How Ship Draft Affects a Ship’s Performance

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If you have ever wondered how a cruise ship can stay afloat and navigate the water with such impressive maneuverability, you should take a minute to understand how a cruise ship’s draft impacts its overall performance. Fortunately, this guide explains everything you should know about cruise ship draft!

Not only will we cover how cruise ship draft can affect the ship’s stability, maneuverability, and speed, we will dive into some other interesting elements.

To help you gain a more in-depth understanding of ship draft, we will compare how the draft of a typical cruise ship differs from that of other types of vessels.

What Is Ship Draft?

In simple terms, it is a measure of the vertical distance between the waterline sits and the deepest part of the ship’s hull. So, you can think of ship draft as the amount of the vessel immersed in the water.

As you may have guessed, the ship’s draft will depend on the type of vessel it is, the ship’s size, operational requirements, and even how much cargo space it needs. While certain types of ships need a larger draft than others, the draft’s measurement and size can seriously impact the ship’s performance.

How Ship Draft Impacts the Performance of the Vessel

The following are just some of the ways that ship draft can affect a vessel’s performance in the water.

The Ship’s Stability

The measure of the ship’s draft will play a critical role in determining how stable it will be at sea. The deeper the ship’s draft is, the more stable it will be because a deep draft lowers the ship’s center of gravity, meaning it is more difficult to capsize.

Vessel Level on Water
Photo Credit: Mascha Tace / Shutterstock

Not only can a deep draft help prevent the vessel from completely tipping over, but it can also reduce movement when the ship is navigating through rough waters and high winds. For cruise ships, a deep draft is essential for the safety and comfort of the passengers on board the vessel.

The Ship’s Top Speed and Fuel Efficiency

One of the drawbacks of a deep draft is that it generates drag when the vessel moves through the water. With that said, if the lower portion of the ship is designed to maximize hydrodynamic performance, it can reduce water resistance enough to allow the ship to move with speed and efficiency.

Basically, there has to be a balance between the draft depth and how hydrodynamic the draft is. Largescale cruise ships can take a while to get up to full speed, but once they reach it, they can navigate the water without burning excess fuel.

The Ship’s Maneuverability and Ability to Access Shallow Waters

Draft also significantly impacts the ship’s maneuverability and turning radius. While a deep draft allows the vessel to turn without leaning too much, which is more comfortable for the passengers and crew, it also means the vessel needs more space and time to turn.

Cruise Ships in the Dominican Republic
Cruise Ships in the Dominican Republic (Photo Credit: Ihor Koptilin / Shutterstock)

In contrast to the deep draft of a full-size cruise ship, a small passenger boat would have a shallow draft so that it would be far more maneuverable in the water.

As you would expect, draft depth also decides how capable a ship is of navigating shallow waters. Since cruise ships tend to have a deep draft, they can only access certain coastlines and ports. This is why cruise lines take such careful precautions when plotting a course and deciding which ports the ship will visit.

How Much Weight the Ship Can Carry

Finally, draft depth also has a significant influence on the payload a ship is capable of safely carrying. In other words, the deeper the depth, the more weight the vessel can carry, as the ship will be more buoyant than a ship of equal length but with a shallower draft.

Carnival Cruise Line Deck Party
Carnival Cruise Line Deck Party

For cruise ships, which are top-heavy and loaded with thousands of passengers, cabins, amenities, incredible propulsion systems, and so much more, a deep draft is necessary to keep the vessel stable and at no risk of overloading.

What Are the Components of a Cruise Ship Draft?

The area of a cruise ship that could be considered the ship’s draft is composed of several areas. The following are the most prominent:

The Ship’s Bilge

The bilge of a ship is the curved sides of a ship’s hull that provide structural integrity to the entire ship. Since it is where the ship’s lowest part meets up with its sides, it needs to be incredibly strong. It also helps with drainage of the ship’s wastewater and soiled fluid from the ship’s engine.

The Ship’s Ballast

Ballast is the weight of cruise ships that helps keep the vessel balanced and stable. It sits at the bottom of the ship and can be increased or decreased depending on the ship’s needs.

It can also be moved around to counteract forces acting against the ship. Typically, the ballast is filled with seawater, which is easy to take in and drain during the voyage.

The Ship’s Keel

The keel can be viewed as the ship’s spine. It is the lowest point of the hull and stretches along the entire underside of the vessel. Often, it is the first piece put in place when a ship is being built. Essentially, the draft of a ship is a measure of where the lowest point of the keel sits to where the waterline hits on the side of the ship’s hull.

