Vibrating shaft at slightly high revs.

[GILow]

Originally Posted by Hoohaa

Won't an aquadrive just deal with an alignment issue?
Would installing a cutlass bearing at the back of the stern pipe, support the shaft and prevent or reduce the wobble?

We have a Cutless (or water lubricated) bearing immediately behind the PSS shaft seal, on the recommendation of a marine engineer at our club, in theory to stabilise the shaft. (In our case I think it was overkill, the shaft is only 4 feet long in total, and 1.5 " thick, with a max shaft speed of 1300 rpm.)

My main concern with it is that it limits the movement of the shaft to the point of interfering with the soft mounting of the engine.

This is something you might need to consider if you do put a Cutless bearing at that point, though in your case the extra distance between the placement of the bearing and the engine might allow the engine to move ok.

As an aside, I assume all of those photos you have posted were taken BEFORE the bearings were replaced? My reason for asking is none of them look particularly new to me.

Matt

[DeepFrz]

The aquadrive (and I use a brand name but there are many) effectively cuts your shaft length in half, or there abouts, as well as dealing with alignment. It allows you to use softer engine mounts as well, which further smooths the engine vibrations.

It may even allow you to get away with a 25 mm shaft diameter, but I think that is still on the edge, looking at the length of shaft outside the hull. That's why I suggested maby you need both. I know that is very expensive but I think in your case you don't really have much of a choice.

The problem with installing a bearing at the stuffing box area is you now have a rigidly mounted shaft and an engine on flexible mounts. That is not going to work well as the engine movement may work harden or even break the shaft at the top bearing. I think the double bearings work okay if the engine is hard mounted and can be precisely aligned. Anyway, that is my opinion and I'm not an engineer so take it all with a grain of salt. Best to get a marine engineer to assess your needs.

[Hoohaa]

Originally Posted by GILow

We have a Cutless (or water lubricated) bearing immediately behind the PSS shaft seal, on the recommendation of a marine engineer at our club, in theory to stabilise the shaft. (In our case I think it was overkill, the shaft is only 4 feet long in total, and 1.5 " thick, with a max shaft speed of 1300 rpm.)

My main concern with it is that it limits the movement of the shaft to the point of interfering with the soft mounting of the engine.

This is something you might need to consider if you do put a Cutless bearing at that point, though in your case the extra distance between the placement of the bearing and the engine might allow the engine to move ok.

As an aside, I assume all of those photos you have posted were taken BEFORE the bearings were replaced? My reason for asking is none of them look particularly new to me.

Matt

Is your cutlass bearing in the forward end of the stern tube?
The pictures with the grey undercoat and shots showing the newly constructed stern pipe replacement are during the replacement work and should show the new bearings. Unfortunately the other pics are before and after. Gets a bit confusing. I have been digging around trying locate any useful pics that i thought might help.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[GILow]

Our cutless bearings are at the extremes of either end of the stern tube. Therefore the forward cutless abuts the pss shaft seal.

Ours is a simpler setup with a single 3' stern tube with the prop at the aft end and the PSS seal at the front. The shaft extends less than 12 inches from the stern tube to gearbox flange. The whole thing feels very rigid.


Sent from my iPhone using Cruisers Sailing Forum

[GILow]

Originally Posted by DeepFrz

.

The problem with installing a bearing at the stuffing box area is you now have a rigidly mounted shaft and an engine on flexible mounts. That is not going to work well as the engine movement may work harden or even break the shaft at the top bearing. I think the double bearings work okay if the engine is hard mounted and can be precisely aligned. Anyway, that is my opinion and I'm not an engineer so take it all with a grain of salt. Best to get a marine engineer to assess your needs.

Absolutely agree with this. It has worried me since we installed the extra bearing in our boat, ironically at the advice of a marine engineer.


Sent from my iPhone using Cruisers Sailing Forum

[Snowpetrel]

Originally Posted by Hoohaa

Won't an aquadrive just deal with an alignment issue?
Would installing a cutlass bearing at the back of the stern pipe, support the shaft and prevent or reduce the wobble?

I was thinking an aquadrive might solve the overlong, overwhippy shaft issue by shortening it, and allowing an intermediate bearing.

Saying that there has been plenty of boats with rigid shafts (lots of intermediate bearings) and flexi mounted engines that have done fine with just a simple drivesaver type coupling, or even a bolted together coupling. Even though in theory this would seam to put a lot of load on the shaft and gearbox rear bearings.



Sent from my SM-G930F using Cruisers Sailing Forum mobile app

[OceanSeaSpray]

Originally Posted by GILow

Absolutely agree with this. It has worried me since we installed the extra bearing in our boat, ironically at the advice of a marine engineer.

The mitigating factor in these situations is how long the exposed shaft is between the coupling and the first support point. Supporting the shaft directly behind the seal is a very bad idea if the engine is very close, but a longer exposed shaft (like we see here) will flex a little and follow with the engine without issues.

With short shafts, like 600mm, it makes no sense at all to support them more than just in front of the prop (it is in fact asking for trouble down the track) and hardly anything else matters. They are too short to develop whirling issues.

[GILow]

I agree that 600 mm would not need the extra support. It what about when the shaft is 1200 mm like ours?


Sent from my iPhone using Cruisers Sailing Forum

[DeepFrz]

e.g. Alignment, balancing, soft coupler, new prop shaft, new prop, PSS to packing gland and even a Pythondrive installation. The final resolve was to up-size the shaft to the next size (1-1/8") which cured the problem.

