How long should a prop shaft last?

Time to pick the thoughts of the collective brain here...

Yesterday I sawed out my cutless bearing (after dropping the rudder and pulling the shaft and prop). I wish I had read the suggestion to drive screws into the bearing and pull, but I sawed it out with only minor damage to the shaft tube, just like last time. It was fairly easy to pull once I had something to get a grip on.

Last time I replaced the cutless the boat had about 650 hours on engine and the shaft, the cutless was noted for replacement on the survey. The shaft was in pretty much pristine condition with minor polishing at cutless and packing gland. That was four years ago, this time having added another 2400 hours to the Hobbs I have a depression at both points on the shaft. No vibration and I am inclined to just re-install with a new cutless and restore or replace the shaft on the next go around.

Is 3000 hours on a 50 hp engine with a 1 1/4" shaft excessive wear or should I consider that routine maintenance? Of note, most of the west on the cutless was on the inboard side of the shaft. I will have the alignment checked before I go back into the water next year...

I don't know the answer to your question but I can offer a datapoint. My stb shaft was replaced at 4000 hours as were the bearings at that time. It and the bearings are still in great shape. My port shaft at almost 5000 hours is still in use. Yes, it has some wear at the bearing locations, but no excessive clearance and no vibration or excessive wear so I see no reason to change or repair it yet. Shafts are 1 3/4" diameter running behind 135 Lehmans.

Ken

My cutless is well supported on the inboard side, the outboard side projects out a couple of inches past the hull so I suspect it can flex a little. That's exactly what I was looking for, data points

Do you have a water feed line to the stuffing box?

I guess I have 8,000 hours on the 1.5" shaft in my charter boat. The shaft turns 1,400 rpm most of that time. Changed the cutlass bearing maybe 500 hours ago. Shaft wear was minor when I changed the cutlass bearing. I have a PSS dripless stuffing box with a water feed line. Have often wondered if there was any benefit to a constant flow of water through the shaft log and cutlass bearing.

Ted

I know I saw sleeves for prop shafts in another thread.

Oh yeh,

http://www.trawlerforum.com/forums/showpost.php?p=597447&postcount=6

I would think you would have a constant flow of water with a dripless, doesn't it flow under pressure at the dripless and out the cutless? I just have a stuffing box with three rings of packing. I turn 2650 rpm's when traveling, 1850 when putting around, 650 when trolling. The packing was hard, and the box was cranked down tight so the wear at that end is probably my fault with 3000 hours on the packing.

The cutless was still fairly tight, with just little wiggle in it. It was much more loose when I picked up the boat in Anacortes just from deterioration from being on the hard all of it's life. It only had 46 hours on the engine, so it wasn't wear...

It makes me wonder if the cutless is under supported in the stern tube.

Just for comparison my shaft , I think it is original, has about 6,200 hrs. Shows some polishing but no serious wear.
It's 2"

I have just pulled our 1.25 inch shaft. It was easy to take it along to a local prop specialist for an eyeball examination. He also put it between centres and found it had some bend so am replacing it.
The quick check was free of charge.

A prop shaft should last till you run on something that bends it ,

and the local shop cant fix it.

I recently replaced my shaft due to a grounding and because of some crevice corrosion. Boat only has 1000 hours but I think it sat at the dock a lot before I bought it. And actually, I don't put all that many hours on it either.

It has a standard packing. What causes that crevice corrosion? Not enough water flow through the packing? Sitting around too much? Something else?

O C Diver said:

Do you have a water feed line to the stuffing box?

I guess I have 8,000 hours on the 1.5" shaft in my charter boat. The shaft turns 1,400 rpm most of that time. Changed the cutlass bearing maybe 500 hours ago. Shaft wear was minor when I changed the cutlass bearing. I have a PSS dripless stuffing box with a water feed line. Have often wondered if there was any benefit to a constant flow of water through the shaft log and cutlass bearing.

Ted

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Ted, I do believe you are onto something. Clean flushing water has worked wonders in industrial pumps for a very long time. Can't see why vessels big and small are any different.

A properly spec'd and installed shaft should last practically forever.
I think you have some installation and/or alignment issues causing premature wear.
Hull flex and engine mounts are part of the equation too.

My hull is only 30' and has a fully encapsulated lead ballast under the engine and almost all the way back to the stern tune, so I am guessing alignment quite probably is an issue rather than hull flex. You wouldn't think a hull that short and stiff would flex much...

Being a true double ender the prop is set forward of the rudder as well, so it should be really stiff. Propeller shaft is only about 6'-7' long, with the engine midship behind the pilot house. I am going to have alignment checked before going back in the water for sure!

