Two seperate cutlass bearings in stern tube?

[Tim Good]

Ok so I have a 6inch cutlass bearing in the p bracket and the same in the stern tube. I have removed both and replaced.

However, the stern tub is about 12 inches long. So there is about 6 inches of stern tube without any bearing.

I could just get another and stick it into the stern tube from inside of the boat, drill and tap a couple of new grub screws holes and then the stern tube would have two bearings giving more support to the shaft?

Am I loosing the plot and should I just keep it how I found it?

Like this maybe:

View attachment 49980

View attachment 49981

 

[Neil_Y]

How far is the engine/gearbox coupling from the forward bearing? in soft mount installations there needs to be a bit of shaft length forward of the last bearing. As the engine moves the shaft flexes but also loads the bearing the closer the bearing the greater the loads and hence wear/heat.

Running an effectively longer bearing if they are close together we would open up the clearances slightly for cooling and keeping a water film.

Questions are what are the distances involved to engine (and bearing to bearing) and does the stern tube have a water feed and where does it come from. Before or after engine.

Do you have a problem with bearing wear?

 

[VicS]

would alignment/ misalignment of three bearings be an issue ?


You would have to provide a source of lubrication ( water) for your proposed additional bearing,


Are there reasons to fit an additional bearing or are you "fixing whats not broken" ?

 

[oldharry]

Same question as VicS _ why do you think you need a third bearing? Remember that bearings absorb power. Not an issue on a big engine, but a small auxiliary it could be significant, and add to fuel cost to maintain a set speed.

If you are noticing vibration in the shaft, then the shaft is bent out of true and no amount of extra bearings will stop it shaking! (Assuming the existing bearings are not badly worn)

 

[Tim Good]

How far is the engine/gearbox coupling from the forward bearing?

From the end of stern tube to the aquadrive coupling is around 12 inches. Add the 6 inches of stern tube before the end of the aft stern tube bearing which makes a total of 18inches currently.

does the stern tube have a water feed and where does it come from. Before or after engine.

No not directly but the stern glad has a water feed which I suppose is the same? That comes from the engine. Directly off the water intake before it enters the engine I think. Might need to check on that.

Do you have a problem with bearing wear?

Same question as VicS _ why do you think you need a third bearing?

No. I am just replacing the prop shaft and wondered if it might be better whilst I'm here doing the job. Maybe you're right. Don't fix what isn't broken.

 

[Tranona]

No need for a second bearing. The current arrangement provides more than enough support. Remember your Aquadrive means that the shaft is is not subject to any movement from the engine so provided the two bearings are correctly aligned it should write well. A well probably over engineered job.

 

[vyv_cox]

No need for a second bearing. The current arrangement provides more than enough support. Remember your Aquadrive means that the shaft is is not subject to any movement from the engine so provided the two bearings are correctly aligned it should write well. A well probably over engineered job.

In theory absolutely correct. However, in practice my shaft knocks on the stern tube in certain conditions despite having an Aquadrive. If motoring at 2000 rpm, which seems to be a critical speed for my shaft, in a following sea, or when manoeuvring using bursts of forward and astern, we often hear shaft knocks. I suspect that the shaft is bending a little, sufficient to close the small clearance that on my boat is not enough for a cutless bearing. I intend fitting a small steady bearing at some stage

 

[Tranona]

However no this case the shaft is 1 1/2" diameter and has an unsupported lenght ofor only 6". Would be really worked for that showed any signs of movement.

 

[vyv_cox]

However no this case the shaft is 1 1/2" diameter and has an unsupported lenght ofor only 6". Would be really worked for that showed any signs of movement.

We may be advising at cross purposes I am suggesting that the existing inboard bearing should remain but no need for another one..

 

[Tranona]

We may be advising at cross purposes I am suggesting that the existing inboard bearing should remain but no need for another one..

Not what you said - you were describing what happened with your shaft. Can't see any suggestion in your post that the OP does not need to do anything except for an oblique reference to perhaps adding an additional bearing to your shaft.

His original installation was designed specifically to give the shaft good support, even though the Aquadrive will minimse any movement of the shaft - one of its selling features. Like most of the rest of his boat, very well engineered.

 

[Tim Good]

Just for those that have commented on this thread already. You have clearly replaced bearings before. I have removed the old bronze and phenolic ones. One of each. I now have two GRP ones to replace.

They slid into the P Bracket and Stern tube with relative ease. I certainly didn't need any threaded rods or the like. Just a light tap with a rubber mallet.

Anyway I have slots for grub screws. Is the proceedure to use a small drill bit and put a tiny indent into the GRP casing and then tighten the grub screws in to secure the bearings?

If so would I be wise to put a bit if Sikaflex on the threads to keep it in place?

 

[Topcat47]

My original engine was an MD1B and my stern tube had cutless bearings at both inside and outside. When i re-engined with a 1GM10, the mountings were much softer and the engine lighter. As the new engine moved around more on the soft mountings, the shaft whipped and the outboard cults bearing wore out in a season or two. I dispensed with the inner cults bearing at the same time ass replacing the outer one and since then, the excessive wear problem has disappeared. I have contemplated fitting an aquadrive thrust bearing in the safe line and were I to do that, I'd go back to using two cults bearings on the shaft.

 

[Tranona]

Just for those that have commented on this thread already. You have clearly replaced bearings before. I have removed the old bronze and phenolic ones. One of each. I now have two GRP ones to replace.

They slid into the P Bracket and Stern tube with relative ease. I certainly didn't need any threaded rods or the like. Just a light tap with a rubber mallet.

Anyway I have slots for grub screws. Is the proceedure to use a small drill bit and put a tiny indent into the GRP casing and then tighten the grub screws in to secure the bearings?

If so would I be wise to put a bit if Sikaflex on the threads to keep it in place?

Any reason for using grp ones rather than brass, or are they composite. No advantage to grp unless you have a steel tube. However if they are the correct size they should be a push fit and no need for any sealant. The grub screw will make its own dimple but some people do use adrill to create one when the bearing is in place. Again no need for any sealant, the screws are often difficult to shift rather than coming loose.

 

[Neil_Y]

The only reason you might add an extra forward bearing would be if you experienced problems with vibration or noise associated with a very heavy prop. (this happens with yachts and the larger heavier feathering/folding props on thin shafts sometimes) As you have an aquadrive the bearing won't be subject to the forces of an engine bouncing around on its mounts which needs some length of shaft fleax to deal with in a normal installation.

So it sounds like you would see no benefit as you don't have problems and a 1.5" shaft on a 40hp (I'm guessing) is quite well matched. There have been some yachts that have had 1" shafts and 40Hp which is getting a bit thin and bendy. Seeing as you're on old fashioned rubber bearings you will get some shaft flex movement as the rubber compresses as the water pressure changes around the shaft.

In Summary probably not needed

 

[Tim Good]

Any reason for using grp ones rather than brass

That was just the advice I was given. Was told GRP tend to be better nowadays. My stern tube is grp and P bracket is bronze.

Appreciate all the advice re triple bearings. Will leave it as two as per general advice. Haven't had any issues to date running 80hp Perkins via aqua drive on a 6ft 1.5" shaft with feathering prop.

 

[Tranona]

Not sure there is a sound basis for saying it is better. Brass shell is compatible with both bronze and grp. Doubt there will be any difference in performance. As I suggested grp has the advantage if you have a steel or stainless tube like modern Beneteaus.