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Old 08-04-2014, 13:29   #1
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Magnetic Drive

Somebody in another thread mentioned the necessity
of a shaft through the hull for an engine.

Has anyone attempted to build a neodymium magnetic
drive coupling? They are damn strong magnets. One on
the inside and one on the outside. No hole.

Comments?
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Old 08-04-2014, 13:37   #2
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Re: Magnetic Drive

The thickness of a boat hull would make the efficiency quite poor if even possible.
There are other ways besides shafts through hulls.
The obvious one is outboards, there are also hydraulic drives and electric drives.
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Old 08-04-2014, 13:43   #3
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Re: Magnetic Drive

I would think that the magnetic field would wreak havoc with the thousands of dollars of sensitive electronics on board. Could be wrong though.
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Old 08-04-2014, 13:43   #4
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Re: Magnetic Drive

It would require a small hull section perpendicular to the axis of the shaft to accommodate whatever diameter coupling you came up with (a source of hydrodynamic drag) and a pretty substantial thrust bearing on the outside of the hull (exposed to sea water). The thickness of the hull section, combined with the thickness of the trust bearing, would hold the magnets at a significant separation, significantly reducing their mutual coupling strength. The thrust bearing could probably be recessed into the coupling, though, to put the magnets closer to each other.

There would be significant torque, putting the coupling of the individual magnets in shear... not sure how well they resist shear forces as opposed to axial forces. In any event, I'm sure a pretty large-diameter coupling would be necessary to have a long enough moment arm, exacerbating the drag problem.

An interesting idea, but this ol' engineer doesn't think it is a very practical one.
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Old 08-04-2014, 13:47   #5
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Re: Magnetic Drive

Quote:
Originally Posted by Ukeluthier View Post
It would require a small hull section perpendicular to the axis of the shaft to accommodate whatever diameter coupling you came up with (a source of hydrodynamic drag) and a pretty substantial thrust bearing on the outside of the hull (exposed to sea water). The thickness of the hull section, combined with the thickness of the trust bearing, would hold the magnets at a significant separation, significantly reducing their mutual coupling strength. The thrust bearing could probably be recessed into the coupling, though, to put the magnets closer to each other.

There would be significant torque, putting the coupling of the individual magnets in shear... not sure how well they resist shear forces as opposed to axial forces. In any event, I'm sure a pretty large-diameter coupling would be necessary to have a long enough moment arm, exacerbating the drag problem.

An interesting idea, but this ol' engineer doesn't think it is a very practical one.
Ya what he said . Because honestly I only understood every 3rd word in that statement.
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Old 08-04-2014, 13:52   #6
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Re: Magnetic Drive

It exists

Tom
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Old 08-04-2014, 14:03   #7
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Re: Magnetic Drive

It exists


That is so cool!

And it can cover enough torque for my boat:
5,000 horsepower! Alright!
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Old 08-04-2014, 14:08   #8
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Re: Magnetic Drive

Interesting stuff, but I will let you try it first,

I think I would rather lap cone plates for a little longer
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Old 08-04-2014, 14:58   #9
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Re: Magnetic Drive

Jongleur,

yes, it's an interesting concept, and we've been using magnetic coupling in pumps for decades to get leakproof designs. The efficiency problem actually could be overcome if you used enough Neodym - once the prop stays in synch with the engine there actually are very few losses in the design. However, a shaft does not just transfer torque, but it also holds the prop both axially and radially. And here is where your proposed design would get rather complicated - instead of one shaft seal you would need at least two radial bearings and a thrust bearing running in sea water, or encapsulated with half a dozen oil seals. That does not sound like a simplification...

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Old 08-04-2014, 15:01   #10
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Re: Magnetic Drive

Quote:
Originally Posted by ontherocks83 View Post
I would think that the magnetic field would wreak havoc with the thousands of dollars of sensitive electronics on board. Could be wrong though.
Not to mention the effect on my tin foil hat!
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Old 08-04-2014, 15:14   #11
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Re: Magnetic Drive

A rim driven propeller could have no penetrations.



The magnetic rotor ring around the propeller tips is driven by a stator inside.

Cheers,
JM.
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Old 08-04-2014, 15:23   #12
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Re: Magnetic Drive

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Originally Posted by NahanniV View Post
A rim driven propeller could have no penetrations.



The magnetic rotor ring around the propeller tips is driven by a stator inside.

Cheers,
JM.
Right, an electric drive...what makes this design special is it requires no bearings or seals, the prop is held in place by the magnetic field.
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Old 09-04-2014, 08:17   #13
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Re: Magnetic Drive

Quote:
Originally Posted by jongleur View Post
It exists


That is so cool!

And it can cover enough torque for my boat:
5,000 horsepower! Alright!
Actually, that design is more of a variable-slip clutch... not really what the OP is looking for. It has a shaft running between the two magnetic coupling assemblies; the OP is looking to eliminate the need for a shaft through the hull.

That rim-driven prop with the built-in electrical motor makes more sense, either in a diesel-electric hybrid drive or a straight electric one.

I'm not saying it can't be done... just that the disadvantages (including cost) outweigh the advantages for the typical boat.
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Old 09-04-2014, 12:17   #14
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Re: Magnetic Drive

Hoofsmit:

I'm not familiar with lap cone plates.
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Old 09-04-2014, 13:07   #15
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Re: Magnetic Drive

Quote:
Originally Posted by jongleur View Post
Hoofsmit:

I'm not familiar with lap cone plates.

Some of the yanmar sail drives have cone clutch plates which slip, the contact surface can be abraded to cause adhesion, with an abrasive compound, this process is called lapping, sorry if it was not clear
My comment was because of the fact that this has to be done every few hundred hours( on some)but I would consider this better than the alternative to the unknown problems of the drives mentioned on this thread... Just my opinion and it was a tongue in cheek comment
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