Intentions are to write-off the monies spent of maintaining the wretched thing (about £5k the past two years) and go for a new or reconditioned SD60 which I can source for about £4k plus fitting/VAT.  

I'm told that 17 years and 1500 hours of use is 'old' and 'very much' and that its no surprise that the leg is failing.  Given the number of such legs in use that surprises me a great deal and I'd be interested in the experience of others with SD50s.  Any views on if the mechanic should have been able to spot the wear given the number of times he's worked on it?

If you can't take a joke - don't buy a boat!

Iain

The case hardening that you appear to have occurs as longitudinal crevasses in the gear tooth face and in outboards and drive legs is usually caused by long periods of sitting at tick over or light loads. In yachts this usually occurs when the engine is left running at tick over or just above in neutral to charge batteries. Under these conditions, the drive chain can experience chatter in the transmission where the gear teeth literally bounce back and forth between gears instead of sliding smoothly across the surfaces of the teeth. If there has been gear chatter between the intermediate and output shafts, it is likely that there has also been chatter between the gears on the input and intermediate shafts.

From what I can see in your photo, it’s not that bad. If it was my boat, I’d be annoyed but I would rebuild it as there is still plenty of life in it. When in use I’d make sure I didn’t leave the engine ticking over in neutral for any length of time and I’d reduce my oil change intervals. Another thing I’d do is modify the existing drain plug or find an alternative that has a magnetic insert in it (I’d be very surprised if there wasn’t a suitable one to use from an outboard or outdrive). A magnetic drain plug will catch and hold any chards or metal flakes that come off the gear teeth and minimise the risk of damage to bearings and other gear teeth.

I’d also keep a lookout to find a spare leg or gear set for future use. Having said that it wouldn’t surprise me if you got years of future service out of your existing leg if you don’t send time in neutral low load situations.

Iain, if you want any further info PM me

Hope this was useful 

Simon 

Since posting the original I'd been toying with the idea of delaying replacement for another season by which time we plan to be in the Clyde area.  However, the Mechanic had been concerned about chards affecting the upper section so your advice about the the magnetised plug is useful in itself.  Thanks also for your explanation of what the picture shows; by itself it meant nothing at all to me!

Iain

As I said earlier, I wouldn’t worry too much about your gears. I actually have a spare leg for my own boat which I picked up on eBay a few years ago for just over £200 and it’s supposed to have only 200 - 300 hours on it. It’s never been antifouled and is in amazingly good condition, so they are out there if you want to get one, you just need to take your time looking to find one.

Simon

no trouble doing the oil change while afloat.  In case it's not too obvious, just need 2 of the (reassuringly expensive) Yammer oil extract tubes in place.  One at the base as normal and another at the alternative connection near the filler cap.  I have the top one permanently connected to a small oil reservoir anyway.  With the drive as warm as possible, put a decent oil extract pump on the lower tube.  Then use a small tyre compressor to pressurise the top of the SD50 gently.  It's never a quick process but I get >2L out in 10-15mins when the drive is warm.  Can take 30 mins if the drive is cold but can still be done.  Easy enough that I do it a couple of times a season (I do that as the GL4 is kinder to the cone clutch but I'm not as convinced of its ability to manage any small seawater ingress (versus the GL5) via the oil seals as they and the propshaft age).

Best of luck with the gears.  Personally, I'd also just go into a phase of refit and monitor.  It's not hard to get the bottom leg open to take a look at oil seals, shaft and lower gears.

Mark

Thx for your posts.  

I recently dealt with oil seals etc on my SD50.  Also changed prop shaft bearings, but could not get to the seat and bearing sitting behind the pinion gear in the pic you posted. Was at a loss as to how to do it without removing the vertical shaft.  Grateful if you could shed some light on how to do it. 

Also interested in your opinion on fitting a gearbox breather using the screw opening near the gear oil filler.  Im going to fit the yanmar supplied oil change barb and attach a length of 8mm PVC pipe.  After reading these forums, I understood one of the reasons for oil seals failing was the  induced pressure/vacuum in the gearbox caused by warming/cooling cycles. By using a breather, it means there is no adverse +/- pressure build up.  Seems logical, but i say to myself, why did the designers make a completely sealed unit? Does it need pressure build up for the gear box oil to circulate?  What is your opinion on this? 

