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Old 17-07-2022, 02:48   #1
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Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft?

I have a mysterious flexible coupling from the mid 90s, probably French since its in a Jeanneau, between engine and propshaft. It has two "CV" joints, one at each end of a short stubshaft. see pic

This will permit engine misalignment due to vibration etc, but how to set it up optimally? I presume minimizing the angles would be the guiding principal.
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Old 17-07-2022, 02:49   #2
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Re: Aligning sailboat propshaft w cv joint between motor and proshaft?

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Old 17-07-2022, 03:10   #3
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

If the engine is flexible mounted firstly check all the mounts ,replace as necessary the double c/v is a good unit but it must not be totally aligned ,it must have some differential of angles to work correctly .⛵️⚓️
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Old 17-07-2022, 04:18   #4
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

First things first. Check to see if the CV joints are working properly. This double joint system is sometimes used to mount the engine flat while the prop shaft is at an angle. If you have rust inside them, they will not run correctly. We would disconnect this whole short shaft and examine it closely. Checking the condition of the motor mounts, as posted above, is an excellent idea. Check that the propeller shaft is not flopping around or that the shaft is bent or not true. Loosen up the shaft seal and check for how the shaft feels.
More information and some photos would help.
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Old 18-07-2022, 01:02   #5
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

Some cv joints need an offset in them to stop vibration
You need to find directions for your type of system
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Old 18-07-2022, 04:57   #6
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

I assume you are talking about 2 × universal joints back to back. These are a 'Cross' with 4 needle roller bearings, ie 1 at each end of the cross. Like on rear wheel drive cars.

These give velocity fluctuations, which are vibrations when driving through an angle. Basically the driven shaft speeds up and slows down compared to the driver.

But if you have 2 of them back to back then the second one cancels the the vibrations caused by the first. Because the first one is an angle, the 2nd one is the opposite angle. This is actually one form of Constant velocity joint.

However the usual one, like in most front wheel drive, and 4wd cars has a set of balls to transfer the drive.

Anyway, getting back to the original post, the least amount of vibrations is, or should be when there is NO angle, and the drive and driven axis is the same, ie 'coaxial'. Ie where the name for shielded antenna cable comes from.

Sure it is true that with 'CV' joints should not vibrate driving through an angle. However they will wear more as the balls, or bearings are going back and forth in their 'tracks' more each revolution.

So doing a propeller shaft alignment, or at least check is the best answer.

This is normally done using feeler gauges in between the faces of your drive coupling faces.

This has some limitations but is much better than a lot of others that dont check theirs. Motor mounts sag over time etc

The feeler gauge method cant check for- bent shaft, eccentric couplings etc.

The best way, for many reasons is using a couple of Dial gauges.

Good luck.
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Old 18-07-2022, 05:17   #7
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

Darned good explanation, Q Xopa.
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Old 18-07-2022, 05:29   #8
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

Make sure the joints are free and not binding. However there should not be looseness in the joints either. The joints should be lubricated with grease. With CV joints at both ends and a spline between, alignment should be a non-issue.
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Old 18-07-2022, 05:47   #9
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and proshaft?

Interesting responses.
I have to say, these things on each do not look like UJs and are not serviceable as far as i can see.
They run smoothly
Motor is 4cyl diesel on new motor mounts.
The motor mounts permit significant adjustment of the down angle of the engine and the height of the engine relative to the propshaft.
Since no adjusmet is necessary (esp NOT with feeler guages) its a puzzle to figure out the optimal alignment when none is really necessary - if that makes sense
its 30yr old and no one can properly ID it.
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Old 18-07-2022, 12:52   #10
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft

Hello
It looks like a "Aqua drive"
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Old 18-07-2022, 13:18   #11
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft

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Originally Posted by CdS2 Roland View Post
Hello
It looks like a "Aqua drive"
Agreed and very expensive. Not a normal fit, must have been fitted by a previous owner.

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Old 18-07-2022, 14:37   #12
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft

Since no adjusmet is necessary (esp NOT with feeler guages) its a puzzle to figure out the optimal alignment when none is really necessary

Hmm I'm not sure I understand how you are arriving at the above conclusion?

Why is "no adjustment is neccessary, esp NOT with feeler gauges"

Feeler gauges are the usual way and are way better than not doing anything, which is a large proportion of boat owners.

The optimal angle is 'Aligned' ie shaft centre lines the same angle and the same axis.

Yes CV 'can' handle an angle, but if you dont have to, ie you can adjust it properly to not have an angle, why would you?

You dont get a CV driving through an angle with no cost. The cost is wear. Ask any front wheel drive owners that have had to replace their CV joints.

The idea of CVs, and othe flex couplings, in a boat is to allow for TRANSIENT misalignments. Ie when Torque is applied your motor twists one way, the other as torque is reduced. When your motor flops around in a seaway. When your hull flexes etc.
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Old 18-07-2022, 15:09   #13
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft

CV joints ( Constant Velocity) don’t act like automotive universal joints and consist of a number of steel balls in a housing full of grease ... usually molybdenum disulfide based.
The thrust bearing part of the CV that is attached to the propeller shaft and supported by a bulkhead or transverse frame does need to be lined up so that the shaft runs true in the centre of the stern tube. Once this is done, the alignment usually stays true but does need to be re adjusted if the rubber insulators fail , harden or are removed to work on the propshaft and shaft seal.
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Old 18-07-2022, 16:12   #14
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft

Quote:
Originally Posted by skipperpete View Post
CV joints ( Constant Velocity) don’t act like automotive universal joints and consist of a number of steel balls in a housing full of grease ... usually molybdenum disulfide based.
The thrust bearing part of the CV that is attached to the propeller shaft and supported by a bulkhead or transverse frame does need to be lined up so that the shaft runs true in the centre of the stern tube. Once this is done, the alignment usually stays true but does need to be re adjusted if the rubber insulators fail , harden or are removed to work on the propshaft and shaft seal.
Agreed,

It sounds pretty much exactly what I said, apart from you also for some reason mentioning thrust bearings.
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Old 18-07-2022, 16:59   #15
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Re: Aligning sailboat propshaft w flexible shaft coupling between motor and propshaft

The CV assembly in the OP’s photo is only half of the installation. CV’s can’t take thrust loads ( neither should UJ’s) so a bearing assembly is provided for that purpose and it needs to be installed correctly.
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