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You Can Move a Ship

A Guide to the unorthodox practice of Ship-Shifting

This may sound like it requires superheroic strength, moving a ship by hand, but it just requires a knowledge of physics, and some perseverance. Moving a ship is like pushing a car, except it's easier! Incredible as it may initially seem, it is actually possible for a person to move a ship which weighs many hundreds of tons. How can that be, when the ship weighs so much? I'll explain...

Moving a heavy object requires overcoming two types of friction. These are known as "static friction" and "rolling resistance". If you're trying to shove a large chunk of concrete along a floor, there's a minimum force below which it will not move at all. It's as if it's glued to the floor. That is static friction. Once it's moving it requires work to be done to keep it moving, as there's some rolling resistance, with the block grinding against the floor. With pushing a car, it's easier because the static friction is smaller, but still significant. And then, when it's rolling along, there's just the rolling resistance to keep ahead of, to keep it moving. Wheels roll much easier than concrete blocks on a concrete floor, obviously.

With a ship, floating in the calm water of a dock, the static friction is ZERO. It is initially frictionless! Even when you get the ship moving, at very slow speeds the rolling resistance is negligible. You're moving a heavy object in an almost frictionless environment.

This is an excellent demonstration of practical physics. It's as near to absolute theoretical conditions as you can get without having to go into space. Go to a dockyard near you, and shift a ship! I recommend starting off with something about a hundred tonnes, and then progressing to a thousand tonnes when you've had some experience and you are confident enough.

You don't need to be strong to do this, or even in good health. It's more a matter of the strength of your belief in physics rather than anything to do with physical strength. This sounds almost shamanistic, as if it's magic, but really it's much more Newtonian.

Newton's Three Laws of Motion:

1. An object travels in a straight line unless acted on by a force.

2. F=MA ... Force = mass x acceleration

3. Every action has an equal and opposite reaction.

It is the second of Newton's laws of motion that's most significant in ship-shifting. As the mass of the ship is large, and the force you have with which to shove it is relatively small, the acceleration is going to be very small. However, acceleration is like interest on a bank savings account, and it soon starts to stack up. The speed of the ship is more related to time, and your observation of the distance which you've moved the ship is related to t2 time squared. This is why in ship-shifting, perseverance is more important than strength.

A practice: Before you go off to a quayside and start asking if you may move some maritime hardware about, let's go for a practice test. What's it like to move a ship? Go along to a brick wall, and put both hands up against it, and give it a sustained gentle shove, for about a minute. Don't put any "work" into it, or you'll give up too soon. Just apply a sustained force of about a tenth of your body weight. You could do this by just leaning against the wall.

Now although the wall won't move, if it had been a ship, it would have done, as it's in a near frictionless environment. If it had been 100 tonnes, and you applied a force of equivalent to 10 kilogrammes, for a minute, the ship would have moved 1.8 millimetres. Admittedly not far, but you would notice it. Make that two minutes, and the ship would have moved 7.2 millimetres.

I have personally moved a relatively small ship, weighing about 50 tonnes, several inches. I was quite young at the time. I have heard reliable stories of people who have personally shifted thousand tonne ships. Speeds of up to a slow walking pace have been achieved.

At first, when you start to shove a ship, nothing happens, and you feel as if it's entirely futile. However, after a while, and the ship starts to move, it feels like a miracle, as if you've done something which is regarded as "impossible".

Now some helpful advice about ship-shifting:

* Don't hurt yourself. There is no point in putting in lots of "effort". Much better to apply a relentless gentle force.

* Ask permission before you shove someone's ship.

* Don't try to shift a ship against the weather. The weather is stronger. Ideal calm conditions are required.

* Don't worry if you see no movement at all in the first few seconds. It's not because it's an "immovable object". It just takes a time to start to move.

* If you succeed in getting a ship moving along at a steady pace, bear in mind that the momentum is not going to stop. You'll need to slow the ship down or to use a proper crumple-zone such as old tyres and drums. I take no responsibility for jetties and quayside equipment becoming all scrunched-up because someone has been so silly as to do unreasonable things with momentum.

If you're wondering WHY do such a thing as moving a ship, well it is in order to get a feeling for real momentum in a frictionless environment. It's very much a hands-on approach to physics visualisation. This has spacegoing connotations.


The picture is obviously only illustrative. It's only fair to mention, though, that it is a derivative work produced by careful adjustment of an image acquired from Easy Cruise, where the images of their ships are made available provided Easy Cruise are credited. So, well done to Easy Cruise! Incidentally, the ship shown is a likeness of the Easycruise Life / Ocean Life, which at a displacement of 1445 metric tonnes, would require a considerable time to get moving by human effort. However, the stick-figure and the ship have been readjusted to make the apparent weight more realistic.

I suppose I'm going to get asked "why do ships have engines?", now that I've explained how you can move a ship by hand. Well the reason is that as the speed increases, there is some rolling resistance. The faster you want to go on water, the more rolling resistance there is. The amount of power required to keep moving increases drastically over 20 knots. So although the power required to move at a snailspace is negligible, it's quite usual to need thousands of horsepower (or alternatively, acres of sailcloth) to go along at a reasonable pace. Also, ships need power for manoeuvrability, to avoid things, icebergs for example.