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Old 02-25-2022, 04:51 PM   #1
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Shaft Droop Math

I've been scratching my head over this for about a week now. If you have any experience or expertise in this realm, I'm grateful for your input. I'm doing my first shaft/xmsn alignment after having changed the engine mounts. DD 4-71s on Aloson MH-30 xmsn. Shafts and props are clean, checked, and straight, cutlesses are new.. The first cutless brg is about 5 - 8 feet from the shaft log. That means there's some unknown droop in the shaft end closest to the engine. Logic seems to say I should pull up on the shaft some amount to compensate for that droop before connecting to the engine. If not, I could be moving the engine around to align it with a drooping shaft, no? How do I calculate the amount of droop, and therefore the amount of upward pressure to put on the shaft to compensate for the droop?
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Old 02-25-2022, 07:54 PM   #2
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A rule of thumb is bearings spaced no closer than 20 times the shaft diameter and no further apart than 40 times the shaft diameter.

Your resilient mounts will allow the engine to move on all axis so you want some unsupported shaft length between the gear flange and the 1st bearing, but not so much that the shaft "whips." That's what the rule of thumb above is for. If your shaft size and bearing spacing falls into this range I doubt droop will impact alignment enough to allow for. There are other factors.

Consider that torque will take you out of your "at rest" alignment and that the engine might move forward 1mm or more under thrust.

Of course the engine should sit on the new mounts for minimum 24 hours, and final alignment should be done in the water. The alignment should be looked at again 6 months to a year after installation. Mounts take most of their "set" in 24 hours. A measurable amount after 6-12 months, and very little per year after that.

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Old 02-25-2022, 08:28 PM   #3
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You could make a guestimate from the weight. You'll have to put in some real numbers but for instance:

2" shaft has πR² area or 3.14 sq in.
If it is hanging out 60" then the volume of shaft is 3.14 * 60 or 188 cu in.
Steel weighs about 0.3 lbs/cu in so 57 lbs of shaft hanging out.
The aft end is being carried by the cutlass, and the forward end by the trans so you need only 1/2 of that or 28 lbs.
You need to add all of the weight of the coupler which is carried almost entirely by the transmission. If you can take it off to weigh it, then you can estimate it by the same methods.

If you want to support it part way along rather than right at the coupler then it is a little more involved but not that much.
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Old 02-25-2022, 11:12 PM   #4
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Thx, good info x 2. I'll give the mounts 24 hrs before aligning.things. it's in the water now, so no issues there. Yes, it's a 2 in shaft, so your calculations are perfect - thank you. Ill weigh the coupler (hadn't thot about that) and add the 28 lbs plus the coupler, pull up that much, then try to use that for the alignment. Thanks again for your help. I'll let u know how it goes.
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Old 02-25-2022, 11:23 PM   #5
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Much of any droop should compensated for by making sure the shaft half of the coupler face is square with the length of the shaft. Where you have multiple cutlasses, there probably isn’t as much deflection as with a single bearing configuration. On my boat I had less than half an inch that I could move the shaft around as I brought it to the transmission coupling half. Once the bolts are in but loose any droop or deflection will show in the gap between the two halves of the coupler. Getting the final alignment right takes time and patience. I found that making a log of how readings change with every x amount of action on which mount made it easier to understand and predict results. Things like going up on the forward mounts will probably lower the coupler. Doing it with someone, one turning wrenches on the port side of the engine, the other person on the starboard and doing the checking, will make the job go much easier on the bodies.
If you can get an experienced technician to work one or both shafts with you it is probably money well spent. I learned from a tech in the 1960’s and have used the knowledge ever since.
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Old 02-26-2022, 11:21 AM   #6
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My method is this:
1 Measure the vertical height of the shaft flange in its unsupported (lowest) position. (fore/aft position of shaft must be close final location)
2 Pull the shaft up by hand as far as it moves and measure the vertical height (movement will be limited by the play in the stuffing box connection or by the stern tube).
3 Take the average of the two heights and use this as the target height for the flange when coupled to the engine.

Same goes for the side-to-side position.

Good luck!
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Old 02-26-2022, 12:54 PM   #7
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I like that - straightforward and simple.
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Old 02-26-2022, 01:33 PM   #8
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https://retmarine.com/propeller-shaf...ng-calculator/


This might be some good reading for you.
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Old 02-26-2022, 02:32 PM   #9
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Quote:
Originally Posted by Nick F View Post
My method is this:
1 Measure the vertical height of the shaft flange in its unsupported (lowest) position. (fore/aft position of shaft must be close final location)
2 Pull the shaft up by hand as far as it moves and measure the vertical height (movement will be limited by the play in the stuffing box connection or by the stern tube).
3 Take the average of the two heights and use this as the target height for the flange when coupled to the engine.

Same goes for the side-to-side position.

Good luck!
Nick
This is the method I’ve used for many years, and while not exactly scientifically accurate, it works!
No amount of perfection in alignment will remain valid once the gear is engaged and torque is applied to those nice cushy mounts.
There are drive systems available that can compensate for some inequities, but I’d like to see something on the lines of a fluid driven torque converter that could absorb alignment discrepancies with minimal power losses.
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Old 02-26-2022, 02:55 PM   #10
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Minor detail added to the side-to-side part of the method:

1 Measure the vertical height of the shaft flange in its unsupported (lowest) position. (fore/aft position of shaft must be close final location)
2 Pull the shaft up by hand as far as it moves and measure the vertical height (movement will be limited by the play in the stuffing box connection or by the stern tube).
3 Take the average of the two heights and use this as the target height for the flange when coupled to the engine.

