Prop Shaft Support

[hobbystuff]

I am looking for some information regarding the shaft setup in my Universal 36 Tri-Cabin.

Currently I have a 1.75" Aquamet 22 shaft, driven by a 120 Lehman, the shaft was installed last year prior to the season, the new shaft was installed exactly as the old one.

There is one cutlass bearing at the stern tube (which was also just replaced), the overall shaft length from stern tube to transmission flange is 130 inches. The shaft exits the tube at a cross-member that is ~ 60" back from the transmission coupling. A PSS shaft seal is mounted at that location. During discussion with the shaft supplier, he mentioned that this is a fairly long unsupported length, but that at our engine rating/rpm it "should" work out fine. My mechanic at the time convinced me of the same.

The boat has run just fine, although I feel like it could run smoother. Occasionally I think the shaft may experience a bit of "whip" at certain, low RPM settings.

This winter I am pulling the boat for several months so that I can do a rebuild on the Lehman. I am using a new mechanic, and during discussions he suggested we may want to take the time to install a cutlass bearing at the PSS seal to provide some additional shaft support. I have some questions:

(1) I have no idea what the original stuffing box arrangement looked like on this boat, since the PSS was installed before I acquired it. Where the shaft exits the cross-member, there is a brass tube with a bolted flange that looks as if the threads were turned down to allow the PSS to slide on. While there is considerable clearance for the shaft in the shaft tube itself, this brass tube narrows the exit to about ~1/4" clearance around the shaft. This leads me to believe that in the original configuration, the stuffing box was located at this point and provided some additional shaft support.


(2) I am considering fabricating a new component that would allow me to install a cutlass bearing at this location, which would reduce the unsupported shaft length from 130" to 60" (transmission to PSS), and 70" (PSS to Shaft Log). My new mechanic seems to think this would be a nice improvement.


Long winded, I know. Any thoughts on this? What I don't want to do is go to all the work, and then find out I just made things worse.....

Thanks in advance.

 

[Xsbank]

Do you know any mechanical engineers?

If your engine needs a rebuild, how do you know its the shaft or the engine giving you rough running?

You can get a rebuilt Lehman from Industrial Engines in Delta.

Or better yet, Aiden Butterfield in Vancouver blueprints them when he rebuilds them.

 

[hobbystuff]

Do you know any mechanical engineers?

If your engine needs a rebuild, how do you know its the shaft or the engine giving you rough running?

You can get a rebuilt Lehman from Industrial Engines in Delta.

Or better yet, Aiden Butterfield in Vancouver blueprints them when he rebuilds them.

XS:

Thanks for the response.

(1) I am a mechanical engineer.
(2) This is a good question. I believe both are contributing to the vibration, but visual observation of the shaft at the PSS shows me that there is some movement that isn't always there, but can manifest itself.
(3) I am already in contact with Industrial Engines and am heading over there in a couple weeks to talk directly. Didn't know about Aiden Butterfield, will have to look him up.

Cheers.

 

[jleonard]

I had an old Mainship I and it had a 103 inch shaft, 1 1/2 diameter. It had a pillow block bearing mounted on a bulkhead about halfway.
When I repowered I had everything apart. Someone had suggested removing the bearing. Since the unsupported distance was "marginal" per the Aq 22 material supplier, I re-assembled without the bearing. As I was tightening up the prop nuts, I decided there was way too much "flex" in the shaft.
I put the bearing back in place.
FYI the bearing was a Torrington/Fafnir "RCJC 1 1/2" . (the 1 1/2 designates the ID).
I am also a mechanical engineer.

 

[hobbystuff]

I had an old Mainship I and it had a 103 inch shaft, 1 1/2 diameter. It had a pillow block bearing mounted on a bulkhead about halfway.
When I repowered I had everything apart. Someone had suggested removing the bearing. Since the unsupported distance was "marginal" per the Aq 22 material supplier, I re-assembled without the bearing. As I was tightening up the prop nuts, I decided there was way too much "flex" in the shaft.
I put the bearing back in place.
FYI the bearing was a Torrington/Fafnir "RCJC 1 1/2" . (the 1 1/2 designates the ID).
I am also a mechanical engineer.

Do you have a photo of that bearing installed? How did the RCJC bearing get aligned with the shaft?

Thanks.

 

[SoWhat]

" I think the shaft may experience a bit of "whip" at certain, low RPM settings."

Not sure I would redesign the drive train on an I think.
MacGyver a piece of angle iron alongside the shaft. Mount a dial indicator. Check TIR every 12". Then you know what has to be done.