The four main items that will affect vibration is the (1) coupler alignment, (2) cutlass bearing, (3) shaft and prop straightness and balance (4) motor mounting condition.

Check all these items. Going to a larger shaft is a total *refit/replacement from the back of the motor to the end of the prop.

*coupler, shaft, prop, stern tube, shaft seal/packing and strut/cutlass.

I've taken the liberty of reposting some of delmarrey's post from post #9.

[OceanSeaSpray]

Originally Posted by GILow

I agree that 600 mm would not need the extra support. It what about when the shaft is 1200 mm like ours?

If you post the distance between the coupling and rear bearing as well as max shaft RPM, I will have a look for a 25mm shaft tonight.

If a floating shaft is going to work at all, it will only be hard coupled, like mine basically.

[GILow]

Originally Posted by OceanSeaSpray

If you post the distance between the coupling and rear bearing as well as max shaft RPM, I will have a look for a 25mm shaft tonight.

If a floating shaft is going to work at all, it will only be hard coupled, like mine basically.

It's a 1.5 inch shaft, not 1 inch. So about 38mm if my maths is not letting me down. Total shaft length is about 1200 mm, say 150 mm protrudes beyond the rear cutless, which is about 200 mm long so therefore from the front face of the cutless to the gearbox flange would be about 850 mm. Max shaft rpm is 1300. I use a polyflex coupling like the OP, with notable difference that the edges have not been chewed off. :^)

I'd be interested to hear what your tables say about that combo.

Matt


Sent from my iPhone using Cruisers Sailing Forum

[OceanSeaSpray]

Originally Posted by GILow

It's a 1.5 inch shaft, not 1 inch. So about 38mm if my maths is not letting me down. Total shaft length is about 1200 mm, say 150 mm protrudes beyond the rear cutless, which is about 200 mm long so therefore from the front face of the cutless to the gearbox flange would be about 850 mm. Max shaft rpm is 1300. I use a polyflex coupling like the OP, with notable difference that the edges have not been chewed off. :^)

I'd be interested to hear what your tables say about that combo.

Matt

This is similar to my shaft, just shorter and somewhat higher RPM. Mine is 1430mm long between supports and I had to couple it hard to stabilise it.

For you, the first critical speed would be around 6400RPM, hard-coupled and discounting engine vibrations etc. You might well get away with the drive saver if you really like it, but it should really just be hard-coupled and supported at the rear end by the cutless bearing, end of story. It is not going to go anywhere, guaranteed. It is too short and big enough to easily stay put.

If you have around 50HP on it, then your safety factor is about 3, conservatively normal...

Eric

[GILow]

Originally Posted by OceanSeaSpray

This is similar to my shaft, just shorter and somewhat higher RPM. Mine is 1430mm long between supports and I had to couple it hard to stabilise it.

For you, the first critical speed would be around 6400RPM, hard-coupled and discounting engine vibrations etc. You might well get away with the drive saver if you really like it, but it should really just be hard-coupled and supported at the rear end by the cutless bearing, end of story. It is not going to go anywhere, guaranteed. It is too short and big enough to easily stay put.

If you have around 50HP on it, then your safety factor is about 3, conservatively normal...

Eric

Thanks Eric, that's a great help.

I was a bit surprised at the recommendation of the extra bearing, but took the advice because it came from someone who works as a shipwright and I was told was a marine engineer.

Hmm...

BUT, does it change things much that the engine is actually 96 HP (though it is limited by the prop pitch and size to 85 HP)?

Matt

[OceanSeaSpray]

Originally Posted by GILow

Thanks Eric, that's a great help.

I was a bit surprised at the recommendation of the extra bearing, but took the advice because it came from someone who works as a shipwright and I was told was a marine engineer.

Hmm...

BUT, does it change things much that the engine is actually 96 HP (though it is limited by the prop pitch and size to 85 HP)?

Matt

Yes, there are all kinds of marine engineers... some proper ones, and many who have just been around boats for a long time. Ask for the shaft calculation and it all sorts itself out nicely.
The larger engine reduces the safety factor on torsional stresses, but thin safety factors on yachts are quite common because a bigger shaft is more expensive, heavier and the shafts seldom/almost never operate at maximum torque. Here you should still have 50% margin at full torque.

Even if you were running hard, suddenly caught a heavy net in the prop and it went really wrong... you could still sail home, the engine is just an auxiliary...

The OP's 1-inch shaft could be interesting in this regard. Pleasure craft are not taken as seriously as commercial vessels and the old "value for money" concept does the rest... the shaft diameter is not what the buyers look at. This being said, we don't hear much about sheared off shafts and lost props.

[Cheechako]

Originally Posted by OceanSeaSpray

Yes, there are all kinds of marine engineers... some proper ones, and many who have just been around boats for a long time. Ask for the shaft calculation and it all sorts itself out nicely.
The larger engine reduces the safety factor on torsional stresses, but thin safety factors on yachts are quite common because a bigger shaft is more expensive, heavier and the shafts seldom/almost never operate at maximum torque. Here you should still have 50% margin at full torque.

Even if you were running hard, suddenly caught a heavy net in the prop and it went really wrong... you could still sail home, the engine is just an auxiliary...

The OP's 1-inch shaft could be interesting in this regard. Pleasure craft are not taken as seriously as commercial vessels and the old "value for money" concept does the rest... the shaft diameter is not what the buyers look at. This being said, we don't hear much about sheared off shafts and lost props.

Yes, here's a boat designed by a whole group of "proper" marine engineers:

https://www.youtube.com/watch?v=8Yau9A7XDHs