How to find correct alignment?

[divezonescuba]

Hello:

I consulted the usual resources (Calder's, etc) and have still come up wanting information on where to exactly align the engine to the prop shaft with just the p strut and transmission as support points. A dripless gland is used.

The specific approach that many resources suggest is applying a lifting force of 50% of the weight of the shaft and flange to the flange end of the shaft. This gives a place to start for the correct vertical position of the flange end of the shaft. The engine mounts can be adjusted for that position.

The bigger problem for me is how to determine the position horizontally, that is port to starboard. Precisely determining this position given that it is measured in thousands of an inch is vexing.

I have seen some internet references to positioning a laser bore sight in the p-strut with the shaft removed and then seeing where the laser hits the existing position of the transmission flange.

Is there a way to precisely find the horizontal alighment with the boat in the water with the shaft in place, but with the coupler flange moved back?

[Jsta_Rebel]

Assuming that your shaft is straight, you should be able to separate the shaft coupler a small amount and use a feeler gauge to insure that the opening at four 90 degree points is the same thickness. Top and bottom will show your horizontal alignment, while half way down on either side will check the vertical alignment.

[TurninTurtle]

You actually need a "dial indicator" (some are now digital)

1" Travel Machinist's Dial Indicator

[capt-couillon]

Originally Posted by Jsta_Rebel

Assuming that your shaft is straight, you should be able to separate the shaft coupler a small amount and use a feeler gauge to insure that the opening at four 90 degree points is the same thickness. Top and bottom will show your horizontal alignment, while half way down on either side will check the vertical alignment.

Dead on the money.... Except reversed... Top and bottom give vertical deviation from center, side and side give horizontal. If you rotate shaft and clearances remain same, you know shaft is straight as well.

[danstanford]

This makes sense to me but I read in another thread where someone warned that you should make sure the shaft doesn't fall out and sink your boat! I think the shaft will stay in place supported by the stuffing box seal and the cutlass bearing and that is the whole point of the alignment, isn't it?
I would certainly make sure when I undo the bolts that the shaft doesn't slide back but imagine it will site right where it is, have I got this right?
Our boat 2GM has some real smooth rev levels and some quite rough ones and so I would like to check this out as a cause along with the motor mounts.
Dan

[TurninTurtle]

How easy it is for the shaft to slip... and end up sinking the boat... will vary.

Its a good idea to give the warning and it not be needed, instead of skipping the warning and someone ending up needing divers with airbags to get their boat back to the surface.

I'd expect it to be pretty rare for the shaft to slip... but do you want to be the guy who never heard of the possibility of it falling out and have it happen to you?

[pcmm]

Originally Posted by TurninTurtle

How easy it is for the shaft to slip... and end up sinking the boat... will vary.

Its a good idea to give the warning and it not be needed, instead of skipping the warning and someone ending up needing divers with airbags to get their boat back to the surface.

I'd expect it to be pretty rare for the shaft to slip... but do you want to be the guy who never heard of the possibility of it falling out and have it happen to you?

Its not unheard of for the shaft to slip out of the coupling. I've never understood why couplings aren't through bolted after they're installed and faced to the shaft.

You can mitigate the problem by storing a spare shaft zinc on the inside bolted around the shaft. This sill stop the shaft from backing out all the way in the even it does disengage from the coupling.

[zeehag]

Originally Posted by divezonescuba

Hello:

I consulted the usual resources (Calder's, etc) and have still come up wanting information on where to exactly align the engine to the prop shaft with just the p strut and transmission as support points. A dripless gland is used.

The specific approach that many resources suggest is applying a lifting force of 50% of the weight of the shaft and flange to the flange end of the shaft. This gives a place to start for the correct vertical position of the flange end of the shaft. The engine mounts can be adjusted for that position.

The bigger problem for me is how to determine the position horizontally, that is port to starboard. Precisely determining this position given that it is measured in thousands of an inch is vexing.

I have seen some internet references to positioning a laser bore sight in the p-strut with the shaft removed and then seeing where the laser hits the existing position of the transmission flange.

Is there a way to precisely find the horizontal alighment with the boat in the water with the shaft in place, but with the coupler flange moved back?

we used feeler gages. was not easy but it was not difficult.
make sure the gap is same all around. make sure all parts are installed well and correctly and enjoy your boat.
alignment protects your shaft, shaft tube and packing gland. in many boats, also the cutlass bearing and shaft log. out of alignment will show wear on the tubes and out of round and or loss of threads on the packing unit.
after engine rebuild, we checked alignment with the flat feeler gages and did it all in water.
we did much of it on anchor.

sailed an irwin citation when the shaft left coupling--that is a wierd situation-- lots of rpm and no go.... more rpm no go--oops..is one good reason for taking bearings while motor sailing and sailing. check your sog with eyeball backup.

[CarlF]

I was taught that you loosen but not remove the bolts. Otherwise the weight of the shaft will throw off the measurement. Obviously with the bolts attached there is no risk of the shaft sliding out.