You are probably at the extreme limit for unsupported shaft. Split Pillow Block is probably a good idea. Might also consider stiffer engine mounts.

 

[boatpoker]

I am looking for some information regarding the shaft setup in my Universal 36 Tri-Cabin.

Currently I have a 1.75" Aquamet 22 shaft, driven by a 120 Lehman, the shaft was installed last year prior to the season, the new shaft was installed exactly as the old one.

There is one cutlass bearing at the stern tube (which was also just replaced), the overall shaft length from stern tube to transmission flange is 130 inches. The shaft exits the tube at a cross-member that is ~ 60" back from the transmission coupling. A PSS shaft seal is mounted at that location. During discussion with the shaft supplier, he mentioned that this is a fairly long unsupported length, but that at our engine rating/rpm it "should" work out fine. My mechanic at the time convinced me of the same.

The boat has run just fine, although I feel like it could run smoother. Occasionally I think the shaft may experience a bit of "whip" at certain, low RPM settings.

This winter I am pulling the boat for several months so that I can do a rebuild on the Lehman. I am using a new mechanic, and during discussions he suggested we may want to take the time to install a cutlass bearing at the PSS seal to provide some additional shaft support. I have some questions:

(1) I have no idea what the original stuffing box arrangement looked like on this boat, since the PSS was installed before I acquired it. Where the shaft exits the cross-member, there is a brass tube with a bolted flange that looks as if the threads were turned down to allow the PSS to slide on. While there is considerable clearance for the shaft in the shaft tube itself, this brass tube narrows the exit to about ~1/4" clearance around the shaft. This leads me to believe that in the original configuration, the stuffing box was located at this point and provided some additional shaft support.


(2) I am considering fabricating a new component that would allow me to install a cutlass bearing at this location, which would reduce the unsupported shaft length from 130" to 60" (transmission to PSS), and 70" (PSS to Shaft Log). My new mechanic seems to think this would be a nice improvement.


Long winded, I know. Any thoughts on this? What I don't want to do is go to all the work, and then find out I just made things worse.....

Thanks in advance.

PM me an email address and I will send you the ABYC Shafting Standard

 

[hobbystuff]

I like the idea of installing a Pillow Block type bearing on the inboard side of the PSS to stabilize the shaft. It is less work overall than putting a cutlass bearing into the top end of the stern tube (and more "reversible" should I need to revert....).

The pillow block should only take radial not thrust loads - those of you that have pillow blocks in your boat, is the bearing locked to the shaft in some way? My mechanic seems to think it is necessary to do that. I believe that the pillow block needs to allow thrust loads to be transferred to the engine, and also allow any forward/aft axial motion of the shaft.

Thoughts? Photos?

Thanks in advance.

 

[Xsbank]

You're welcome!

The reason I asked about engineering is because I assumed that shaft support was a fairly defined science. Isn't there a relationship between shaft diameter, length and speed of rotation that relates to (I don't know the proper term for "wobble") that might guide you?

I think pillow blocks do not attempt to capture the shaft in any way. Having said that, I would investigate some sort of "boot" to cover it to protect from getting any sort of grit in it. You will also need a serious mount for the block, one which will not transfer any sound or vibration to the structure of the boat. Being a boat, but not one that is used very much compared to one that is in commercial use, you will need to find out if a sealed bearing or one with a grease fitting is best. Make sure its easily maintained or replaced. Also, depending upon the numbers you find as above, perhaps a better solution might be a larger-diameter shaft? Coupler, stuffing box, cutlass and wheel dimensions will have to be addressed.

I will PM you Aiden's number.

 

[DNT99611]

Another option would be either a python drive or an aquadrive. They are a little spendy but sure are nice. You could shorten your shaft and use really soft engine mounts to help kill vibration noise.

 

[LarryM]

" I think the shaft may experience a bit of "whip" at certain, low RPM settings."

Not sure I would redesign the drive train on an I think.
MacGyver a piece of angle iron alongside the shaft. Mount a dial indicator. Check TIR every 12". Then you know what has to be done.

You are probably at the extreme limit for unsupported shaft. Split Pillow Block is probably a good idea. Might also consider stiffer engine mounts.

My last boat was a Fales 30 which had a 13'-4"x1.25" prop shaft that was supported by two Dodge split pillow blocks that were rigidly mounted on transverse fiberglass supports under the shaft. The shaft was free to move fore and aft as thrust was absorbed solely by the engine mounts. They basically prevented shaft 'whip' which occurred at certain engine/shaft speeds as a result of harmonics and balance.

