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Old 10-14-2016, 07:56 PM   #1
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Cutlass bearing clearance for lubrication

I was going to put two zinc collars (double zinc) on my prop shaft and that left me with about 1/2 inch clearance between the cutlass bearing and the zinc and someone told me that wasn't enough clearance for bearing to get sufficient water lubrication. Does this sound correct? How much clearance is enough?
Thanks all!
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Old 10-14-2016, 09:23 PM   #2
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Humm - I would think plenty water would get to cutlass bearing... even with fairly close zinc tolerance of 1/2" Be interesting to hear from others on this subject.
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Old 10-14-2016, 09:45 PM   #3
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Speed plays a big part...less than 10 knots and I would say minimum clearance is no big deal...as long as no cavitation is occuring, which it shouldn't at slow speeds, you should be fine.
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Old 10-15-2016, 02:34 AM   #4
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Speed plays a big part...less than 10 knots and I would say minimum clearance is no big deal...as long as no cavitation is occurring, which it shouldn't at slow speeds, you should be fine.
Dittos, but it doesn't leave much room for unexpected circumstances like picking up some rope, fishing line, or other debris. Such things like to accumulate in "low spots", so why create one?
How about a prop nut zinc, or a shaft brush on the interior wired to a big zinc on the transom?
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Old 10-15-2016, 02:38 AM   #5
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Quote:
Originally Posted by freshalaska View Post
I was going to put two zinc collars (double zinc) on my prop shaft and that left me with about 1/2 inch clearance between the cutlass bearing and the zinc and someone told me that wasn't enough clearance for bearing to get sufficient water lubrication. Does this sound correct? How much clearance is enough?
Thanks all!
Personally, I would not be comfortable with 1/2" or less of clearance. Even at trawler speeds.

Plus I can't see why you'd need two zincs.
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Old 10-15-2016, 07:34 AM   #6
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Maybe use one of those line cutter zincs and see how it works.

In my experience.....a wider gap just allows more line to get wrapped up in there till you get to msny inches wide.
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Old 10-15-2016, 07:42 AM   #7
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Does your boat have a strut with both ends of the bearing exposed? If there is, at displacement the speeds, you should be fine, imho.

As C. Bill asks, why add an extra zinc?
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Old 10-15-2016, 08:56 AM   #8
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Does your boat have a strut with both ends of the bearing exposed? If there is, at displacement the speeds, you should be fine, imho.

As C. Bill asks, why add an extra zinc?
Personally... Aboard boats... I do like redundancy. Twin screws, two pilot stations, two heads, two fuel tanks, two water tanks - [two gals - oops; scratch that one! lol].

But really, IMO it is good to have two zincs per shaft. Especially if the uw portion of boat goes considerable time between dive-checks. I have seen zincs fall off of uw placements before. Of course if your shaft is not long enough to readily accept dual zincs then one must suffice. That said, I still doubt that a zinc being 1/2" away from cutlass bearing would impede water contact to the bearing unless maybe if traveling at a zillion mph.

Happy UW Zinc-Anode Daze! - Art
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Old 10-15-2016, 09:10 AM   #9
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I've seen distances all over the map. - Anywhere from 1/4" to 1-2x shaft dia

Here's one link w/ some specific recommendations
How To Properly Install A Shaft Zinc
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Old 10-15-2016, 09:58 AM   #10
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I've seen distances all over the map. - Anywhere from 1/4" to 1-2x shaft dia

Here's one link w/ some specific recommendations
How To Properly Install A Shaft Zinc
In that article... "A good rule of thumb is between 1-2 times the shaft diameter in the front, and at least ’’ behind the bearing."

With his eight years of experience... I wonder where he got those numbers from?? They do sound perfectly acceptable as rule-of-thumb.

I'd like to see controlled glass tank tests to clearly view what happens to water lubrication on cutlass bearing with shaft zinc placed at different distances from the bearing... and at different shaft rpm as well as speeds through water. Maybe the 1/2" away in the front (as mentioned in op) would interfere with water flow... maybe not??
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Old 10-15-2016, 12:14 PM   #11
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I have a fast planing boat and there is about 1/2" between back of zinc and front of strut. No issues.

It does not take a lot of flow to cool and lube the bearing, just can't be zero flow.
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Old 10-15-2016, 12:15 PM   #12
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I don't like the 5200 idea because if you have to remove it, good luck!

My friend's boat was hauled for a survey last week and his zincs were half worn but we had to remove the old to replace them with new.

Prepare your new zincs before you install them. Sitting in front of the tv with masking tape and a small can of Tremclad or any other leftover paint, mask the areas around the mounting holes and while watching Donna on TV, paint that area. Two coats are best. This will prevent the zincs from deteriorating where you painted and they will last a lot longer. The zincs fall off too soon because they corrode around the mounting screws where the metal is very thin. This will prevent that.
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Old 10-15-2016, 01:01 PM   #13
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Shape of zinc and speed of boat are what determines if there would be water starvation to the bearing. A given distance is totally arbitrary.

Using a shaft brush and making sure you don't have any stray currents is big.

Especially for single screws with little exposed shafts.

My 1 -1/2 " shaft only has about 3 inches exposed abd the donut zinc takes up a little over an inch.

Every year It is replaced with very little wastage that I attribute to normal conditions and use of a shaft brush.
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Old 10-15-2016, 01:54 PM   #14
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To try and answer a few of the questions asked. First off I do not have a strut going to the shaft or cutlass bearing. I rarely travel over 7 knots.
My new shaft zinc was almost completely gone after 10 weeks. I never had the boat plugged into shore power and spent little time in marinas.
I think I must have some stray electricity going on within the boat and to figure out where it is coming from and get it stopped is the best solution. Although I have little experience with tacking down stray electricity I will give this a try.
Thank you all for all the good information.
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Old 10-15-2016, 02:17 PM   #15
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I'll say you do have some stray current issue!
Good thing you spotted it right away, your propeller is the next anode in line.
The "12 Volt Doctor's Practical Handbook" explains in terms that even I can understand how to test for stray current, as well as how to install the proper amount of zinc to protect your boat.
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Old 10-15-2016, 02:43 PM   #16
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One other concern about a close fit is the amount of flex in your drive line, including engine mounts.
I'd be a bit worried about that 1/2" clearance at rest becoming zero clearance under full power.
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Old 10-15-2016, 03:24 PM   #17
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All good points and great forum overall. What did we do before the internet. Now the past seems like the dark ages.
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Old 10-15-2016, 06:31 PM   #18
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If your Schucker, below the water line, looks like this one, I think with a 1/2" you should be fine.
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Old 10-15-2016, 09:45 PM   #19
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I'll say you do have some stray current issue!
Good thing you spotted it right away, your propeller is the next anode in line.
The "12 Volt Doctor's Practical Handbook" explains in terms that even I can understand how to test for stray current, as well as how to install the proper amount of zinc to protect your boat.
Just ordered one - sounds good to have aboard! - TY Art
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Old 10-15-2016, 09:47 PM   #20
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One other concern about a close fit is the amount of flex in your drive line, including engine mounts.
I'd be a bit worried about that 1/2" clearance at rest becoming zero clearance under full power.
Good note!
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