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Old 12-05-2016, 06:16   #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.

Comments?
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Old 12-05-2016, 17:46   #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, 15:27   #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, 15:32   #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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Old 07-06-2019, 18:54   #5
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

My SD 20 saildrives are 13 years old
And I pulled the outer rubbers off (bottom)on them for inspection, those alone were $125 each ..from what I could see the main rubber diaframs both look like new...no leaks.. ..the boat has been in the water about 6 months a year ,every year and sailing in the Med. About 3500 hours each on them.
The top band inside is gone and after careful examination of that so called seal, it's hard to imagine it holding any water out, seems worthless..I'm going to restrap it with large wire ties anyway as soon as possible..

Instead of changing the diafram at this time I have decided on and installed 2000gph bildge pump with a water witch sencer auto switch.ive also connected an alarm to it so when it turns on it will sound..the engine room has a bulkhead that goes a few inches above the waterline..so hopefully if a major breach occur the boat does not sink..
I've also installed 4000gph pumps set up the same way in both hulls..

The only leaks I've ever encountered is from the raw water pumps , when there worn
I rebuild them with new seals and impellers about ever 3 years or so
The shut-off valve for the incoming water on these drives suck so I replaced them with a ball valve that works well....
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Old 09-06-2019, 13:17   #6
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

I installed larger bilge pumps back there as well. Our boat has a bulkhead forward of the engine, too, and other than an impact tearing out the SD, I can't imagine a breach causing more than a slow leak, at least one that the pump could handle.

Our SD20s (and diaphragms) are almost 20 years old.

This would be a good time and place to hear from others about their experience in this regard....
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Old 25-11-2020, 09:13   #7
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Ram - did you used wire ties? How did they work? New bands are now $63 each and do not seem to last long. I am thinking to use large heavy duty zip ties (wire ties) with a tool to tighten them.
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Old 26-11-2020, 19:29   #8
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

I replaced a band that broke with an extra large hose clamp.

In a pinch, you could put a couple of smaller ones together.
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Old 08-01-2021, 12:01   #9
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Volvo saildrive owners also welcome!

Can anyone else chime in on their experience with the lower rubber diaphragm (boot, seal or whatever) that keeps water from entering the hull?

Did you have a leak due to failure of this item? What was the nature of the failure? How bad was the leak?

Was there an impact to the saildrive that caused the leak?

Did you change this boot because of age? If so how old was it and how did it look?

I'm at 20 years on the original boots and don't want to mess with something that is still intact...unless there is a reason to. If an impact is the most likely cause of a leak, it seems that would happen regardless of age.

As many saildrives as there are, there should be a lot of knowledge out there.

Another thing. That upper rubber diaphragm/seal; I'm not sure how much comfort I have that it would contain water pressure from a leak in the lower diaphragm. It's awfully thin, and not secured sufficiently for much water pressure, IMO. So I'm counting on the strength of the lower boot, good sized bilge pumps, and water tight bulkheads.
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Old 11-01-2021, 05:52   #10
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Wow, nobody?

With the cost/difficulty of replacing the diaphragm---- the consequences of a failure--- the numbers of saildrives being installed in all these catamarans---and considering that Yanmar says to replace it every 6 or 7 years, I thought there would be a few comments.
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Old 11-01-2021, 06:09   #11
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Quote:
the cost/difficulty of replacing the diaphragm

I did this when I first bought the boat, had to remove the engines to get the saildrives out, not easy ( I did it myself). The ones I took out looked fine (unknown number of years). I am doing it again but the 5 year old ones look fine but 2 of the expensive SS bands had failed. You can be sure I won't be getting another boat with saildrives.
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Old 12-01-2021, 18:16   #12
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Since you have done the job, you know more about it than I do!

You mention the two ss bands. Is that what secures the lower diaphragm as well? Or are you speaking about the upper diaphragm?
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Old 13-01-2021, 06:17   #13
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

ggray - correct, one for each, i will post a pic when I replace each one to show you, maybe next week.
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Old 13-01-2021, 09:13   #14
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Thanks for anything you can share!

So the reason for doing this is the failure of the SS bands? Were they both on the same motor?