Same goes for the side-to-side position, but you must hold the shaft at the target height while determining the side-to-side.
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Old 02-26-2022, 05:16 PM   #11
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Got it, makes sense. And you've done this successfully? While I still have 1 more mount to replace (they've been taking about 3/4 of a day each) I tried pulling up on the shaft to see what it did. I might have gotten a 1/2" or so of movement, and that with a lot of grunting...


I expected there to be a more movement than that.
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Old 02-26-2022, 05:26 PM   #12
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I should add that it’s important that your cutless bearings are in good shape, or preferably new before alignment is attempted.
Both coupling faces should be verified flat as well.
All the alignment in the world won’t do any good if the shaft is bent or the propeller is tweaked, or is incorrectly installed on the shaft.
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Old 02-26-2022, 07:39 PM   #13
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It isn't just the radial play in the cutlass bearing. A 2" shaft is surprisingly flexible. Especially in the context of an alignment spec that typically calls out for <0.002 across a 5" face.

If you ever do any machining, you learn quickly that everything is made of rubber. Some of it is stiffer than other, but still rubber. A 2" shaft cantilevered 60" will deflect about 0.050 with a 30 lb load at the end. Now, there's usually a lot of other things in a boat propulsion system that move more - but you don't need a feeler gage to see 0.050.
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Old 02-26-2022, 08:09 PM   #14
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Quote: I might have gotten a 1/2" or so of movement, and that with a lot of grunting...

I expected there to be a more movement than that. Quote

I would expect less than 1/2". Dont go crazy. You should pull up with, say, double the weight of the first lift movement. In this way the up force against the stern tube would be the same as the downward force of the weight when the shaft is resting in the down position. The limit of up movement should be fairly obvious when you pull up on the shaft.

Yes, I have done this !

The other comments about the flange face being "true", etc are all valid, but the generailty of coupling height is what I describe. (This, of course, assumes that the cutlass bearing is aligned with the stern tube but that is a whole dimension of possible alignment issues.)
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Old 02-26-2022, 08:48 PM   #15
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This is one of the things I let someone else do for me. That said the advice given here make it possible to consider in future.

Question. Post one speaks about droop. There are no measurements discussed for the shaft from stern tube, which I thought supports the shaft. There is only a 5-8' from first cutlass. A shaft hanging free 5-8 feet as opposed to a shaft hanging free x? feet.
The suggested halfling the up/down side/side movement still makes sense. OP moved 1/2", so is that then 1/4" held up connection point?
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Old 02-27-2022, 06:10 AM   #16
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On some mounts the studs are removable. This is true with Cushyfloat (below) mounts for example. If you are doing a direct replacement same/same the removable stud makes the change easy. You jack the engine enough to get a sawsall or other tool and cut the old stud between the engine bracket and the old mount. Slip the old mount out, the new one in, and thread the new stud in from on top of the bracket.



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While I still have 1 more mount to replace (they've been taking about 3/4 of a day each)
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Old 02-27-2022, 04:54 PM   #17
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Thx, Keys. It's averaging so long for each mount mostly cuz I'm working alone, and figuring it out as I go. Lifting the engine by hoisting it isn't feasible, so I've been using three mounts to raise it so I can replace the 4th mount, each in turn. Unfortunately, despite the fact I'm replacing with the same mounts (DF-238 M1s), sometime over the years they lengthened the DF238 mount's shafts, so they're about an inch taller. So my old mounts don't go up high enough to slide the new mounts in. Had to get creative to get the 2 fwd mounts in. Now I've figured out how to get the aft ones in (used a bottle jack to slightly raise the aft end, then removed the engine-mounted bracket and mount from the engine). 3 down, 5 to go (two engines)!
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Old 02-27-2022, 05:14 PM   #18
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Old 03-04-2022, 02:36 PM   #19
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Shaft Droop

Shaft droop is a common issue and there is no reason for embarrassment. And thanks to modern technology it isn't something you have to just accept with aging.

Proper alignment of course is also very important. Even a young shaft with no droop will cause problems if it isn't aligned correctly. No judgement there, you can align it however you like but it is very important for BOTH sides to agree on where the alignment will happen.

Removable studs can help but again it is important for both sides to be in agreement before the stud is inserted, even if it is to be removed quickly. Removable studs are sometimes frowned on in some contexts and safety is very important when using them. Latex is recommend when quick removal is important. For long term use however a properly placed ring, usually gold, will keep the stud in place.

But to the OP from an OG it is important to look into why there is droop in the first place. As mentioned there will always be some droop, it happens to all shafts at some point.

You may have to adjust your bearings rather than addressing your shaft. For many the shaft will dictate everything else. Often however when droop becomes an issue it is easier get your bearings straight first. In older boats good bearings become more and more important and shaft droop is less of an issue.

So just keep a stiff upper lip, go with the flow and take advantage of all those online instruction videos that take up 90% of all internet traffic.
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Old 03-04-2022, 04:22 PM   #20
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Step by step alignment here https://stevedmarineconsulting.com/w...ignment-05.pdf

For an in-depth analysis https://stevedmarineconsulting.com/t...aft-alignment/

None of this is a 'black art' it's all scientific and measurable.
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