I replaced the original stuffing box with a PSS dripless shaft seal, but when I did, I actually lost some shaft support as a result. The conventional stuffing box and stiff, short rubber connection to the shaft log actually provided some support to the long, flexible shaft. I ended up with some vibration at certain engine speeds that was not evident before the 'upgrade' so I performed the bearing spacing calculations and found that the now-longer un-supported shaft length between the last intermediate shaft bearing and the cutless bearing was slightly longer than the calculated maximum bearing spacing.

I sold the boat before I got around to that item on my to-do list, and the boat has since covered thousands of miles, so I guess it wasn't a serious problem.

Here is the chart you can use to calculate the bearing spacing for your Aquamet 22 shaft. It has a modulus of elasticity of 28. I already plotted the 1.75" shaft with Aquamet 22 so just furnish your shaft rpm and follow the instructions to determine the maximum shaft spacing.

If you want to perform the long calculations, I have attached that info as well.

Easy peasy

 

[hobbystuff]

My last boat was a Fales 30 which had a 13'-4"x1.25" prop shaft that was supported by two Dodge split pillow blocks that were rigidly mounted on transverse fiberglass supports under the shaft. The shaft was free to move fore and aft as thrust was absorbed solely by the engine mounts. They basically prevented shaft 'whip' which occurred at certain engine/shaft speeds as a result of harmonics and balance.

I replaced the original stuffing box with a PSS dripless shaft seal, but when I did, I actually lost some shaft support as a result. The conventional stuffing box and stiff, short rubber connection to the shaft log actually provided some support to the long, flexible shaft. I ended up with some vibration at certain engine speeds that was not evident before the 'upgrade' so I performed the bearing spacing calculations and found that the now-longer un-supported shaft length between the last intermediate shaft bearing and the cutless bearing was slightly longer than the calculated maximum bearing spacing.

I sold the boat before I got around to that item on my to-do list, and the boat has since covered thousands of miles, so I guess it wasn't a serious problem.

Here is the chart you can use to calculate the bearing spacing for your Aquamet 22 shaft. It has a modulus of elasticity of 28. I already plotted the 1.75" shaft with Aquamet 22 so just furnish your shaft rpm and follow the instructions to determine the maximum shaft spacing.

If you want to perform the long calculations, I have attached that info as well.

Easy peasy

Perfect, thank you. I'm going to sit down with some coffee and run the numbers.

 

[FF]

"Perfect, thank you. I'm going to sit down with some coffee and run the numbers."

Or get a copy of Skenes Elements of Yacht Design and look at the tables .


Great reading for tech folks, local library or cheap used.

Skene's Elements of Yacht Design, Eighth Edition: Francis S. Kinney ...

https://www.amazon.com/Skenes-Elements-Yacht-Design-Eighth/dp/0396079687


Skene's Elements of Yacht Design, Eighth Edition Hardcover – September, 1983. ... Elements of Yacht Design: The Original Edition of the Classic Book on Yacht…. ... Noteworthy are the clear negative implications of some features of modern racing yacht design in terms of stability and ...

 

[jleonard]

I like the idea of installing a Pillow Block type bearing on the inboard side of the PSS to stabilize the shaft. It is less work overall than putting a cutlass bearing into the top end of the stern tube (and more "reversible" should I need to revert....).

The pillow block should only take radial not thrust loads - those of you that have pillow blocks in your boat, is the bearing locked to the shaft in some way? My mechanic seems to think it is necessary to do that. I believe that the pillow block needs to allow thrust loads to be transferred to the engine, and also allow any forward/aft axial motion of the shaft.

Thoughts? Photos?

Thanks in advance.

The inner race of the bearing is locked to the shaft with 1 or 2 set screws.
I had to change the location of the pillow block when I did my repower and they re-aligned the new engine. Our yard mechanic (who is very good) aligned the engine and cutlass with the bearing removed from the bulkhead. After he got it right I re mounted the bearing in it's new location. It worked out perfectly. Thrust loads were not an issue in my case.
Unfortunately I do not have any pictures, it was about 16 years ago.

One issue with adding a pillow block bearing....aquamet shafts come in different dimensions. Mine was ordered specifically to be 1.500 +0, - (whatever the tolerance was) inch. It slid onto the shaft so nicely.
My friend ordered randomly and it came 1.500 + tolerance, -0. As a result he had to grind the ID of the bearing to get it onto the shaft. Something to look out for ahead of time.

Hope this helps.

 

[nmuir]

I have pillow block bearings on my boat. When looking for replacements I was told to try V.M. Dafoe. They had some in stock though unfortunately not exactly the size what I needed. They said they were common on fishing boats around here.


