Saildrive Header Tank to Protect Against Seal Failure

[David B]

In searching this forum I see a similar question has been raised before, but as it was nearly 15 years ago now, I thought it well worth reviewing for members input and experience since.

The issue is a saildrive output shaft being under positive pressure due to the the depth at which it is immersed, compared to the top of the saildrive (still usually below water level, but not by much).

Once the shaft seals wear, seawater (helped by that pressure) will slip past the worn seals and/or shaft, emulsify the gear oil, and so corrosion and wear issues can develop.

Over-filling the gearcase is also not a good idea. As I understand it, this is because there is reduced air volume to allow for expansion as the oil heats up, and therefore if oil is forced out when hot, seawater can be drawn in to make up the volume, once things cool down again (and perhaps the transmission oil could also be forced past the engine output shaft to mix with the engine oil).

So, what about a header tank supplying the gearcase. With the header tank high up in the engine bay, it will be above water level, and therefore if there is a leakage, oil will run out past the saildrive shaft seals, rather than seawater running in. With a breathable cap on the header tank, there would also be no pressure build-up as the oil heats.

The gearcase dipstick is removed, a hose nipple screwed in, and a flexible hose from there up to the bulkhead-mounted header tank is installed.
With a breathable cap on the tank, a shut-off valve on the bottom for service, and a capacity of 1-2 litres, it would be a simple installation. Then if the oil level at cold was just to the top of the tube, it would be very obvious if a leak developed, seeing the oil level drop in the clear tube.

A simple upgrade - if that is indeed what it would be.

What do we think, and what are the experiences of those who have done this - does it prevent seawater entering our saildrives?

[ggray]

Pretty much it.

Except the tank I installed 18 years ago holds only about 8 oz, what about 1/4 liter?

I drilled a hole in the existing fill cap where the dip stick was, and pressed a ss tube through it. Not sure you will find a hose nipple of same thread.

[ggray]

I've not had water in sail drive since.

[tkeithlu]

You may be solving a problem that doesn't really exist. You're dealing with at best 2 PSI, and ordinary oil seals are good for a minimum of 3 PSI and more usually 7 PSI.

Have you found water in the oil? Consider a schedule of replacing the seal after a certain number of years if you suspect aging.

[tkeithlu]

Two other wet blanket thoughts (sorry). If the gearcase has a dip stick, it is vented and pressurization from thermal expansion will not happen. Second, the header tank will overfill the gearcase. That's why you have a dipstick.

I'm a real source of joy this morning.

[HeywoodJ]

It’s been done, and the people who have done it swear by it. One aspect, from a purely technical perspective is that the oil seals in the drive leg should then be reversed. The seals are uni-directional and intended to keep the high pressure liquid (salt water) away from the low pressure liquid (the oil). By making the oil the high pressure liquid you are reversing the pressure gradient and thus should reverse the seals. The pressures are so low that I’m guessing you can get away without doing so, but from a purely design perspective it should be considered.

[ggray]

Well, the proof is in the pudding.

I've had no more water in the oil. I'm very hesitant to say this, but it appears that the last time I changed the seals was January 2010.

Installing the header tanks 18 years ago was FAR easier than changing the seals, and was done without hauling, which in itself is a big deal.

To me it is not only a simple solution, but how it should have been done in the first place.

And the dipstick itself is sealed. It is part of the fill cap which screws in and has an o-ring. In fact one solution considered was just to leave the cap loose. But then the obvious addition was to raise the "point of venting".

I wouldn't be surprised if the housing is full of oil. I inserted the SS vent tube through the fill cap down to the level marked on the dipstick, so theoretically, it will fill no higher than that. But it wouldn't surprise me if air in that pocket hasn't somehow leaked out during the usual two years between haulouts and oil change. It doesn't really concern me much as the oil will not ever be under significant pressure...it's vented!

Incidently, I first ran my idea of doing this by a major Yanmar certified service manager, and he thought it was a good idea.

[Thumbs Up]

Originally Posted by tkeithlu

You may be solving a problem that doesn't really exist. You're dealing with at best 2 PSI, and ordinary oil seals are good for a minimum of 3 PSI and more usually 7 PSI.

Have you found water in the oil? Consider a schedule of replacing the seal after a certain number of years if you suspect aging.

