# Why not a shaft universal joint?



## hertfordnc (Sep 10, 2007)

I'm sure there is a good reason and I have no desire to re-engineer what works but the question has been bugging me;

Why is the shaft on most boats connected with a rigid coupling that has to be lined up perfectly straight rather than a universal or CV joint? 

Of course, if this is an idea whose time has come and if 100 years of convention was just wrong, i plan to install a shaft with a universal joint coupled to a perpetual motion engine I've been working on.......

thanks


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## celenoglu (Dec 13, 2008)

You do not need to use a rigid coupling. There are a lot of couplings whch work similiar to universal coupling with elastic material in between. This type of coupling allows some degrees of freedom and reduces the noise due to propellor or engine.


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## deniseO30 (Nov 27, 2006)

there is an outfit that makes a shaft coupling made up with UniVersal joints so engines can be mounted level. I'm sure someone on the site will remember the name.


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## Maine Sail (Jan 6, 2003)

deniseO30 said:


> there is an outfit that makes a shaft coupling made up with UniVersal joints so engines can be mounted level. I'm sure someone on the site will remember the name.


The Evolution Company in Rockland Maine..

*Evolution Company (LINK)*


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## hertfordnc (Sep 10, 2007)

So there's is not some compelling reason for the rigid coupling? 

I just figured that given the challenge of lining it up and the consequences of not having it lined up that there must be some reason why thios is the convention.

THanks


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## WouldaShoulda (Oct 7, 2008)

Look how long it took to get dripless stuffing boxes!!

I'd think the engine would lube itself more effectively as well.

Mouting brackets, everything would work better (it seems) if made veritcle.


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## tommays (Sep 9, 2008)

They do make them agua something. 

The first thing i can think of is the lenth it takes on installs that are allready pretty cramed to fit under steps. 

I guess if you have a bigger boat with the motor under the kitcken sink there might be more space. 

If you really look at one and see all the extra support bearings required you might feel a bit different about it. 



I dont know what happens on a sailboat BUT when the fail on a powerboat they do some pretty wicked dammage


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## boatpoker (Jul 21, 2008)

Universal joints and thrust bearings have been around for decades. Two reasons you don't find them on most boats are 1.) Cost. 2.) Space.


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## TomL (Jul 21, 2008)

*why..*

...complicate
and your perpetual motion engine...who wants to hear an engine running 24/7?
KISS


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## bubb2 (Nov 9, 2002)

The reason you don't have universal joint on a boat is in most applications it is not needed. In a car your drive shaft is connected to a axle that is moving up and down constantly. Thats the flex the universal makes up for in a car which is not needed for a boat.


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## Maine Sail (Jan 6, 2003)

hertfordnc said:


> So there's is not some compelling reason for the rigid coupling?
> 
> I just figured that given the challenge of lining it up and the consequences of not having it lined up that there must be some reason why thios is the convention.
> 
> THanks


One main reason is SPACE!! Lot's of boats lack the physical space between the tranny & the shaft log...


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## US27inKS (Feb 6, 2005)

bubb2 said:


> In a car your drive shaft is connected to a axle that is moving up and down constantly. Thats the flex the universal makes up for in a car which is not needed for a boat.


While this is true in most cases, but not all. There are many rear wheel drive cars with independent rear suspension that have the differential rigidly mounted, but still need to have universal joints. The universal joints in this case are useful to reduce the need for perfect driveshaft alignment, and allow for a small amount of movement in the engine and transmission mounts. Overall, they allow for a much smoother and quieter installation.

The problem that a boat has, and a car doesn't have, is thrust being applied to the shaft. Now we have the shaft trying to drive itself into the trans, or be pulled out of the trans. In order to get a good quiet installation, you would need 2 joints, the associated flanges, a thrust bearing, and it's associated flanges. You would need the joints to be far enough apart to allow for the needed movement or misalignment to be compensated for without binding. On an 80 foot boat it would probably be worth it, but on a 30 footer I doubt it.


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## deniseO30 (Nov 27, 2006)

pray tell, where are thrust bearings on boats? in the transmission?


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## timebandit (Sep 18, 2002)

You have to remember just what a sailboat engine is.

It is the same as an industrial stationary engine like the ones on a trailer that is hauled out to a job site and started and left to run for hours on end driving a water pump, air compresser or gererator bolted to it.

