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

In summary: Let's assume that floating in the water provides an almost frictionless surface.If you exerted a force of 50 newtons against the hull of the ship, you would move the ship an increment of 0.5 meters in 1 hour.
  • #1
thetexan
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11
I was watching a program on the worlds largest cruise ship, the Oasis of the Seas. It displaces 243000 tons. They we're discussing the sophisticated maneuvering systems and I got to thinking.

Let's say the ship is docked, within a foot of the dock and untethered. If I sat down with my legs dangling over the side of the dock, put my feet against the hull and pushed against the hull bracing my back against the pier, and pushed as hard as I could, would I be able to move the ship away from the dock?

How much force would it take and would a human be able to do that? It might move slowly but would I be able to move it noticeably?

How would you go about figuring that out?

Tex
 
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  • #2
I have no idea. My first thought is figure out what acceleration you define as perceptible and then apply Newton's law to see what net force you would need. Then maybe figure out what the range of forces from the water and air pushing on the ship in various amounts are. If the force from your legs would just get lost in that value as "noise" then you would not be able to do it. If the force from your legs can rise above the typical forces, then you could do it.
 
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  • #3
Newton's ##F=ma## will do the trick. You know the mass ##m##, you can come up with a pretty good estimate of the force ##F##, calculate the resulting acceleration.

This is ignoring the effects of friction with the water - include these and the ship will act as if it's solidly aground instead of floating, no more likely to move than the average mountain.
 
  • #4
Nugatory said:
This is ignoring the effects of friction with the water - include these and the ship will act as if it's solidly aground instead of floating, no more likely to move than the average mountain.

That would be true for static friction. Can water exert something similar, that prevents any acceleration below a certain force?
 
  • #5
Let's separate theoretical answer from the practical one.

In practice pretty small currents or winds (ones that will require sensitive sensors to be detected) will create forces that you are not able to overcome, just because they work on a huge surfaces.
 
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  • #6
Nugatory said:
Newton's ##F=ma## will do the trick. You know the mass ##m##, you can come up with a pretty good estimate of the force ##F##, calculate the resulting acceleration.

This is ignoring the effects of friction with the water - include these and the ship will act as if it's solidly aground instead of floating, no more likely to move than the average mountain.
It also ignores the fact that a large mass of water needs to be moved as it sloshes round from one side of the ship to another. Its effective mass needs to be included in the "ma" thing. Even a light paddle would take quite a lot of pushing if it were as big as a ship's cross section.

That said, it is possible to shift some pretty hefty barges on canals with the available power of just one person.
 
  • #7
sophiecentaur said:
It also ignores the fact that a large mass of water needs to be moved as it sloshes round from one side of the ship to another. Its effective mass needs to be included in the "ma" thing. Even a light paddle would take quite a lot of pushing if it were as big as a ship's cross section.

That said, it is possible to shift some pretty hefty barges on canals with the available power of just one person.

In the absence of any perturbation, the acceleration would be F/m, where m will be just the mass of the ship. F will be the hydrodynamic resistance, that is a function of the cross-section of the ship perpendicular to its motion, the velocity squared, the density of the water, and some coefficient of hydrodynamic drag...
 
  • #8
So...help me out here please. If I pushed with 50 lbs of force against the hull, and assuming that water resistance or wind or waves don't play a factor, what distance of movement could I expect in 1 hour of pushing? Let's assume that floating in the water provides an almost frictionless surface.
 
  • #9
thetexan said:
So...help me out here please. If I pushed with 50 lbs of force against the hull, and assuming that water resistance or wind or waves don't play a factor, what distance of movement could I expect in 1 hour of pushing? Let's assume that floating in the water provides an almost frictionless surface.
Just use the third equation:
http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#mot1
 
  • #10
thetexan said:
So...help me out here please. If I pushed with 50 lbs of force against the hull, and assuming that water resistance or wind or waves don't play a factor, what distance of movement could I expect in 1 hour of pushing? Let's assume that floating in the water provides an almost frictionless surface.

It's an impossible situation, but that's easy to calculate. 50 lbs = 222 N. If the ship has a mass of 243000 metric tons = 2,43 * 108 kg

a = 222/2,43 * 108 = 9,14 * 10-7 m/s2

e = ½ * a * t2 = 0,5 * 9,14 * 10-7 * 36002 = 0,00165 m = 1,65 mm

Sounds plausible... And that, with zero water resistance...
 
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  • #11
NTW said:
In the absence of any perturbation, the acceleration would be F/m, where m will be just the mass of the ship. F will be the hydrodynamic resistance, that is a function of the cross-section of the ship perpendicular to its motion, the velocity squared, the density of the water, and some coefficient of hydrodynamic drag...
I would agree that is possibly the case if the ship is out in the open water but, against the quay, the situation is different, surely. Water must be moved from the open sea to the gap in-between the hull and the wall. It has to be accelerated. (and a lot more than the acceleration of the ship am). I don't see how the acceleration of that water is less important than the acceleration of the boat itself. During the operation, there will be a difference in hydrostatic pressures on with sides of the ship because the level on the quayside will be lower. I realize we are trying to get as 'ideal' as possible but can you really ignore my point?
 
