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Old 22-02-2015, 03:54   #1
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Too many flexible bits.

Hi Cruisers,

I met up with the club engineer recently, and asked him if he was available to check the alignment on my engine, now that I had replaced the old hard mounting system with soft mounts. During our discussion he expressed concern at my current mounting configuration, which is soft engine mounts (Vetus), a flexible coupling (Polyflex) and a PSS shaft seal. He pointed out that the PSS manual does not recommend this configuration, and suggested that the PSS should be stabilised with a cutless bearing at the top of the propeller log.

Bugger!

So I cannot find the PSS manual, and the online versions I could find made no mention of this limitation (though the online version is much shorter than how I remember the manual that came with the seal), but a bit of reading does suggest that he is correct and that only two of the three points should be flexible, so something has to change.

Well, I was about to set the wheels in motion to arrange a new cutless for the front of the drive shaft, when I remembered that the polyflex coupling is the last original component in the drive train which I was intending to replace anyway, and it occurred to me that if I were to replace the polyflex with a rigid coupling, that might be the solution.

Full technical details of the system are all of the above flexible bits, attached to a Velvet drive gearbox at one end, a 4 foot long 1.5" drive shaft ending in a conventional cutless bearing housed in the back of the keel. Shaft maximum speed is 1300 RPM, engine is 96HP.

The engine mounts are new, as is the PSS shaft seal, the cutless at the stern and the drive shaft itself.

With all this in mind, would you forgo the polyflex coupling for a solid coupling, or go with the cutless at the top of the shaft log to support the PSS? FWIW, if I go the cutless will have to pull the boat out of the water, due in the next few months anyway, whereas the solid coupling can be safely installed in the water.

Over to the brains trust.

Matt
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Old 22-02-2015, 04:59   #2
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Re: Too many flexible bits.

Well, I can't help but I'm interested in the replies you get.

I recently had a new cutlets bearing installed in the end of the tube to support the pss seal too. And I have a solid coupling.
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Old 22-02-2015, 05:59   #3
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Re: Too many flexible bits.

Matt,

I can see what your friend is saying here, and it seems to have pretty good merit... Sounds to me too like some stabilization would go a HECK of a long way to avoiding or eliminating possible wear problems...

Can ya make a quick diagram of the shaft layout, and basic distances between components???

I've got an Aquadrive in the middle of my setup, and really like the layout... You could do the same thing with a glassed in plate with a pillow block bearing supporting the inboard end of the shaft before you get to the flange... It will offer lots of adjustment, and when tightened up, a nice support..

Seems WAY better to me than adding another cutless???
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Old 22-02-2015, 06:01   #4
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Re: Too many flexible bits.

Jayzuhs...

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Old 22-02-2015, 09:26   #5
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Re: Too many flexible bits.

Hi everyone.

There is NO NEED for cutlass bearing. The carbon part is centring the shaft in the pss shaft seal. It will move freely with the shaft, should it move. So, if you have installed it according to the installation manual, it is OK. You say the engine is 96 HP, so the shaft should be at least 35mm (1 3/8"). Even if the shaft is free for about 1,2m (4'), there should be NO problem, However, if the distance between the PSS shaft seal and shaft coupling is bigger, it would be a good idea to stabilise it by external bearing as shown on HappyMdRSailor's pictures, as close to the SS rotor of the PSS shaft seal as practical.
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Old 22-02-2015, 16:06   #6
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Re: Too many flexible bits.

Quote:
Originally Posted by Dolphinblue-BOB View Post
Hi everyone.
So, if you have installed it according to the installation manual, it is OK.
The total distance between the PSS shaft seal and the transmission coupling is less than 12 inches, probably 10 inches at a guess. The whole shaft is only four feet long, and that includes the bit in the log, plus the bit sticking out past the cutless for the prop.

The problem is, the engineer thinks the installation is NOT in accordance with the PSS manual, he thinks the manual states a maximum of two of the three connections should be flexible. I wonder if anyone has the paper version of the PSS manual hanging around that they could check this?

Anyway, given I reckon it a good chance he's right, I am still keen to hear people's opinion on a solid coupling to replace the polyflex.

The pillowblock bearing option is interesting, but won't work for me, there's just not enough space between the end of the shaft log and the back of the gearbox. Also, it would be a heck of a lot more work than the cutless solution, which is at least pretty simple.

Matt
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Old 22-02-2015, 16:57   #7
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Re: Too many flexible bits.

GILow

I would think that if your Engine is soft mounted then the connection to the Shaft should also be soft, why are you changing that too hard? Sounds like you are trying to centrally locate the engine in respect to the cutlass bearing and seal via the shaft.


I just replaced flange bolts on a Perkins engine Non-Flex coupling, due to misalignment problems (All bolts broke, vessel near the Port Phillip Heads), think sometimes flex couplings and soft mounts are used as a cure all for alignment adjustment issues.
Shaft should be a very reliable piece of equipment.

I tentatively agree with Dolphin Blue.