Once everything is alinged there is nothing to move.

Ten to forty hp is what most of these engines seem to be so stationary mounts is all the have.

One Ujoint still requires alignment so you might want to use something like this.

http://www.dodge-pt.com/pdf/catalog/pt_components/2004_pdf/dodge_para_flex_fea.pdf

I can't explain why boats dont use these they have been around for over 40 years. You just keep a replacement doughnut around just incase.



hertfordnc said:


> I'm sure there is a good reason and I have no desire to re-engineer what works but the question has been bugging me;
> 
> Why is the shaft on most boats connected with a rigid coupling that has to be lined up perfectly straight rather than a universal or CV joint?
> 
> ...


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## boatpoker (Jul 21, 2008)

The polyurethane "drive savers" are mounted between the shaft flange and gear reduction flange. They are designed to self sacrifice on impact so that the transmission is not damaged (or shaft). They will permit a miniscule amount of misalignment but this is not what they are designed for.

A thrust beraring is rigidly mounted between the shaft and gear reduction unit and is dsigned to absorb all of the thrust and vibration from the shaft and with the u-joint allows the use of very soft motor mounts.which in turn greatly reduces the engine vibration being transmitted throughout the boat.

Transmissions tend to last a lot longer with this setup. They are normally found on powerboats of about 55' and up. (eg. Fleming 55) and on very large sailboats (70' and up) with 6 cylinder diesels.
I recently surveyed a Reliant 44' with such a unit and it was very hard to tell when the engine was running from the cockpit or dock.


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## tommays (Sep 9, 2008)

The dodge and most coupplings do a good job ON twist BUT are NOT for the and PUSH or PULL of a prop


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## Valiente (Jun 16, 2006)

boatpoker said:


> The polyurethane "drive savers" are mounted between the shaft flange and gear reduction flange. They are designed to self sacrifice on impact so that the transmission is not damaged (or shaft). They will permit a miniscule amount of misalignment but this is not what they are designed for.
> 
> A thrust beraring is rigidly mounted between the shaft and gear reduction unit and is dsigned to absorb all of the thrust and vibration from the shaft and with the u-joint allows the use of very soft motor mounts.which in turn greatly reduces the engine vibration being transmitted throughout the boat.
> 
> ...


Good summation. It's not that Reliant 44 that's been for sale for 18 months, is it? That's a nice looking boat.

I have purchased and will shortly be installing my AquaDrive UJ coupler and "soft mounts" for precisely the reasons listed above. I also have a VariProp and a steel sailboat, so it is relatively straightforward to weld in a thrust bearing and there's space to do so. The reasons were many: 1) Vibration/noise reduction, 2) Less critical that alignment be maintained, so wear is lessened when motorsailing at a heel, 3) Transmission is spared from the torque-y effects of feathering blades and gear shifting with a feathering prop.

Knowing some people with AutoProps and hearing about cone wear convinced me that this was a great idea for long-term passagemaking, but the truth is that we will probably appreciate the noise reducing even more. When the engine and the tanks are out shortly, I will be painting the engine bay with sound-absorbing paint as well, so this will, I hope, make everything noticeably more quiet and "smooth".


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## boatpoker (Jul 21, 2008)

Valiente .... Not the one thats been for sale. This one will be leaving PCYC for Scotland this summer.


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## TSOJOURNER (Dec 16, 1999)

> Originally Posted by hertfordnc View Post
> So there's is not some compelling reason for the rigid coupling?
> 
> I just figured that given the challenge of lining it up and the consequences of not having it lined up that there must be some reason why thios is the convention.
> ...


Cars that have U-Joints must have TWO U-Joints, not one. So a boat drive shaft may not have just one U-Joint. It must have two just like a drive shaft; one at the front, another at the back.

That's because a single U-Joint must accelerate and decelerate as it revolves around in an elliptical circle. The second U-Joint is mounted 180 degrees displaced from the first and the result is that the accel/decel movement cancels out as long as input angle (front) and output angle (back) are the same.
Ever wonder what the "CV" joint is that is found in front wheel driven vehicles? That's a _constant velocity_ joint, and is so named because it is, in essence, two U-Joints joined together and overlapping. There is no accel/decel phenomenon in a CV joint.

