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Old 02-01-2014, 06:43 PM   #1
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Engine Alignment with Dripless Shaft Seal

I have done engine alignments on all my previous boats with good success, however they all had conventional stuffing boxes and I did it with the boat in the water - as I believe you should.

With a stuffing box I was able to separate the shaft coupler, then tighten up the stuffing box to keep the shaft centered.

With a PSS dripless shaft seal how do you determine if the shaft is properly centered in the shaft log?

Thanks in advance.

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Old 02-01-2014, 07:55 PM   #2
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There will still be some "shaft droop" (start another thread about that topic!) which you would still get as the shaft's own weight will bend it, depending upon the diameter. Somebody on here might be able to tell you how to measure it, but on my last boat with dripless shaft seal (GB 32), I ignored it. I realigned the engine and got great results, resulting in virtually no vibration and I was very pleased. Apparently the cutlass bearing held the shaft adequately.

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Old 02-01-2014, 08:27 PM   #3
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Are you single engine? If so, do you know if your shaft has an upper cutless bearing in the shaft log near the dripless?

If single with an upper cutless and a shaft diameter greater than one inch (and I would guess it is) and not much more than a couple of feet to the tranny flange...I doubt you have very much sag....if really concerned...I have used a spring scale to try and center it but never had much luck with that method.
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Old 02-01-2014, 09:33 PM   #4
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If 2 or more shaft bearings it's not a problem as the shaft will be centred unless the cutlass bearingare excessively worn. If single bearing what I do is make a saddle the same size as the shaft diameter in a block of 2x4 or suitable sized timber. Measure the height of the coupling from the bilge whilst unsupported then lift the coupling and measure the height again. Put the timber underneath with the shaft sitting in the saddle and tap it back until the height is half the difference of your 2 measurements. Side to side movement can be taken care of by the same sort of measurement and simply moving the block sideways.
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Old 02-01-2014, 10:00 PM   #5
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I've thought about the spring scale. One would need to know the weight of the shaft at the 2 ends and the middle. That could be easily computed from the shaft weight per foot specs. I think half of the shaft weight would be borne by the center support so if your shaft weighed 60 lbs the shaft would be as straight as possible w end supports and a center support. And that center support re the example given (60lbs) should require 30 lbs of support if I've got my head around this correctly.

So in the above example putting a spring scale on the center of the unsupported shaft w a 30lb pull should make the shaft very close to straight. This would seem to be a good method to position the shaft so it will be in the center of the stern tube. I personally think the shaft positioned in the center of the stern tube w a PSS is more important than w a standard stuffing box as the stuffing box acts much like a bearing and centers (approx) the shaft (to the stern tube) whereas the PSS does little to accomplish that. And movement of the bellows could cause the bellows to fail prematurely and fill your boat w water.

Also any tendency for the shaft to "whip" (move around in very small circles like jumping rope w a very long rope and a very short guy) may be greatly affected. Because the unsupported shaft length will be much greater w the PSS. So the engine may excite the shaft at cruising rpm w the PSS whereas it may not have before and vice-versa. Sort-of like changing the length of tuning forks.

So w the PSS the droop of the shaft should be involved in the alignment and w the stuffing box it may be not necessary ... especially if there is little droop like a 2" shaft w 6' between supports.

I have a PSS but may change back to the stuffing box.

North Western Washington State USA
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Old 02-01-2014, 10:19 PM   #6
City: Port Vila
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Having the shaft in the centre is the starting point. Any shaft droop is taken care of when the spigot on the coupling halves is engaged during the alignment process. This supports the front of the shaft and eliminates droop. Minor deviation from centre is OK on a single bearing set up as long as the seal stator surface is parallel to the shaft and this can be achieved by adjusting the bellows where they are secured to the stern tube. The PSS seals allow for deviation from centre by having an over large inside diameter.

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