Plumbing the Plimsoll Line

The Plimsoll Line

 



I saw this on the side of a rusty barge in the Elizabeth River. If you’ve been paddling around commercial rivers and harbors, you most likely have seen this on the side of ships and barges, but unless you are involved in shipping, you probably haven’t understood what it represents. This is called a Plimsoll Line or Mark, named after Samuel Plimsoll, who in 1874, 1875, and 1876 introduced three pieces of legislation before the British Parliament that became known as the Merchant Shipping Act. The goal was to standardize the amount of freeboard a vessel should have under different sea conditions. (Freeboard is the height of the side of the ship from the waterline to the vessel’s deck.) Plimsoll’s name stuck because his standards became British law, which in turn became the international standard in 1930 with the International Load Line Convention.

The Plimsoll Line was necessary because insurance companies, ship owners, captains and crews have interests that are naturally diametrically opposed to the interests of shippers. The owners, captains, and insurance companies would like to see the vessel arrive intact at its destination. The crews seem to favor not drowning at sea. Shippers, on the other hand, want to load the vessel as heavily as possible. It’s all about money. The more they can get aboard, the more money they make. If loading it too heavily causes the ship to sink, well, the load is insured, so they get paid anyhow, so regardless of what the shipping companies say about their devotion to safety, I know from personal experience that some shippers will often use threats of contract cancellation, tantrums, intimidation or anything else to force a captain into taking on more load. The Plimsoll Line gives the captain final authority. When the relevant line is reached, and the shipper doesn’t concur that loading is finished, the captain can call the owners, insurers, or the Coast Guard, any and all of which will back his decision to stop loading.

Plimsoll wasn’t the first to realize that there was a problem. Overloading destroys a vessel’s stability, and this problem has existed since the first raft was pushed into the water. The result can be capsizing (rolling over) or being overcome by seas because of inadequate buoyancy. This results in the vessel flooding and sinking, or foundering. The first known regulations date back to 2,500 BC in Crete, and the Romans and other civilizations followed suit.

Here’s how you read the Plimsoll Line. On the left side of the symbol, there is a circle with a bisecting horizontal line. In the summer, a vessel must have freeboard that extends from the line to the level of the vessel’s deck. The two letters represent the authors of the enforceable standards under which the vessel operates. Here, the letters AB represent the American Bureau of Shipping. LR would be Lloyd’s Registry (British), IR for Indian Registry, and so on.

Since fresh and salt waters have different densities, a vessel with a given load will settle deeper in freshwater. The two lines to the left of the vertical staff are for freshwater, and the lines to the right are for saltwater. TF is for tropical freshwater, and F for fresh. On the right, T is tropical saltwater, S for summer saltwater, W for winter saltwater, and WNA is for winter North Atlantic. Besides water density, the lines take into account the likelihood of encountering heavy seas at different times of the year, or in different regions. The end result, hopefully, is that the ship, captain, crew, AND LOAD, will get safely from point A to point B.