  • #12
sophiecentaur said:
I would agree that is possibly the case if the ship is out in the open water but, against the quay, the situation is different, surely. Water must be moved from the open sea to the gap in-between the hull and the wall. It has to be accelerated. (and a lot more than the acceleration of the ship am). I don't see how the acceleration of that water is less important than the acceleration of the boat itself. During the operation, there will be a difference in hydrostatic pressures on with sides of the ship because the level on the quayside will be lower. I realize we are trying to get as 'ideal' as possible but can you really ignore my point?
That's always the case of an object moving in a fluid A force of drag appears, and opposes the movement. I have mentioned the equation used for drag in aerodynamics, but I feel pretty sure that it's generally valid for water also.

I agree that the 'vacuum' formed between the hull and the quay may add some additional resistance, but not all quays are walls that reach the bottom. There are many pole-supported quays, and perhaps even cantilevered ones...
 
  • #13
NTW said:
In the absence of any perturbation, the acceleration would be F/m, where m will be just the mass of the ship. F will be the hydrodynamic resistance
What do you mean by "In the absence of any perturbation"? Without the push by the human? Your statement only makes sense if hydrodynamic resistance is the only force acting.
 
  • #14
A.T. said:
What do you mean by "In the absence of any perturbation"?

I think he refers to things I have mentioned - that is, no current and no wind (impossible in practice).
 
  • #15
Borek said:
I think he refers to things I have mentioned - that is, no current and no wind (impossible in practice).

Of course...
 
  • #16
NTW said:
Of course...
Then your statement was wrong. If the man still pushes, then hydrodynamic resistance is not the only force on the ship, and therefore it doesn't determine its acceleration in the way you claimed in post #7.
 
  • #17
NTW said:
1,65 mm
We can safely use this as upper limit for a central push (it is a bit more if we push at one end). The additional amount of water moved and its velocity will depend on many factors, including the geometry of the walls and ground around the ship.
 
  • #18
A.T. said:
Then your statement was wrong. If the man still pushes, then hydrodynamic resistance is not the only force on the ship, and therefore it doesn't determine its acceleration in the way you claimed in post #7.

There is a net force that produces the acceleration. The ship will only move if the push by the man is greater than the hydrodynamic resistance. But that hydrodynamic resistance is zero (because the ship's velocity is also zero) when the man starts pushing, As the speed builds up, the hydrodynamic resistance will grow, and if the man wishes to keep the acceleration constant, he will have to push harder and harder...

In the case of zero hydrodynamic resistance, contemplated in post #9, things are quite different, and even more impossible...
 
  • #19
thetexan said:
I was watching a program on the worlds largest cruise ship, the Oasis of the Seas. It displaces 243000 tons. They we're discussing the sophisticated maneuvering systems and I got to thinking.

You must be careful here. Cruise vessels are advertised on the basis of their Gross or Net Tonnage, which is not the same as displacement tonnage.

For example, the Oasis of the Seas has a Gross Tonnage of 225,282, but an actual displacement tonnage of about 100,000 metric tons. Gross tonnage is a measure of the internal volume of the vessel, and not necessarily its weight (or displacement).

http://en.wikipedia.org/wiki/MS_Allure_of_the_Seas
 
  • #20
mfb said:
We can safely use this as upper limit for a central push (it is a bit more if we push at one end). The additional amount of water moved and its velocity will depend on many factors, including the geometry of the walls and ground around the ship.
I think it can't be even an upper limit when there exists hydrodynamic resistance.

That distance of 1,65mm (one hour of pushing with a force of 50 lbs) was the result for a very unlikely case, namely, that the hydrodynamic resistance was zero... A weird condition stipulated by thetexan in post #8...
 
  • #21
How can you get faster motion if you increase resistance?
 
  • #22
NTW said:
e = ½ * a * t2 = 0,5 * 9,14 * 10-7 * 36002 = 0,00165 m = 1,65 mm

This is wrong. You multiplied with 36002 and not with (3600)^2. The real answer is 5.92 metres.
 
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  • #23
5.92 meters in one hour is substantial! I would not have guessed it would be that much. That's about 10 centimeters per minute or one centimeter every 6 seconds. That would be an observable movement.
 
  • #24
willem2 said:
This is wrong. You multiplied with 36002 and not with (3600)^2. The real answer is 5.92 metres.

Right. It was a mistake with my calculator...
 
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  • #25
5.92 meters is also more than the typical length of a human. Okay, so a human can move the ship - in the absence of wind and water currents.
 
  • #26
mfb said:
5.92 meters is also more than the typical length of a human. Okay, so a human can move the ship - in the absence of wind and water currents.

And of hydrodynamic resistance, nothing less. Just like if the ship glided on a frictionless surface...
 