Once loose, use feeler gauges top, bottom and each side - do this four times rotating the shaft 90 degrees by hand each time.

Have I been doing it wrong all these years?

[darylat8750]

It appears to me that the OP is asking how to determine if the shaft is centered in the tube with a dripless seal. Not how to align the center line of the transmission shaft with the centerline of the prop shaft. With the traditional packing the packing can hold the shaft close enough to center to make the system work. The only way that I can see to check for this in the water with a dripless seal is to use a dial indicator as mentioned above. Out of the water a laser would be my preferred method.

[pcmm]

Originally Posted by darylat8750

It appears to me that the OP is asking how to determine if the shaft is centered in the tube with a dripless seal. Not how to align the center line of the transmission shaft with the centerline of the prop shaft. With the traditional packing the packing can hold the shaft close enough to center to make the system work. The only way that I can see to check for this in the water with a dripless seal is to use a dial indicator as mentioned above. Out of the water a laser would be my preferred method.

Centering in the dripless seal is pretty easy as it doesn't have to be within "thousandths of an inch" just remove the dripless seal assembly so you can see the shaft log. then block up the shaft until it's centered in the log.

From there you can align the motor to the shaft flange, then reassemble the dripless seal. I have it easy as I have a support bearing within 18 inches of the engine.

[darylat8750]

Originally Posted by pcmm

Centering in the dripless seal is pretty easy as it doesn't have to be within "thousandths of an inch" just remove the dripless seal assembly so you can see the shaft log. then block up the shaft until it's centered in the log.

From there you can align the motor to the shaft flange, then reassemble the dripless seal. I have it easy as I have a support bearing within 18 inches of the engine.

That makes sense to me. I've never done a dripless seal one. If done in the water will a normal bilge pump keep up with the volume of water coming in around the shaft?

[divezonescuba]

Originally Posted by darylat8750

It appears to me that the OP is asking how to determine if the shaft is centered in the tube with a dripless seal. Not how to align the center line of the transmission shaft with the centerline of the prop shaft. With the traditional packing the packing can hold the shaft close enough to center to make the system work. The only way that I can see to check for this in the water with a dripless seal is to use a dial indicator as mentioned above. Out of the water a laser would be my preferred method.

That is sort of correct. I am interested in determine how you know if the shaft is centered with respect to the p-strut and cutless bearing. That's why some places put the laser in the p-strut. Then again, the laser would have to be perfectly aligned in the pstrut.


The function of the dripless seal is just to keep the water out. The seal can be adjusted so it doesn't leak, so that is not the true problem.

I don't think it really matters if the shaft is not perfectly centered in the tube as long as it does not rub. You are really aligning the engine to the p-strut and cutless bearing not the tube.

If you don't have the shaft aligned through the p-strut and cutless bearing than you cannot determine if the engine is properly aligned.

[Lake-Effect]

Originally Posted by divezonescuba

Is there a way to precisely find the horizontal alighment with the boat in the water with the shaft in place, but with the coupler flange moved back?

I apprenticed a summer with a boat mechanic, and his practice was to move the coupler flanges together, and use feeler gauges. The goal was to have no part of the flange gap larger than 4 mils (0.004"). I don't recall anything different when there was a dripless seal... having the coupler bolts in loosely seems reasonable.

[edit] actually, I recall the coupler faces had a concentric protrusion on one and a corresponding depression in the other, so when they were pushed together , these ensured they were centered, so you could make the final call with the feeler gauges.

Originally Posted by pcmm

Its not unheard of for the shaft to slip out of the coupling. I've never understood why couplings aren't through bolted after they're installed and faced to the shaft.

You can mitigate the problem by storing a spare shaft zinc on the inside bolted around the shaft. This will stop the shaft from backing out all the way in the event it does disengage from the coupling.

My mechanic mentor would drill indents into the propeller shaft for the coupling's shaft clamp screws to "grab" into. And these square-headed screws had small holes in the head that we'd run stainless steel seizing wire through, which prevents the screws from backing out.

But yeah, that zinc (either inside, or positioned ahead of the strut) is a nice bit of insurance.

[Terra Nova]

Originally Posted by Lake-Effect

...drill indents into the propeller shaft for the coupling's shaft clamp screws to "grab" into. And these square-headed screws had small holes in the head that we'd run stainless steel seizing wire through...

This is normally how the keyed shaft coupling is retained on the shaft.

OP--if you're in the yard, you have many options. But, even in the water, it is easy to determine the nominal center position. With the engine coupling bolts removed, and the shaft slid back far enough to clear the coupling halves, the shaft will naturally lie on the athwartships centerline.

To determine the vertical centerline, you need only wiggle the shaft up and down; the center of that travel will be your nominal vertical location.

Now adjust the engine's location such that, when the shaft is slid forward, engaging the coupling's shoulder and counterbore, the shaft remains on center.

Then you can begin feeler gauge-ing the space between the coupling halves and adjusting the engine/trans mounts, taking great care that you don't move off the centerline.