Maximum spacing between propeller hub and cutless bearing/strut?

[footrope]

Has anyone ever heard of this advice or rule? I don't recall seeing it before. In the February-March 2018 issue of Boat U.S. magazine there is a sidebar on page 100 that states the spacing between strut and prop hub should not be more than the diameter of the shaft. "Otherwise the shaft could eventually break from the prop's stress." The picture appears to be of a sailboat with a 2-blade fixed prop.

For most of us that distance would be an inch or less spacing. I am not at all worried about this because mine has been as pictured below for almost as long as I've owned the boat - more than 10 years. Before that the prop was about an inch further away on the same shaft. I'm wondering if this statement is crazy conservative or is based on older advice before stainless steel shafts were commonly used in small boats? I was surprised to see such a statement.

Here is a recent picture of my prop and strut. The shaft is 1" diameter.

 

[toddster]

I heard that rule loud and clear when I posted pics of my old prop and (worn out) shaft a few years ago. The gap was four inches or so. It was clearly a previous owners patch job, but I've seen pictures of other Ericsons with similar spacing. I ordered the new shaft a little shorter and the new prop was a bit smaller diameter so it maintained spacing from the hull. Sort of split the difference.

 

[Christian Williams]

Here's ours:

 

[kmac]

Looks like a long cutlass bearing which makes that rule less important. Do your cutlass bearings wear out relatively fast? Do you have a lot of vibration? If not it's probably OK but not ideal.

 

[Kenneth K]

My surveyor commented that my prop was unusually far away from the cutlass bearing (in his opinion). He said the danger was "whipping" of the shaft (transverse motion) which could stress the shaft strut. I suspect a PO pushed the shaft further aft when installing this soft shaft coupling, rather than having the shaft shortened as he probably should have.



I have no data or engineering specs, but I can't see how a normal sized prop on a 20-30 hp engine is going to damage a 1" steel shaft (especially if it hasn't damaged a nylon coupling after several decades). Sideways stressing of the strut I can understand, but I've seen no evidence of wear or play on my strut. I'm happy with mine as is.

Makes me wonder if that whole line of thought comes from the power boating world. Think of a 300-hp ski boat going immediately from idle to full throttle on a regular basis. That's a lot of shaft stress.

Comparatively, I'm guessing most of us operate our throttles pretty gingerly.

 

[kmac]

"Whipping" of the shaft is much more likely in v-drive configurations because of the longer shafts involved with that arrangement and also if the shaft is marginal in diameter. Short shafts and shafts well supported with bearings don't have that issue. Alignment is critical regardless.

 

[footrope]

Good responses and I appreciate the pictures. I'm not having any problems. I have heard of shaft "whipping" but I don't recall the remedy. The cutless bearing space in the E38 strut is long, around 6 inches. In fact you can populate it with about one and a half bearings. Before I changed the bearing in 2009, the old one stuck out the aft end of the strut about an extra inch. When I changed it I economized by only buying one and put it at the aft end of the strut. So far so good.

It is important to wet the bearing when turning the shaft very much when you're out of the water. The alignment is critical and I aligned it myself while on the stands. That is a task that begs for a helper, at least on this model boat. Measure-adjust cycles are pretty good stair and bicep exercises. I had a diver check the alignment after we were back in the water for awhile and did not have to do any further adjustments.

 

[Frank Langer]

This has been a very interesting thread, but I'm puzzled, Footrope, about your alignment process and would appreciate some further information. You mentioned that you aligned the shaft while the boat was in the stands, but I have always heard that one should wait until the boat is in the water again because the hull takes on a different shape on land from in the water. You mentioned that you had a diver check it after launching, but I always understood that one can only really check the alignment by removing the coupling and checking on all sides for tolerance, hoping for .002 inch or less. I can appreciate that turning the propeller in the water to ensure it runs freely might give an indication that alignment is approximately correct--is that what you meant?
Thanks for any clarification!

Frank

 

[907Juice]

Yeah!

Well, I was looking for a thread like this and eureka I found it! When I pulled my engine and reinstalled it, I moved it forward about an inch so I could epoxy the old holes and drill new ones. That in addition to the new and longer shaft seal it prob moved it about where it needed to be. My prop shaft is 3/4” and extended quite a ways out. I haven’t dove on it yet (well cause the water is literally freezing) but I’m guessing now it looks more like yours now. I didn’t know what the distance should be but I think It is prob fine. I’ll definitely be paying attention to what you say.

This is how it looked when I pulled the boat a couple summers ago.

Juice

 

[footrope]

This has been a very interesting thread, but I'm puzzled, Footrope, about your alignment process and would appreciate some further information. You mentioned that you aligned the shaft while the boat was in the stands, but I have always heard that one should wait until the boat is in the water again because the hull takes on a different shape on land from in the water. You mentioned that you had a diver check it after launching, but I always understood that one can only really check the alignment by removing the coupling and checking on all sides for tolerance, hoping for .002 inch or less. I can appreciate that turning the propeller in the water to ensure it runs freely might give an indication that alignment is approximately correct--is that what you meant?
Thanks for any clarification!

