Could a person do this? -- Push a cruise ship by hand

[Noah Diamond]

I have pulled on one of the mooring lines for a large ship and it moved. It took about 10 seconds of pulling before the ship actually started to move to the point where I could notice it moving. When I pulled on the line in the other direction, it slowed, then came back. I was astonished that this was happening at the time, but now I understand why.

I was 16 years old when this happened. I think I weighed about 140-150 pounds at the time. The ship was the Fun Ship Sensation of Carnival Cruise lines. The mooring lines would stop the ship if it had any significant motion as that's their purpose, but they do have some play to allow for changes in tide and the effects of wind.

If you are anchored on the deck, and you pull on the rope of a non anchored ship, it will actually move. I asked myself about the docks gangways that connect the ship to the dock itself, but that also has some play in it, so there isn't much that would prevent the ship from moving.

The only problem is once the ship is moving in one direction, it wants to keep moving that way until something stops it, so changing it's course while in motion probably won't affect it much. If the ship is at rest though, and the ship were close enough, you could probably move it with your legs... However standing and using the weight of your body to lean against a mooring line transfers much more energy via weight to the line, giving you a better chance of moving the ship.

If you were sitting on a dock, trying to push it, you would probably slide away from it and push it slightly away from you. When you push with your legs, you are applying lateral force in which you are pushing against all the water along the other side of the ship. The ropes are meant to keep the ship from floating away or crashing into the dock. If you pull on the mooring lines in a longitudinal manner you have much less water in front of the ship. The bulbous bow also helps by further reducing drag effects of moving water. The water in and near the dock is usually pretty calm as well.

I don't have the maths for it. I just simply did something similar.

 

[DaveC426913]

I have pulled on one of the mooring lines for a large ship and it moved. It took about 10 seconds of pulling before the ship actually started to move to the point where I could notice it moving. When I pulled on the line in the other direction, it slowed, then came back. I was astonished that this was happening at the time, but now I understand why.

I'm afraid 10 seconds of pulling by a 150 pound person is too little by several orders of magnitude.

You were almost certainly wtinessing the influence of current or wind. Any movement in one direction from any source, will cause the tightening of mooring lines, and the restorative movement will be automatic. It is normal for large ships to ebb an flow in the tide/current/winds.

It is possible in principle to drag a large ship by hand, but you'd need perfectly calm waters and a lot more time.

 

[diogenesNY]

It is worth noting as a historical example, that (in the 19c for this example, but by all accounts this was a very long lived practice) fully ship rigged three masted squareriggers would often leave enclosed harbors, particularly those with troublesome navigation or shelter from wind by 'warping'. That is pulling the ship along out of the harbor by way of the crew pulling it along a fixed chain set up for the purpose. Another practice for getting out of a remote harbor when sheltered or becalmed was to use a ships boat to paddle out several hundred yards with a 'kedge' anchor and a length of chain. The boat would drop the anchor and return with the end of the chain, which would be attached to the ship's windlass and the ship would then pull itself ahead several hundred yards. Repeat as necessary until the ship was in open water.diogenesNY

 

[sophiecentaur]

It is very easy to pull manually any size of barge that will operate in inland canals in the UK but the relative size of a thousand tonne vessel would take it out of the bounds of possibility to produce any measurable effect (at least, the effect would be swamped by other forces, as mentioned above). Having said that, the LIGO system would be more than capable of measuring that sort of thing, I am sure. You would have to be looking for a very small statistical trend in the presence of some massive perturbations.

 

[sophiecentaur]

a 'kedge' anchor and a length of chain

Done it myself with my 4 tonne sailing cruiser (even smaller scale though).

 

[anorlunda]

You were almost certainly wtinessing the influence of current or wind.

I agree with @DaveC426913. Since it is probable that the ship was in salt water, tidal currents are significant. The period of slack tidal currents when motion stops, is very brief.

To test this hypothesis in real life would be very difficult because there wind and/or waves and/or currents are almost always present. Even oscillations allowed by the elasticity of the docking lines easily overpower a human.

Nevertheless, the theoretical question is fun. I think the 5.92 meter answer is credible for the theoretical question.

Does anyone have contacts in NASA? I would not be surprised to hear they measured displacement of the ISS caused by a space walking astronaut pushing on it.