Not stronger. Just patient
How Does a Tiny Tugboat Move a Huge Ship?
A tug cannot outmuscle a container ship. It does not have to — on water, a small push held for long enough moves anything.
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Step 1 of 5
Floating removes almost all the effort of moving something.
The ship's weight is already held up by the water. Nothing has to be lifted or dragged along the ground, so only the water in front has to be pushed aside.
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Step 2 of 5
And water pushed aside slowly hardly resists at all.
Resistance grows steeply with speed. Move a ship at walking pace and the water objects mildly. Move it at sea speed and it objects enormously.
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Step 3 of 5
So the tug does not push hard. It pushes for a long time.
A few minutes of steady thrust builds the ship up to a slow drift. All the tug is doing is spending time where it cannot spend force.
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Step 4 of 5
The harder job is turning, and that is done with leverage.
A tug at the bow and another at the stern push opposite ways. Working at the two ends of a long hull turns a modest push into a large twist.
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Step 5 of 5
Stopping is the reason tugs are there at all.
A ship has no brakes and takes kilometres to stop on its own. In a harbour there are not kilometres, so tugs hold it back — and that is the dangerous part of the work.
A tug's propellers can usually swivel right round, so it can push sideways as easily as forwards. That is why it can hold station against a moving hull.
The short version
A tug moves a giant ship because floating removes the weight and slow water barely resists — so a small push, held for minutes and applied at the ends, is enough.
Try it yourself
Push a friend on a swing with one finger, once per swing. You are never strong, but the swing goes high because you keep spending time.