The situation is completely different for saildrives and outboards where the gearbox is immersed in water and they don’t have breathers. Firstly the heat build up is significantly less and consequently the pressure build up is also considerably lower so oil retention at seals and gaskets is more easily achieved. But once the seals have wear on them they do leak small amounts of oil and when the gearbox cools down after use there is also a small amount of water drawn into the gearbox which in turn causes the oil to turn milky. It is remarkable how tolerant gearboxes are to emulsified oil, but the situation needs to be addressed before it gets too bad. The reason that these marine gearboxes are sealed is for the situation when seals are starting to weep. As a sealed unit the boat is still water tight. But if there is a breather fitted, when the seals weep the boat has a leak and if left long enough the water will continue to fill the gearbox and the oil will flow out of the breather until such time as there is no oil left and water is flowing out into the boat. If the breather is fitted and raised to be above the waterline, the gearbox will still take a considerable amount of water before the fluid equilibrium has been reached. In this situation there will be an additional 250 - 500ml of water in the box. Too much for the oil to emulsify and still work which results in a lubrication failure and scraping the internal gears and bearings. 

To summarise, I personally wouldn’t recommend having a breather on a saildrive or outboard gearbox for the reasons outlined above. Just keep an eye on it when doing your engine checks and if the oil is a bit milky, don’t worry too much about it but take a note to change the seals at the end of the season.

Hope this was useful 

Simon

Simon

Think of a 1m pipe with 1dm2 area. This will hold 100 liters. If its filled with gear oil the weight will be 90 kg. Submerged into water this oil filled pipe (assuming pipe material density is equal to oil density) will float with 10 cm sticking up, pushing away 90 liters of water witch is equal to its own weight (this is called Archimedes teori).

Thank you for detailing the logic behind sealed gearboxes and in particular, for this marine application.

  

Jesper, 

Thank you for your post detailing the alternative viewpoint. 

The engineer in me can see the benefits of both.  

I found a place that does seals for $10 compared to the $150 yanmar option, so my initial thoughts are to change the SD oil and shaft seals same time as engine oil and annual haul out. In the meantime, i install the breather extending to the very top of the engine compartment (0.5m above WL).  

I have another question.  I recently purchased a Gori 3 blade folding prop to replace the fixed prop installed by previous owner.  Gori weighs around 15kg.  When i asked the mechanic to fit it, he said i think it is too heavy for the SD50 leg.  The supplier that sold the Gori said nope, it's the right one, im wondering if anyone could shed some light on this from their own experience. 

Thanks for feedback re: the prop weight. 

The Gori I have is an 18x13 too.  

When a gearbox alternates hetween running and stationary with a breather, every time it cools down it draws cooler air into the gearbox and it is quite common for condensation  to occur which is particularly destructive in a marine environment with salty air.

When it is a sealed unit there is almost no condensation at all. Yes, there is a build up in pressure,  but the effects of that are taken into account by the SD designers ( typically ZF), who have thousands of specialist desigbengineers and 100+ years of experience producing millions of gearboxes for every application imaginable including ships, planes, helicopters, military applications, yachts, automotive etc.

If you choose to fit a breather then it is important to use a desiccant type, or Gortex type which prevent atmospheric  moisture transfer into the saildrive.

If you use a simple breather you reduce the issue of pressure build up  but increase the serious issue of corrosion through water+salt increase due to condensation.  Nothing kills lightly loaded case hardened gear teeth faster than corrosion. 

I will disclose that many years ago I was National Technical Manager for ZF in Australia, and have many decades involvement in mechanical power transmission (gearboxes) and as such have seen the effects of condensation related corrosion many times.

Mark

I don't  really see much advantage in a separate header tank, as it just adds more  air (if it is sealed) or more air and more condensation if fitted with a breather.

