STARTING Engine Alignment- shaft centered in sterntube?

[sanibel sailor]

Redoing my drive train a bit. Cutlass bearing seized to prop shaft and wallowed out the aftermost portion of the sterntube. I am not quite sure why but the bearing had been very slightly loose in the tube previously.
I have now sleeved the sterntube and replaced the bearing. Checked shaft for straightness. I replaced one engine mount that was bad.
I am getting ready to realign the engine to the shaft. My shaft is about 42"(1.1m), guessing 6" (150mm) outside and a foot (300mm) inside, so tube is probably about 24" (600 mm) long or so. It is a 1" (25 mm) shaft, 1-1/2" ID tube When I connect it to the transmission flange, the shaft is slightly off center in the tube, about 1/16" (1.5 mm) closer on one side compared to the other. Perfect is better of course, but likely unobtainable or necessary. With a 4" (100 mm) long bearing, the offset at bearing itself must only be .010" or 0.15 mm. I am not even sure I was able to sleeve the tube with that accuracy. There must be some allowance? I have never seen anything written on this.

So... How closely centered does the shaft have to be in the tube at the inboard end?

[senormechanico]

You have 1/4" on each side, static. Sounds like 1/16" isn't a big deal.
IMHO, it's close enough.

[SailingHarmonie]

I am going to refer you to this article posted by Transmission Marine in Fort Lauderdale. These guys are the experts, and really know what they are doing. The procedure is a bit daunting when you first look at it, but I'll tell you up front, 1/16" off axially (0.0625") is not a good place to start.

You should easily be able to shift the engine mounts by that much and get the transmission and propshaft coaxial. Then you have only started. The faces of a 4" flange should be parallel to within 0.002". Just so you have an idea, that's about 1/2 the thickness of a piece of printer paper.

If you don't get this right, problems will plague you. Vibrations, and rapid wear of the seals and bearing in the transmission, not to mention the cutless bearing that you just replaced.

I know this is not what you want to hear. Doing this for the first time by yourself is a challenge, but if you approach it one step at a time, and don't rush, you'll get it right.

[sanibel sailor]

Originally Posted by SailingHarmonie

I am going to refer you to this article posted by Transmission Marine in Fort Lauderdale. These guys are the experts, and really know what they are doing. The procedure is a bit daunting when you first look at it, but I'll tell you up front, 1/16" off axially (0.0625") is not a good place to start.

You should easily be able to shift the engine mounts by that much and get the transmission and propshaft coaxial. Then you have only started. The faces of a 4" flange should be parallel to within 0.002". Just so you have an idea, that's about 1/2 the thickness of a piece of printer paper.

If you don't get this right, problems will plague you. Vibrations, and rapid wear of the seals and bearing in the transmission, not to mention the cutless bearing that you just replaced.

I know this is not what you want to hear. Doing this for the first time by yourself is a challenge, but if you approach it one step at a time, and don't rush, you'll get it right.

Thanks for the response.
The article does not address position of the shaft within the sterntube.
All the references I have seen suggest .001" per inch of flange diameter, a tolerance of .004" in my case. Given the flexibility of the hull, the flexible engine mounts, flexible rubber cutlass bearing, engine movement and vibration while running, flexibility of my small diameter shaft and associated dynamic shaft runout, etc, the article's suggested goal of perfect coupling alignment is probably unrealistic.

[Bowdrie]

I can't say you must "Do this or do that", as there are various techniques.
I can only offer one that I've used many times on new installations.
The article linked above is a good one, but the idea that fiberglass boats don't have a "working" problem is false.
A fiberglass hull may not "work" in the same sense that a wooden hull does, but on a sailboat they do "bend" when the rigging is tuned vs loose, and this does cause some misalignment.
If you have enough length to work with between the transmission and the stuffing box/packing gland, and with the boat hauled, (so the Cutless is dry,) you might be able to rig-up a notched board to support the inboard end of the shaft when it's decoupled from the engine.
With a dry Cutless you can easily "feel" any binding if the shaft is cocked in it.
Moving the notched board up/down/left/right and you can get an initial "center" where the Cutless is not trying to "grab" the shaft and it turns easily with little effort.
Note; you want to do this with the rubber hose/packing gland released from the stern tube.
Then make the engine match the coupling position, working down with your feeler gages.
It's a bit frustrating, and the closer you get the more patience is needed.
But once you get the shaft so that it just "floats" in the Cutless than you can address issues with the shaft being "off-center" in the tube, and they are really not so hard if not "way out in left field", so to speak.

[sanibel sailor]

Thanks for the input. I tried shimming the shaft inside the tube. I didn’t feel any difference if it is centered or off a quarter, but I wasn’t focusing my attention on that specifically. Boat is hauled out.
I will give it another go tomorrow.

[markrsimon]

What type of packing or dripless seal are you planning on using? Typically there is a heavy duty rubber tube/hose from the stern tube to the packing gland that helps compensate - to a degree. I would vote to have the shaft aligned within the cutlass/strut, align the engine, and then deal with the stern tube. Old school packing glands will give you more leeway than the newer dripless ones.

[sanibel sailor]

I am going with the PSS dripless seal. It is off for now, simplifies the alignment, allows better access/visualization.

No strut, prop in aperture.

