Plimsoll line: what is the draft measurement for?

Being an important activity for at least half a century, shipping with cargo ships has spent all these years adapting and creating new ways to increasingly optimize the loading and transport of goods.

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That is why there are so many terms, processes and indicators that point out the best way to make this journey with the least losses for those who buy and the highest possible profit for those who sell.

The Plimsoll Line is one such example, essential for the safety of vessels and maximum use of storage on each chartered vessel.

Do you want to understand more about what this measure is? With the special participation of Franklin Galdez, operations manager at GMS Brazil, we will explain what it means for the sector. Follow up.

What is the Plimsoll Line?

From the point of view of exporters and importers, it is clear that the will in the routine of maritime chartering is always to carry the maximum amount of cargo that each vessel can hold in volume. It is a search that is part of the navigation perspectives, with a view to growth.

But that is not always possible. Even if that vessel supports a certain weight, there is a buoyancy limit to the hull that increases the risk of grounding when the draft is completely submerged.

To avoid any incident of this type, which can cause damage to cargo and even interrupt operations in ports, there is a measure adopted worldwide to standardize this safety margin. It is called the Plimsoll Line, as Franklin defines it:

“The Plimsoll Line is a marking made on the side amidships in order to determine the safe freeboard for loading merchant ships on their respective voyages. It bears the acronyms of the Classification Society issuing the “Freeboard Certification”.

How was it created?

The Plimsoll Line is named after its creator, Samuel Plimsoll. Member of the British Parliament and negotiator in the coal market at the end of the 19th century, he was active in the reforms of English maritime laws, especially the excessive loading of ships, which caused so many accidents at the time.

The parameterization idea is to inhibit this overload and to increase safety and optimize the use of the supported weight. The denomination “line” has to do with its marking on the hull of the ship: a circle cut by a line.

Franklin points out that this determination was instituted in 1876. “The Merchant Shipping Act made freeboard mandatory, guaranteeing loading limits to merchant ships.”

The adoption of physical markings on the hulls took place in 1894 and, during the 20th century, it was improved with more precise observations, with markings on the side of the circle according to each possible floating scenario:

• TF: tropical freshwater;
• F: fresh water;
• T: tropical salt water;
• S: salt water in summer;
• W: salt water in winter;
• WNA: Winter North Atlantic water.

How to measure a ship’s draft?

The determination of the Plimsoll Line has a direct link with one of the most important measures for maritime transport: the ship’s draft.

Objectively, the draft is the distance between the ship’s keel (its lowest point) and the surface line of the water. Therefore, it is the entire part of the vessel that is submerged. This measurement is made with visual markings on all sides of the hull, in meters or, for older ships, in feet and inches (imperial system).

“This work is normally carried out by surveyors in conjunction with the ship’s deck officers, where the verified drafts are recorded for the purpose of calculating the load on board the vessel or even to measure the amount of cargo that has been unloaded from a vessel”.

Franklin also tells us that, currently, there are sensors installed on the hulls capable of making this measurement and shown on a dial (gauge). “However, its accuracy changes considering several factors, with the direct reading method being the most recommended.”

What is a ship’s draft for?

The ship’s draft is a universal measurement for all international trade and especially important for determining cargo volumes and port logistics.

The main reason this conference exists, of course, is for the safety of the ships and their crew. So much so that this was Samuel Plimsoll’s motivation when creating the indicator.

When a ship is overloaded, its draft is either wrongly measured or simply ignored. Thus, there is a risk of sinking or even running aground, which occurs when it submerges to the point in which the keel hits the sea bed.

This type of incident not only puts people and material property at risk, but it can also mean a complete stoppage of a port or a canal, depending on the grounding conditions (you probably saw what happened to the Ever Given vessel in the Suez Canal in March 2021). So, this is also an economic issue.

Calculating and respecting the draft of each ship is a way to increase efficiency, especially in ports. Each terminal has its own depth, and the management of vessels that make the most of each of these values speeds up loading and unloading operations and always allows the maximum to be extracted from each one of them individually.

How to guarantee this and other compliances in maritime transport?

With the discussion of the previous topic, we can clearly see that, like many other details of the maritime transport operation, the respect for the draft and the introduction of the Plimsoll Line are critical issues in an increasingly relevant subject for the sector: port intelligence.

It is one of those points of attention that need to be part of the routine of those who import and export. Hence the importance of having the best possible shipping agency.

“Wilson Sons, in its wide scope of services, works with these draft figures, both for ships in ballast (without cargo) and for ships that will unload in certain ports”, explains the Operations Manager.

“We need to constantly be aware of the limitations of each port, anchorage and terminal in order to provide correct information to ships and customers.”

This is vital data for the efficiency of the operation, such as:

• correct areas to anchor;
• maximum draft limits per berth or navigation channel;
• maximum loading limit considering limitation in a certain port of destination;
• impeditive loading conditions that must be observed — which sometimes impose on the vessel the need to ballast a cargo hold temporarily, in order to allow the ship loader to reach the limits of the holds safely.

In this sense, the Plimsoll Line is further proof that attention to detail and proper planning make shipping a routine of optimizations that benefit both buyers and sellers.

Why, then, don’t you get to know more about the best Maritime Agency in Brazil? Visit Wilson Sons’ website now and get in touch!