Shaft Droop Math

[Juliet 15]

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?

 

[Keysdisease]

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.

 

[DDW]

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.

 

[Juliet 15]

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.

 

[Pcpete]

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.

 

[Nick F]

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

 

[Juliet 15]

I like that - straightforward and simple.

 

[DNT99611]

https://retmarine.com/propeller-shaft-bearing-spacing-calculator/


This might be some good reading for you.

 

[kapnd]

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.

 

[Nick F]

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.

 

[Juliet 15]

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.

 

[kapnd]

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.

 

[DDW]

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.

 

[Nick F]

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.)

 

[SteveK]

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?

 

[Keysdisease]

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.

While I still have 1 more mount to replace (they've been taking about 3/4 of a day each)

 

[Juliet 15]

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)!

 

[rgano]

You're getting there!

 

[Andiamo2018]

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.

 

[Steve DAntonio]

Step by step alignment here https://stevedmarineconsulting.com/wp-content/uploads/2021/04/TaskSheet190-EngineAlignment-05.pdf

For an in-depth analysis https://stevedmarineconsulting.com/the-ins-and-outs-of-engine-and-shaft-alignment/

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

 

[AdamC]

Alignment

You can set up a wood v-block to hold up the prop shaft end. Find the up/down/port/stbd position that allows the easiest rotation and least binding. This will position the prop shaft about where you want with respect to aft running gear. A few thousandths one way or other won't matter.

Move the transmission up/down/port/stbd to match roughly but don't worry too much about the vertical or horizontal alignment.. What you really want is to have the faces parallel.

Strive to have the face-face gap on top/bottom/port/stbd all the same or within a few thousandths. it's this angular alignment that must be correct to prevent bending stresses on your shaft, coupling and transmission output shaft. Most couplings are rigid and will create large bending stresses if non-parallel faces are drawn up tight.

You can install a couple coupling bolts loosely to center-up the coupling faces (radially) and draw up to a small gap that is convenient to measure with a thin feeler gauge. Then check gap at 12, 3, 6 and 9 oclock at the rim. Move the front of the engine up/down/left/right to equalize the gaps. Rotate the assy (both sides of coupling together with the loosely installed bolts) 90 degrees. Check gaps again. Repeat a few times til you're confident you get repeatable readings where all gaps are the same within about .001" per inch of coupling face diameter. It doesn't matter what the gap is, just that they are equal.

If you can't get repeatable results. Something is bent or "cocked". There are ways to find out what that is but let's hope none of us need to cross that bridge!

Adam

 

[Fish53]

I have two R&D drive savers and a bobbin so I don't need to unbolt my flange to check alignment, consequently I don't have to worry about the shaft drooping.

 

[ronobrien]

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?

Some years ago I heard an old timer say that you use a fish scale and pull up on the engine end of the shaft to set the scale the same weight as the 1/2 coupling that is on the shaft. It seemed to make sense to me.

 

[Osprey69]

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.

Andiamo...that was one well written and funny double entendre!

Well played sir...well played.

 

[ronobrien]

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.

I left the post about the old timer talking about supporting the end with a fish scale and loading it 1/2 the coupling weight. Maybe I misunderstood the topic he was referring to.

 

[Steve DAntonio]

I left the post about the old timer talking about supporting the end with a fish scale and loading it 1/2 the coupling weight. Maybe I misunderstood the topic he was referring to.

A fish or hanging scale is a valid approach, a phot of one being used is shown in the article link I shared.