AKDoug said:

My hull is only 30' and has a fully encapsulated lead ballast under the engine and almost all the way back to the stern tune, so I am guessing alignment quite probably is an issue rather than hull flex. You wouldn't think a hull that short and stiff would flex much...

Being a true double ender the prop is set forward of the rudder as well, so it should be really stiff. Propeller shaft is only about 6'-7' long, with the engine midship behind the pilot house. I am going to have alignment checked before going back in the water for sure!

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Your shaft bearing tube is not aligned with the shaft.
Either the hull has changed since it was built, or more likely, it was not installed correctly from the getgo.
Just being a double ender with a bunch of lead in the keel does not insure that things cannot be be out of kilter.
Fiberglass shrinks, wood swells, metals corrode, and **** happens!
Just replacing the bearing has already proven that there is a problem, now compounded with a worn shaft.

Agreed! My suspicion is that it was never aligned correctly.

75 years old, so far. My shafts are monel, 2.75" made in 1942 for 850 hp engines. They have 20,000 hours I know of on the current 200 hp engines. They're still in excellent condition. For 35 years the boat was licensed as a passenger carrier so the shafts were removed for routine inspection, last in 2005. They'll still be good in another 75 years. Monel used to be the common metal for shafting. I've never owned a shaft that wasn't monel. It's about 2/3 nickel 30% copper and traces of other metals like iron. Stainless, whatever the alloy, is not ideal for underwater purposes, but is cheaper. Most yacht shafts are too small.

"What causes that crevice corrosion?"

The selection of a SS shaft.

cardude01 said:

I recently replaced my shaft due to a grounding and because of some crevice corrosion. Boat only has 1000 hours but I think it sat at the dock a lot before I bought it.

It has a standard packing. What causes that crevice corrosion? Not enough water flow through the packing? Sitting around too much? Something else?

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Something else: Electrolysis.

From my experience, crevice corrosion is a result of 'someones' shore power/ bonding system being faulty. It's hard to pin down, because is could be as far as 100' from you.
You mention the boat being at a dock. Alarm bells ringing.
When I bought my boat 3 years ago a throughhull was corroded. She was marina kept also. Last winter when I pulled the shaft to replace cutlass I found 3' long corrosion pitting along shaft where it's inside shaft tube. Scandia Prop made a new shaft and fitted the old coupling to it.

So, Electrolysis is a big cause. Make sure all throughulls, engine and generator are bonded. And make sure you have correct amount of zincs.

Crevice corrosion and electrolysis are two different animals. Crevice corrosion happens to stainless when the oxygen around it is depleted, and surrounding water becomes acidic, setting up localized corrosion within the metal. Happens to fasteners all the time. It happens at a microscopic level, so any defects in the surface of an underwater metal can be a starting point. Flow of new water helps the situation.

I had the shaft inspected yesterday, it's true, and the dye inspection revealed no cracks. The technician said I had an alignment issue, marks on the shaft at the flange connection from the flange made that clear to him. It had only .020 scoring and they did not recommend replacement until it went past .045, new shaft is $480 (good to know for the future).

I have the name of a good technician to re-align and I will be looking over his shoulder while he does the job so I am clued in as to how to make it a regular check for my maintenance schedule.

Thanks for all the tip, opinions, and information. I learned a lot from this thread and hopefully at least a few other people will benefit from it as well

question for the group...if the shaft needs replacing due to wear or corrosion what does that typically cost ?

I forget the name but there was a company that created "inside out " cutlass bearings.

If a shaft was too scored the shaft would have a sleive with the rubbers attached slipped on and a bearing surface installed where the old bearing was fitted.

My understanding that with all the parts easy to R&R a scored shaft could be run "forever"

A good deal in shallow sandy water.

FF said:

A prop shaft should last till you run on something that bends it ,

and the local shop cant fix it.

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Or corrosion sets in. Crevice corrosion is alive and well, particularly where little water movement occurs, such as in a standard packing arrangement. Car dude apparently encountered this issue. Many others have too.

I did have to replace my shaft recently due to crevice corrosion. Boat was very lightly used before I bought it, and I don’t use it enough either. Standard packing gland.

I think the shaft was like $800. I didn’t really price shop. Probably paid too much but wanted to get it done and go on a trip.

FF said:

A prop shaft should last till you run on something that bends it ,

and the local shop cant fix it.

Click to expand...

My 2.75" monel shafts are 1942 models. Shafts can be welded and turned to size and there are coatings that can build the shaft back to spec.