How does a tugboat work?

Question

how are they built that they can pull so much weight?

Answer 1

They have large engines and low gearing - they are like the 4X4 of the sea.  A trawler has an equally difficult job dragging large nets.

Answer 2

Tugs are relatively small boats, built around large Engines, and props that are very large diameter and low pitch.  That means that a lot of force is applied for each revolution of the prop.  The Tug sacrifices speed for power.  Tugs have evolved in to specialty craft.   Some designed to move ships around in harbor.  Others to move barges up and down rivers. More and more are designed to put to sea, moving giant oil rigs.