View Full Version : Propshaft Question
12-17-2008, 12:46 PM
I was looking over Spinner's thread on the repowering of ORCA and the great work that Dave was doing and I saw this picture
With my limited exposure to sailboat drivelines I beg to ask, how common are universal joints?
I can see real advantages to them to compensate for a bit of misalignement if you have a carrier bearing supporting the shaft and the engine is at a different angle. Also, the propshaft on BS is off center but the motor is on center
When we repower BS I am concerned about the new motor to shaft angles so this caught my attention.
12-17-2008, 12:50 PM
U-joints are not common at all. I have never seen one. Might be logical to think about using them there...but they are not.
I can't explain the reason but someone will be along shortly to furnish the answer.
12-17-2008, 01:58 PM
Very strange. Standard U joints aren't designed to transmit thrust, which is what a propshaft does. In order to turn smoothly they also have to be used in pairs. There are some systems that use CV joints (Aquadrive is one) but that's not what's in the pic.
12-17-2008, 02:23 PM
Looks like some lashup made by an amateur from an automotive component. I have seen a very clever improvisation using the UJ from a front wheel drive car, which can take thrust as well as misalignment.
12-17-2008, 02:25 PM
i never thaought about that . It makes sense all the pressure is on the u joint itself two little round points . thats a lot of pressure
U-joints are acceptable IF they are isolated from the thrust from the propeller. In other words, the tailshaft (prop to inside the boat) has a thrust bearing fitted so that the prop thrust is taken by the thrust block attached to the ship structure. This unloads the inboard end of the tailshaft so that a U-joint can be fitted between it and the intermediate shaft. Problem with this is that it both takes up space and costs money, both of which are usually in short supply in pleasure boats.
U-joints don't need to be used in pairs, but they should be placed such that there is at least 2-3 degrees of misalignment between the connected shafts.
If what is shown in the above photo is the entirety of Orca's shaft system, I'm not too fond of it. The U-joint becomes the weak link, and can be very weak, indeed. There are other options...
12-17-2008, 02:58 PM
Grantala has what Michael just outlined - a thrust bearing on the inboard end of each shaft and then a short two-piece sliding shaft of 18" or so with a large universal joint at each end.
12-17-2008, 04:33 PM
I would just get rid of it.
Check the alignment of shaft. Put in a urethane disk in case the prop ever hits something.The problem is corrosion. because it is all metal. Just one more thing to go wrong.
And it is not designed for thrust - JUST rotational force.
12-17-2008, 07:58 PM
There was a boat that used to pass through St. Augustine on a seasonal basis, name of Old Timer, back when I was living there on my Gazelle. The owner-builder was a retired machinist from Maine and he designed and built Old Timer for his retirement years. She started as a 40 footer but that wasn't big enough so he fabricated a ten foot section during one of his summers in Maine and after wintering in Florida he had Old Timer hauled the next time he was in Maine. He cut the hull in half with a torch (she was steel), pulled the sections apart 10 feet and slipped in the section he'd fabricated the previous year then welded it up. He and his wife were off to Florida the following winter.
But this is about u-joints on boats.... Old Timer had a machine shop in her forepeak which was powered by her main engine, also in her forepeak. There were simple pulleys, belts, and adjustable idlers that provided power as needed to the lathe and mill. The main engine also turned the propeller at the stern, of course, through a series of shafts and u-joints that ran along the starboard side of the hull.
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