Displacement Tonnage, sometimes just called displacement, is just one way a ship is measured by weight. Naval architects who design all kinds of vessels have goals to build the ship as close to the designed weight as possible. This helps ensure that it performs as expected in all sorts of conditions, and can carry loads or maintain desired cruising speed. Creative, right? If we take our box out of the water and fill the inside with water up to the waterline we can measure how many gallons it takes. Then we can multiply that number of gallons by eight because we said our water weighed exactly eight pounds per gallon. Let’s say it took 100 gallons to fill our box to the Water Line. The total weight of that water is 800 pounds and if we weigh our box we will see that it weighs exactly the same, 800 pounds. Organizations who set rules and standards for ships use displacement tonnage as a way to classify different sizes of ships. Ports and harbors use displacement tonnage as one of the criteria when determining berthing charges. To understand the concepts related to displacement we will use a simplified example. The first thing we need to know is that water has weight and for the example, we’re going to say eight pounds per gallon because of its close to 3.5 kilos. In the real world, water varies a little bit if it is fresh or salt water and weighs less when it gets hot since it expands slightly. Our ship is going to be a simple box with an open top and flat bottom. Now we float the box in some water. Because it has the weight it will push some of the water out of the way as it floats. On the side, we mark a line where the water comes up the sides of the box. So displacement means; what is the weight of the water displaced by the hull of a ship up to the waterline. If the vessel is a cargo ship the waterline may change and be measured with Load Lines but displacement tonnage is always measured with a ship completely empty of cargo. The word tonnage is just another way to say weight-in-tons. In a simple hull design called a displacement hull, the waterline is easy to place and it can change according to load. Almost all big cargo ships have displacement hull designs so they can carry the maximum cargo. Another type of hull has multiple chines, or levels, that the vessel rides on at different speeds. These hulls lift the boat out of the water to reduce resistance and increase speed. Many small recreational boats have this design but it’s also found on warships like the Littoral Combat Ship. In the case of these hulls, the desired waterline must be carefully calculated to achieve the desired performance and angle of attack at any speed.