Slight Oil Slick Showing with Boat at Mooring (1988 Cobra ..

The oil appears to be coming from the lowest point of the housing as
it emerges from the stern. Based on the size and depth of the slick,
something is weeping slightly. To give a basis for comparison, a
finger dipped in engine oil and immersed in the water produces a
slick of equal or greater magnitude.
I am not obviously losing oil from either the engine or the stern
drive. Neither oil shows any sign of cloudiness.
I am guessing the source is one of the seals on either side
of the universal, either the engine side or the drive side.
Other possible sources are: hydraulic tilt or hydraulic trim tabs,
but the location rules out the tabs.
(1) Where do you think the oil is coming from?
(2) Can the source be checked with the boat in the water?
(3) Is the source so slight that the shop will have trouble
finding it even with the boat in dry dock?
(4) How often should seals be changed as part of preventative
maintenance?
Many thanks for your kind assistance?
Frank

> (1) Where do you think the oil is coming from?
> (2) Can the source be checked with the boat in the water?
> (3) Is the source so slight that the shop will have trouble
> finding it even with the boat in dry dock?
> (4) How often should seals be changed as part of preventative
> maintenance?
> Many thanks for your kind assistance?
> Frank

Fwiw ... I had a slight slick coming from one of the small lines to
one of the trim cylinders on an older boat. Since the boat was
trailerable, was easy to spot once out of the water. Also fwiw, seems
to me there was some sort of seal/o-ring on the top of the mercruiser
outdrive that with time or overfill (lube needs room to expand when
hot else blows the seal) might leak ... allowing water into the
outdrive and lube to exit which is not a good combination.