The pillow blocks that match my boat are from Split Journal Bearings | Royersford Foundry and Machine Co. Inc. - the gibbed joint ones (naturally the most expensive!)

 

[FF]

"The pillow blocks that match my boat are from Split Journal Bearings | Royersford Foundry and Machine Co. Inc. - the gibbed joint ones (naturally the most expensive!)

Expensive , BUT they allow the bearing to be replaced without pulling the shaft.

Very worthwhile!!!! And a spare can be kept on board, just in case.

 

[scott2640]

Going through the same calculations right now on our build. Dave Gerr says and I believe this is a general rule, that a shaft should not exceed an unsupported span greater than 40 times the shaft diameter. That's the rough calculation, Dave also provides a more accurate mathematical formula in his boat mechanical systems book that will generally give you a little more length. However I needed my teenage son to calculate it for me because the formula is rather complex.

R.E. Thomas has an intermediate shaft bearing/stuffing box combo that may meet your needs. Good luck.

https://www.retmarine.com/

I'm not affiliated with RE Thomas, just fyi.

 

[bglad]

Photo is from 1979 34 Mainship. The owner needed to pull his propeller shaft to change it. I suggested while the shaft was out he install a split coupling on it since it may not be the last time it will happen. Straight fit couplings tend to lose clearance each time they are worked or seizing on the shaft complicating disassembly next go around.

A shelf and pillar block bearing could also be used versus the flange bearing in the photo. The bearings in both types have a rounded outer race which can be swiveled about so the platform or bulkhead does not have to be exactly vertical or horizontal.

I like the idea of installing a Pillow Block type bearing on the inboard side of the PSS to stabilize the shaft. It is less work overall than putting a cutlass bearing into the top end of the stern tube (and more "reversible" should I need to revert....).

The pillow block should only take radial not thrust loads - those of you that have pillow blocks in your boat, is the bearing locked to the shaft in some way? My mechanic seems to think it is necessary to do that. I believe that the pillow block needs to allow thrust loads to be transferred to the engine, and also allow any forward/aft axial motion of the shaft.

Thoughts? Photos?

Thanks in advance.

 

[sunchaser]

With a rebuilt engine, new motor mounts and a proper alignment there may not be an issue. Add to it a top flight prop scan and rudder post PM. This is basically in support of XSB's post #2.

As mentioned by others, several additional support arrangements can be engineered in, but to solve what problem? A good BC tradesman with a dial indicator and strobe light can assess current movement.

PS, these ramblings are from another engineer.

 

[Keysdisease]

The photo in post 18 is the same bulkhead bearing J Leonard mentioned in his post re his repower.

One thing to consider regarding cutlass bearings at the stern tube, the distance between the gear flange and the first bearing will be a consideration for how much deflection to allow via engine mounts. If the distance is very short an appropriate mount selection for smooth operation may need to be adjusted as there may be too much shaft movement at a bearing 18" (for example) from the gear flange. This is why many packing glands or shaft seals are mounted on hose and not rigid.

Also re: "I believe that the pillow block needs to allow thrust loads to be transferred to the engine, and also allow any forward/aft axial motion of the shaft."

I agree. Look at any installation with thrust on the engine and resilient mounts and you will see axial movement as thrust is applied. This can often be observed at the packing gland as when thrust is applied you can often see a small band of "polished" shaft appear.

Before embarking on a solution for this vibration be sure you have eliminated all other potential sources. I just hate fixing something that's not broken only to find out the chosen "solution" was not addressing the source of the problem.

And then again, I may need coffee

I like the idea of installing a Pillow Block type bearing on the inboard side of the PSS to stabilize the shaft. It is less work overall than putting a cutlass bearing into the top end of the stern tube (and more "reversible" should I need to revert....).

The pillow block should only take radial not thrust loads - those of you that have pillow blocks in your boat, is the bearing locked to the shaft in some way? My mechanic seems to think it is necessary to do that. I believe that the pillow block needs to allow thrust loads to be transferred to the engine, and also allow any forward/aft axial motion of the shaft.

Thoughts? Photos?

Thanks in advance.

 

[bughermarine]

The maximum recommended unsupported distance for a 1.75" shaft is approximately 72". Yes, you will have significant shaft whip if you have an unsupported length of 130". Since this is not underwater, I might suggest fabricating a shelf and installing an intermediate greasable bearing at the mid point.
Best Regards, Blair Bugher

 

[hobbystuff]

Alright, thanks everyone for the input. We have spent a bit of time running the boat at various speeds and observing the shaft. When running at lower speeds, we do occasionally (but not all the time) get a bit of whip in the shaft observable at the PSS shaft seal - the recommendation from my mechanic and the shaft supplier is to install a pillow block just ahead of the PSS. That will reduce the unsupported shaft length to within recommendations.