[SailingHarmonie]

Originally Posted by sanibel sailor

Thanks for the response.
The article does not address position of the shaft within the sterntube.
All the references I have seen suggest .001" per inch of flange diameter, a tolerance of .004" in my case. Given the flexibility of the hull, the flexible engine mounts, flexible rubber cutlass bearing, engine movement and vibration while running, flexibility of my small diameter shaft and associated dynamic shaft runout, etc, the article's suggested goal of perfect coupling alignment is probably unrealistic.

If you know it all already, wtf are you asking for?

Just to be clear, if you have to push the prop shaft to one side to put the bolts in that is NOT aligned.

I know I have no trouble aligning my shaft and transmission to the point where a 0.001" feeler will not fit inside any part of the flange.

It's your boat, do as you think best.

[skipperpete]

Originally Posted by sanibel sailor

Thanks for the response.
The article does not address position of the shaft within the sterntube.
All the references I have seen suggest .001" per inch of flange diameter, a tolerance of .004" in my case. Given the flexibility of the hull, the flexible engine mounts, flexible rubber cutlass bearing, engine movement and vibration while running, flexibility of my small diameter shaft and associated dynamic shaft runout, etc, the article's suggested goal of perfect coupling alignment is probably unrealistic.

I didn’t read the article in the link BUT its a given that the shaft is exactly in the centre of the stern tube…. Always. You can use a length of electrical cable or stuffing box packing wrapped around the shaft and pusherd into the stern tube or even get a bushing made from 1”id plastic pipe…. Whatever it takes to centre that shaft in the stern tube, its not negotiable and is the starting point for any engine alignment, the engine is then adjusted to where the shaft dictates it must be regardless of how difficult that may be.

[Bowdrie]

Originally Posted by skipperpete

Whatever it takes to centre that shaft in the stern tube, its not negotiable and is the starting point for any engine alignment

That is only true if the Cutless is exactly centered and in plane with the stern tube.
You can center the shaft in the tube all you want, but if the Cutless does not have the same center and plane you end-up with what the OP spoke of, siezed up bearings and a wallowed bearing support.
The shaft must run "true" in the Cutless, what's between the Cutless and the coupling flange has to be adapted to that.
You can't use a stuffing box as a fulcrum to hold a shaft where you want it.

[sanibel sailor]

Originally Posted by SailingHarmonie

If you know it all already, wtf are you asking for?

Asking because I haven't (still) seen the answer to my question addressed anywhere.

[Bowdrie]

Posts #2 and#7 answered you pretty well.
How much hose you have that's "free span"?
The amount of hose between the aft end of the packing gland and the forward edge of the stern tube,, the unsupported length.
If you've got even a few inches, it's no big deal to align that set-up.
Have all the hose clamps slacked off.
You can easily wiggle around the hose-on-tube, and the hose-on-gland to get a 1/16th or even more accommodated before tightening the clamps.

[Cheechako]

Originally Posted by sanibel sailor

Redoing my drive train a bit. Cutlass bearing seized to prop shaft and wallowed out the aftermost portion of the sterntube. I am not quite sure why but the bearing had been very slightly loose in the tube previously.
I have now sleeved the sterntube and replaced the bearing. Checked shaft for straightness. I replaced one engine mount that was bad.
I am getting ready to realign the engine to the shaft. My shaft is about 42"(1.1m), guessing 6" (150mm) outside and a foot (300mm) inside, so tube is probably about 24" (600 mm) long or so. It is a 1" (25 mm) shaft, 1-1/2" ID tube When I connect it to the transmission flange, the shaft is slightly off center in the tube, about 1/16" (1.5 mm) closer on one side compared to the other. Perfect is better of course, but likely unobtainable or necessary. With a 4" (100 mm) long bearing, the offset at bearing itself must only be .010" or 0.15 mm. I am not even sure I was able to sleeve the tube with that accuracy. There must be some allowance? I have never seen anything written on this.

So... How closely centered does the shaft have to be in the tube at the inboard end?

If I understand your situation correctly, it's the cutlass bearing that guides the shaft you should be concerned with. You want the shaft to be concentric to the cutlass bearing inside diameter. Of course, cutlass bearings are not a tight fit so you will have to shim temporarily or guesstimate dead center with the shaft hanging free. One wonders why the shaft would be off center? But 1/16" isn't too bad. In theory I would adjust the engine location to match the free standing shaft.

But if it's too far off center to the tube, it's going to bind on the packing on one side. You dont want to have the shaft binding on one side of the cutlass bearing either, you want it "floating free". You can often loosen the stern tube rubber hose and adjust packing area concentricity a bit.

In a perfect world with enough shaft length I'd like to align the shaft to the engine without the stern tube hose clamped. Then slide it on and clamp it after.

[MikeHoncho]

I had a situation where my shaft was rubbing on the shaft log tube as the shaft exited the hull. This was after changing out motor mounts. It appears that the exhaust hose that connects the shaft gland to the tube on the inside of the hull had, over time flexed down with the collapse of the motor mounts and kept the bend when the shaft was disconnected from the transmission. I ended up raising the gland/tube up with a clamp then aligning the motor to the shaft. The shaft alignment is dependent in the log on the is dependent not only with the struck/cutlass but also the hose that the gland is connected to.

The symptom I had was under hard throttle there would be a clunking which sounded like propwash hitting the hull. It was the shaft flexing a little and hitting the shaft log. By moving the shaft up a quarter inch, it resolved all of my problems.