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YANMAR SD40 SAILDRIVE - DIAPHRAGM REPLACEMENT

Updated: Oct 7, 2023






We’ve recently returned from France, where we did some work on our 42 foot aluminium sailboat, Kadash, AKA “Kim”. The boat is ashore at the Port Napoleon marina in southern France, near the mouth of the Rhone.

The boat is fitted with a 3 cylinder Yanmar diesel engine, which drives the propeller through a Yanmar SD40 saildrive. The saildrive is similar to the lower part of an outboard motor, although it does not swivel, and it is bolted to the back of the engine.

The engine and saildrive are supported on flexible mounts, and to allow for movement of the saildrive, the hole in the hull is slightly larger than the saildrive “leg”. The gap between the leg and the hull is filled by a pair of flexible rubber diaphragms. A water sensor in the space between the two diaphragms sounds an alarm in the event of leakage.

Our insurance company insisted that the diaphragms should be changed. The local Yanmar dealer in Port Napoleon declined to do the work as he deemed the access to the engine to be to difficult, leaving us with no option but to do the work ourselves. As I have not been able to find much information about diaphragm replacement, and the Yanmar manual is not very useful, I thought it might be helpful if I set out how I set about the work.

Note - this is only an account of my experience with my particular boat, and is by no means an instruction manual

1. Remove engine box and calorifier

2. Remove throttle and gear selector cables (easier to remove fuel lever from engine rather than try and extract split pin}

3. Remove engine stop solenoid, air filter, sea suction hose

4. Remove the propeller – this is on a spline

5. I removed the saildrive anode (4 x M8 capscrews) and the propeller shaft unit (2 x M10 capscrews). I improvised a puller using M10 and nuts to pull the propeller shaft assembly out of the leg, but I think that the leg would have fitted through the hull aperture with the propeller shaft in place

6. Remove the fairing or seal which fills the gap between the leg and hull

7. Remove the plug from the bottom of the saildrive and drain the oil (about 2 litres of rather smelly hypoid gear oil. If the oil is emulsified check cause)

8. Pull the bottom of the sterndrive forward to take the weight off the spline between the engine and gearbox. I used a rope tied to the boat’s keel

9. Support the weight of the engine (I used “Spanish windlasses” – ropes twisted by a lever)

10. Remove the M10 bolts which attach the engine mounts to the boat’s engine bearers

11. Remove the 8 x M8 hex screws around the gearbox bellhousing which attach it to the engine

12. Insert a wedge between the bellhousing and engine, and then leave engine forward using a suitable bar. It does not need to move very far. The saildrive is likely to drop down when the spline comes out

13. Remove the saildrive engine mount nut and the nuts on the studs which pull the saildrive down onto the diaphragm. (8 x M10 Nyloc nuts)

14. Lift the saildrive up into the boat. It is quite heavy for one person, but not impossible

15. With the saildrive on the bench, remove the 8 x M10 capscrews which attach the upper vertical shaft bearing housing to the gearbox

a. NOTE: Do not remove the 2 x M8 capscrews which hold the shaft bearing housing together

b. NOTE: The female spline coupling which connects the vertical shaft to the gearbox output will fall out. Make sure that it is not lost

16. Carefully remove the lower diaphragm from the gearbox case. Examine for cracks or damage. My diaphragm had shallow crazing cracks and I am glad that then insurance company insisted on its replacement

17. Remove the upper diaphragm. This is held in place by two narrow overcentre tightened stainless steel clips

18. Replace the lower diaphragm. Make sure it is fully home in the gearbox housing

19. Replace the upper diaphragm. I suggest that the old clips are used to practice tightening the overcentre mechanism

20. Assembly is the reverse of the above steps, but the following points may be of interest

a. Make sure the sea suction valve closes properly. Mine had clearly never been closed, and it was a struggle to get it seated. I tried to remove it to replace it with a plastic valve, but couldn’t. The copper alloy valve and aluminium saildrive is an unhappy combination

b. Replace any O rings and seals which look at all secondhand. I replaced the shaft lip seals as well as the propeller shaft assembly O rings

c. It is easier to fit the forward two M10 nuts holding the saildrive onto the diaphragm if the unit is tilted up using the keel rope

d. In my case sliding the engine back so that the splines engaged was unexpectedly easy, using judicious adjustments of the engine slings and keel rope

e. Fill the saildrive with the recommended 80W-90 gear oil

f. Check for oil leaks

g. AND – when the boat is launched, for check for water leaks!















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