How does a tug boat work?

How does a tug boat work?
I see this little tiny boat pulling 700 ton ship.
It totally amazes me.
Daniel … Toronto

Tug boats are powerful for their size. Also, they’re not trying to lift the ship; just push it. Consider a car can way a ton or two, and yet a person can push it.

Funny, I was going to ask this exact same question a few weeks ago after watching the movie Titanic and seeing the ship being towed out to sea by these relatively tiny pre-1912 tugboats. I did some research on it and got some answers but I still don’t really understand it fully. Lots of modern tugboats use one or more diesel locomotive engines so the raw horsepower and torque are very powerful but I can’t quite wrap my mind around how that gets fully translated into that much towing power on the water as opposed to land. They also work in teams for the very large ships like supertankers and giant passenger ships.

That is what I was able to find but it still isn’t intuitive for me as well.

That is a good point. The water they work in is usually going to be level and probably not all that choppy out of the harbor. However, I have seen tugboats pull some huge ships upstream in the lower Mississippi River and the current there is pretty strong and they have to be able to fight against that.

As noted you start with a big engine.

If the tug is using a traditional prop instead of something more modern/exotic (see wikipedia link), the prop is probably pitched to deliver maximum thrust at a relatively low forward speed, like single-digit knots.

Tugs are basically a giant engine with a massive propellor with a small hull and cabin for the crew built around it. They are designed to do what they do.

Imagine 90 percent of your car/truck/semi’s volume was engine and it had a really low gear setting. It could push or pull some massive stuff around as well, albeit slowly.

I think 700 tonnes is a VERY VERY VERY conservative estimate of what a tugboat can push. I would expert it to be around if not more than 5 to 10,000 tonnes, no? Off to google I go.

I think 700 tonnes is a VERY VERY VERY conservative estimate of what a tugboat can push. I would expect it to be around if not more than 5 to 10,000 tonnes, no? Off to google I go.

I am probably pretty far off. Can anyone give a proper value to this?

Look, a person can move a pretty good sized boat along just by pulling on a rope. I’ve moved sailboats and cabin cruisers into dock very easily. A tug is pushing a much bigger ship, but it also has considerably more power than I can provide.

And if the ship is especially big, they use more than one tug.

Anything truly worthy of being called a ship is probably at least a couple thousand tons. A large ship (e.g. an aircraft carrier) would approach or exceed 100,000 tons.

This. I’ve poled a 20-ton river barge (this one) and once it’s moving you can keep it moving. Mass resists acceleration, not movement - it’s friction that opposes movement, and there’s precious little of that in water. True, you need your push to be predominant force over wind or current, but if you’re patient you can soon give a big vessel more of a velocity vector than is offset by current.

For the biggest oil tankers, half a million tons isn’t out of the question.

Why are the engine sizes measured in kW?

Why *not *use the SI (kilo-) unit of power?

Tugboat = big engine, little ship. Slow but effective.

Basically the use of a tugboat is a matter of attaching another engine to the ship to move it along in the water. As noted upthread, moving along in the water doesn’t involve that much resistance.

Also think of those contraptions that push passenger jetliners away from the gate. They are heavy with big, wide tires for traction. They are nowhere near the size of the plane or as powerful. They are little more than a power assist and can’t do much else. They could never get the plane off the ground but they sure can move it around (slowly). They are the land version of a tugboat.

Tugboats are mostly used for barges. The barges have no engines. Attach the tugboat and you have effectively turned the barge into a ship. Why build barges with engines and all that expensive complicates stuff when you can just get tugboats to move them around.

And curiously the brand name for them is Tug. You can actually get them going pretty fast if you remove the governor, although they don’t handle worth a damn.

There are different types of tugboats for different purposes. I don’t have my Primer of Towing here at home, but basically there are harbor tugs (that push ships around a harbor and up protected channels), river tugs, and ocean-going tugs. Harbor tugs are used for maneuvering in narrow channels and harbors where a large freighter can’t maneuver well enough or go fast enough to maintain steerage-weigh. Often the ship’s engines themselves will be running at some fraction of power and the tugs are used more as thrusters to guide it through the channel. River tugs are basically big engines that tow the barges upriver (going downstream is easier for obvious reasons); since rivers don’t have much in the way of wave action, they don’t have to be particularly seaworthy, just maneuverable enough to deal with river currents and bends. Ocean-going tugs are primarily meant to pull (or sometimes push) barges, and so are considerably more sea-worthy, although they tend to pitch and roll quite a bit more than a full-sized ship.

As previously stated, these boats are basically all engine, and in their towing capacity not especially fast, which means that their hull characteristics don’t have to be refined for speed the way a yacht or liner would be. Since barges aren’t going “uphill” the way a truck or train is on land (although for a river or harbor tug, they do need to be able to overcome current speed) the amount of power they need to maintain mobility is rather modest in comparison to their size or displacement; essentially enough to overcome prismatic drag of the hull at low speeds, and maintain speed against wave motion. The later is important, as it provides the limiting case as to what sea conditions are safe for a tug to pull an ocean-going vessel. (Generally the WMO Sea State criteria, along with the tug captain’s experience, are used to determine the suitability of conditions.)


I am merely curious. I have usually seen engine size described by horsepower.

In the US, engine power is described almost exclusively in terms of hp. In the rest of the world, it is in kW. It’s also a little confusing because the say net power output is specified and rated (SAE-certified vs. DIN 70020 and ECE R24) is different, so it isn’t just a conversion factor between American and European/Asian net power output ratings. This causes no end to problems in homologating engines from and to the US.


I’ve canoed on the lower parts of the Missouri River where it is a huge river with a very solid current. We always manuvered to the side when a tug with barges was coming. You don’t realize that these things will actually suck the water out from under you from about a mile away. It was amazing to watch the water level of the entire river drop by at least half a foot starting a mile ahead of the barge. These things have the power to move a hell of a lot of water

I have been on the bank of the Arkansas when barges came by, they must not have been as large as the ones you saw. The huge ones I’ve only seen from a bridge over the Mississippi.
Being in the water with them probably gives one a very different viewpoint, too. :slight_smile:

Stranger - Great info! I love it. Good job.

Now wait for a thread about tugs with some questions of my own instead of hijacking this one. Stay tuned.