Magnetic Couplings - the ultimate dripless stern-tube

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Magnetic Couplings - the ultimate dripless stern-tube

How can it drip when there's no hole?

The dripless stern tube is something of a holy grail in marine engineering. There are a lot of dripping flax-packed glands out there, and there are a lot of patent "dripless" alternatives. And, just check the sailboat forums - there is a lot of debate as to the effectiveness of them.

I was recently pondering this subject when my eye fell upon my high-tech electric toothbrush, which uses a non-contact mechanism to agitate the brush. Now, if they can swing a toothbrush without touching it, how hard would it be to swing a propeller in a similar manner?

A little internet research turned up the idea of a Magnetic Torque Coupling. Magnetic Torque Couplings are used in heavy industry to accomplish exactly what the sailor wants from a dripless packing: Allow me to put the spinning thing on the wet side of the problem, and drive it from the dry side of the problem, without any of the water coming through.

In industrial applications this can be extremely important, because it's usually not water that is involved: Magnetic couplings are used for such tasks as stirring vats of acid. Not something that we want to have leak!

A magnetic coupling involves two spinning plates, with magnets on their rim. Imagine a disk-brake rotor... two of them in fact, only a short distance apart. Put magnets on the perimeter of them, so that they attract each other. Now spin one of them: The other one will follow.

Now put a piece of plastic between the two disks: Nothing changes really, the 'slave' disk will still follow the 'master' disk because the magnetic field will happily penetrate the plastic.

The effectiveness of the coupling depends on issues of diameter and magnet strength. To make this work in a boat application, of course I am suggesting that the 'plastic barrier' will actually be the boat hull.

Click here to see one example of a commercial magnetic coupling. [ ] I like this firm's radial design. I have seen another unit with an axial design that will take sailboat-like power levels (1" shaft diameter) and has a gap between the magnetic disks of 1/4 inch. I can't tell what the thickness is of the nonmagnetic barrier on the Dexter unit. But surely we can make a composite diaphragm thinner than 1/4 inch which will withstand the hull pressure. And then we have a drive shaft that can never leak.

Also note that these couplings will act as a "drive saver" because if you pick up a rope or strike and object the coupling will simply slip.

Of course, we will need to engineer an appropriate shape into the back of the boat, but I envision this as a sort of 'internal appendage' - it would something like as if you had epoxied a 5-gallon pail into the boat, with the drive shaft passing through the center of the pail. The hydrodynamic skin of the boat would be 'where it belongs' and would simply have a hole in it to allow the shaft to pass through. We will also need to engineer a thrust bearing on the wet side of the coupling, since the thrust bearing is normally included in the gearbox, and a magnetic coupling doesn't provide for this. We will also of course have to provide proper support for the (wet side) tail shaft, via one or two shaft brackets (one of which would be inside the 'bucket.')

I think that Magnetic Couplings could revolutionize the dripless packing market. And dripless packings are SO expensive - and from what I've heard are a bit finicky too. Why not go magnetic and never worry again?

Anybody want to call me to develop one of these?

Chris McKesson chris -at- mckesson.US

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