Double universal joint style drive systems

[Bowdrie]

In a double U-joint drive you actually want a small amount of angularity, you don't want perfection in the alignment that you strive for in a standard/normal transmission flange to prop shaft flange.
Without a little angularity the bearing cups don't move the needle bearings in their races, and they tend to wear in one spot under torque pressure.
Just a couple of degrees allows the cups/needle bearings to rotate a little bit, this distributes the grease and wear pattern(s) and gives longer life.

[nuku34]

To the OP:
On my Cavalier 32 with Bukh 20 engine, I have a drive system using a short shaft with a universal joint at each end. The shaft has a splined section so it can change length. The prop shaft has a double roller thrust bearing (available through any good bearing shop) which clamps on the shaft with an asymmetric collar. The thrust bearing is mounted on a transverse web frame abut 100mm past my PSS shaft seal. The beauty of the system is that once you have the thrust bearing properly orientated on the web frame, you will NEVER have to do an engine/shaft alignment. The prop shaft is held rock steady between the cutlass bearing and the thrust bearing. The engine is free to vibrate (they ALL do) and the transmission does not take any axial load. As to U joint angles: yes, perfect alignment is not good for cup wear. 3 degrees misalignment is usually recommended. I grease the thrust bearing and the U joints once a year. The original thrust bearing lasted 30 years.


My previous boat with 80hp Mercedes used an Aquadrive. They use CV joints, not universal joints. Great units, but quite expensive compare to the above system.

[Wotname]

For other readers.

While the there should be some angular offset when using a single uni joint, there should be no total angular offset when using a double uni joint in you wish to maintain a constant velocity between source and load i.e each uni joint should have the same angular offset so that the input and output shafts remain parallel.

A constant velocity between transmission and prop is a good thing. A single uni joint does not provide for a constant velocity, a double uni joint can do so provided the angles are equal but opposite.

[GILow]

This is all gold. Thank you folks, I’m now itching to try this setup in the new boat. [emoji106]

[roryboy]

What benefit do you hope to derive from adopting the complication and expense of a double u-joint drive versus adding another 1/2 meter of prop shaft? A 1 1/4" shaft that is only 1.5 meters / 5 ft long will not have a problem with 'whipping'. Particularly at the lower shaft speed you are turning.

[Cyrus Safdari]

This isn't just about alignment but also thrust force transmission
CV joints are used on sailboats that have prop shaft thrust bearings, which incidentally require complete alignment. The benefit of installing a thrust bearing between the prop and the engine to take the thrust force of the prop and push the boat in the water which is considered a better arrangement than relying on the engine feet to push the boat, if you have the room to install it.
I'd like to install one but don't have the space between the stern tube and the engine
I could probably fit a SigmaDrive provides help for misalignment issues but doesn't involve a thrust bearing as does Aquamarine

[skipperpete]

Originally Posted by roryboy

What benefit do you hope to derive from adopting the complication and expense of a double u-joint drive versus adding another 1/2 meter of prop shaft? A 1 1/4" shaft that is only 1.5 meters / 5 ft long will not have a problem with 'whipping'. Particularly at the lower shaft speed you are turning.

Ok, there are several compelling reasons to adopt a uni joint or a CV shaft.
1. It’s an established and well proven technology
2. The engine is able to move independently
3. Softer engine mounts can be used.
4. The shaft seal benefits from having a rock steady thrust bearing.
5. Easy to remove for gearbox access....AND....
6. No need to dive and move shaft anode to slide the propshaft back.
7. No need to fight with a delicate shaft seal to slide a propshaft back
8. No need to haul out to install a longer propshaft
9. No need to buy a new longer propshaft (and throw an oldie away)
10. Alignment less critical ( but still required.... to a lesser extent)

I admit that a longer shaft and a beautiful compact Sigma thrust capable CV is a seductive proposition but there’s no guarantee that shaft whip won’t be introduced with a longer shaft and requiring a cooper bearing or pillow block to be installed.
My 4WD has 2 ujoint shafts, one of which spins at 3,000 rpm at highway speeds in overdrive and no detectable vibration.

[GILow]

Originally Posted by roryboy

What benefit do you hope to derive from adopting the complication and expense of a double u-joint drive versus adding another 1/2 meter of prop shaft? A 1 1/4" shaft that is only 1.5 meters / 5 ft long will not have a problem with 'whipping'. Particularly at the lower shaft speed you are turning.

I think Skipperpete covered just about anything I could possibly think of, but in my case it’s mainly about noise and not having what I feel is an overly long prop shaft. I will end up with a much bigger gap between the shaft seal and the engine, this approach seems to bridge that gap nicely, while having the added bonus of reducing noise and vibration.

I admit, I started this thread fully expecting tales of serious problems with the setup, but to the contrary, it seems most have had nothing but positive things to say.

[col50]

Originally Posted by wholybee

It is very important to get the angles right. In order for a u-joint to work, the U joint on each end must be at exactly the same angles. In a car, that is done by rotating the rear axle, easy enough to do. But you don't have that easy adjustment on the boat. You can adjust the engine mounts somewhat.

The "exactness" is probably less important on a boat than a car, but I think still needs to be as good as you can get it. If off, the speed of the prop will change every half revolution. (or is it every 1/4?) This will turn into a vibration in the driveline and the engine.

