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Discussion Starter #1
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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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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One of None
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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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Discussion Starter #5
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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AEOLUS II
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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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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 :eek:
 

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Dirt Free
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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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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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Midwest Puddle Pirate
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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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pray tell, where are thrust bearings on boats? in the transmission?
 

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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.


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

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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.

THanks
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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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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