Prop shaft alignment

[pcatterall]

The new shaft and stern gear fittings will be ready for fitting next week. The job is being done by an engineer and shipwright in Poole.
The shipwright was discussing allignment and recommends that the boat goes in the water for a couple of days to settle. She is a long keel Heavilly built Neptunian and should not flex too much but I can see that it is important to ensure there is no distortion when she is on the hard.
I just wonder about the process; If the boat is in the water with prop,shaft and all the bits in place then how do we know that the shaft at the back is fitting centrally in the cutlass bearings? wont the shaft 'droop' so that it is no longer central in the bearing? or is the shaft suspended vertically to take the shafts weight at the engine end ( I guess that a spring balance could be used loaded with the calculated weight of the shaft?)
I guess that the shipwright would then have to adjust the engine mounts to bring the coupling faces together within tolerances?
I've tried to google this process and got some bits of info but not this bit of the process.
Enlightenment will be appreciated!!

 

[GrahamM376]

Much depends on your setup. Some stern tubes have another cutless bearing which holds the shaft central, no need to worry about droop.

If you have a flexible tube holding the stern gland assembly to the stern tube and no second bearing, I would slide the rubber forward off the stern tube and insert temporary packings to centre the shaft. Slide the shaft forwards to mate the propshaft flange and the gearbox and adjust the engine mounts until it's aligned within 5 thou max. Then remove the packings, refit the rubber tube and bolt everything together.

Assuming you have a flexible coupling, one of the bolts has a dome head to allow feeler gauges to be inserted between that and the gearbox flange. Easy job once the boat has settled in the water to rotate the shaft, check clearances every 90 degrees and finally tweak the mounts until it's aligned properly.

 

[Burnham Bob]

When the engineers fitted my new engine they suggested that the alignment was checked after 25 engine hours to allow for everything to settle down. If you think about it there are lots of things to move about which will become clear when the engine has run for a while.

Seems more sensible than a check after a few days, unless of course the engineer is doing the first 'proper' alignment in the water so its more accurate.

 

[vyv_cox]

I think you are confusing yourself somewhat. Presumably with the boat you describe, the cutless bearing is at the aft end of the stern tube? In this case, provided the bearing is correctly located in the tube, it dictates the location of the shaft, which by definition must be central in the tube. Even if you had a P bracket the same should apply - the shaft will (should) be central at the aft end.

Unless the shaft is long and spindly the droop will be insignificant. My 1 inch shaft is about 4ft 6ins long and does not appear to droop at all. The important part is to ensure that the shaft is central in the tube at the forward end before aligning the flanges between shaft and gearbox. It is possible to have the latter perfectly aligned but the shaft rubbing on the tube inside the stern gland, either all the time or when the engine moves on its mountings, perhaps in a seaway. This can be more difficult to arrange with the boat in the water, as the gland needs to come off to see it. The preference is therefore to set it up ashore first, then do the final coupling alignment on the water.

 

[webcraft]

.
As stated above, you need to centre the propshaft i the stgerntube at the inboard end. Using ordinary wedges is one answer, but it is not ideal. If you ahve the tools, make special hemio-conical wedges and the job is much easier, holding the shaft rock steady in the exact centre olf the tube while you finish the job.

Get a cone of wood that tapers from greater than the sterntube diameter to smaller than the sterntube diameter but bigger than the propshaft diameter.

Drill a hole centrally through the cone the same diameter as the propshaft.

Split the cone in two laterally.

You now have two semi-conic wedges. Place them over the shaft so you have a complete cone again and slide this conic wedge down the propshaft into the stern tube. This will centralise the shaft in the tube.

Now align the gearbox coupling or flexible coupling flange using feeler gauges as described.

- W

 

[pcatterall]

Thanks guys, I think something is penetrating the brain at last.
When I spoke about 'droop' I was thinking partly about the weight of the shaft ( 1.5" and 5 feet long) but also that the rubber in the cutlass bearing would be ditorted by the weight of the shaft.
I now understand that the centrallity in the bearing can be acheived by wedges and that the shaft itself will not 'droop' so with the stern end held secure and central the 'front ' end can be aligned?
Have I got there yet?!!
Thanks

 

[GrahamM376]

Thanks guys, I think something is penetrating the brain at last.
When I spoke about 'droop' I was thinking partly about the weight of the shaft ( 1.5" and 5 feet long) but also that the rubber in the cutlass bearing would be ditorted by the weight of the shaft.
I now understand that the centrallity in the bearing can be acheived by wedges and that the shaft itself will not 'droop' so with the stern end held secure and central the 'front ' end can be aligned?
Have I got there yet?!!
Thanks

You have no need to try to centralise the shaft in the cutless bearing, if it's sloppy enough to move around , you have the wrong bearing.

All you need to do is centralise the shaft in the front end of the stern tube before aligning the engine.

 

[Neil_Y]

We advise people to check carrier alignment by chocking the shaft centraly in the bearing / carrier as this is where the shaft will be when running. A stationary shaft will sit in the bottom of the bearing and if it's rubber it will give/sag.

The engine is designed to move on its mounts where as the shaft is meant to run true in the bearings. The movement comes from shaft flex between the foreward most bearing and the engine.