Saildrive Diaphragm Replacement Schedule

[Sonosailor]

Seems to be no thread specifically on this. How often do saildrive diaphragms fail, how often should they be inspected and how, and what is a reasonable replacement schedule for each manufacturer and each model?

Cat Tales (1996 FP 35) has the original diaphragms in her two Yanmar SD-20s. She also has an interior boot with a water sensor to give us some alternative protection. I really do not hear much about these things failing, and have even asked repair people about them with vague answers (Mechanics at Le Marin, Martinique suggest I show up when one fails, not before). Still, I have a surveyor's report now telling my insurance company that the manufacturer recommends inspection every year and replacement every five years. I know that a decade ago they pushed for replacement every two years; so this seems to be under review from time to time.

Any comments, any experiences? I'd love to hear from Yanmar, but can't seem to find the right contact.

[Sonosailor]

Well, I just checked again, and the newest on-line manual says now to replace the diaphragms every six years! I wonder what my surveyor used?

[DotDun]

Thanks for the heads up on the Surveyor. I now know how to answer the question when he asks!

Mine is 12+ years old and I'm thinking the next bottom job might be a good time to replace them.

Like you, I have not heard of any problems from anyone no matter the age.

[colemj]

The replacement "schedule" for your drives would be whatever was recommended at the time of their manufacture/installation. I think it is every two years for yours. Volvo replacement recommendation is every 7 years.

Your inner seal is very lightweight, and I would not count on it holding in a total breach. It is only meant to hold a slight leak at best. The outer seal is your main, and pretty much only, seal. Volvo doesn't even use an inner seal.

I have the same question on replacement as you do, and have searched high and low to find an actual case of seal failure that was not caused as a secondary event to something catastrophic - punching a saildrive on a rock does not count as seal failure, for example.

I have not found a single case, nor have any of the professionals and manufacturers I have spoken to about this ever heard of a failure. I found one person who changed theirs after 14 years and they said their old seal was a bit hard and had some small shallow checking on the underside.

Ours are 17yrs old and I plan to replace them at our next haul out (which could be 2yrs from now). They show no obvious signs of being old. I have the new ones in hand and they are very thick rubber.

I suspect failure of this seal due to old age would manifest mostly in weeping of water through small cracks - possibly stopping when not underway, and leaking when moving. Sort of like how exhaust or thick water hose fails. I don't expect a catastrophic and total failure mode, with huge ingress of water to sink the boat. So one would probably have ample time to haul and replace if they notice water from the seal.

Mark

[Jim Cate]

Mark, I wonder if one reason for replacement is that as they age, it takes less of an impact to cause them to rupture?

Jim

[colemj]

There really isn't a way for them to be impacted. They are deep within the hull and the hole around them is covered with a rubber gasket. It would take a 1/4" wide by 18" long shiv jammed up hard in just the right orientation to rupture them. A sharp pointed shiv. After it has punctured the fairing gasket.

As for the saildrive itself being punched up and causing a seal rupture, or wrapping a line in a way that violently twists the drive leg and ruptures the seal, any force great enough to break the drive/engine mount(s) will rupture even a new seal.

I have both a Yanmar and a Volvo seal on hand and can see absolutely no difference between them. The fact that Yanmar recommends they be replaced every 2 years and Volvo every 7 years seems to me to be a combination of CYA and guess work. Maybe the parts sales department had a hand in it also?

As an analogy, these seals are far thicker and more robust than bellows used in dripless shaft seals, and those don't fail after 2 or 7 years.

Mark

[DotDun]

We replaced the outer rubber (at the outer hull) on our Yanmar SD20s with a solid plastic material ~4mm thick. Easy to make, it's the same size as the original and follows the profile of the leg leaving a gap of ~4-5mm. Cut a slit in it and bend it around the leg and screwed to the hull.

It would take a precision strike to get between the plastic outer cover and the sail drive leg to get up far enough to damage the main seal.

