Magnetic couplings

Has anyone seen or had any experience with the use of magnetic couplings to transmit torque through a hull?

As far as I can see there are a number of advantages. The biggest is that you avoid a hull penetration, but also they give you some alignment tolerance, help to damp vibration and can be designed to slip somewhere north of the peak engine torque output.

The biggest challenge seems to be how to then transmit the thrust to the hull - requiring some sort of thrust bearing that's happy working in seawater.

Interested in others thoughts.

 

I've googled around that theme recently and got nowhere for my application: it turned out the industrial magnetic couplings for torques like i'd need (~1000 nm) get huge, and they use really small gaps, like 1mm. So i ended up more scared about the discs grinding out a giant hole in case of impact or some alignement problem, than i could ever be about a dripless shaft seal breaking.

Anyway here's a twist that may make the idea even more attractive in lower power applications:
Contactless gears:
X Shore Eelex 800: Could this electric boat become the Tesla of the seas? https://www.mby.com/reviews/rib-and-sportsboats/x-shore-eelex-800-electric-boat-tesla-inspired
or magnetic gears like used in some wind turbines:
magnetic gears - Google Search https://www.google.com/search?q=magnetic+gears&tbm=isch&ved=2ahUKEwjRuq24lM7rAhWmBLkGHZ1aCXAQ2-cCegQIABAA&oq=magnetic+gears&gs_lcp=CgNpbWcQAzICCAAyAggAMgYIABAIEB4yBAgAEBgyBAgAEBgyBAgAEBgyBAgAEBgyBAgAEBg6BAgAEEM6BggAEAUQHlCmjgFY25kBYKWbAWgAcAB4AIAB6AGIAdcIkgEFMC42LjGYAQCgAQGqAQtnd3Mtd2l6LWltZ8ABAQ&sclient=img&ei=7n5RX9GLAaaJ5OUPnbWlgAc&bih=903&biw=952&rlz=1C1CHBF_frFR808FR808

I'll be curious where you end up, good luck and keep us posted !

 

It's possible. All it takes is a truckload (in your case a lorryload) of cash. The coupler will cost you more than if you buy a high end yacht with a regular shaft, and probably leave money left for beers.

 

I have two 10KW inboards on my catamaran right now, coupled to props using 3:3 MGT-200 by https://www.mgt.com.au/ — 20cm in diameter, 12 wheels in total. Supposedly can transfer at least 43Nm of torque in that configuration; my motors can go up to 76Nm, but I don't think I'll ever need that on a sailing catamaran at < 1500RPM.

The cost was around $1500 for everything. You can get it cheaper and likely with more torque transfer if you go with larger wheels in 1:1 or 2:2 configuration, my bilges are just very narrow, I'm limited to about 22cm in diameter for both the motors and couplings.

Here's the hubs that connect those wheels to shafts: Printables https://www.printables.com/model/510571-catamaran-propulsion-system#preview:file-qL0Xf & Printables https://www.printables.com/model/510571-catamaran-propulsion-system#preview:file-Jey15

Dangerous stuff by the way, make sure you're careful working with those, don't want your fingers to get caught between the discs.

 

Undersizing the coupler by over 40% is a bad idea. They should be able to transfer the maximum torque with a margin of safety.

 

They can transfer more torque, perhaps more than 76Nm, once spinning. I'll see how they perform, but again, I have less than 1500 RPM and feathering propellers, it's a sailing catamaran, I really don't think I'd ever need more than 30Nm, but of course I could be wrong.
This type of couplings would have to be way larger and stronger for motors with high RPMs, especially those that need to start fast, like jet drives.

One of the benefits of magnetic couplings is they start slipping if a rope wraps around the propeller, so the less torque they can transfer, the easier it'll be on the motor, if the prop gets hit or stuck for any reason.

 

Interesting concept.
76 NM is only 54 foot pounds torque.
I can't see this working except for small boats with small engines.
And what happens when stormy weather, winds, waves currents are working against hull and prop.
Slippage.
And how about slippage, it seems like an Auto Trans in a car with a torque converter, that also slips and has an impact on fuel economy. A torque converter uses oil and impellor turbine blades to transfer torque, the magnetic coupler, is it really useful? Why dont they use Torque converter transmissions in boats?
Well the prop in water is the slip factor, no direct connection like a car tire on a road.
How can a magnetic coupled prop shaft not slip, it must slip when conditions are met.

The only advantage might be the frictionless concept of gearing not needing oil, bearings, gears.

 

Motor shaft Hull penetration can be more easily avoided in other ways, like using the mounts associated with an Asian long tail, or an outboard motor design. The Asian long tail can also be designed to use water lubricated bushings at the prop end or even run unsheathed and thus avoid the need for leak proof submersible bearings and submerged gearbox seals.

As far as the slippage aspect to protect the propulsion system and propeller from tangles and collisions, that can be done with mechanical /adjustable slip clutches similar to what is used to adjust torque with cordless screwdrivers. But one advantage of a magnetic coupling clutch would be much better torque release setting reproducibility and lack of frictional wear degradation. The slippage concept may not work well with high RPM Motors or large boats with big motors.

My one man shallow running Creek boat can run in water down to four inches, even while hitting bottom and encountering collisions with small rocks, as well as being able to shed some degree of tangling weeds. It uses an Asian type design motor and mount, plus an unsheathed and unsupported flexible shaft, with an improvised cordless screwdriver mechanical slip clutch to help with any tangles or hard collisions. The motor mount allows the propeller to be tilted up out of water to some degree so it can run surface piercing; or the lightweight system can even be lifted up by hand out of water while sitting in the boat, to spin off weed bundles. Of course this is a very limited Niche application, not likely to be used by very many people. But it sure beats walking miles of shallow water Flats carrying paraphernalia, while having easy Portage options, when fishing..

 

Another advantage of a long tail shaft is the spinning prop shaft stays out of the water. When a shaft spins in water it creates a lot of drag.

 

A prop spinning at the end of a long shaft is generally more efficient compared to an inboard or outboard because it operates further away from the shadow of blocking/ disturbed water in front (from the boat/ appendage), plus the leading section of shaft can have zero drag profile (in surface piercing mode).

Magnetic decoupling might be particularly helpful for minimizing Driveline and motor damage resulting from prop Collisions, especially when using the more efficient larger props at lower RPMs. At least, the use of a mechanical clutch in my one-man boats has reduced fragile RC prop breakage down to zero- over the several years where it has been used.