Cutlass Bearings

[angus99]

Had Stella hauled yesterday for a bottom job. Nice to see no blisters and no visible damage—especially after the flotsam we encountered on the Mississippi two years ago.

There is some extremely minor movement—just enough to “click” slightly—when I pull on the shafts. Also, I notice a “ticking” sound from one of the struts while underway. These are long shafts with three cutless bearings on each.

Questions:

—Is any amount of shaft play acceptable?
—Is there a predictive lifespan for cutless bearings?
—Should I just have them changed as preventive maintenance while we’re on land?

One slightly jarring find: the cotter pin and castle nut on the starboard shaft were gone. Not the way I’d have preferred to run the boat over the last 120 miles!

 

[Russell Clifton]

That shaft should have two nuts on it. Hopefully one was removed before the picture was taken? Hard for a properly installed cotter pin to fall out.

 

[angus99]

That shaft should have two nuts on it. Hopefully one was removed before the picture was taken? Hard for a properly installed cotter pin to fall out.

Yes, my point exactly. That prop was last removed and reinstalled by a highly respected shop in 2018 during our half-loop. Makes me wonder . . .

 

[kchace]

Movement from pushing and pulling on the end of the shaft is play in the thrust bearing inside the transmission. A very small amount may be acceptable but you’ll have to consult the transmission manual for that number.

Side to side movement is controlled by the amount of clearance between the cutless bearing and the shaft. I’ve seen a few numbers thrown around but I think up to about 1/8” is ok. There’s no real predictable lifespan on cutless bearings. Depending on water conditions they can easily last 10 years or even more, but some experience considerably shorter lifespan.

Ken

 

[Northern Spy]

Table V of ABYC P-06 has strut bearing clearances.

Max clearances are:
0.007" for shafts under 1.25"
0.009" for 1.5" shafts
0.010" for 2" shafts
0.011" for 2.5" shafts
0.014" for 3" shafts
0.015 for 4" shafts

It doesn't clearly say if this is radial or diametric clearance. I would assume diametric at the 12 o'clock position.

If it has longitudinal grooves (like staves). You can easily see wear.

I wouldn't be immediately concerned of it were a few thou (or more) out of the recommend clearances. But I'd measure and order one for next time.

If it thunks when you grab the prop and shake it, it is probably too much.

1/8" clearance would be way too much for me.

That said, you're missing your nut and cotter. If I had any unusual vibration underway, I'd change it.

 

[angus99]

Thank you both.

These shafts are maybe 12-15 ft long with three bearings per shaft. The minuscule amount of play I feel is in the mid-strut only where the force I’m applying is at the max. I doubt it approaches a 32nd, but I think I’ll ask the yard to measure any gap and just go from there. They’re 1.75” shafts so it looks like there’s not much to allow for.

Appreciate your feedback.

 

[Comodave]

When we had our current boat surveyed the surveyor measured the side to side and up and down movement. He had a set of plastic feeler gauges so the blade would bend and fit in between the ridges in the cutlass bearing and the shaft. The measurements on our 2” shaft went from .002 to .009. If you are in the 1/16” then it is probably too big so the bearing may be worn.

 

[kchace]

Table V of ABYC P-06 has strut bearing clearances.

Max clearances are:
0.007" for shafts under 1.25"
0.009" for 1.5" shafts
0.010" for 2" shafts
0.011" for 2.5" shafts
0.014" for 3" shafts
0.015 for 4" shafts

It doesn't clearly say if this is radial or diametric clearance. I would assume diametric at the 12 o'clock position.

If it has longitudinal grooves (like staves). You can easily see wear.

I wouldn't be immediately concerned of it were a few thou (or more) out of the recommend clearances. But I'd measure and order one for next time.

If it thunks when you grab the prop and shake it, it is probably too much.

1/8" clearance would be way too much for me.

That said, you're missing your nut and cotter. If I had any unusual vibration underway, I'd change it.