A single U-Joint will set up a nasty vibration. That's why they are not used.


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## Valiente (Jun 16, 2006)

Selkirk, you are of course correct.The AquaDrive I mentioned is a CV coupler, and I used the term "UJ" to mean "universal joint" in the sense that it could pivot under load in any direction to about 10 degrees of offset without damage.

This is what I have:










A U-joint is the forked thing you see under delivery trucks driving the rear wheels. I assume that is what you mean..


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## Omatako (Sep 14, 2003)

OK so here's my take from an engineering perspective.

A universal joint (UJ) has to operate at an angle of not less that 1.5 degrees to prevent "brinelling" which is wear caused by the needle rollers not being allowed to oscillate. It must never run straight. This is not my invention, it's a fundamental engineering principle.

The UJ must also operate at an angle of not more than 8 degrees else it will start to cause a vibration.

A UJ on any normal vehicle has only torsional moments on it, it doesn't push things, it is not designed to take thrust. This is even more true of CV joints. If you look at the picture in Valiente's post, you will see that the thrust bearing carrier is mounted *behind* the CV unit (between the UJs and the prop) and that takes up the thrust. That's the "compelling reason" for not having them on a boat without a thrust bearing carrier.

If the shaft comes in at an angle directly to the gearbox flange, it will produce lateral stresses on the engine/gearbox that are not there when a rigid shaft pushes straight into the rear of the engine. This will play hell on your engine mountings, especially if they are soft cushions (unless you have a thrust bearing in between as detailed above).

A fixed engine on hard mounts with a straight, directly coupled shaft, once set up correctly will work for many years with zero attention and zero vibration. A shaft with UJs, thrust bearing and soft engine mountings is a recipe for saturation maintenance and will probably fit well into the vast lists of maintenance we have seen in other threads. If you like fixing things, this is for you.


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## Gramp34 (Oct 5, 2006)

Omatako said:


> A fixed engine on hard mounts with a straight, directly coupled shaft, once set up correctly will work for many years with zero attention and zero vibration.


As long as you have an engine that doesn't vibrate. 

Tim


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## timebandit (Sep 18, 2002)

The thing I always forget is that these are auxilery engines and not the prime sorce of motovation.

They are not made to work much and as such do not make use of a lot of things that the automotive industry has available.

These systems are just expensive ballast.


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## boatpoker (Jul 21, 2008)

timebandit said:


> The thing I always forget is that these are auxilery engines and not the prime sorce of motovation.
> 
> They are not made to work much and as such do not make use of a lot of things that the automotive industry has available.
> 
> These systems are just expensive ballast.


In a survey of 350 northern (from the north) boats in the Bahamas and throughout the Keys I found that from the Great Lakes to southern waters the average time with sails up was 6hrs......auxiliary ?


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## NICHOLSON58 (Feb 22, 2009)

A single universal joint will allow the shafts to meet at a slight angle. This will not account for parallel offset errors. The Joints used in vehicles are applied in pairs. Also it is planned to have the output shaft and drive shaft roughly parallel with the center floating shaft in an offset. It is important that like ears on the two joints must be in line. If they are asseembled 90 degrees off you will risk large vibration with increasing angles. This is because a single joint bent will cause the speed of the output shaft to oscillate sinusoidally around the average rpm of the driving shaft. Properly aligned pairs of joints correct for this so that only the floating (center) shaft is unsteady. Next, if you install a shaft with a floating section and two universal joints you will also need to install a substantial bearing on the shaft mounting the prop. This is because the normal bearings used will almost certainly not be designed to resist the possibility of the shaft bending about the center line of the single bearing and seal normally on your boat. 

There are alternative marine flexible couplings as mentined above. They need to maintain a bit of lateral rigidity in order to resist the tendency of the shaft to try to whip. Typical sailboat power plant low rpms helps a lot. The unballance force increases with the square of the rpm.

Your best bet (lets assume some room) is to push the coupling aside and mount a dial indicator on the shaft. Rotate the shaft with the indicator around the mating but disconnected shaft. Shim the engine mounts to minimize the dial +/- excursions. Use the indicator in conjunction with a straight edge. Proper alignment is to minimize parallel offset and angular error. If you can get your hands on a laser aligner including a trained operator you can get things really straight. Re-connect and check again.

