Trojan Boat Forum

I am going to ask a simple question as I would like to know the answer and I am sure it will make sense.

Why in typical marine applications do the manufacturers not use a universal joint or a double universal joint such as constant velocity type joints. IO drives sometime use a constant velocity joint. It seems the allignment would be less of an issue, important, but not down to the thousandth of an inch.

Some of my tractor power take off driven equipment uses a constant velocity joint in it and there is no allignment issues and no vibration.

Me thinks of in marine use it is because of space requirements but I defer to the experts?

Larry

"most conventional" Universal joints are not designed to carry thrust loads. When the prop pushes the boat- all that counter force is transmitted up the shaft and actaully pushes on the transmission which has special thrust bearings to absorb the load.

Think about what would happen if a universal joint failed and the shaft started wipping while hundreds (or thousands) of pounds of force are pushing on it. Carnage and very possibly lots of water where it don't belong.

Bingo that is a good explanation. seems a system could be designed thrust bearings but would require more complexity.

Guess keep it simple is best.

Thx

Larry

I wonder how many rpm a u-joint could do before vibration showed up.

U-joints on IOs often run at an angle and at WOT.

Big D wrote:U-joints on IOs often run at an angle and at WOT.

There are limits though... On an IO, NEVER run at anything higher than an idle speed if you are in 'trailer trim' mode. The range you can move an outdrive up or down regular via power trim is designed to ensure you can't put too much load in the universals (and for safety -- trim that drive out of the water at 4000 rpm and somebody's gonna get hurt!). Too much tilt + any more than just a little bit of power is trouble.

As for turning side to side, most drives don't turn any more than about 30degrees in either direction.

I agree, just putting out there that they can spin pretty fast with no vibration. Wonder if anyone has ever maxed them out in a test. I always recommend that when past "trim limit" and in trailer range, one shouldn't be running the engine period; definately putting a lot of stress on the u-joints then.

Alignment is very important on ujoints or you can get a lot of vibration . Thats one reason you won't see one ujoint . when you have 2 joints both joints should be close in their angle that way one cancels the other . when 3 joints are used you will find 1 joint at near zero angle .

on I/Os ypu will notice they use 2 joints

one reason you may not notice that on a tractor is the PTO spins at a relatively low speed , but they still use 2 joints.

anyone that has taken a basic automotive course , may remember a set up where you had a drive shaft that you spin at a constant input speed you could watch the shaft sped up and slow down , and yet the out put was the same as the input when when both joints are at the same angle

Paul, you are correct regarding ujoints there has to be two and they have to be at correct angles to each other or all kind os vibrations startup. Regarding durability of them, in auto use they generally run at the same speed (1:1 ratio) as the engine when 3rd gear for a 3 speed, 4th for a 4 speed. Other higher gears are overdrive and cause the joint and shaft to turn faster than the engineto conserver fuel. Example in a NASCAR 4 speed output of tranny is one to one with engine and the run for hours at VERY high RPMs using a very tall or high differential gearing to obtain speed. (of course they have 800 HP engines to pull that kind of load)

U Joints are however very durable turning many revolutions in vehicles and machinery at many different speeds. They could be used in an inboard but would make the drive line mechanism more complex and space consuming as the thurst would have to be transferred to the boat through alternative means.

I do not believe the rpms would damage a set of ujoints designed for the application, they do work in IO applications.

Sorry did not mean to digress into automotive and machinery world. What is in use today in marine world is most likely the simplest and most durable method.

Just curious why we never see the application.

LARRY

universal joints can spin at any speed which they are designed,
remember formula one engines/ transmissions, spin in some cases 18000 rpms, so
it can be done,
this is something I can comment on, as I deal with this every day,
marine is different, and In my opinion, and not just my 77, Marine is leaps and bounds behind the automotive world,
but I dont know what the modern yachts are using for props, but I dont think its even close to what I see on todays modern high end auto's

as others have state the big differnence is thrust pressure. Design something to take the propeller thrust pressure then link shaft to power and you have it. But I suspect more complex and would not produce and additional power or energy transfer, but would definetly make it easier to set up.

Remember cars are built on fast moving assembly lines. Boats are built basically one at at time or hand built.

I wish it were like autos but boats are made on a much slower methodology.

Larry

Check out this web site. http://www.evolutionmarine.com You build a lateral stringer and install one of these. Then add a short shaft with a U joint and a flange at each end between the trans flange and the thrust assembly. May even be able to use parts from a U joint assembly for an I/O.

The engine would have to have room to move forward however it can sit a little more level in the bilge. Since the thrust is removed from the engine & trans mounts and all their left to deal with is torque, you can use softer (less vibration) mounts.

OK, I'll stop now.

Thanks Paul I knew it could be done, however the current design is simple, durable and gets the job done.

If the industry was building boats by the 10s of thousand then u joints and what your afrticle suggests would be cost effective.
As I see it, and that can be questioned the current systems are simple and cheap.

Larry

Re universals : going back to the 'thrust load' Jav originally mentioned and the concept of 'stern drives use them so why don't inboards'... Look at what boats stern drives are used on and what they have in common... typically smaller and lighter runabouts, daycruisers, and express cruisers to 30'.

Aside from heavy duty commercial installations, I think the biggest boats I have ever seen stern drives on are offshore performance rigs (maybe 50' in length but that weigh at most 20,000 pounds (and all mechanicals are rebuilt or replaced pretty much yearly), houseboats (up to 40 or 50' perhaps) that are running low horsepower and are only designed to do 8-10 knots, and a handfull of 36-40' cruisers (max) that might weigh 20,000 - 25,000 pounds at most (I have seen them a few low-budget express cruisers, and on a make called (I think) an 'Innovation', which is essentially a 36 tri cabin clone).

For heavier high speed boats, they just aren't used. If space is an issue, you tend to see V-drives instead. The more fancy a drive system, and the more 'connecting' parts you have, the more potential failure points you have. A solid steel shaft is just that much more reliable when pushing a big load with hunky horses. The marine load curve is just SO much different than any automotive load curve. Not sure what history will eventually say about the newer Zeus and IPS pod drive systems.

talk about beatig a subjec to death,,lol