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Old 12-05-2016, 07:16   #1
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Why Change out diaphragms in a Yanmar SD20 Saildrive

I intend to start two separate threads: Why or why not change out your diaphragms, and how I did mine. This is the first.

From my own experience and from posts here, I suggest the reasons we change them out are:

1. They have begun to let water either into the boat or at the sensor that says there is water between the two seals;
2. They are old, and either an insurance surveyor or the manufacturers' agents says it is time.

Number one, is a no brainer! I am not sure what could create a leak in the lower diaphragm - they are outrageously hardy - but it is obvious from some other posts here that it can happen. Of course, leaks in a boat must be addressed.

Number two, well, I now have trouble with that. I just changed the diaphragms on the two saildrives on Cat Tales, a 1996 FP Tobago 35. It was dreadfully hard to do, and the equipment appears to be in fantastic shape - like new - after 20 years.

The best information on the web seems to suggest that Yanmar recommends inspection every two years and total replacement of the stuff every 6 years. As a result, if you have your boat surveyed every 5 or so years for insurance purposes, and the surveyor is worth anything, you have to demonstrate your effort to stay on the manufacturer's schedule.

Inspection is really difficult. I guess you could sacrifice the lower boot and inspect the lower section with a mirror and good lighting; and remove the straps on the upper diaphragm and inspect the upper part of the lower section and the upper diaphragm completely. This method might only cost you $500 US or 400 Euros, as the boot usually rips, and the two straps are not allowed to be used after removal.

Not that I'd do it, but I can see how someone might simply say to himself that he has tested the water sensor, and since there is no water between the two diaphragms, I have inspected the installation - and say to the surveyor that he has done so.

Replacement after 6 years? That means a new lower, upper, two straps, and the boot that provides fairing against the outer hull. That costs ~1000 Euros per saildrive, if you do it yourself.

If I was not attempting to maintain my insurance, and knowing what I know now, I would not spend the money. I swear, all parts were in excellent shape, no different from the new parts.

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Old 12-05-2016, 18:46   #2
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

I should add that there are significant opportunities to do damage while the work is being done, for example when removing the controls, damaging the finish on the saildrive, causing leaking that was not happening, ruining threads on bolts and the bolt holes, misaligning the drive train, and of course ruining your back.
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Old 13-05-2016, 16:27   #3
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

I hear you, and appreciate your sharing what you found with your old diaphragms.

We need to have a sail drive diaphragm support group. It would be good to know who has had a leaking diaphragm other than from a substantial hit to the lower unit, or a careless shifting of the engine.

I get a certain amount of comfort from the fact that our engines are located behind (practically) water-tight bulkheads. I have increased the size of the bilge pump back there on one side, and will do so on the other side as well.
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Old 17-06-2016, 16:32   #4
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

More Diaphram Blues. Amongst others. I will try and explain!
Top Diaphram large band has come off. I am hoping to replace while in the water , also I would like to replace the water sensor.
I am thinking this can be done also.
Reasons.
I was away from the boat for about 3 weeks. I did a pre engine check before the last sail , so possibly 3-4 weeks ago.
Everything appeared fine , including bilge pumps.
On my return and pre inspection I found my engine half under water. Will not start. Just another issue.
Bilge pump failure . After pumping out the water , spare pump , I have found a constant drip in the water pump.
My thoughts were not enough to have so much water, but then realised maybe when engine was previously running , the pump could be pumping enormous amounts of water into the bilge. Ok , so look into the water pump also.
On further inspection , I noticed the top Diaphram , large metal band , had come off , with the Diaphram now only held in place by the small band. Also found one wire on the alarm broken . Only thing stopping further water getting in must be the lower Diaphram . Doesn't sound like I do much maintenance does it . I am usually quite persistent with all engine room checks.
Back to top . Maybe should start a new post.
I am writing before thoroughly going through all posts as I simply want to get this Diaphram back in place for obvious reasons.
Cheers Darren
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