Ever tried weighing a ferry? Neither have we. It’s probably best left to the experts. 

Luckily, calculating the weight of a ship may be easier than it seems. In fact, according to our research, there are two main methods of doing this. 

The first involves weighing all the parts that go into building the vessel. This can be a lengthy, complicated process. While the ships’ architects can make these calculations, the method often sees inaccuracies. 

But, there is a better way...

Archimedes’ Principle

The Archimedes Principle states that the volume of water displaced by an immersed body is exactly equal to the weight of the immersed body. 

So, the volume of water displaced needs to be known to us. And, because the saltiness of the seawater will impact the ship’s buoyancy, the density of the water also needs to be measured.

Thanks to Archimedes and his relatively basic theories on floatation, we can actually predict the weight of ships using pinpoint accuracy.

The numbers on the side of a ship’s hull

Next time you see a ship you will notice numbers painted on the side of the hull. These displacement marks indicate the displacement caused by the ship under different load conditions. The marks allow us to calculate how much water is displaced which in turn gives us the weight.

Maximum draught is marked with a ring with a straight line in the middle. That mark, called a plimsoll line, is named after Samuel Plimsoll, an English politician who worked to make life safer for seafarers. The clear mark on the side prevents overloading of the ship.

How much do ferries weigh?

That depends on a number of factors but especially the overall size and carrying capacity. For example, the Stena Estrid is a passenger and vehicle 'Ro-Pax' ferry which sails for Stena Line on its Holyhead–Dublin route. She is the first ferry in the E-Flexer class, and was delivered to the company on 15 November 2019. She has a gross tonnage of 41,671. 

The Stena Hollandica, operating on the Harwich – Hoek van Holland route on the North Sea, has a gross tonnage of over 64,000t. They have respectively 3,100 and 5,566 lane metres freight capacity.

Using Archimedes Principle, the process of determining the weight of a ferry becomes much simpler.

What is the heaviest ship in the world?

The world’s heaviest boat is the Pioneering Spirit. 

Launched in 2013 and originally named Pieter Schelte, the Swiss owned Pioneering Spirit is a crane vessel whose task as the largest construction vessel in the world is to install and remove oil and gas platforms as well as undertaking pipeline installations. She has two barges that assist her named Iron Lady and Bumblebee. 

As the heaviest ferry ever made, the Pioneering Spirit has a gross tonnage of 403,342t. 

Meanwhile, as of December 2023, the largest cruise ship in the world, Icon of the Seas, has a gross tonnage of 248,336, is 365 metres (1,198 ft) long and holds up to 7,600 passengers.

That covers the methods used to weigh ferries. Now, it’s time to book your next crossing! As one of Europe’s biggest (and highest-rated) ferry ticket agents, we can help with that. Save money when you book your freight ferry tickets with us.