Again, I haven't put my hands on anything regarding the lower diaphragm, but if the band securing the lower diaphragm is like the one securing the top diaphragm, that is a concern. One of mine fell off years ago. I think I bought a new one, but having no means to crimp it, I just bought an extra large screw hose clamp. I think it's much stronger anyway,
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Old 13-01-2021, 10:06   #15
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Re: Why Change out diaphragms in a Yanmar SD20 Saildrive

Greetings fellow mariners,

Below email is a cut and paste of a response I proffered in a thread a couple years ago. I am lazy and it is easier to do this than to retype it......


Greetings all,

After growing up on power boats and monohulls, as well as owning several of each myself, I was a staunch shaft only guy and nary had a good word to say of saildrives. When we made the switch to cats I even specifically chose to only look at catamarans with traditional shaft drive and we ended up buying a TPI Lagoon 35 with twin Yanmars and shafts. Loved that boat but sold it two years ago to “upsize”.

Due to many circumstances far too long and boring to type here we ended up purchasing a 1993 Solaris Sunrise 36’ sport catamaran with twin yanmars and SD20 saildrives. Being anti-saildrive made it a difficult decision but the price was far too enticing to pass up and being recently retired I needed a project. Got it for a steal as it has been sitting unused for about 15 years and is in need of a complete retrofit. The engines have just over 600 hours each so barely used. Spent the first six months on the hard, pulled both engines, saildrives, rudders, every thru-hull......etc.etc.etc and either rebuilt or replaced it all.

The port side diaphragms, which I am certain were both original from 1993 as I had to chip the factory paint to get the bolts out, were in pristine condition. I replaced them all anyways, and kept them for spares, as I went through the entire saildrives including gears, bearings, seals......etc.

The starboard side was not so good. Through no fault of the saildrive design or equipment it was a gooey soft mushy mess and was actually slowly allowing water to seep though prior to us hauling out. The previous owner, or perhaps one of the many brokers or fender kickers that had looked at it prior to us buying it had ran it with the diesel return line removed from the fuel rack and as a result the bilge had filled with about 6 inches of diesel oil. It was full when I went for our initial inspection and helped the owner, actually I pretty much did it for him, bail it out and clean the bilge. I also pointed out to him the source of the diesel in the bilge and repaired it.

Long story short, the diesel bath didn’t do any favors to the diaphragms and had turned them into a gelatinous goo.

After removing and rebuilding the saildrives I am now a convert. I am no longer at all concerned about the “big hole in the boat” or the craftsmanship of the design. We do have water tight bulkheads well above the waterline for each engine bay which also boosts my confidence.

If you read all that and are still here........or if you have skipped to the end of my ramblings then the condensed version is....

Don’t let any oil/diesel or any solvent or petroleum products come into contact with the diaphragms. I have always been very diligent about clean engine rooms, I was a machinery technician in the Coast Guard, but I am now OCD, as my wife puts it, about keeping the bays spotless and the diaphragms free of any contaminants.

Just one poor mans prose,

Safe Journeys all,
~Jake


Like the OP’s vessel, ours also has water tight bulkheads in each “engine room” which extends well above the waterline, even when heavily loaded or full of water. I also have installed large bilge pumps with Ultra float switches/alarms in each engine space, as well as the forward bilge compartments so that we now have a 3500GPH pump in each engine space plus a 2000GPH in each hull, amidships, as well as a portable manual whale gusher 30 and a spare 2000GpH pump with a 25’ wire lead/alligator clips for emergency backup if and when a 12vdc power source exists. I have a coil of 10’ discharge hose with it in my “damage control kit” which, in my boat, is sufficient in length to reach at least one port or hatch from any spot in the boat. You may perceive the above preparation as excessive and paranoid. No argument from me, do as you wish. After spending time in the USCG it has taught me to be over prepared in many ways. Not sure if any of that is valuable or useful, and I have no doubt that everyone here is much smarter than am I, so their opinions are of much more value than any I might offer, assuming I am of course allowed an opinion. I have been told that only those that meet some members here high level of intelligence and experience are worthy of posting or providing guidance. With that, and upon further reflection, simply ignore the entire prose found above in favor of those that are willing to share the vast, impressive, unparalleled and amazing knowledge they possess as they are happy to tell you how much smarter, stronger, faster, more productive, and clearly more well liked, and how in general, have little or no use for us commoners of the world.

Safe Journeys to most,
~Jake
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