The Secret Markings on Cargo Ships

Look closely at the sides and stern of the cargo ships docked at Redwood City as you sail to and from San Francisco Bay, and you’ll see cryptic markings in a language filled with history and drama. The painted symbols provide critical information to tugboat captains, harbormasters and San Francisco Bar Pilots. Deciphering these maritime hieroglyphs requires our own Rosetta Stone, and can be a lot of fun as you skipper your small sloop past a vessel the size of a football field.

The most famous or perhaps notorious mark on a cargo ship is the Plimsoll line, sometimes called a load line, which is a circle with a horizontal line through it. It shows the maximum depth of the ship when fully loaded with cargo in typical waters. If that line is submerged, the ship is carrying too much cargo to be safe. 

It’s named after Samuel Plimsoll, a member of the British parliament who, in the 1870s, raised alarms about what he called “coffin ships”— cargo vessels that were so overloaded that they predictably sank, taking the lives of sailors with them. Plimsoll chalked it up to greed on the part of the owners, since the ships but not the sailors were often heavily insured.

His Merchant Shipping Act of 1876 gave the British government the power to inspect these vessels and required a prominent loading line on the hull showing the safe amount of cargo they could hold.

The two letters on the mark (in this illustration, GL for Germanischer Lloyd) are the abbreviation for the authority that made the necessary calculations. The most common ones you’ll see around here are AB for American Bureau of Shipping and LR for Lloyds Register. There are Plimsoll lines on both the port and starboard sides so that observers can easily see if the load is balanced.

In this illustration, the cargo can typically be loaded until the bottom of the ship’s keel is eight meters (8m) below the surface. But a single load line is a bit simplistic since water density changes in different places and seasons. That’s where that ladder-like symbol to the side of the Plimsoll line comes into play, showing the different load levels for different types of water. TF on top is for tropical fresh water—think Amazon River. F is for other fresh water like the Great Lakes. WNA is for winter in the North Atlantic. A ship loaded in Stockholm in January (WNA) will sink considerably when it enters the less-dense waters of Toronto’s Lake Ontario (F). The numbers on the left show the maximum keel depth. You derive that from the number that’s right at the water level. For example, if the ship is fully loaded for winter in the North Atlantic, the keel would be about 7.7 meters below the surface.

You might also see some other marks on the bow. What looks like a distorted number 3 (or sometimes the bottom half of a number 5) is a warning to tugboat captains that hidden danger lurks below the surface: a “bulbous bow” that’s underwater when the ship is fully loaded. It’s designed to reduce drag and save fuel.

An X or a Maltese cross in a circle tells tugs that the ship has a bow thruster, which can shoot high-velocity water off to the side and helps maneuver the cargo ship in tight situations. 

On some cargo vessels, you’ll see what look like the letter T with an arrow pointing down. These indicate strong points where a tugboat can safely attach a line to a bitt that’s firmly welded to the hull. The tugboat can use these lines or cables to maneuver the much-larger cargo vessel.

Next to each bit is a notice of the safe working load of the bitt—in this case, 50 tons. Apply more pressure than this, and you run the risk of the bitt flying out of the boat, which can have deadly results on the tug.

Those are the most common marks you’ll see on the cargo vessels we sail by in this part of the country, but they’re only a small part of the secret language of the maritime world.