You’re right, 1/8” would be too much play. Don’t know what I was thinking.

I believe those are recommended clearances for new bearings. I have not seen a chart of numbers as to when to replace.

https://www.pacificmarine.net/engin...rudder-bearings/cutless-bearing-clearance.htm

Ken

 

[Northern Spy]

I believe those are recommended clearances for new bearings. I have not seen a chart of numbers as to when to replace.

Ken

Agree.

Water lubricated bearings are funny things. I have seen fantastically worn cutlass bearings on sub pumps that had no vibration. Vertical shaft though.

I've been tangentiality involved with stave bearings on runners on hydro plants and stern bearings on a couple of naval ships. They tend towards conservative replacement.

I think a lot depends on bearing load and shaft speed so a fair amount of variables particularly with our little boats. If the bearings are short or long has an effect too.

As they wear, the surface area increases when the groove gets narrower, which may actually increase load and excite vibration.

My understanding is that water lubricated bearings aren't nearly as well understood as oil lubricated bearings. Probably why I was taught about lignum vitae bearings in school not that long ago. (OK. 35 years ago).

There may be a certain "if it ain't broke don't fix it" take for me. Just be prepared for next time. Easy to buy the parts now and replace later, than to wait for parts now or later.

 

[kchace]

My stb shaft has been replaced maybe 10 years ago and it’s fairly snug in the bearings. My port shaft is original and has some shaft wear so that even with new bearings there is a small bit of play. I haven’t measured it but maybe I will just for fun. I haul out every October (for now) so I get to check them every year and so far neither side has gotten looser.

Ken

 

[boatpoker]

keep in mind, the more it's worn the faster ... and faster it wears

 

[angus99]

Update

So, I decided to have the four cutless bearings housed in struts replaced. (The two embedded in the hull appeared to be OK.) The new bearings are about an inch shorter than the struts could accommodate. They are mounted flush with the aft lip of each strut, but there is a 1-inch opening at the front of each strut where the cutless bearing would otherwise be. Otherwise the install looks fine.

These are fairly long struts (see older photo) so I’m not sure how much an inch matters.

I only examined how the bearings previously fit inside the struts on the aft side <slaps head> so I’m not sure if this is normal/acceptable or likely to be an issue down the road. The yard manager thinks they’re fine. Would appreciate any thoughts from those in the know.

 

[angus99]

Anyone?

 

[DDW]

Sorry, still recovering from the headache of thinking about how 4 cutlass bearings are aligned.

Certainly there are many boats with a cutlass bearing mounted at the end of a long shaft log and don't have issues. Only thing I can imagine is trapping something in the gap which wears away on the shaft. But I suspect it will be filled with marine growth quickly.

 

[angus99]

Sorry, still recovering from the headache of thinking about how 4 cutlass bearings are aligned.

Certainly there are many boats with a cutlass bearing mounted at the end of a long shaft log and don't have issues. Only thing I can imagine is trapping something in the gap which wears away on the shaft. But I suspect it will be filled with marine growth quickly.

Sorry if I confused things. It’s three cutlass bearings per shaft. Only two of which per shaft are being replaced.

 

[Alaskan Sea-Duction]

I think lessons learned here is to pull the boat annually or simi-annual for a bottom inspection. I pull ASD every spring.

 

[angus99]

I think lessons learned here is to pull the boat annually or simi-annual for a bottom inspection. I pull ASD every spring.

Thanks, but I’m not sure how pulling it yearly would—or wouldn’t—impact whether these cutless bearings are correctly sized. Can you explain what you meant?

 

[Alaskan Sea-Duction]

Thanks, but I’m not sure how pulling it yearly would—or wouldn’t—impact whether these cutless bearings are correctly sized. Can you explain what you meant?

So other than checking zincs, you also need to check the condition and clearances on your cutless bearings. You would have also found the nut and cotter key missing, saving you a few boat $$ on new props.

