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Old 09-12-2017, 11:21 AM   #1
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Shaft logs and stern tubes

Hi All,

My sailboat had a hole in the boat with a molded "tube" on the inside that a stuffing box or PSS would clamp onto. Simple. Picture below is the new trawler. Its got a big bronze housing bolted to the boat (that has the "cutlass" bearing). My question is this: Is the boat from that point on just a molded hollow space? Then there is the "stern tube"? that is stuffed into the mold? I have a cross member with a tube coming out of it (see other picture - the clamps are to put the required tension on the PSS bellows before tightening the set screws on the puck). I have a drip coming from UNDER that cross member. I have no idea what's back there and why I have a drip. Obvious the water is coming in from the "cutlass", but don't know how it's getting into the boat. Need some structural expertise. Thanks.

Dave
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Old 09-12-2017, 11:55 AM   #2
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Not enough info here.
Hard to tell from your pics exactly whats going on.
Is the water coming from under the bronze gland, or out of the fiberglass?
This might be a good candidate for professional help, if you aren't familiar with these things.
That said, it isn't rocket science, and since you look to be drydocked anyway, take the whole mess apart, clean it up, and reinstall correctly.
Bear in mind that misalignment, bent shafts or props can cause wobble that will eventually create leaks.
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Old 09-12-2017, 04:24 PM   #3
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Usually a separate tube that the cutless bearing holder and stuffing box end screw onto.

Some bronze, some composite, some who knows?

Yes they are a source of leaks if ybey are holed.
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Old 09-13-2017, 07:30 AM   #4
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Below are a couple of pictures, one is the typical set up of the marine shaft set up, the second is showing what can happen with older age stern tube's and corrosion, the other issue is sealing the end bearing to the hull so no water leaks into the stern tube surround aperture.

Trust this gives you an insight as to the normal set up,

Cheers Steve
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Old 09-13-2017, 08:22 AM   #5
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Good pix. What's the brass-looking piece sitting on top of the packing gland . . . where the raw water injection port is on my boat. I've been looking for a bronze 3/8" NPT to hose barb for that fitting but I don't think they are made.
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Old 09-13-2017, 08:31 AM   #6
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That my friend is an old style grease cap, (from days gone by i guess) fill it with grease and give it one turn every week or so and squeezes grease into the gland packing's,

They are still available(believe it or not!)

Cheers Steve
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Old 09-13-2017, 08:36 AM   #7
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Most hardware stores will have the npt/hose tail, only issue is that quite often brass and you really want bronze, especially in that location, a small machine shop will only take a hour to turn one up if you cant locate , use only BRONZE

Cheers Steve
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Old 09-13-2017, 08:42 AM   #8
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I went with SS as a temporary solution, promising myself I'd replace them annually to prevent crevice corrosion. Good idea about the machine shop. Thnx.
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Old 09-13-2017, 09:28 AM   #9
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Yeah, just remember your promise as screwing SS nipple into a bronze fitting with sea water (and under the water line) is an accident looking to happen,

This is a typical case (not of crevis corrosion but galvanic corrosion) as

1) Two metals with different corrosion potentials Direct metal-to-metal electrical contact
2) A conductive electrolyte solution (e.g. water) must connect the two metals on a regular basis. The electrolyte solution creates a "conductive path". This will occur when there is regular immersion,
3) Stainless Steel (304/316) is approx 0 to 0.-0.1 More Noble on the Galvanic Scale where as the average Manganese Bronze is -0.3 to -0.4 less Noble
4) Thus meaning the Bronze will act as an Anode where the interface as the threads can very quickly eat away leaving the SS FITTING TO FALL OUT
5) If there is any stray electrical current current involved as well the failure can happen in weeks/days
6) If there is no alternative for a temporary (VERY Temporary ) then the SS fitting should be fitted with DURALAC jointing compound
as (https://www.fisheriessupply.com/sadd...nting-compound)

Cheers Steve
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Old 09-15-2017, 02:54 PM   #10
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Have a friend that had a CHB 34 with the exact same problem. He described how the shaft log screws into the stuffing box (which is mounted on a cross member) and his was only screwed in with 3 threads and leaked as well. What ticks me off is I was on the hard with the prop off, the shaft almost all the way out... Really disappointed with the surveyor I had.

Any way, I'm leaking about 4.5-5 gallons a day.

Dave
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Old 09-15-2017, 03:54 PM   #11
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Quote:
Originally Posted by CaptSteve53 View Post
Yeah, just remember your promise as screwing SS nipple into a bronze fitting with sea water (and under the water line) is an accident looking to happen,

This is a typical case (not of crevis corrosion but galvanic corrosion) as

1) Two metals with different corrosion potentials Direct metal-to-metal electrical contact
2) A conductive electrolyte solution (e.g. water) must connect the two metals on a regular basis. The electrolyte solution creates a "conductive path". This will occur when there is regular immersion,
3) Stainless Steel (304/316) is approx 0 to 0.-0.1 More Noble on the Galvanic Scale where as the average Manganese Bronze is -0.3 to -0.4 less Noble
4) Thus meaning the Bronze will act as an Anode where the interface as the threads can very quickly eat away leaving the SS FITTING TO FALL OUT
5) If there is any stray electrical current current involved as well the failure can happen in weeks/days
6) If there is no alternative for a temporary (VERY Temporary ) then the SS fitting should be fitted with DURALAC jointing compound
as (https://www.fisheriessupply.com/sadd...nting-compound)

Cheers Steve
I agree there is a significant risk and will likely have the SS hose adapters replaced this winter--with something. (I will note that the adapters I removed -- and had been there since I bought the boat in 2014 -- were brass and showed no signs of deterioration.) I've looked everywhere for 3/8" bronze--and even Marelon--with no luck. I'll see if somebody can make me a couple.
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Old 09-15-2017, 05:08 PM   #12
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Use Nylon....PSS uses them in their shaft seals....no real force, no real pressure and if worried about breaking put a cover on it.


SS and bronze lasts a pretty long time unless something else is wrong. Heck we put bronze props on stainless shafts...yes different alloys have different characteristics but holy cow...not that much different.


Even brass in bronze, isolated from most everything else by a mile will last a long time, ask all of us that have found them on our 30 year old trawlers.


Sure there are better materials, but lets not panic yet...check them regularly (not even daily or weekly...but regularly) and when you can break it off in you hand with a sharp blow, it's time....hopefully you have the replacement in your hand.


If your boat can sink from just that fitting, you may want to rethink your bilge pump system.
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Old 09-15-2017, 10:31 PM   #13
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https://gigharbormarina.com/roughwat...ew-shaft-tube/
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