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Yanmar saildrive diaphram

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Hi guys


We have recently discovered that the diaphragm B in our yanmar sd20 sail drive has split (see pic). It isn't leaking any water, so I assume diaphragm A is still solid. I have a few questions:


Have any of you replaced these diaphragms? If so how much did it cost and how difficult was it? 


How risky do you think it is to continue using the boat like this?


Do you think the local Yanmar dealer should have picked this up in their recent service?



diaphram sd20.png

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OK, so the alarm isnt working? 

If there is water in between the seals, the alarm should be sounding.

The inner seal is more basic and less strong than the outer. Its held to the inside lip frame by a simple clamp.

Sorry but replacement of the outer seal is a boat out of water job. Its not onerous, but it takes a bit of time. I'd fix the alarm at the same time....

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The inner seal is the one that is cracked. I assume its purpose is some kind of backup for the outer seal.


I haven't checked the alarm yet, but there's no water to trip it at this stage. We are hauling out next week to anti foul, so that could be an ideal time to do it.


Is it a case of pulling the prop off the sd, then the sd off of the engine, then sd out of the boat, fix it and reinstall?

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You will find in the manual that they are to be inspected every 2 and replaced every 6 years or the likes

This I learnt when I replaced a motor and sail drive that was 20+ years old with original seal

Edited by Jon
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Quite risky to continue using it not so much because it’s likely to fail but mostly because if the boat sunk because if this your insurance would most likely not pay unless you could prove you had replaced / inspected it within recent past and it was all ok. Your latest inspection has shown it’s not ok - and so your insurer would require you to take action reasonably soon rather than ignore it and hope for the best.

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Thanks for the replies. The boat had just been serviced by the yanmar agents, so I think that insurance would have a hard time saying that we were negligent. Also I would rather base my decisions on actual risk instead of hypothetical situations. Some other brand sail drives only use 1 seal and seem to be surviving ok.


After having a further look today I think that the inner diaphragm was never installed properly as I cant find the inner steel band. I'm not sure if this contributed to the tearing, but we have carried this risk since buying the boat and it has been ok.


Regardless, I think the boat was repowered in 2007 so 12 years life is reasonable for what is supposed to last 6 years. I think I would rather know that the outer seal is 100% so that I can feel confident in taking it up the coast this summer. It scares me a bit that the only thing in these saildrive boats between floating and sinking is a bit of rubber that is attached to a hunk of metal with explosions and rotating parts inside. 


After having a look into it, the pros reckon its a case of pulling the saildrive out from above, giving it a birthday and putting it back in. Ill do a few other jobs on it while its out and then at least we should get a few more years before having to worry about it again. Good thing we were planning on hauling out for antifouling next week anyways.


Oh well, we didn't really need that $1200 and spare weekend before Christmas I suppose....

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  • 4 weeks later...

The inner seal doesn't do much, it's mainly just to keep dirt off the main seal and to stop water collecting in there causing corrosion.


It's often wrongly described as a backup to the main seal but the clip holding it on is pretty useless and it would pop off if there was any pressure behind it. Other saildrive manufacturers don't even have a secondary seal.


The main seal should be replaced after 7 years if it has been heavily used or had sharp shells growing on it. When it gets very old and stiff it fails very gradually (cracks let in a very little water) over a long period of time. It is very unlikely to fail catastrophically.


If the main seal is getting old then it would be best to replace it on your next haul out, which involves splitting the leg from the gearbox. and removing both. You can remove them from above as a single unit before splitting them or remove the leg from below and then the gearbox from above, whatever works best for your install. It's not as hard as it sounds. Unbolt the ring clamping the seal down to the engine bed first and the bellhousing from the engine.


I wouldn't bother replacing the secondary seal, as long as it keeps the dust and dirt out and you can keep it reasonably dry in there.

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