SD50 - Life Expectancy

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   Each to their own. But if you do want to go down the breather and oil reservoir route the height of the surface of the oil in the reservoir from the propshaft  would need to be approximately 1.3 times the depth of the propshaft to the waterline measurement. It’s simple maths to work out hydrostatic pressure that the seal experiences at the propshaft.
Simon

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sgrhma2 Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 20 November 2021 Location: Northern Irelan Status: Offline Points: 271	Post Options Post Reply Quote sgrhma2 Report Post Thanks(1) Quote Reply Posted: 26 March 2025 at 10:26
	Each to their own. But if you do want to go down the breather and oil reservoir route the height of the surface of the oil in the reservoir from the propshaft would need to be approximately 1.3 times the depth of the propshaft to the waterline measurement. It’s simple maths to work out hydrostatic pressure that the seal experiences at the propshaft. Simon

jeb Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 05 May 2014 Location: Sweden Status: Offline Points: 339	Post Options Post Reply Quote jeb Report Post Thanks(1) Quote Reply Posted: 27 March 2025 at 14:04
	Simon is correct, the oil level of the reservoar need to be well above the waterline. Since gear oil is around 900 kg/m3 in density, the distance from prop-shaft (or where leakage can happen) to reservoar should be waterline to prop-shaft x 1/0,9 or approx 1,11, so reservoar should be 11% of the prop-shaft to waterline above the waterline. For my boat with approx 90cm from waterline to prop-shaft this translates into 10cm above waterline. 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).
	Jesper Hanse 400e

asimo Members Profile Send Private Message Find Members Posts Add to Buddy List Lieutenant Joined: 08 October 2024 Location: MelbourneAUS Status: Offline Points: 22	Post Options Post Reply Quote asimo Report Post Thanks(0) Quote Reply Posted: 28 March 2025 at 00:10
	Simon, 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.
	H400e - DE HANJ0344K708

jeb Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 05 May 2014 Location: Sweden Status: Offline Points: 339	Post Options Post Reply Quote jeb Report Post Thanks(1) Quote Reply Posted: 28 March 2025 at 08:21
	Asimo, I have a Flex-O-Fold 18x13 3L on my SD 50. I can put in on the scale tomorrow and check weight, but I would guess somewhere between 10 and 15 kg.
	Jesper Hanse 400e

asimo Members Profile Send Private Message Find Members Posts Add to Buddy List Lieutenant Joined: 08 October 2024 Location: MelbourneAUS Status: Offline Points: 22	Post Options Post Reply Quote asimo Report Post Thanks(0) Quote Reply Posted: 29 March 2025 at 12:28
	Jesper, Thanks for feedback re: the prop weight. The Gori I have is an 18x13 too.
	H400e - DE HANJ0344K708

DJgun Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 04 April 2020 Location: SydneyAustralia Status: Offline Points: 287	Post Options Post Reply Quote DJgun Report Post Thanks(1) Quote Reply Posted: 01 May 2025 at 00:02
	One of the reasons gearboxes such as SD units are sealed rather than fitted with breathers is to prevent internal condensation which is extremely destructive. 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.
	DJ.....Sailor Ordinaire HIN DE-HANJ0331J708

Mark_J1 Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 12 March 2013 Location: Dover&Medway UK Status: Offline Points: 690	Post Options Post Reply Quote Mark_J1 Report Post Thanks(0) Quote Reply Posted: 01 May 2025 at 10:12
	Interesting DJ. Thoughts on a separate header tank, rather than a simple filter? Mark
	Hanse 400e "Grey Goose" Hull #31

DJgun Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 04 April 2020 Location: SydneyAustralia Status: Offline Points: 287	Post Options Post Reply Quote DJgun Report Post Thanks(1) Quote Reply Posted: 01 May 2025 at 10:52
	Hi 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.
	DJ.....Sailor Ordinaire HIN DE-HANJ0331J708

Ratbasher Members Profile Send Private Message Find Members Posts Add to Buddy List Vice Admiral Joined: 31 May 2017 Location: Cyprus Status: Offline Points: 818	Post Options Post Reply Quote Ratbasher Report Post Thanks(0) Quote Reply Posted: 23 May 2025 at 10:04
	Update on my original post. After 21 hours of operation this season I noted small quantities of oil had leaked down the side of the gearbox and puddled across the base. Having had a breather tube fitted as part of the upgrade kit I noted some oil had been forced out of it. There were small areas of discolouration in the oil below the dipstick (see photo). I had seen such discolouration at the end of last season (~70 hrs) and reported it to my mechanic but there had been no oil leakage visible then. 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! Edited by Ratbasher - 23 May 2025 at 10:05
	H400 (2008) 'Wight Leopard' Gosport, UK

Mark_J1 Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 12 March 2013 Location: Dover&Medway UK Status: Offline Points: 690	Post Options Post Reply Quote Mark_J1 Report Post Thanks(0) Quote Reply Posted: 29 May 2025 at 22:05
	Iain - as you say, that’s over filled. A fat syringe (Turkey Baster?) will let you suck some oil out easily. At a rough guess that’s about 150ml too much in there based on the replacement long dipstick. 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
	Hanse 400e "Grey Goose" Hull #31



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