Prop shaft alignment

[J. Mark]

Working on rebuilding a true inboard. I have read the prop shaft alignment guide from Borg Warner and they are very specific and the tolerances are .003 of parallel between the output shaft and the prop shaft couplers.

To align, the engine has to be moved at all 4 corners.

The motor mounts allow up and down adjustment via a sandwiched nut arrangement . . . but the motor mounts also use a locking shim to keep the lower nut from turning once it is set.

Am I correct in assuming I need to bend these flat to set the alignment and once I am done bend them back to lock the lower nuts in place?

The specs call for the final adjustment to be done with the boat in the water. To turn the nuts I need to lift weight off of the engine and I don't see how to do that with the boat in the water.

Are these specs overly rigid? I don't imagine boat manufacturers put every boat in the water for final adjustment, but if I need to, I will.

If you have reinstalled an inboard set up I'd like to know how you did it and your thoughts on the process.

Thanks

 

[Ned L]

Re: Prop shaft alignment

I have done a number of inboard installations. Yes, .003" is the maximum recommended allowance (closer is better). Can you be farther off than this(?), sure, but it will result in more vibration and more bearing wear in both the cutlass bearing in the strut and in the reverse gear. How much more (?), depends on how far out of alignment it is, a bit out & it may not cause an issue for as long as you own the boat, to far out & you might be replacing parts every couple of years.
I'm not familiar with the 'locking shimes' you are describing, but by your description I'd say you are right on. As for lifting the engine to make the adjustment, that is not necessary. Unless there is a problem you should be able to turn the bottom nuts quite easilly with a wrench without lifting the engine. When you are done with the alighnment there should be pretty much even weight on all four bottom nuts (equal difficulty to turn), and then you tighten the top nuts.
You didn't say how large the boat is, which will affect the need to re-allign after launching. The smaller the boat the less the issue. If the boat is blocked up well you should not have a problem. At the very least I would recommend pulling the flange bolts a day or so after launching and checking the allignment (chances are pretty goo it will be fine).

 

[Ned L]

Re: Prop shaft alignment

You asked for thoughts on the process didn't you. ---- It isn't difficult when you understand the process, can be a bit tedious though. (front up, back end left, whole engine up, front end right, whole engine left, etc, etc, etc, until you get it tuned in). Don't be in a hurry, it can easilly take an hour or more of this tweeking a couple of thousanths at a time back & forth. It is satisfying when you get it and a piece of paper won't fit between the flange faces.

 

[J. Mark]

Re: Prop shaft alignment

Thanks Ned, it is a 19 foot Marlin Ski boat. I know I need to be close and intend to be dead nut on if possible. My front motor mounts have sliding pins (as do the rears) that are frozen in place. I'll pull them off and get them freed up before I try and align things.

When I got the boat the alignment was so bad that the bottom of the shaft log was eaten away. Hopefully I can do better than the PO!

 

[Ned L]

Re: Prop shaft alignment

On a 19' skiboat things shouldn't shift much when you drop her in the water, but it would still be a good idea to pull the flange bolts & check. Given it is just a small V8, you should have no problem at all making the adjustments without having to lift the engine.
--- "When I got the boat the alignment was so bad that the bottom of the shaft log was eaten away" I'd say things were pretty for out.

Oh yes, a good size pry bar (or even a piece of 2"x4" lumber) will help with the left, right, back & forth part. Good Luck.

 

[J. Mark]

Re: Prop shaft alignment

I'll post an update in my restore thread when I get it done.

http://forums.iboats.com/showthread.php?t=292744