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Old 05-22-2018, 07:28 PM   #1
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Prop Chord Thinning and Other Things

I'm looking at vessel in Portland Oregon (fresh water most of the time) and obtained an older survey. A couple things caused some concern, I'm just not sure how much concern they should be. First is a notation that the Props show prop chord thinning due to dressings. Not sure what this really means and is it a concern or pending prop replacement? Next was a port exhaust tube and elbow that was leaking and had a shoddy repair that hasn't really worked. In the two year the current owner used it he hadn't bothered to have it repaired. In the photo I can see cracks in the hose as well so , yes it should all be replaced, but, why just the damage on the port side? The port side shaft coupler also showed quite a lot of corrosion in that servey and apparently still does. Are there concerns about shaft alignment etc?
The engines are 2001 era Cummins 6BTA 370 hp. Unknow at this time if the aftercoolers were serviced every two years as required though being in fresh water might help a bit. Any advice, thoughts, are appreciated.

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Frank
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Old 05-23-2018, 04:17 PM   #2
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Shaft couplers are usually steel. If unpainted, gather surface rust. It doesn't affect it's strength or purpose. The mixing elbow (where raw cooling water enters the exhaust stream) if cast iron or steel, tend to rust out especially in salt water. Usually they are not repairable, but replaced. If rusted out, on some engines it's possible for water to enter the cylinders. Most elbows get changed over time or the owners make a stainless steel one like I did.
As far as hoses or maintenance, for long life without trouble change all the hoses and keep one of each size as an emergency backup. Do the aftercooler and all other maintenance and keep the schedule after it's your boat.
In ships, we used a natural fiber door mat on the shaft. Placed on top where it was in good contact and moved a little every day. Just gravity applying pressure, it kept the shaft shiny. Same can be done to the corroded coupler.
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Old 05-23-2018, 08:19 PM   #3
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The aftercooler should be taken apart and cleaned. Yes, the fresh water will help but that means instead of the every 2 years in salt water they should be done every 4 years, maybe a fifth but the first time generally at 4 and then think about extending the interval based on condition observation.

What happens is where the cooling tube bundle cartridge joins the housing there will be some corrosion, fresh or salt. If it is let go long enough the corrosion will freeze the tube bundle in the housing.

If unlucky the tube bundle can be wrecked by poor removal techniques OR being to late.

If stuck and If really lucky it can be removed by long soaking with a good penetrating oil such as Kroil. Maybe some gentle heating.
SOmetimes the penetrant must be poured into the housing through the air holes so it acts inside out. Then turned over so the other end gets treated/soaked also.

GENTLE tapping may get it to move a bit at a time. Do not just keep hammering in one direction but reversing the direction to slowly break the corrosion up. Use a thick block of wood, built up plywood or something as the tube ends will not withstand direct hammer blows. The block needs to almost entirely cover the tube bundle end. Lots of relatively gentle tapping is better than a few heavy blows.

Once out then the bevel on the housing must be inspected. Any pitting must be corrected. Often entails some light machining. One fellow here used a router with the appropriate 45o cutter and it worked. He only needed a light clean up. Failure to do this will result in seawater misting of the engine
leading to serious damage. Those pits will allow water to get past the O rings into the air stream.

Reassembly will require some heavy waterproof grease such as ALCO METALUBE and lots of it. on the various sealing surfaces.

RECOMMEND: Or you can go to Seaboard Marine, www.sbmar.com , and look up Tony Athens' article and get more tips.

Alco Metalube and Kroil are available from Seaboard.

ANother plug. GO to Seaboard and do some serious time reading. He is a guru/expert on the Cummins engines, 6BT and 8.3 among others. What makes him terrific is on his site he has offered a huge amount of info for free. You don't have to do anything except read and learn. Then use what he has offered as you see fit. He is not the only one to be extremely helpfull but his articles have made the info available to anyone who looks.

And by the way although he has written the info for the Cummins engines most of it also serves any other engine.

As for the coupler rusting, that can often be simply inattention to the stuffing box. A simple old beat up fender can be cut to cover the stuffing box shaft entry. Those stuffing boxes will throw a fine spray around a wide area which will cause rusting.
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Old 05-24-2018, 01:25 PM   #4
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Thanks guys, some great info and tips. Much appreciated.

Frank
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Old 05-24-2018, 04:16 PM   #5
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I still want to know what prop chord thinning is.

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Old 05-24-2018, 04:17 PM   #6
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Is anyone else concerned about buying a boat from an owner who found out about the leaky poorly repaired elbow 2 years ago and didn't address it....or who runs with visibly cracked hoses ??? Hopefully there is a large discount compared other similar boats.....you can guarantee that other systems were neglected if he/she was willing to neglect his running gear like that.
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Old 05-24-2018, 05:14 PM   #7
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Quote:
Originally Posted by Tator View Post
I still want to know what prop chord thinning is.

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Chord is the thickness of a wing or in this case a prop blade. If it has been dressed or ground aggressively the metal may be thinned enough to present a potential failure point.
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Old 05-26-2018, 02:26 PM   #8
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Originally Posted by Benthic2 View Post
Is anyone else concerned about buying a boat from an owner who found out about the leaky poorly repaired elbow 2 years ago and didn't address it....or who runs with visibly cracked hoses ??? Hopefully there is a large discount compared other similar boats.....you can guarantee that other systems were neglected if he/she was willing to neglect his running gear like that.
Hear, Hear!
The very first thing Id do is pull the exhaust elbow to have a peek down the turbos throat, looking for corrosion from exhaust water entry.
Also great advice from C Lectric re Tony Athens website.
Such items can often pave the way to a sweet deal, especially if youre handy with a wrench.
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