There is a lot of traffic on the ocean and in various harbors. The traffic includes cargo ships, cruise ships, and many other things. Getting all those things moving and where they should go isn't always as easy as it looks. This is especially true in places like harbors, where the big ships can't maneuver and can't use all their engine power to get where they need to be. For getting things moving and maneuvering the ships in the harbor a tug boat is necessary. But, how can a small tugboat move something like a huge cargo ship around? After all, the tugs are much smaller than the huge ships; even the biggest tugboat in the world is many times smaller. There are a number of things that make tugboats work the way that they do.
One reason that tugboats can do what they do is that they have very powerful engines. The engines are much bigger and more powerful than would be found in another boat of comparable size. Those powerful engines give the boats more power to pull or push things like those big ships.
Another thing that makes it possible for tugboats to do their job is the way that the hull is shaped so that water is funneled and directed toward the propellers. That action makes it easier for the boat to move through the water. It can also help the propellers to work more efficiently. Both of these things make it easier for the boat to move through the water, letting more of the engine's power go toward letting the tug move the ship instead of just having to move the tug.
Water resistance also helps tugs move ships. If you have ever pushed something in the water, you know that you have to put a lot of effort into it when you first start because the water does have some resistance. But, once you get the object moving, then there is much less resistance, and the laws of physics are used to help keep the ship moving.
Without ocean travel, shipping goods around the world would be difficult, as would troop movement. Without tugboats like a ranger tug boat, the large cargo ships, aircraft carriers, and cruise ships would have a hard time maneuvering in harbors or difficulties in getting moving. Barges that are used on rivers wouldn't be able to go against the current and would have a harder time getting where they need to go.