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Old 30-07-2020, 09:14   #1
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Saildrive: does ATF float?

If I have a leak from my lower saildrive seal, would I see oil residue on the water's surface? Or do you think ATF would sink so I wouldn't see anything on the surface?
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Old 30-07-2020, 09:18   #2
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Re: Saildrive: does ATF float?

Oil floats and ATF is oil, but I wouldn’t expect it to be such a large leak so that you could see it, or it would very soon be empty.
Often times it seems water gets in and turns the fluid into a pink milkshake.
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Old 30-07-2020, 09:24   #3
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Re: Saildrive: does ATF float?

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Oil floats and ATF is oil, but I wouldn’t expect it to be such a large leak so that you could see it, or it would very soon be empty.
Often times it seems water gets in and turns the fluid into a pink milkshake.

Good idea. I should siphon some fluid out and see if it looks like a milkshake.


My problem is probably not the saildrive leaking, but it made me curious if one of the symptoms of a lower seal leak is residue on the water's surface.
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Old 30-07-2020, 09:31   #4
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Re: Saildrive: does ATF float?

Typically there are two seals, one to keep the oil in and another to keep the water out. The seals only work in one direction.

Often the water seal in the lower unit will be compromised, allowing water to come into contact with the inner oil seal and as the unit cools after being run, water is drawn in because of the contraction of the air and oil contained in the outdrive. No oil will leak out, but water will 'leak' in.
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Old 30-07-2020, 10:34   #5
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Re: Saildrive: does ATF float?

AFT floats. But there would be too little to see. A small leak while running would emulsify and be invisible.

ATF III - Mineral automatic transmission fluid
Temp. [°C] Dyn. Viscosity [mPa.s] Density [g/cm³]
0 217.29 0.8783
10 118.06 0.8718
20 70.04 0.8655
30 44.70 0.8591
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Old 30-07-2020, 10:53   #6
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Re: Saildrive: does ATF float?

Thanks all.



The ATF doesn't look milky and you've convinced me I wouldn't see any residue if the saildrive were leaking so it must be something else.


I'll keep looking.
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Old 30-07-2020, 11:17   #7
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Re: Saildrive: does ATF float?

You should put 15w40 engine oil in that saildrive not ATF. Only fill to the minimum mark on the dipstick with the dipstick not screwed in. If you fill to the top of the line or overfill you are more likely to suck water in past your seals.
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Old 30-07-2020, 11:34   #8
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Re: Saildrive: does ATF float?

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Originally Posted by Thumbs Up View Post
You should put 15w40 engine oil in that saildrive not ATF. Only fill to the minimum mark on the dipstick with the dipstick not screwed in. If you fill to the top of the line or overfill you are more likely to suck water in past your seals.

I really don't know why the saildrive manual says ATF or DEXRONII instead of normal engine oil.


They're seaprop 60 saildrives I bought from Beta marine. Not much info online about them.
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Old 30-07-2020, 13:03   #9
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Re: Saildrive: does ATF float?

The advice I gave was for a Volvo Penta, which also says ATF in the manual but they issued a service bulletin switching to engine oil. You should stay with the ATF.
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Old 14-08-2020, 06:00   #10
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Re: Saildrive: does ATF float?

ATF is lighter than salt water. However, if the fluid level in your drive extends above the waterline, this means the ATF fluid will then be under a slight head pressure relative to the level of the salt water. Also; during operation, the fluid heats up and if the sail drive case is no open vented to the atmosphere the case will slightly pressurize. Some transmissions use an open vent while some use a check ball with a spring to maintain a very slight pressurization. Either way, water seems to find its way into the case. The high speed rotation of the prop shaft can sometimes create a pumping action that will push water past seals.
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Old 14-08-2020, 06:06   #11
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Re: Saildrive: does ATF float?

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ATF is lighter than salt water. However, if the fluid level in your drive extends above the waterline, this means the ATF fluid will then be under a slight head pressure relative to the level of the salt water. Also; during operation, the fluid heats up and if the sail drive case is no open vented to the atmosphere the case will slightly pressurize. Some transmissions use an open vent while some use a check ball with a spring to maintain a very slight pressurization. Either way, water seems to find its way into the case. The high speed rotation of the prop shaft can sometimes create a pumping action that will push water past seals.
Thanks for the info. I'm glad I started this thread. I've learned a little more about saildrives which have always been black boxes to me.
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