Unsolved Engineering Problem: Runaway Anchor Drops

[hutchphd]

I considered that. These are incredible things - they can stop very, very, very heavy objects on a dime. However, I think in principle they suffer from the same problem as mechanical brakes - what do you do with the energy?

Clearly I have no "feel" for this size force. That being said three things mitigate for magnetic brake in this circumstance:

1. Water will not directly affect braking force by lubricating
2. Heat is delivered throughout disk...not just the surface
3. The force increases with speed (to saturation current I guess)

I believe the temperature dependence is not large (Curie temperatures may need consideration for permanent magnets). One worry is that all the new rare Earth magnets seem to oxidase very easilly and deeply. Salt air.
Surely there is an effective way to exract heat when an unlimited cold bath is meters away...directly submerge the entire brake somehow?

 

[anorlunda]

1) Assume a worst case hanging mass. The anchor is just above the bottom of the deepest water in which the ship will be anchored.

2) Assume a maximum allowable speed of drop at that point. This value may be iterated.

I like that. It's a rational way to rate the system based on a knowable worst case.

But what you called a "hard stop" and a "weak link" are contradictory, and the hard stop seems to fill the same purpose of the water brake.

This system should allow both fast drops and controlled drops, and allows for operator error on the brake. This system will allow manual control of drops with a failed water brake. It will allow anchoring with a mechanical brake that failed in the open (unbraked) position by letting the chain run all the way out, then winching back to the desired length.

Music to an engineer's ears, fault tolerance and contingencies.

A weakest chain link is supposed to be built into the chain near the end.

Yes. The last link is called "the bitter end". Often it is not attached to the ship at all. No attachment and a weak link are approximately the same thing.

Surely there is an effective way to exract heat when an unlimited cold bath is meters away...directly submerge the entire brake somehow?

Yes, a water bath for cooling should be doable. I thought about both water brakes and magnetic brakes. Both may be viable.

But when I read that the resistance of the water brake can be controlled by a valve on the water feed, that tipped the scale for me. Then I could control that valve proportional to speed, and adjust the proportionality constant to change the "steepness" of the brake force versus speed curve.

With magnetic brakes, how does one make the braking force adjustable, and the slope of force versus speed adjustable?

Ship speed should be zero when dropping anchor.

No. That is discussed in #17.---

Great discussion everyone, I think we are zeroing in on a consensus solution. Too bad nobody wants to hire the PF community to design ship systems.

 

[hutchphd]

With magnetic brakes, how does one make the braking force adjustable, and the slope of force versus speed adjustable?

My stationary bicycle (only as a last resort) has magnetic drag. I think the adjustment is to move the magnet away. In any system I think it involves lowering the effective magnetic strength (and probably force it is quadratic in B). There are many simple ways to do this...I do not see it as a problem. The slope may be a little trickier but maybe it is as simple as radius of the disk? More thought required.

 

[Vanadium 50]

1. Water will not directly affect braking force by lubricating
2. Heat is delivered throughout disk...not just the surface

1. No, but salt water will interfere with the EM fields.
2. Heat is actually delivered by (and energy to) a dump resistor. The problem is what happens if the dump resistor overloads or otherwise fails.

 

[hutchphd]

1. No, but salt water will interfere with the EM fields.

A static B field? I will need to think about this, but I think it would just ad to the braking slightly

2. Heat is actually delivered by (and energy to) a dump resistor. The problem is what happens if the dump resistor overloads or otherwise fails.

I am not sure we are considering the same system. I am thinking about permanent magnets inducing eddy currents in a rotating disk. The braking is from the intrinsic conductivity (resistivity) of the disk (or drum)
My thought was to have a reversable electric motor drive to raise and lower the anchor. In addition there would be a passive permanent magnet brake. This has the happy characteristic of being (passively) more effective at high speed and probably would need no active components. Sort of like dropping a magnet through a conducting tube. (maybe that is the appropriated geometry but it is not obvious how to make that work at scale). Perhaps separate windlass with a cooled disk and magnets. One would need to look at the numbers of course.

 

[anorlunda]

My stationary bicycle (only as a last resort) has magnetic drag. I think the adjustment is to move the magnet away. In any system I think it involves lowering the effective magnetic strength (and probably force it is quadratic in B). There are many simple ways to do this...I do not see it as a problem. The slope may be a little trickier but maybe it is as simple as radius of the disk? More thought required.

If you use a flyball governor to move the magnets, then you have the adjustment of the slope and intercept of the force versus speed curve.

 

[hutchphd]

If you use a flyball governor to move the magnets, then you have the adjustment of the slope and intercept of the force versus speed curve.

For the bicycle I think the fixed magnetic drag supplies a simulacrum of the air drag, although I simply think of it as a midievel rack so who cares. However I still enjoy the real two wheeled variety on early summer mornings...(not dead yet!)

 

[Rive]

I too thought of more sophisticated solutions, but at the end what I would add is really just an (oversized) elevator-style speed limiter. The kind which not only doing speed limiting, but making a full stop till further intervention. (With optional usage as secondary brake - so it could be initiated manually too, not just by the overspeed.)

 

[Stormer]

Seems pretty simple to solve. Just use a eddycurrent brake. It will brake the chain more the higher speed it gets. So it is self regulating.