Repair or Replace Shafts?

[angus99]

Stella Maris is on the hard to correct a misaligned strut causing excessive vibration. I think the misalignment has been there for years but, as the cutless bearings wear unevenly, it becomes more pronounced.

I’m bracing myself for what they’ll tell me about the shafts, which AFAIK are original 1987 vintage. We had significant corrosion on our rudder shafts several years ago and I had a company plasma spray them with Inconel, restoring them to like-new condition (see pix). The same thing is done routinely with industrial pump shafts, many of which run continuously under heavy loading. Wondering if anyone’s done this with prop shafts and what the criteria might be for either spraying or replacing them. Any recommendations for an unbiased expert to help me decide whether to repair or replace would be much-appreciated.

Ian McLeod
DeFever 44 Stella Maris
Deltaville, VA

 

[O C Diver]

I would say it depends on how close the shaft is to the failure point. If you had a 450 HP engine running through tall gears (2.5:1) and plan to intermittently run the engine ar WOT, there is an incredible amount of torque on the shaft, and it should probably be replaced. On the other hand, the typical trawler might never see 66% of total power and have little risk.

I could be mistaken, but it was my impression that spraying metal on the shaft was about restoring the surface for a bearing or seal, it doesn't restore the strength of the shaft. Your pictures appear to be significant electrolysis. It would be difficult to determine how much strength was lost in that area.

If I were going to make a determination regarding that pitting, I would want to know how close to the minimum I was with the shaft in good condition. As an example, I swapped a 450 HP engine with a 135 HP engine. Used the same shaft, transmission, and propeller diameter. Could probably have gone from a 2.5" shaft to a 1" shaft, theoretically. 1/2" deep pits on the 2.5" shaft probably wouldn't have been an issue.

Ted

 

[angus99]

Thanks, Ted. On the rudder shafts, the shop did a dye penetration test and were able to determine that the pitting was not deep, with 95%+ of the original metal left. Most of the on-line providers talk about restoring both the strength and surface dimensions of the shafts they spray, but I’m hoping to hear from folks without income on the line. The prop shafts are 1.75” and you’re getting at my basic question: how much of the original metal needs to be intact before a replacement is necessary? Probably a question for metallurgists.

 

[twistedtree]

Thanks, Ted. On the rudder shafts, the shop did a dye penetration test and were able to determine that the pitting was not deep, with 95%+ of the original metal left. Most of the on-line providers talk about restoring both the strength and surface dimensions of the shafts they spray, but I’m hoping to hear from folks without income on the line. The prop shafts are 1.75” and you’re getting at my basic question: how much of the original metal needs to be intact before a replacement is necessary? Probably a question for metallurgists.

Do you have access to the same shop? I think fix or replace will be heavily influenced by the skills and capabilities of the shop. Also the grade of SS for the original shaft. Unless it's already A22, I would lean towards replacement.

 

[angus99]

Do you have access to the same shop? I think fix or replace will be heavily influenced by the skills and capabilities of the shop. Also the grade of SS for the original shaft. Unless it's already A22, I would lean towards replacement.

Not easily accessible, I’m afraid. One of my reasons for asking is to avoid being ripped off. Even though this yard has a reputation for fairness, previous experience with boatyards had me feeling I’d left my wallet on the sidewalk. One advantage here is that another DeFever much like ours just had similar work done by the same yard and the owner has been very candid about cost and quality.

Good thought about the SS. I’ve read the quality used back in the day was inconsistent to say the least.

Thanks.

 

[O C Diver]

Thanks, Ted. On the rudder shafts, the shop did a dye penetration test and were able to determine that the pitting was not deep, with 95%+ of the original metal left. Most of the on-line providers talk about restoring both the strength and surface dimensions of the shafts they spray, but I’m hoping to hear from folks without income on the line. The prop shafts are 1.75” and you’re getting at my basic question: how much of the original metal needs to be intact before a replacement is necessary? Probably a question for metallurgists.

I generally cringe when I hear "good as new". If it were me, I get the price of new versus repaired, straightened, and retrued to the coupling. Then factor in the cost of labor and maybe reconditioning the props. Don't know if this is the only reason you're hauling out, but I would want to put the whole job cost in perspective. If the difference was less than 20%, I wouldn't hesitate to have new and peace of mind.

Ted

 

[High Wire]

My shaft didn't look nearly that bad and the shaft shop said to replace it. It had runout, wear and corrosion but didn't look like that. Those holes could go halfway through the diameter.

 

[angus99]

I generally cringe when I hear "good as new". If it were me, I get the price of new versus repaired, straightened, and retrued to the coupling. Then factor in the cost of labor and maybe reconditioning the props. Don't know if this is the only reason you're hauling out, but I would want to put the whole job cost in perspective. If the difference was less than 20%, I wouldn't hesitate to have new and peace of mind.

Ted

I’ll definitely be comparing. On the rudder shafts the difference was >50%, IIRC.

 

[angus99]

My shaft didn't look nearly that bad and the shaft shop said to replace it. It had runout, wear and corrosion but didn't look like that. Those holes could go halfway through the diameter.

The pix aren’t my prop shafts. They’re rudder shafts and dye testing showed the cavities were superficial. Thnx.

 

[jleonard]

Id replace and never think about it again.
That is the best repair.