OK let's try to make this good.
russ_watters said:
I don't understand what you are saying here: why would the anchor brake need to stop the ship? ...and why as soon as it touches bottom?
If the anchor catches on a rock or coral on the bottom and the ship is still in motion (forward or backward), then as soon as the chain becomes taught with ship's momentum versus anchor hold force producing tension. Unfortunately, chain has no elasticity to absorb shock. That shock force can be enough to yank the brake clear off the ship. Slippage in the brake in this case prevents the ship from trying to stop in near zero distance. So max brake force and slippage is a necessary property to allow for this contingency.
The ship should never be at a dead stop when the anchor is dropped, because after the anchor hits bottom and chain keeps coming, it would make a pile. Piles of chain can make knots or make loops around the anchor flukes; both very bad. The total length of chain, let's say 10 times water depth, needs to be played out as the ship moves. Ideally the chain should lie evenly distributed on the bottom in the catenary shape. So the first 1x water depth, the anchor falls by gravity. After that, 9x water depth more chain is pulled out by letting the ship drift with no propeller power. A runaway causes the chain to make a pile on the bottom.
Once full chain is deployed, the catenary curve give the whole system elasticity because chain needs to be lifted from the bottom before it forms a taught straight line. Most of the forces keeping the ship from further movement comes from the heavy chain lying on the bottom, not from the anchor. Vulnerability to sudden shock loads is highest when the anchor first touches down and the chain is nearly vertical.
Brakes:
Yes those brakes are subject to all the bad features as any brake. They could be cooled with water. But the momentum of a runaway chain is so great that an overly effective brake might cause a sudden stop to a runaway. The resulting forces could be so big, the brake housing and everything near it to tear right out of the deck, or conceivably tear off the whole bow of the ship. A runaway must be stopped by
gradual deceleration. IMO, it should be much easier to prevent a runaway than to stop one.
jrmichler said:
The brake operator would slowly release the brake, the chain would accelerate, the brake operator would quickly start tightening the brake, and the chain would continue to accelerate while the operator continued to tighten.
Exactly. But the chain may also stop temporarily with no braking because of the motion of the ship due to wind and currents. Remember, that after the anchor reaches the bottom, more chain is pulled only by moving the ship. My theory is that stoppage fools the operators. They think it stopped because of braking, so they turn the wheel more and more to release the brake. Extra turns just move the brake pads further from the disc. They turn so many turns, that when the chain starts moving again, it takes many seconds for them to turn the control wheel in the other direction to start braking once again. In that interval, a runaway starts. That is very clear in more than one of those cases, that as long as the chain is stopped, the operators keep cranking that control wheel more and more turns. If the officer in charge had a GPS in his hand, he could better judge if the chain stopped because of braking or because the ship stopped moving.
russ_watters said:
There's another possibility: While I think it's worth discussing, I suspect the issue is plenty solved, but the old way continues largely due to tradition.
That must be at least partially true. The brake control wheel turns a threaded shaft to move the brake pads. That is selected to give sufficient leverage with zero auxiliary power. A hydraulic brake control could be much faster and have zero deadband. I suspect that it is just conservatism and tradition, to not depend on systems like hydraulics or electric that need aux power. After all, one reason to initiate an emergency anchor drop could be loss of onboard power.
Lnewqban said:
Was not Elisha Graves Otis who found a practical solution (US patent No. 31,128) to a similar problem?
After seeing the stuff above, including the catenary, do you still think an elevator is a good analogy?
Good thinking everyone. This problem is not at all simple. Keep thinking. As I said, nobody anywhere has yet to devise a better system that fits all the contingencies.