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Prop shaft to Flexi plate flange connection


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Not at the boat but trying to formulate a plan to pull prop shaft on next haul out.

l guess flange screws onto shaft then machined true on lath.

I don’t remember any locknut.

Otherwise  have to move motor forward and pull ps into boat minus anode and prop.

Stern gland is behaving erratically.

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That makes sense. Must be a shallow taper as flexiplate isn’t that thick.

I guess if I remove the anode and prop and flexiplate I should have enough play to inspect shaft at packing area.

Im keen to fix in the water if possible as don’t want to haul boat until end of summer.

Replaced the packing a couple of days ago but the old looked ok.

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Several points, if your packing gland is leaking too much, there could be several things. Our shaft was 'necked' fairly noticably. Nothing a new shaft wont fix ;-)

Any play / vibration in the whole system wont help. That would include cutless wear, but also engine alignment, and the condition of your engine mounts. Don't forget engine mounts have a limited life, and do need replacing occasionally. Alignment might look good at rest, but if the mounts are stuffed, there will be misalignment when underway, which flogs out the flax and leads to weeping.

How to get the half coupling off depends entirely on what you have on. We had a half coupling that went to an R&D flexi coupling (white, thin ish disk of 10mm, with offset bolts on each side. That half coupling had a locking nut on the engine side, with a thread on the end of the shaft. You will need a blow torch - and be in an angry mood... I've got new spare R&D couplings in 4 inch and 5 inch if anyone is interested. 

We replaced that with a CentaFlex. That uses a parrallel (straight) ended shaft, and uses a concentric tapper that pulls down on it to create a clamping force (pulls down along the length of the shaft. To get it off there are threaded bolt holes to push the concentric tapper back off the shaft (put bolts in and gently wind).

We ended up going for a dripless kiwi seal, but then we had a whole new drive train when we put in the new engine. My missus still brings up the amount we spent on all of that...

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Just a thought, if you want to see if the shaft is necked, if you remove the flexi-coupling, you may be able to slide it forward enough to see any necking. Depending how far your prop is from the cutless of course. Could be a pragmatic way to diognose andexclude an issue and plan for fixing it before your haul out

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🙂 some shafts inboard are tapered some are threaded ( " inside") the flange and lots are just straight.

Any of the above may have some kind of grub screw as a safety as well. These can obvious or flush and hidden under paint, rust or grease

The Flexi type couplings have captured nuts. Often they have been loc tite which is an absolute no no. They can then be impossible to remove because they strip their captured surface . If so you then cut them off and get a new coupling. ( Use a torque wrench only to do them up) .

Don't load up the output shaft of your gearbox when dealing with a stuck prop shaft. You can do major damage to the output shaft, rear bearing, or seal. 

Excessive dripping from the stern gland.

The prop shaft may have crevice corrosion (don't use graphite impregnated packing) or scoring under the packing. The engine alignment may have changed, or the mounts become too flexible. The prop may be vibrating or out of balance for many possible reasons . Stern tube bearings can be a problem here. 

Replacing packing can sometimes cause confusion because it can be difficult and take time for new packing to "slow down".  Particularly if the stern gland is of the no grease type. 

There are good reasons for "non traditional" stern glands ! History shows the change from hard mounted engines to quieter flexible mounts. This greater movement of the engine output alignment ( ovaling or spiralling) is not a friend of a fixed centre sealing system. Python drives, crude uni joints and even rubber shaft sections have been used. Now the most common is the flexible disc coupling. But...with all of these systems, if there is no stern tube bearing just outboard from the packing gland then the packing gland has to " hold" the shaft.  That is why PSS seals work in this case. They can take up the concentric movement of the shaft. Oil and O ring type seals or lip seals ( Volvo seals) next best. 

Last.. and sadly.. some boats l have worked on.. you can only pull the prop shaft forwards enough to see what's going on if you undo the engine off its mounts.. then you have to go through a whole engine realign process when ( if) you can fix the problem. The big upside of trying to fix the problem while in the water is...well you are in the water. 

...and you can never tell if it's leaking untill you are. !!

Hope it goes well.

 

 

 

 

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