As long as the angles are correct, it should be pretty simple to set-up. U joints are not maintenance free. They need to periodically be greased and eventually changed.

This rule does not apply to CV joints; and that's why they are used on front wheel drive cars, and on all cars with independent suspension.

[myketch42]

I used the aqua drive system on my aluminum ketch .....I did it to reduce the noise from vibration . the other benefits have been mentioned previously . I couldn't really tell if the noise was less in my case .

[Mike Burch]

+1 for the Aquadrive. It has a self-contained rubber-mounted thrust bearing combined with a Cardan shaft (a shaft with a proper Constant Velocity Joint, NOT a common or garden universal, at each end).

Taking the prop thrust on a separate thrust bearing means that the engine mounts have to contend only with weight and torque, not with thrust. This means that they can be softer, thus transferring less vibration to the boat, resulting in a smoother ride with less noise. The Cardan shaft can be had in various different lengths, to suit most existing installations. (When I fitted mine I had to saw about six inches off the prop shaft.)

The manufacturers recommend a small amount of deliberate parallel misalignment for best running.

If I were ever to build another boat I would fit an Aquadrive without hesitation.

[Nicholson58]

Originally Posted by GILow

A few weeks back a fellow yachtie wandered past clutching a short drive shaft with a universal joint at both ends. Just the sort of thing you seen on live axle 4WDs. It is part of his drive system, connecting the engine to the prop shaft. (His boat is a fascinating thing, with all sorts of technical ideas including a pushpit that doubles as a jockey pole to lower his tabernacle mast.). In his case the setup is needed because his engine is actually lower than the propellor shaft. Unusual to say the least.

Anyway, I didn’t think much more about it until this week when I was considering what to do with the new boat. I am removing the heavy old Perkins 154 with VelvetDrive transmission and replacing it with a smaller, lighter and, above all SHORTER Beta 50 with a TMC manual transmission. The shorter bit is a problem as it would sit way aft in the engine bay and cause problems with fore and aft trim. I am fitting a new prop shaft so one option was to make the prop shaft longer (with all the associated issues of whipping).

But now I’m thinking that the double uni joint idea might be the way to go. I’m assuming I’ll need a thrust bearing on the prop shaft (should be easy enough to arrange) but has anyone had experience of such a setup and if so can you comment on it in terms of reliability, noise etc?

Engine is a 50 HP Beta with a 2:1 reduction gear. Shaft will be 1.25” with a maximum rpm of 1350. Shaft itself is currently pretty short, I think a bit over a meter. The extension with the uni joints would likely need to be a bit over half a meter.

Matt

Edit: for the record, I know there are some commercial versions of this out there (python?) but they are waaay out of my budget.

Ours is Aqua Drive. One end is universal (cardan joint). The other is a CV joint that allows length variation as well as offset. There is a thrust beating integral to the aft end to land the prop shaft thrust. My opinion is that this arrangement is the only proper and easy way to couple a prop and engine.

[GILow]

Originally Posted by Nicholson58

Ours is Aqua Drive. One end is universal (cardan joint). The other is a CV joint that allows length variation as well as offset. There is a thrust beating integral to the aft end to land the prop shaft thrust. My opinion is that this arrangement is the only proper and easy way to couple a prop and engine.

That combination of uni joint and cv joint has me scratching my head a little. From what I’ve read, it would produce a non-constant output speed. The trick seems to be to have two uni joints offset in such a way that they eliminate the speed variation produced by each joint, resulting in constant speed at the output but non-constant in the connecting shaft.

Thoughts?

But very glad to hear you also endorse the setup. It does seem those who have it or something like it are happy with the arrangement.

[Wotname]

Originally Posted by GILow

That combination of uni joint and cv joint has me scratching my head a little. From what I’ve read, it would produce a non-constant output speed. The trick seems to be to have two uni joints offset in such a way that they eliminate the speed variation produced by each joint, resulting in constant speed at the output but non-constant in the connecting shaft.

Thoughts?
.............

This is true. With one uni joint and one CV joint, the resulting change in the velocity output is the same as that of having only one uni joint.

However the change in the non linear velocity is dependant on the angular displacement of the uni joint. If the angular displacement of the uni joint is small, the change in velocity is small. Dependant on the prop characteristics and shaft rpm, a small velocity change could be unnoticeable.

FWIW - I expect (but don't know) that it would less noticeable for a large blade area, 3 blade low rpm prop than on a small blade area, 2 blade high rpm prop.

[Nicholson58]

Originally Posted by Wotname

This is true. With one uni joint and one CV joint, the resulting change in the velocity output is the same as that of having only one uni joint.

However the change in the non linear velocity is dependant on the angular displacement of the uni joint. If the angular displacement of the uni joint is small, the change in velocity is small. Dependant on the prop characteristics and shaft rpm, a small velocity change could be unnoticeable.

FWIW - I expect (but don't know) that it would less noticeable for a large blade area, 3 blade low rpm prop than on a small blade area, 2 blade high rpm prop.

Good points, correct.

Without a CV component it would not permit length variations.

Go to the vendor’s site. They will give you better hear-say than we professionals.