how can a tiny tug boat push/pull huge tankers and cruise ships?


I get that plane tugs have huge tyres and tonnes of torque, but a boat? A tiny boat? 🤷‍♂️

In: Physics

The size of the boat doesn’t matter. The only things that matter are the engines and the propellor.

It makes no difference if you put the propellor and engine in the big boat, or a small boat that tugs it.

The engines and propellers of those tugboats are comparable in size to those of the huge tankers. The tugboats are smaller not because they have smaller engines but because they do not have any cargo holds or giant fuel tanks. A tugboat is literally just a giant engine and propeller in the smallest boat possible. Not unlike plane tugs.

The shape of the hull defs matters.. tub boats act like a lever against the water and they halos the giant props do work

Tugs are designed to partially submerge themselves to “grip” the water and allow them the leverage needed to push. Couple that with a beefed up engine with lots of horsepower that’s precision controlled with on board computers and there’s no ship too big for them to move around.

They have a lot of power stashed in that little package. Big cargo and cruise ships also have a lot of power, and might hit say 35 knots, while a tug boat has enough power to move them 8-10 knots. That’s perfectly okay, because we aren’t looking for speed when bringing a ship to port, we need agility and the ability to react a little less slowly, which tugs offer.

Most ships are trying to do multiple things. They want to move people/cargo, they want to go a long distance, and they want to move quickly. This means most of the ship is allotted to cargo/passenger space, and lots of food/fuel/water for the long voyage, and a pretty powerful engine so they can do 20-30 knots in relatively rough seas.

A tugboat has a single job


It doesn’t need to go far, it doesn’t need to carry anything, it doesn’t even need to go fast, it just needs a massive engine so tugboats have a far higher percentage of internal volume dedicated to engine space.

Moving a ship in water isn’t terribly difficult, but the faster you want to go the more difficult it gets, if you just want to go fast you’ll need tens of thousands of horsepower but if you just want to go at a few knots then a few thousand horsepower will do the trick. A big propeller to efficiently get that power into the water at low speed and a tug with 3000 HP is plenty to guide a big cargo ship at just a few knots.

For a sense of power density here, a modern tugboat will have 3000-6000 HP and weigh in around 100-300 tons while a WW1 era *battleship* generally had 30,000 horsepower while weighing in around 20,000 tons, but the tugboat never really has to leave its harbor so it can be 100% focused on tuggin

Water has very little friction, so all they have to overcome is the inertia of the boat, not the friction between the boat and what it’s sitting on.