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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 02-23-2007
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Prop shaft alignment

I need to align my prop shaft. I know there are ways to check the alignment, but I already know it is slightly out of wack because, on the hard, I can see the shaft is resting lightly on the shaft log. It seems to me that all I need to do is raise or lower the engine a little on the mounts. Anyone have any advice? I've never actually messed with this type of problem before.
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Old 02-23-2007
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The advice I've read is that course adjustments can be made on land, but fine adjustments should be made in the water. Raising the engine so the shaft is centered in the shaft log is the beginning. Once in the water, you'll need to unbolt the shaft from the flange and then use feeler guages to look for gaps. I think the rule of thumb is to try and get it within .001' for every inch of flange diameter.
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Old 02-23-2007
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shaft alignment

The amount of deflection when on the hard compared with the amount of deflection in the water will vary by how thick your hull is. You don't say what boat you have. I would not even attempt it till it is floating for a couple of days.
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It is a Catalina 27. The thing is I can see that it is touching the shaft log exit while it is out of the water. Wouldn't it be better to try it now while I can see the prop end rather than when it is in the water and I can't?
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Old 02-23-2007
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It is hard to figure while not there of course. If I were there.... I would first look to see how much the hull is deflecting in the stands. If it is touching the shaft log outside while on the the hard because of deflection due to the stands, then once it is in the water it is not going to be accurate any more when the hull goes back to the shape it is supposed to have. The second thing I would check (with feeler gauges) is the allignment right now and see if the alignment (whether high, low, left, or right) is consistent with what you see outside the hull.
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Here's an idea, and a hopeful one at that. Maybe the shaft is touching the shaft log exit from deflection alone. The reason I say this is because there are no rubbing marks on the log from the shaft, which makes me think that perhaps in the water the shaft is aligned. Otherwise, I think there would be some sort of mark from the shaft rubbing against the log while under power.
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Old 02-23-2007
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Shaft alignment really should be done in the water. I'ts very difficult to ascertain that the boat is sitting true when on the hard. I've seen Cat27s that were severely flexed while sitting on cradle, so I'd really wait til you're afloat.

The process itself is a bit complex as you have adjustments on all mounts (3 or 4 depending on installation) and you have few fixed references on the other end. You are working on several axes at once.

While afloat you can still judge the "midpoint" of the shaft log by lifting the decoupled shaft and find the limits of motion, then block the shaft in the middle of that range. Then adjust the engine mounts to roughly match. This is followed by measuring the face to face distances at the coupling to get it the same all around. Initially this can be done with a piece of keystock, sliding it between coupling faces and set things up for the same fit (gap) everywhere. Feeler gauges can be used to get it more precise.
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That is what I was alluding to, Jotun. While a bit complicated, alligning a shaft is mostly tedious and time consuming.
It took me 4 hours to go from .065" out of allignment to .0015". Good luck.
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The criteria is not the position of the shaft in the shaft log, but how the flange on the shaft mates with the flange on the engine. The shaft flange will be at right angles to the shaft to a high precision. If it mates with the engine flange then the engine and prop form a straight line and there will be a minimum of vibration. Remember you are trying to get the gap between the two flanges to vary by less than a couple of thousandths around the circumference. That is a pretty tight tolerance. Also when a boat is moved from stands on the hard to the water, it will take several days for it to completely reform to the new forces.
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There are three steps to shaft alignment.

1. Center the shaft in the stern tube. Usually done with the coupling disconnected. The goal is to place the centerline of the shaft as close to the centerline of the stern tube/stern gland as possible. Concentricity is the goal.

2. Adjust the elevation and lateral position of the transmission output coupling flange by adjustment of engine and/or transmission mounts so that its centerline coincides with the shaft centerline and with coupling halves approximately matched for bolting together.

3. Bring the coupling halves together loosely and use a feeler gauge to check parallelism of the mating flange faces. (They must be clean.) Check in the vertical (12 o'clock and 6 o'clock positions) and in athwartship (9 o'clock and 3 o'clock) directions. Make additional adjustments (fine) to engine/transmission mounts as necessary. If significant adjustments are made, separate the flanges and repeat step 2 so that you don't disturb the position of the shaft centerline.

4. When flange faces are parallel within the limits specified by the engine maker, tighten all mounting bolts and check for parallel one more time. If problem found, repeat #3.

5. With all mountings secure and flanges parallel, install and tighten coupling bolts.

This is a lot of work and it should be done when the boat is in the water so that it will assume its normal hull shape. Otherwise you may get to do it again.

There is an option. You could get a flexible coupling and that will eliminate some of the really fussy work on the flange faces. You will still need to pay pretty close attention to the rest of it.

Last edited by Goodnewsboy; 02-23-2007 at 10:24 PM.
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