Frank

Hi Frank,
I wrote it up in the Forum back in 2009, I'm pretty sure, but I'll hit the highlights in case that thread isn't there. I have a dripless shaft seal, which is important to my approach.

I reasoned that I should align the shaft as close to the center of the hole in the hull and also straight through the strut and bearing. I did all this work outside and along the way positioned the faces of the couplings very close to each other inside to facilitate the measurements. Note that a dripless shaft seal allows the shaft to move around and the shaft coupling rides high unless you restrain it. I used cork and or cardboard or wood wedges (no pictures, sadly) in the gaps to get the shaft securely in place and positioned where I wanted it. The dripless shaft seal bellows doesn't do the job the same as a flax-packed shaft log, so I felt it was futile to start in any other way.

Then I jockeyed the heights of the three mounts and nudged the engine side to side to get the desired (nearly) equal clearances at four spots on the couplings. It took me parts of two days to finish.

After we splashed, while still in the straps, I checked in idle reverse and forward for vibration and watched the shaft and coupling on the inside. No apparent problems. The E38 with the engine under the companionway steps is a bear for access to the shaft couplings after the muffler and exhaust is all back together, by the way. It is hard to see the coupling and dripless seal from any access point, at least in my case. For the next couple operating hours I checked the rotation and listened/felt for vibration and found no issues. We cruised and day sailed some and eventually the zincs were ready to be replaced 6-8 months later, so I had a diver out and he confirmed that the shaft was aligned and centered and that the play was nil on the cutless bearing.

Conclusion - I may have gotten away with a shortcut - not checking the alignment of the couplings once we were back in the water. I had heard that after a few days in the water that alignment should be checked, to make sure the hull hasn't changed shape. Was I lucky? Does the TAFG in these hulls make it so stiff that the alignment can be done on the stands?

Other questions come to mind - with the dripless seal how does the mechanic determine the proper/desired shaft coupling position while in the water? It must be pretty "easy" because my engine mounts were replaced by an engine shop while the boat was in the water in 2004. Subsequent cruising and haulouts for bottom cleaning and paint showed the alignment was done properly. It never occurred to me to ask how they did that then, and I've not taken the couplings loose since my alignment work in 2009. So I don't know how the alignment, or an alignment check, is done while in the water. How far do you loosen the coupling bolts? What is the procedure for a conventional shaft log vs. the dripless? So many questions.

 

[Frank Langer]

Footrope,

Thanks for your very detailed and informative reply (as yours usually are!)

It sounds from your description as if your alignment is in pretty good shape, despite perhaps having taken a shortcut.
If you have no vibration, your cutless bearing is not showing signs of wear, you don't see or feel whipping when the prop shaft is turning, these all sound like good signs. In one of his books Don Casey actually says that if you can turn the prop shaft fairly easily by hand without it binding, then it's likely ok--to me that seems insufficient. Similarly, there is a controversial surveyor in the southern US (can't remember his name right now) who says that alignment is a waste of time because the diesel engines move/vibrate so much while running that this motion far supersedes any alignment efforts.

I asked my question because I have worked very hard at engine alignment on our boat, a 1984 E30+. All the books and one Ericson manual indicate that it should be within .002 inch, which is about the thickness of a sheet of normal paper. I finally achieved that after much work, but it wasn't easy. A mechanic who replaced my transmission years ago got it to .004 and said that was good enough, but I did have some vibration and decided to fix it myself.

I am a bit puzzled about your emphasis on the dripless shaft seal making a difference. I also have a PSS, but I don't think it really affects alignment, as the prop shaft goes directly from the transmission coupling through the PSS and the strut/cutless bearing. I don't think either a PSS or a normal stuffing box is intended to support the prop shaft or minimize flexing, but I could be wrong.

I have placed marks around the engine mounts so I can see if there is any movement there over time, but so far after a couple of years they don't seem to have moved at all.

Thanks again for your detailed reply!

Frank

 

[footrope]

Hi Frank,
I should clarify the impact of the dripless. When the couplings are disconnected the weight of my feathering prop causes the shaft coupling to rise due to the little bit of slop in the cutless (and the shaft exit hole, etc.). I have a 4" cutless, rather than a full 6" and that may be why the rising of the disconnected coupling is pronounced. That is why I braced the shaft in the proper alignment before adjusting the mounts. The steadier you can make the shaft coupling, the easier it is to make those measurements in the thousandths of an inch.

And the impact is not a criticism of the dripless. I like the dripless so much that I replaced the old one that came with the boat and the memory of the struggle to install it has all but disappeared after 9 years. Honest! It has always been trouble free when in the water.

Once the shaft is aligned and all bolts are tight the dripless presents no challenges. It is when the couplings are disconnected that the dripless requires some caution and thought, compared to the conventional shaft log. I have been cautioned against disconnecting the couplings (to change the engine or engine mounts) while in the water, as an amateur mechanic, because the dripless bellows requires backward pressure to prevent a seawater leak into the boat. So, if I'm doing any work like that, the boat will be hauled.