  • #27
There is no threshold force that needs to be overcome to move a ship in the absence of wind or current. In fact it is remarkably easy for an unassisted person to move a large ship. This can be explained in terms of kinetic energy (E) and momentum.
Consider a ship with a mass of 20,000 tons. If the ship is given a velocity of 0.5 inch per second then its energy, E = 1/2 mv2 is about 1,000 joules. A thousand joules is a very modest amount of energy. It is the energy expended by a 120-pound man climbing up a 6-foot-high flight of stairs.
At 0.5 inch per second the ship’s momentum, mass x velocity = 2 x 105 Newton-seconds.
The 120-pound man can impart this to the ship by applying his full weight for 400 seconds. If he moves the ship by standing with full weight on one of the mooring lines, he will have descended by 6 feet by the time the ship is moving at 0.5 inches per second.
Actually when a ship is set in motion, a comparable mass of water is also set in motion at a comparable speed.
Consequently the kinetic energy and momentum calculated above have been underestimated by a factor of two or so. However, the main conclusion stands: an unaided person can easily move a ship.
The ship will move. Fluid forces don’t have a limiting static friction. We can think of these frictional fluid forces as being directly proportional to the speed of the ship. They are close to zero when the speed is close to zero, and so on.
 
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  • #28
Great to see some numbers in a question like this.
 
  • #29
In order for the ship to move forward water molecules at the leading edge of the ship would have to be separated, is there enough of a bond to prevent motion starting below an initial starting energy level?
 
  • #30
Uridium said:
In order for the ship to move forward water molecules at the leading edge of the ship would have to be separated, is there enough of a bond to prevent motion starting below an initial starting energy level?
You are simply re-wording the property of viscosity of water. Yes, it is taken into account.

Note, BTW, that moving the ship bow-first would be the easiest way to do it, since it presents the smallest area and that area is streamlined, thus providing the least resistance (after all, it is the very core of ship design: to facilitate forward motion, and resist lateral motion).

It would be much harder to move a ship broadside - you are, in your parlance, "separating" far more water molecules, and making them move a lot larger distance - and yet as explained above, it can certainly be done.
 
  • #31
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.
 
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  • #32
Noah Diamond said:
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.
 
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  • #33
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
 
  • #34
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.
 
  • #35
diogenesNY said:
a 'kedge' anchor and a length of chain
Done it myself with my 4 tonne sailing cruiser (even smaller scale though). :smile:
 
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<h2>1. Could a person actually push a cruise ship by hand?</h2><p>Technically, yes. However, it would be extremely difficult and unlikely for a single person to push a cruise ship by hand. The average cruise ship weighs around 100,000 tons and requires powerful engines to move through the water.</p><h2>2. How much force would it take to push a cruise ship by hand?</h2><p>The amount of force required to push a cruise ship by hand would depend on the size and weight of the ship, as well as the resistance of the water. It would likely require an immense amount of force, far beyond what a single person could generate.</p><h2>3. Has anyone ever successfully pushed a cruise ship by hand?</h2><p>There are no recorded instances of a single person successfully pushing a cruise ship by hand. However, there have been cases where groups of people have pushed smaller boats or vessels for short distances.</p><h2>4. Would it be easier to push a cruise ship in still water or moving water?</h2><p>It would be easier to push a cruise ship in still water, as there would be less resistance. The movement of water around the ship creates a force called drag, which would make it more difficult to push in moving water.</p><h2>5. What other factors would affect a person's ability to push a cruise ship by hand?</h2><p>Aside from the size and weight of the ship, other factors that would affect a person's ability to push a cruise ship by hand include the surface of the ship's hull (smooth or rough), the person's physical strength and endurance, and the weather conditions (wind, waves, etc.).</p>

1. Could a person actually push a cruise ship by hand?

Technically, yes. However, it would be extremely difficult and unlikely for a single person to push a cruise ship by hand. The average cruise ship weighs around 100,000 tons and requires powerful engines to move through the water.

2. How much force would it take to push a cruise ship by hand?

The amount of force required to push a cruise ship by hand would depend on the size and weight of the ship, as well as the resistance of the water. It would likely require an immense amount of force, far beyond what a single person could generate.

3. Has anyone ever successfully pushed a cruise ship by hand?

There are no recorded instances of a single person successfully pushing a cruise ship by hand. However, there have been cases where groups of people have pushed smaller boats or vessels for short distances.

4. Would it be easier to push a cruise ship in still water or moving water?

It would be easier to push a cruise ship in still water, as there would be less resistance. The movement of water around the ship creates a force called drag, which would make it more difficult to push in moving water.

5. What other factors would affect a person's ability to push a cruise ship by hand?

Aside from the size and weight of the ship, other factors that would affect a person's ability to push a cruise ship by hand include the surface of the ship's hull (smooth or rough), the person's physical strength and endurance, and the weather conditions (wind, waves, etc.).

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