More info please, where are these place you descibe as "top of the propellor log" and "top of the shaft log" and did you change to soft engine mounts from solid and why.
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Old 22-02-2015, 17:47   #8
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Re: Too many flexible bits.

OK, to clarify a bit.

The engine was hard mounted, and the vibration was horrendous. A 3.5 litre four cylinder diesel is very, very rough. A healthy engine, with even compression on all four cylinders, just the reality of four big slow pistons doing their thing. So I changed the engine to soft mounts, and it is just lovely now. Very smooth. I am very, very glad I have made this change, and I am happy to make modifications to the drive train to keep the soft mounts. The boat is much more pleasant to use as a result of the change.

In our boat, with the original hard mounted setup, the connection to the propellor shaft from the back of the gearbox was(is) through a polyflex coupling. This one to be precise.



From the coupling to the front of the propellor shaft log is a distance of about 10 inches. At this point I have a PSS seal. (To my understanding, the propellor shaft log is the tube that houses the propellor shaft for the transition between inside and outside the hull, and I also understand the norm is for the log to be full of water in normal operation. If I am misusing the log name that may be causing some confusion.)

Here is a picture from the web that is very similar to my setup, with the exception that in this case the coupling between the shaft and the gearbox is a hard connection, not a polyflex. In this photo I am assuming that the egine is soft mounted for the simple reason that most engines do seem to be soft mounted. In this photo they have a solid connection between the propellor shaft and the back of the gearbox, which is what I am considering as an alternative to the polyflex and is, apparently, what is recommended in the PSS manual.





The alternative offered by the engineer is to fit a cutless bearing just inside the shaft log at the PSS end, to support the PSS.

I assume this extra support is to stop odd harmonic movements developing in the shaft when running, such as might happen from movements in the engine combining in strange ways with movement in the polyflex coupling and causing some kind of skipping rope effect or goodness knows what. I confess I feel that such a short, thick shaft should have no problems, but I respect his instincts on this matter, and also those of PSS who (apparently) discourage a setup like my current arrangment.

Has that cleared things a little?

Matt
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Old 22-02-2015, 18:05   #9
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Re: Too many flexible bits.

I successfully used the same system to what you describe with a coupling similar to the Polyflex, and a Las Drop shaft seal. However, my bronze stuffing box has a long bearing that provides a minimal clearance that is sufficient for water flow. It all boils down to how much clearance you have at the forward end of the shaft log i.e., twixt the shaft and the shaft log/stuffing box. I can see where it could put a lot of lateral strain on the Polyflex, and even though it might not be necessary, based on how much clearance there is, I would consider supporting the forward end of the shaft just to be safe, but then again, I overbuild everything. : o)

Good luck!
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Old 22-02-2015, 18:16   #10
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Re: Too many flexible bits.

Do you have an actual problem, or just the concern of the engineer?

I would be very leery of adding an additional cutlass bearing at the front end, because of inducing too much rigidity into the system. A shaft fixed radially at both ends, attached to an engine with soft mounts and a cast iron coupling sounds to me like an invitation to vibration at best or a broken shaft at worst.

According to everything I could find on the PSS seals, their strong point is absorbing lateral movement, their weak point absorbing fore and aft movement. The problem comes from hard reversing pulling the shaft back and moving the sealing collar forward on the shaft, when the reversing situation is over, the bellows does not have enough tension to seal the carbon against the steel. This problem appears to be generally more common in high powered applications.

Regarding changing the polyflex coupling, I'm not really sure that I would call that a 'flexible bit'. While it's certainly softer than steel, even the softest, at Shore hardness A scale 80, is for all practical purposes, rigid.

And you lose the electrical isolation of your prop and shaft from the electrical system if you change it.

Since this is a new installation, if it were me, I'd make sure the alignment is good, run it a little, tie it off at the dock and throttle it up maybe if you're worried about it, just generally keep an eye on it. Meanwhile, find the manual you're seeking (ask the engineer, after all, he brought it up) and verify if there are or are not any restrictions or guidelines on allowed limits of movement and numbers and types of mountings.

You've kinda got my curiosity up now, if I get time I may call'em tomorrow. If I do I'll let you know what I find....
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Old 22-02-2015, 18:45   #11
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Re: Too many flexible bits.

Quote:
I would be very leery of adding an additional cutlass bearing at the front end, because of inducing too much rigidity into the system. A shaft fixed radially at both ends, attached to an engine with soft mounts and a cast iron coupling sounds to me like an invitation to vibration at best or a broken shaft at worst.
I second this. Its not as though you have a very long shaft. Rigidly supporting the shaft at the forward end only 10 inches or so from the coupling could put undue strain on the shaft and the coupling even with the flexible coupling installed.
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Old 22-02-2015, 18:49   #12
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Re: Too many flexible bits.