Yes, I am an engineer and I design vehicle and transmission test cells and dynomometers. We have a nut-job client who wants to test 500 hp at 15000 rpm. I have a Nicholson 58 with a really big and mostly rigid coupling. It is made more complicated by the Hundested variable pitch prop control that passes through the drive line.


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## boatpoker (Jul 21, 2008)

Thought you guys might like to see this Logan 33 with its engine under the V-berth and a very long two section shaft with a U-joint.



















I thought I shuold add this statement from my survey report .....

Shaft system -- A 1 ¼" stainless steel shaft is led through a carbon fiber shaft log and dripless type stuffing box to a thrust bearing which is bolted to an encapsulated floor. The forward side of the thrust bearing connects to an automotive type u-joint and hollow steel pipe shaft with a second u-joint at the gear reduction flange. The u-joints show no play and grease nipples are readily accessible for maintenance.


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## JHJensen (Sep 9, 2008)

*Very cool... that is a driveline!*



boatpoker said:


> Thought you guys might like to see this Logan 33 with its engine under the V-berth and a very long two section shaft with a U-joint.


Boatpoker,

Very cool! Love the stuff you post.

John


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## Valiente (Jun 16, 2006)

The inside of the boat looks cleaner than the outside.

Guys, various opinions of the worth of a CV coupler aside, I got it to solve or improve specific problems and conditions aboard a motorsailer heading offshore. I expect to do around 1,000 hours of motoring per year, and that's the outside estimate. The shaft will need to be pulled, measured and cut to fit the new coupler (and to confirm it is still true), but the engine is going back in the same place and the same angle, just with somewhat softer mounts. The engine and shaft are currently well-aligned, and will be with the coupler, but the coupler's function in part is to preserve alignment should I need to motor at a heel (and oil circulation puts a limit to this trick, I know) or into waves for an extended period.

The thrust bearing and the coupler together isolate the transmission from the effects of shifting a bigger, four-bladed prop. Other feathering prop owners have cited this as about the biggest issue with their feathering props, but one greatly outweighed by the advantage of such props while sailing (which I already know from my other boat) and from close-quarters maneuvers among docks, in canals, etc. Given this is a 30,000 lb. full keel, that's a real plus, but so is preserving my transmission. Lastly, the noise reduction will be welcome.

Talking with working fishermen and SAR folk indicate that this type of coupling and mount set-up is extremely common and well-attested. Frankly, most recreational craft are used so seldom by contrast in "engine mode" that this is not a necessary accessory. For us, proposing a longer trip, it was thought to be so.

Some of you won't like me chucking out my syphon break plunger vent in favour of a straight hose running out to the deck, either, but again, my type of sailing is about reducing points of potential failure, and that device, unlike the coupler, has failed on me on two boats now.


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## Omatako (Sep 14, 2003)

Valiente said:


> The engine and shaft are currently well-aligned, and will be with the coupler, but the coupler's function in part is to preserve alignment should I need to motor at a heel (and oil circulation puts a limit to this trick, I know) or into waves for an extended period.
> 
> . . . . my type of sailing is about reducing points of potential failure, and that device, unlike the coupler, has failed on me on two boats now.


Normally I wouldn't take a contradictory view to your posts but. . . .

Firstly, how does the setup you are proposing preserve alignment when your at a heel? I thought I understood that you're putting soft mountings in, Would that not potentially increase misalignment at a heel?

Also, you're introducing a new bearing and two constant velocity joints that were never there before. How does this reduce points of potential failure? Where you have all this moving, wearing, requiring maintenance stuff, I have a solid SS shaft.


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## jhaley66 (Nov 4, 2008)

boatpoker said:


> In a survey of 350 northern (from the north) boats in the Bahamas and throughout the Keys I found that from the Great Lakes to southern waters the average time with sails up was 6hrs......auxiliary ?


That is a fascinating factoid... 6 hours over what time period? compared to how many engine hours?


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## boatpoker (Jul 21, 2008)

between 350 and 400 hrs one way from Great Lakes to Bahamas. I know that seems like a lot but you have to realize that those making the trek generally take about two months to do the trip as most of us like to explore the Delaware, Chesapeke, St.Johns and every other nook and cranny on the way.
We have taken as long as three months when the cold weather wasn't chasing us.


