

Cheers,
Mark

 

[Mark L]

Just by way of an update, I took the saildrive out at the weekend, and I think I may have found the problem. As I was lifting it, the rear engine mount (one of three) fell apart. It turns out that the engine mount did not have a nut on the stud, beneath the bracket (i.e. the lower one that supports the weight of the engine). Tightening the upper nut would have done nothing but put tension on the engine mount which must eventually have failed, putting any rotational forces (from vibration or thrust) onto the seal? In the photo below, the upper plate of the engine mount should be flat!
The good news is that the aluminium of the saildrive is in very good condition so this may not be as expensive as I feared. A bit annoyed with myself for not spotting it before lifting the saildrive out, but probably no bad thing to change the seal anyway.

Thanks again for all the advice.