I tend to over engineer a bit myself and sometimes simpler works.
Your Engineer wants to make the shaft thru the Sterntube stiff. It would also make alignment checks easier.
Now, Dolphinblue has already said that the flexing of the shaft won't matter to the seal, as its self centering. That could be handy in a rolling sea.
I would think having flexibility in the shaft is good.

In a vehicle where flexing occurs in a shaft this is accommodated by a spline and twin universals, the beauty of such a system as that speed whip harmonics are eliminated. But I don't have the writing skill to explain it succinctly here.
Its enough to say that if your shaft that passes thru the stern tube is parallel to your gearbox output shaft and in line at even keel, close enough, then all will be good.

However your first sentence "asked him if he was available to check the alignment on my engine" suggests you have no idea if the shafts are parallel and in line, and if you asked me to do it, (I'm an Engineer also) I'd probably avoid you, as it would be extremely difficult to raise lower, move front of engine left right, and how much do your squat in the engine mounts has occurred, no idea I bet.
If I were doing it, I'd probably want a solid coupling and stern shaft supported centrally so I can measure the misalignments and use a bit of trigonometry to work out the adjustments required, chech the results and when happy fit your soft coupling then.
There are many reasons why I'd prefer a solid mount over pussy foot. Nothing wrong with a good bit of Red Gum, heart wood. At least the engine will stay where it supposed to when you are upside down.
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Old 22-02-2015, 18:49   #13
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Re: Too many flexible bits.

1. My PSS manual (circa 2000) has nothing to say on the issue. Online manual is identical to the paper manual, no surprise there.

2. For accurate engine alignement with a flexible coupling, you should replace the coupling with a solid spacer while aligning. There is too much "squish" in the flexible coupling to enable fine measurement. Do you have a solid spacer? If not, you can get one made up at a machine shop, complete with bolt holes, etc.

3. There is a view that flexible mounts + flexible coupling can lead to harmonic vibration at certain revs. Seems to depend on the boat and length of prop shaft. There have been threads on CF. I have never seen anything set down in books or elsewhere. With your solid spacer, you can try both solid and flexible coupling to see if there is any difference.

4. My boat is the same as yours, with identical drive train setup. There is some vibration at high revs, evident with both solid and flexible coupling. Its in a rev range that I never use. I have stuck with the flexible coupling.

5. As a previous poster noted, the PSS is designed so that the faces remain in contact and the bellows takes up any slight movement. If under drive there is visible movement of the PSS and/or leakage, it would suggest there are problems to me.

6. A cutless bearing inside the boat I am not sure of, how would it be lubricated? If you decided to positively locate the shaft I would have thought a proper bearing block as suggested by others would be the way to go. If I was to go to that sort of trouble , I would do a proper job and put in an aquadrive.

Don't know if this helps, Lee
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Old 22-02-2015, 19:19   #14
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Re: Too many flexible bits.

I concur with HappyMdR sailor and Banjoship. Several thousand hours engine ago I installed a Yanmar with flex mounts in the horizontal position rather than at 14 degrees, if it I had been trying to make a straight shot. To make this possible, and since I look for inexpensive ways to make improvements, I used an automotive CV joint, connected to the shaft that was/is supported by a commercial flange mounted thrust bearing. I also did the best I could to align the engine and shaft, but I didn't have to be that fussy. And I have only just removed a very old Las Drop (one of the first) shaft seal with one from PSS since there isn't enough room twixt thrust bearing and stuffing box to allow a longer Las Drop unit.

Incidentally, at the time, Aquadrive based the longevity of the buns used in their device subject to the angle twixt engine and shaft. I am not sure what the situation is now. By selecting an automotive CV joint, my great grandkids will only have to continue to periodically lub the CV joint and the thrust bearing.
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Old 22-02-2015, 19:45   #15
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Re: Too many flexible bits.

Ha, I have just gone through all this myself. I was told that it was simplest to use a floating shaft, with only the one bearing near the prop, and let the rest become flexible, but to do this properly a rigid shaft connection was needed to help reduce the chance of the shaft whipping. For this to work the shaft lengths and diameters needed to be within tolerance. Looking at the Nanni N100 guidelines it seems yours looks ok, subject to your exact rpm and reduction ratio.

If you go a solid shaft with two (or more) bearings then the polyflex drive saver coupling is needed/advisable to reduce the loads on the gearbox output bearings.

This all seemed logical to me and so far I have had no issues with my floating shaft and rigid coupling, but then I haven't done much running yet.

Interested to hear how you go with it all, as it seems like a bit of a dark art. for me it was a no brainer to avoid the polyflex drivesaver as they cost money, and in my case the advice was that they wouldn't work well with my floating setup.

At any rate the system should be setup so that it can be direct coupled if the drivesaver does it's job and shatters, so you should be able to easily try it with and without, and for alignment the drivesaver should be out, and the flanges directly coupled and aligned anyway. Just make sure the couplings are both actually true, it's quite common for them to be a couple of thou out, and some orientations of the shaft vs gearbox couplings can make things worse, and others can cancel out or reduce any small misalignments.
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