I'll post my progress but it won't be until December at the earliest!

 

[hobbystuff]

Here is what we ended up doing. A Royersford split journal bearing was purchased from the shaft supplier and installed about 6" forward of the dripless shaft seal. This was accomplished by glassing pads onto the stringers and then fabricating a metal support structure that was then powder coated and installed. Our shaft and prop (which are both only a year old) were pulled and checked, then the whole thing was reassembled.

We now have a very smooth running shaft and I can say it is definitely an improvement.

The split journal bearing has a grease nipple on top of it that needs to be attended to occasionally. In my install, once things were closed back up, the journal bearing sits about 18" forward of the access hatch. For ease of greasing, I installed a 24" grease whip and permanently installed a grease gun to the side of the access hatch. Now all I have to do is reach in and give it a pump every once in a while.

Quick side note - I am observing the journal bearing temperatures with a temp gun. Initially I was concerned about the bearing running a bit hot. A quick phone call to my shaft supplier and they told me that these bearings will run warm until it settles in, or if over greased. Looks like they were correct, as the temperatures are settling substantially.


Cheers.

 

[Nomad Willy]

Some consideration could be applied to the PSS.

These shaft seals allow about 1/4” runout w very little resistance. If a prop shaft is prone to whip some the PSS probably won’t control it at all or only to an extremely small degree.

A stuffingbox on the other hand typically has a stiff hose supporting the packing gland and the shaft. The hose type/design, stiffness and length will vary the amount of shaft support but it will be many times as much support as the PSS.

But there’s a strong possibility better support for the shaft would result if a stuffing box type seal was employed. And IMO water would provide lubrication enough for fairly long lived packing life. Also boaters tend to think the closer to no leaking is best and run the packing too tight. Kinda like a car burning no oil. But as long as the amount of water leaking in is not a problem I’d be wanting considerable leaking .. within reason.
Also maybe I’m an idealist but I reason the there is a reason a prop shaft whips. Alignment and balance should stop whipping but controlling it with bearings is probably never a 100% solution. So IMO one should check balance and alignment first before adding bearings.

 

[kapnd]

Installing an additional bearing ahead of a dripless packing will further complicate matters when you need to do any repairs to the dripless.
This will only happen on rough crossings in the middle of the night, and is a blatant violation of the KISS code.

 

[bogranjac1]

prop shaft support

I am looking for some information regarding the shaft setup in my Universal 36 Tri-Cabin.

Currently I have a 1.75" Aquamet 22 shaft, driven by a 120 Lehman, the shaft was installed last year prior to the season, the new shaft was installed exactly as the old one.

There is one cutlass bearing at the stern tube (which was also just replaced), the overall shaft length from stern tube to transmission flange is 130 inches. The shaft exits the tube at a cross-member that is ~ 60" back from the transmission coupling. A PSS shaft seal is mounted at that location. During discussion with the shaft supplier, he mentioned that this is a fairly long unsupported length, but that at our engine rating/rpm it "should" work out fine. My mechanic at the time convinced me of the same.

The boat has run just fine, although I feel like it could run smoother. Occasionally I think the shaft may experience a bit of "whip" at certain, low RPM settings.

This winter I am pulling the boat for several months so that I can do a rebuild on the Lehman. I am using a new mechanic, and during discussions he suggested we may want to take the time to install a cutlass bearing at the PSS seal to provide some additional shaft support. I have some questions:

(1) I have no idea what the original stuffing box arrangement looked like on this boat, since the PSS was installed before I acquired it. Where the shaft exits the cross-member, there is a brass tube with a bolted flange that looks as if the threads were turned down to allow the PSS to slide on. While there is considerable clearance for the shaft in the shaft tube itself, this brass tube narrows the exit to about ~1/4" clearance around the shaft. This leads me to believe that in the original configuration, the stuffing box was located at this point and provided some additional shaft support.


(2) I am considering fabricating a new component that would allow me to install a cutlass bearing at this location, which would reduce the unsupported shaft length from 130" to 60" (transmission to PSS), and 70" (PSS to Shaft Log). My new mechanic seems to think this would be a nice improvement.


Long winded, I know. Any thoughts on this? What I don't want to do is go to all the work, and then find out I just made things worse.....

Thanks in advance.

Hobbystuff,
Here are a few pics of how my P.O. addressed possible shaft wobble. That bearing is bolted to a rubber block glued into the bilge directly above the keel.
Hope you have found a remedy by now if not something for your consideration.
regards