[out42]

Just had a summer dogged by this problem. 9 year-old catamaran with two SD20s, on the way from South of England to NW Spain when the stbd warning light goes off and removing the sensor shows water inside. A slow leak but still a leak from the exterior to between the seals.
There followed a long saga of a return to Plymouth and looking for a haul-out and replacement. No evident damage, just a failure of the exterior diaphragm. So it does happen, and I have no idea how you could predict the timing for a change. I certainly wouldn't expect either Yanmar or Volvo to be able to tell you with any certainty as I guess it ultimately depends on use. It's unlikely to cause too many sinkings either, so I doubt the insurers know.
I think now I would probably change mine again in 5-6 years during a routine haul-out so that risk of a ruined summer is minimised. If the boat is out of the water, the diaphragm is known to be old and you have doubts, change it/them.

[a64pilot]

I don't know anything about a sail drive diaphragm, but it sure sounds like the diaphragm that used to be in old design inboard / outboards? I've seen those sink boats in my youth, I assume they are all gone by now?

Service life is often based solely on an opinion, weighted heavily on what the possible failure outcome is, aircraft about the best you can get on a critical part is a 7 to 1, that is replacement interval is 1/7th the expected life in general usage. I wouldn't be surprised if something that could possibly sink a boat has as conservative replacement interval

[2Hulls]

Originally Posted by colemj

I have not found a single case, nor have any of the professionals and manufacturers I have spoken to about this ever heard of a failure.

Nor have I.

I am just completing changing the seals for the first time on my 14 year old Volvo 120S-E drives. After removal, the old ones look fine and are not cracked or brittle. I believe they could have gone a lot longer. I had been concerned that a surveyor would ask when had I changed them - none have asked so far - and I'm in a period of extended refit, so I decided to do it. Note: when installing the Volvo seals, orient them so as you can easily see the embossed year of manufacture to show the surveyor when they get smart enough to ask...

Next up, timing belts and water pumps......

Dave

[colemj]

Originally Posted by 2Hulls

After removal, the old ones look fine and are not cracked or brittle. I believe they could have gone a lot longer.

And now your wallet has been exercised, so it won't be cracked and brittle either.

I wonder if it would be easier just to palm the surveyor the $500 and tell him you just replaced them…

Mark

[colemj]

Originally Posted by out42

A slow leak but still a leak from the exterior to between the seals.
There followed a long saga of a return to Plymouth and looking for a haul-out and replacement. No evident damage, just a failure of the exterior diaphragm.

Was there a noticeable crack or hole in the exterior diaphragm?

The Yanmar dual diaphragms are notorious for false water intrusion that sets off their alarms. Condensation can cause water accumulation, as well as water leaking from the raw water valve through the drive. Also, if you regularly have water around in the engine compartment, the seal between the two seals is not very water-tight and it can leak between them. All of which looks like an exterior seal failure (especially when that warning alarm goes off).

Mark

[Emerald Sea]

Next questions ... where does one purchase these seals? They must all be different sizes for various boats. Do the engine manufacturers produce them? And what is the material grade/specification?


Steve
L450
SV Emerald Sea

[colemj]

Well, if you have Yanmar drives, then you get them from a Yanmar parts company. If Volvo, then from a Volvo parts company. Etc for other types. They are sized for the drive leg, not the boat, so there are only one size for each manufacture.

I have both a Yanmar and Volvo seal on hand, and from a cursory glance at the two, they look identical to me.

The material is some type of rubber - probably synthetic nitrile. I don't know the grade/specification, and one doesn't have any choice of that anyhow.

Mark

[2Hulls]

Originally Posted by colemj

I wonder if it would be easier just to palm the surveyor the $500 and tell him you just replaced them…

For sure easier, but a lot less than $500 to DIY. Better look to see if the date of mfg can be seen on your seals.

By the time I'm done I'll be really good at this and will supervise anybody else for some good rum...

Originally Posted by Emerald Sea

Next questions ... where does one purchase these seals?

...and (following Mark) they're probably both produced or procured by ZF and can maybe be sourced separate from Volvo/Yanmar.

For any Volvo owners - do NOT buy the "all inclusive" kit from Volvo that includes the rubber seal and a few other things. This kit is intended for all Volvo drives and includes some stuff I didn't need plus, as a bonus, it included the wrong gasket for between the adapter plate and tranny. I had to get two of those separately.

Dave