A salutary (saildrive) tale.........

John_Silver

John_Silver

Well-Known Member
Joined
19 Mar 2004
Messages
1,268
Location
St Mary's Island
allatseawithstargazer.blogspot.com
LoResP1010938 (1).JPG

When Stargazer was first launched (in 2010) Yanmar recommended biannual inspection of the SD20 saildrive diaphragm, plus annual testing of the water ingress alarm probe. No finite service life was given. Some time later Yanmar stipulated a replacement interval of five years. Which, to be frank, I took to be a creative revenue generation enterprise, concocted between their Spares Department and Dealer Network. Fellow owners and forumites confirmed my belief that 'no one's ever heard of one failing.'

Today, during the annual engine service, I finally had the 15 year old (aprox 1500 hour) saildrive dropped. Visual inspection had shown no problems and the (diligently tested) water ingress alarm had not sounded. BUT, once out of compression, splits were evident in the diaphragm. The chamber, between the inner and outer skins, had a 'tide mark' due to water ingress.

I think that we had a lucky escape........
 
Last edited:
I put off my last Volvo change to ten years, which I still felt was a year or two too soon, especially after I got the bill. As I understand it, you would be very unlucky to suffer catastrophic failure and would at worst end up with a very wet bilge, not that this is a recommendation to ignore the potential risk.
 
It is certainly painfully expensive! Though, perhaps, not as costly as having to arrange an ‘emergency’ lift and remedials, mid cruise, speaking in a second language. At least some of the costs (lift out and a portion of the engine service labour), if done preventatively during a routine spring refit, are already factored in.

Probably well known: but Simon (Diesel Mechanic) was explaining that, whilst the Yanmar saildrive diaphragm has twin membranes, with a water ingress alarm probe sandwiched between, the Volvo diaphragm is single skin. With less margin for error and no alarm, in the event of a split. He is an agent for both - so I believe him.

Think I will be adding shaft drive to my "dream spec!"
 
Last edited:
It is difficult to tell from the photograph whether the diaphragm has perished or been damaged. As it isn't a weight bearing part and isn't subject to much movement I cannot see why it would have any wear and tear unless very poorly aligned.

I bought a Volvo SD diaphragm service kit and fitted it to our boat last year. It took me a day to remove and refit the diaphragm, plus about half a day preparing as I didn’t know what to expect. Overall it wasn’t too tricky and the only help needed was a second pair of hands to turn the prop when realigning the splined shaft between gearbox and engine.

The old diaphragm was about ten or twelve years old, and other than the date stamp and a little dirt, it was indistinguishable from the new one.
 
In the photo, the diaphragm material is being lightly pulled out of its natural position. The split area was lying, hidden in a fold, which fed on up into the clamping ring.

Possibly the material becomes less flexible with age, and the movement of the drive leg pulls and pushes at the diaphragm, causing chafe?

The original diaphragm was a factory instal by the Hallbery-Rassy yard. Who are quite particular people and have a fair amount of practice. So I'm not suspecting misalignment, in this case. Just that there is a better rationale, behind the change to the recommended replacement cycle, than I had given Yanmar credit for.

EDIT Amongst many nuggets of ‘inside information,’ Simon told me that both the Yanmar and the Volvo diaphragms are manufactured by the same supplier, using the same material.

EDIT 2: Wonder how much of a factor engine hours are? Stargazer’s diaphragm made it to around 1500 hours….
 
Last edited:
John_Silver said:
View attachment 190279

When Stargazer was first launched (in 2010) Yanmar recommended biannual inspection of the SD20 saildrive diaphragm, plus annual testing of the water ingress alarm probe. No finite service life was given. Some time later Yanmar stipulated a replacement interval of five years. Which, to be frank, I took to be a creative revenue generation enterprise, concocted between their Spares Department and Dealer Network. Fellow owners and forumites confirmed my belief that 'no one's ever heard of one failing.'

Today, during the annual engine service, I finally had the 15 year old (aprox 1500 hour) saildrive dropped. Visual inspection had shown no problems and the (diligently tested) water ingress alarm had not sounded. BUT, once out of compression, splits were evident in the diaphragm. The chamber, between the inner and outer skins, had a 'tide mark' due to water ingress.

I think that we had a lucky escape........
Click to expand...

The current manual for the SD25 seems to have extended the recommended interval to 7 years

https://www.yanmar.com/marine/wp-content/uploads/2020/05/0ASDM-EN0032_2019.06.pdf

It may be because I was reading on a mobile but I couldn’t spot any reference to the shaft seals?
 
I have always regarded sail drives with great caution. It seems nonsensical to rely on a diaphragm sealing such a large hole in the bottom of the boat and it makes me wonder at any sailboat designer who would contemplate such a design.
I can already hear the howls from those who have saildrives how hundreds exist with no(?) problems. But me I would never contemplate a boat with one.
 
dansaskip said:
I have always regarded sail drives with great caution. It seems nonsensical to rely on a diaphragm sealing such a large hole in the bottom of the boat and it makes me wonder at any sailboat designer who would contemplate such a design.
I can already hear the howls from those who have saildrives how hundreds exist with no(?) problems. But me I would never contemplate a boat with one.
Click to expand...
I wonder how many people thought that putting a hole through the hull for a prop shaft was a bad thing many years ago ?
 
The elastomer material will degrade naturally, it can't be stopped. For a sail drive elastomer mechanical degradation through movement cycles will likely be the primary failure method as it is quite isolated from chemicals, UV and significant temperature charges. I would have thought that 15 years was pushing it well into failure territory. I use elastomers in critical sealing applications and while not relevant to this application, they come with defined life expectancy in storage and in use. For example, in storage, controlled environment, 5 to 10 years from cure date, depending on the elastomer properties for the required function.
 
Drive shafts are not trouble or service free items either. I heard about one poor fella who's shaft fell out leaving a hole in the most awkward place in his pride and joy.
 
RivalRedwing said:
a rather small hole compared to a sail drive diaphragm... Sail drives are an easy cop out for a designer and probably cheaper overall for boat manufacturers too
Click to expand...
When a sail drive seal fails the whole drive doesn’t fall out, non does the whole seal disappear. It is a significant leak but probably a smaller area and therefore flow than a prop shaft being lost. Both are rare and mitigated by maintenance. The design weakness is the time and effort required to do the change not the need to change the part. Saildrives may of course have advantage in terms of other design parameters - like everything on boats they are a trade off.
 
You must log in or register to reply here.

Share this page

Top