Being sea water cooled on the lower leg, sail drives should not have significant temperature rise in  normal operation. I haven't actually measured the SD60 on my 400e, but I will put a thermal strip on in the near future to monitor max temp in the upper housing.

Gearbox overheating sometimes occurs due to aeration of oil. That happens if too high viscosity oil is used, or oil with unusual frothing, which I have Sen with one or two industrial gear oil brands.

With SD50 units there is the potential overheating due to clutch slip if the clutch isn't functioning properly (a known issue).  That would increase temperature in upper hosing significantly.

Theoretically a small circulation pump, circulating oil from cool leg to  top housing would give improved thermal capacity and consistency, by vertical circulation.

One takeaway is that changing oil in the SailDrive at recommended intervals is actually important. Mineral oils can hold a small amount of water in suspension, but the best thing is change oil on regular interval to get rid of suspended water.

Checking the oil I was surprised to see that the dipstick (old, short version) was filled to the upper line and not the lower as per the Yanmar advisory, something which I had told the mechanic about years before - although a professional firm should have been aware of this anyway.

The gearbox functions correctly ahead and astern.  I intend to change as much of the oil I can get out while afloat and refill to the correct level, but in the light of the posts above am unsure whether to remove the breather and re-seal the gearbox.  However, having had the shaft and oil seals changed at great expense last year I'm concerned about further damage. 

Comments from anyone more knowledgeable than I most welcome!

There’s great info in this thread from DJgun.  I think all I can add to your decision process is that I use a breather tube that’s longer than the engine bay is high. I create a large loop in it so that there’s still plenty of expansion space. The loop retains some oil. My thinking is that limits the exposure to air. I don’t get any overspill of oil. 

All that said, I think I’ve now got a leaky oil seal on the input side of the gearbox. Not large but I’ll need to do it end of season. The SD50 does occasionally drive me nuts! 

Mark

However, I wanted to check further into the discolouration visible on the surface and managed to extract some 800mls with the syringe and hose.  It all appeared clear with only some bubbles forming after aspiration that resembled the streaks seen in the photo.  Perhaps the discolouration was bubbles formed after the expanded oil in the breather tube returned to the gearbox.  I'm sending a sample off to a lab anyway when I'm able.

I'm thinking that boats in general are designed to drive us nuts - or occupy our minds to such an extent that we can't get into any other form of trouble.

800ml is quite an over-fill, and would have been causing quite a lot of foaming/frothing, which makes the gearbox run significantly hotter. And create more pressure in the saildrive. Getting the sample tested is a good idea as it will show up water content well before it becomes visible to the naked eye.

Maintaining the correct oil level of the right oil is 1st step to happiness. If there seems to be a problem first check is oil level.

A suggestion is to take a sample soon after running the SD, and just look to see if the amount of aeration is less. Then squirt it back into the SD after visual.

I can say that over 45 years involved with machanical power transmission, almost every time a client complained about gearbox temperature it was found overfilled, and often with wrong oil viscosity too.  Surprisingly many tradesmen operate on the false premise that more and thicker = better, when in fact more and thicker = worse.

Hopefully we will hear a nice report that your oil test is satisfactory.

Cheers

DJ

I'll do as you suggest about the sampling though; thanks again.   Unsurprisingly, however, I've completely lost what little faith I had in the engineers as I'd made them aware ages ago about the Yanmar advisory which they should have known about anyway.  To be told that I need a new saildrive largely because "1800hrs is a huge amount for a saildrive" (nonsense) and that a new fresh water pump was needed when the cause was their underfilling of the coolant by over two litres is bordering on fraudulent, let alone incompetent.  That said, I should have checked all the levels myself instead of just blindly trusting them.

All the best

Iain

Unfortunately we do need to check work done on our boats by others, simply because they don't have anyone else to check them before signing off.

One of the biggest causes of problems is "interrupted operations" and it can strike even experienced and competent tradesmen and professionals alike.

Good luck

If anyone is considering fitting the upgrade kit, I've found Gideon to be most helpful and quick to respond to any query.  

Iain