Seven Seas Grandeur Keel Laying
Photo Courtesy: Fincantieri

How Is the Draft of a Ship Measured?

When measuring a ship’s draft, there are several ways to do it. Each method offers varying degrees of accuracy. Since cruise ships are so massive in scale and have to carry such a tremendous number of passengers, they are typically equipped with the necessary equipment to take draft measurements in more than one way.

For cruise ships, these methods can include the following:

Measuring Load Lines or Plimsoll Marks

Load lines, also referred to as Plimsoll marks, are markings made along the sides of a ship’s hull. These highly visible lines act as a sort of ruler, so observers can easily see how deep the waterline is sitting. Since these lines are carefully plotted from the deepest point of the keel, they provide a quick, accurate, and convenient way to measure the current draft.

Draft Line Measurement
Draft Line Measurement (Photo Credit: Denys Yelmanov / Shutterstock)

While they offer an accurate way to measure a ship’s draft, they can be challenging to read if the vessel is not in port. Not only is it difficult to view them while onboard the ship, but rough waters can also make it difficult to see where the water line actually hits.

Electronic Draft Sensors

Most modern cruise ships have electronic sensors and cameras that take up-to-the-second measurements while the ship navigates the water to provide more accurate and current measurements.

The sensors measure pressure against the hull, then average that pressure to find where the water and air meet. This is incredibly beneficial, as it considers water conditions and provides instantaneous readings that the ship’s bridge can use in its navigational efforts.

Draft Survey Cameras

Some cruise ships also have downward-facing cameras that can measure the vertical distance between the waterline and various points of the ship’s hull where they are pointed.

Draft of a Ship
Docked Cruise Ship (Photo Credit: bepsy / Shutterstock)

By taking measurements at multiple points across the hull, they can provide an accurate average so the ship’s bridge crew knows where the draft sits at all times.

How Does a Cruise Ship’s Draft Differ from the Draft of Other Types of Vessels?

Now that you understand how it can impact the performance of a vessel and the comfort of those on board the vessel, it is easy to understand why cruise ships need a unique draft depth and shape.

Passenger comfort is critical on a cruise ship, unlike other vessels, like shipping freighters. Cruise ships have such a deep draft. This deeper draft helps limit rocking and pitching while the vessel is in rough waters, which means passengers enjoy a more comfortable voyage and reduce the risk of seasickness.

Ship Draft
Ship Draft (Photo Credit: donvictorio / Shutterstock)

Cruise ships also need numerous decks, so they are relatively tall vessels. They need separate cabin quarters for every passenger on board, which takes up a significant amount of space compared to the shared basic quarters that crewmembers are given on cargo and naval ships.

Cruise ships must also accommodate spacious and heavy amenities, like bars, restaurants, swimming pools, and countless other entertainment venues. All of this added weight and height requires a deep draft.

In other words, without a deep draft, the cruise ship would be too heavy and at risk of toppling over in rough waters.

Frequently Asked Questions

Can the draft of a ship change during a voyage?

Yes, a ship’s draft can vary during a single voyage. Given that the ship’s weight will impact how much it will sit below the waterline, a ship’s draft can lower as it unloads passengers and cargo.

Water conditions can also temporarily affect the draft, as the vessel will bob up and down in the water if it is navigating through rough seas. The fact that ship draft can change is why taking accurate and current measurements throughout a voyage is so important.

Can cruise ship passengers see draft markings?

It is possible to see the markings on the side of a cruise ship, but only when viewing the ship from dry land or another vessel. Typically, they look like small markings that sit just a little bit above the waterline. They tend to be at the bow or stern of the ship.

What is the average draft of a cruise ship?

The average draft of a cruise ship is roughly 20 feet. As you would expect, larger cruise ships have a much deeper draft, while smaller ships can afford a shallower draft.

Royal Caribbean’s Oasis-class cruise ships have some of the deepest drafts of any. The draft of these enormous vessels will measure nearly 31 feet in most conditions.

Final Words

While ship draft is one of the less visible parts of a cruise ship, it is integral to its overall safety and performance. Not only does it impact the ship’s stability and buoyancy, but it also plays a significant role in determining its maneuverability and fuel efficiency.

Read Also: How the Ship’s Rudder Works – The Basics

A ship draft also determines where a ship can visit, as a deep draft prevents a ship from visiting shallow coastal waters and low-depth ports. This explains why it is so essential that the ship’s crew can access accurate and instant measurements of the ship’s draft.

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