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## Valiente (Jun 16, 2006)

_Firstly, how does the setup you are proposing preserve alignment when your at a heel? I thought I understood that you're putting soft mountings in, Would that not potentially increase misalignment at a heel?_

As I understand this (the part is purchased but has yet to be installed), the shaft stays put due to the shaft end of the coupling being bolted into the hefty thrust bearing. The soft mounts aren't about alignment so much as vibration dampening. The engine's "flex" is compensated for at the CV joint, which is essentially like a shoulder socket in that there is flex in all directions. So while you obviously go for the usual "at rest" ideal alignment of all the parts from transmission to prop as one would with a solid coupler, you have some give, plus the thrust bearing, meaning that should shearing forces come into play in rough seas or at an angle of heel, these forces are not transferred within the transmission itself, because the coupler is a sort of rotating shock absorber. The same principle as a drive saver keeps the feathering prop from exerting undue force when shifting.

A fellow building a Westsail 42 posts this in his blog:

Building a Westsail 42 Sailboat, From a Bare Hull: 3/5/06 - 3/12/06
*
"What is an "Aquadrive"?*

Traditional sailboat engine installations usually are just a engine+transmission coupled to a straight shaft that goes directly to the propeller. In this setup, the force from the thrust of the prop is transferred, via the shaft to the transmission/engine, then via the engine mounts to the hull of the boat. That is how the boat gets its forward motion. There are some downsides to this setup. Wear and tear on the transmission for having to bear the forward thrust can cause transmission failure (rare, but it happens). The engine mounts need to be pretty stiff in order to sustain the shear forces from the shaft. This means that noise and vibration from the engine are also more directly transferred to the hull, which accounts for about 50% of the "engine noise" one hears throughout the boat. Finally, the shaft alignment must be very precise. This requires precise alignment of the engine as well. Now moving a 600 pound engine up/down/left/right in 1/8th inch increments while inserting shims can be difficult if not annoying.

The Aquadrive solves these issues by isolating the engine/transmission from the prop shaft. It splits the traditional straight shaft into two shafts. The prop shaft pushes on a thrust bearing and plate that is attached directly to the hull. This eliminates the wearing forces on the transmission in a straight shaft setup. The thrust bearing is then attached to the transmission via a shaft with two CV (constant velocity) joints. The CV joints make the engine placement less critical. And you can actually dilberately offset the engine up/down/left/right a few inches without issue. Alignment of the propshaft is still critical, but it is easier to achieve since the engine does not need to be aligned with it. By isolating the engine from the thrust forces, softer, more vibration absorbent engine mounts can be used, thereby significantly reducing engine noise and vibration from the hull."

_Also, you're introducing a new bearing and two constant velocity joints that were never there before. How does this reduce points of potential failure? Where you have all this moving, wearing, requiring maintenance stuff, I have a solid SS shaft. _

The points of potential failure would be within the transmission itself, as I have seen at first hand via the experience of others with feathering props. The movement of the CV joints themselves is pretty friction-free, as physically playing with the thing out of the box has told me. With my set-up and engine access, a broken CV coupling wouldn't be half the problem of a broken transmission part, and I know we have a lot of engine hours ahead of us. This just solves (or to be fair promises to solve) a few issues for us and seems logical to me.

If I've made a horrendous error, I can carry a big steel spacer, a feeler gauge and a few Drive Savers and rectify it!


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## Gramp34 (Oct 5, 2006)

Aquadrive is owned by GKN who also owns Lobro, who makes the CV joints. It looks like the Aquadrive units use the same CV joints as aircooled VWs and Porsches:










Measure the OD of your joints, count the teeth on the splines, and you should be able to match up replacements at an autoparts counter.

It looks like the Aquadrive units also use the "aerodynamic" boots used by racers:










For the price of the parts (~$100 for the joint and $40 for the boot), it'd be easy to carry spares.

Tim


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## Valiente (Jun 16, 2006)

Good tip! Before I install it, I'll get some "extras" at a auto parts place.


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## copacabana (Oct 1, 2007)

Sorry to drag up an old post .... 

Valiente, did you install the aquadrive unit in the end? I'm asking because I have one in my boat and I'd like to ask you a few questions about parts. Thanks in advance.


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