 

[angus99]

Can anyone offer a knowledgeable opinion on whether there is a problem having cutless bearings that are an inch or so shorter than the struts that house them? Please see post 12.

 

[angus99]

So other than checking zincs, you also need to check the condition and clearances on your cutless bearings. You would have also found the nut and cotter key missing, saving you a few boat $$ on new props.

I check the aluminum anodes at least twice a year with the boat in the water. The cutless bearings are checked every two years when the boat is hauled. Think that’s probably OK for bearings that tend to last up to 10 years. Not to be argumentative, but I don’t know if the nut and cotter pin fell off last year or last week.

 

[Comodave]

I am not sure if an inch shorter matters or not, but why not put in the correct ones to start with? You don’t have as much bearing surface so I would expect that it will wear out sooner.

 

[angus99]

I am not sure if an inch shorter matters or not, but why not put in the correct ones to start with? You don’t have as much bearing surface so I would expect that it will wear out sooner.

Thanks but the yard has already installed them. They are an inch or so shorter than the struts that house them. Before I go to war with the yard, I’m trying to find out if this is actually a problem.

 

[Comodave]

I am not an expert like I said but I would still want to know why not the correct bearing. Maybe ask Steve D.

 

[Northern Spy]

Bearing length:shaft diameter is usually around 4:1 by the prop and shorter further away.

The advantage of a shorter bearing would be less friction. Fairly negligible factor for a recreational boat.

The disadvantage would be less load capability. Again, probably negligible. But may lead to shorter life.

Think about it. The same boat can have a range of horsepower and different prop pitch and diameters. No one thinks about the cutlass bearing load.

Really the modulus of elasticity of the shaft and distance between bearing centres would be in there too. Other variables would be the elastomeric material, groove size and diametrical clearance relative to the old ones.

I would imagine there is a fair amount of thumb rule and over sizing for our applications, and not much actual engineering.

I would venture the design is mostly empirical and based on experience.

I know this isn't much of an answer, but I really don't have a lot of practical experience. None of the rotating engineers are here at work this shift.

If your boatyard mechanic doesn't have issue, and seems like a reliable source of opinion, then there probably isn't much risk.

I would venture it would be like fitting tires to your car. Some are better, some are worse, but they will all more or less function. And the measure of success or failure is also subjective, depending on what one wants.

 

[Northern Spy]

As I now see that they have already been changed, ask for the old ones back.

If things go wrong, it would be easier to point out the difference with them in hand. It would also be easier to source new ones before you pull the boat in a few years.

 

[angus99]

As I now see that they have already been changed, ask for the old ones back.

If things go wrong, it would be easier to point out the difference with them in hand. It would also be easier to source new ones before you pull the boat in a few years.

Good thoughts, all, Spy. Thank you. Pretty sure the old ones were tossed, but I’ll ask.

 

[Alaskan Sea-Duction]

So who manufacturers cutlass bearings. Can they give you some insight? But agree comparing to old ones is an excellent suggestion.

Also sounds like periodic inspection is right on.

 

[Alisske]

Can anyone offer a knowledgeable opinion on whether there is a problem having cutless bearings that are an inch or so shorter than the struts that house them? Please see post 12.

I recently redid a cutlass bearing. My strut is about a foot long. (Behind keel)

The cutlass bearing was 5 inches. Works fine.

 

[angus99]

I recently redid a cutlass bearing. My strut is about a foot long. (Behind keel)

The cutlass bearing was 5 inches. Works fine.

Thanks, Alisske. Others I’ve heard from are pretty much saying the same thing. I’m going to stop obsessing now.

 

[Comodave]

I recently redid a cutlass bearing. My strut is about a foot long. (Behind keel)

The cutlass bearing was 5 inches. Works fine.

It looks like your prop nuts are reversed. The full height nut goes on second and the half height nut goes on first. As you tighten the second nut it unloads the first nut somewhat. Unless I am not seeing the photo correctly, it is hard to see exactly.