The problem absolutely exists. The dipstick is sealed with an O ring. The water gets sucked in past the outer seal on the cool down. The outer seals wear a groove on the shaft allowing this to happen. Venting the case is the solution (header tank or not). A common mistake is overfilling, thus increasing the suction on cool down. Venting eliminates both suction and pressure. One piece of monofilament or one dropped line is all it takes to ruin an outer seal. Volvo and Yanmar should endorse and provide header tanks. Mercruiser outdrives use them. The psi is irrelevant. I have pressurized mine with air and no oil leaks out. It is the negative pressure (vacuum), that sucks the water in. Once again (unless you have a vent or header tank), NEVER overfill these, not even slightly. To be safe, keep the oil level slightly BELOW the minimum level on the dipstick. If you are already leaking, LOOSEN the oil cap or dipstick each time after shutting down the engine. The Volvo dipsticks break. You can install a header tank for less than the price of a new dipstick.

[David B]

Originally Posted by tkeithlu

You may be solving a problem that doesn't really exist. You're dealing with at best 2 PSI, and ordinary oil seals are good for a minimum of 3 PSI and more usually 7 PSI.

Have you found water in the oil? Consider a schedule of replacing the seal after a certain number of years if you suspect aging.

Yes we did have a seal failure, which caused the oil to emulsify and overflow.
Regular seal replacement would be a thing, but just in case ....

[David B]

Originally Posted by tkeithlu

Two other wet blanket thoughts (sorry). If the gearcase has a dip stick, it is vented and pressurization from thermal expansion will not happen. Second, the header tank will overfill the gearcase. That's why you have a dipstick.

I'm a real source of joy this morning.

Dip stick is a screw down type with 'O' ring seal.
Overfilling - well the header will be vented, so that part would not be an issue, and the oil level is pretty well 'up there' right at the top anyway..
That was my concern - is more oil in there a problem or not?

[David B]

Originally Posted by HeywoodJ

It’s been done, and the people who have done it swear by it. One aspect, from a purely technical perspective is that the oil seals in the drive leg should then be reversed. The seals are uni-directional and intended to keep the high pressure liquid (salt water) away from the low pressure liquid (the oil). By making the oil the high pressure liquid you are reversing the pressure gradient and thus should reverse the seals. The pressures are so low that I’m guessing you can get away without doing so, but from a purely design perspective it should be considered.

Now there's a point, but I guess if the header tank is only at exhaust elbow height, there will be little pressure - compared with the bottom of the sail drive being over a metre into the water.

[David B]

Originally Posted by ggray

Well, the proof is in the pudding.

I've had no more water in the oil. I'm very hesitant to say this, but it appears that the last time I changed the seals was January 2010.

Installing the header tanks 18 years ago was FAR easier than changing the seals, and was done without hauling, which in itself is a big deal.

To me it is not only a simple solution, but how it should have been done in the first place.

And the dipstick itself is sealed. It is part of the fill cap which screws in and has an o-ring. In fact one solution considered was just to leave the cap loose. But then the obvious addition was to raise the "point of venting".

I wouldn't be surprised if the housing is full of oil. I inserted the SS vent tube through the fill cap down to the level marked on the dipstick, so theoretically, it will fill no higher than that. But it wouldn't surprise me if air in that pocket hasn't somehow leaked out during the usual two years between haulouts and oil change. It doesn't really concern me much as the oil will not ever be under significant pressure...it's vented!

Incidently, I first ran my idea of doing this by a major Yanmar certified service manager, and he thought it was a good idea.

Well that is very reassuring then. How high above water level do you think you have the tank?

[David B]

Originally Posted by Thumbs Up

The problem absolutely exists. The dipstick is sealed with an O ring. The water gets sucked in past the outer seal on the cool down. The outer seals wear a groove on the shaft allowing this to happen. Venting the case is the solution (header tank or not). A common mistake is overfilling, thus increasing the suction on cool down. Venting eliminates both suction and pressure. One piece of monofilament or one dropped line is all it takes to ruin an outer seal. Volvo and Yanmar should endorse and provide header tanks. Mercruiser outdrives use them. The psi is irrelevant. I have pressurized mine with air and no oil leaks out. It is the negative pressure (vacuum), that sucks the water in. Once again (unless you have a vent or header tank), NEVER overfill these, not even slightly. To be safe, keep the oil level slightly BELOW the minimum level on the dipstick. If you are already leaking, LOOSEN the oil cap or dipstick each time after shutting down the engine. The Volvo dipsticks break. You can install a header tank for less than the price of a new dipstick.

Good to know - thank you. Especially re the Mercruiser. My brother in law has a small Bayliner with Mercruiser, so I will check that out. Might also be the source of the header tank through spare parts division.