On the return trip the factory ship’s speed is much less than when she started out--and not just because her tanks are full. In June her hull was smooth and freshly painted, and it slipped easily through the water. Now in February she has barnacles all over the hull under water--such a rough coat of barnacles that she’s held back a great deal. Barnacles are tiny sea creatures that grow by the millions. They attach themselves to anything under water and form hard little shells. They hold so tightly to the ship that they must be chipped off. That’s a job to be done in a place called drydock. DRYDOCK All ships go to drydock for regular cleaning and repairing and painting. This is what happens: The ship noses into a place surrounded by three concrete walls. Huge water-tight gates swing shut behind her, penning her in. Mooring lines hold her steady in the exact center of the dock, and pumps go to work taking out all the water in which she floats. Slowly the ship settles into a sort of cradle that has been prepared on the floor of the dock to fit her hull just right. When the water is all out, there she stands, balanced and braced. Now men can work under her and all over her--and inside. They scrape off the barnacles, paint the hull, and repair any parts that have begun to wear out. To reach some parts of the hull painters use long-handled brushes--really long. They’re often three times as tall as a man! [Illustration] Experts go over the ship as carefully as doctors examine people. But many men work at top speed in shifts around the clock, and a ship often spends only twenty-four hours in drydock. Then the gates open. Water flows back into the dock. The ship floats again, ready to go to sea. Sometimes a ship can’t get to drydock. Then a floating drydock comes to the ship. It works the same way as a regular one. Floating drydocks have traveled to distant parts of the world, pulled by seagoing tugs. TUGS A tug is a vessel that looks small but has an enormously powerful engine--an engine almost as big as one that moves a cargo ship. In fact, the tugboat’s job is to push and pull cargo and passenger ships around. Big ships need help getting in and out of the narrow spaces between piers in a harbor. If they used only their own power, they might either smash themselves up or crush the piers. Tugs, working together, can push a little here, pull a little there, and ease a huge vessel gently into place. A tugboat captain must have a great deal of knowledge about the harbor in which he works. In order to pass his captain’s examination, he has to draw a map of the harbor from memory, showing every pier and marker and even the rocks, hills and valleys underwater. Most important, he must have a feel for what a ship is going to do when he nudges her at a certain point or when he reverses his propeller and pulls.