What is the Draft of a Ship?

To those that are unfamiliar, nautical terms can be confusing. For those passionate about cruise ship vacations, understanding this terminology can make the entire experience more enjoyable. We will explain in-depth what the draft of a ship or draught of a ship actually means.

Not only will we define what it means, but we will also explain why a ship’s draft is so important when it comes to navigating the shallow waters that can be found in certain ports and surrounding various high-traffic coastlines.

What Is the Draft of a Ship?

Before exploring the term’s relevance for cruise ships, it is essential to establish a basic definition of what it means. Essentially, the draft of a vessel is a specific measurement that defines the distance between the waterline and the lowest point of the ship’s hull.

While it is often measured in both feet and meters, the measurement has to be incredibly precise, as any degree of inaccuracy could have dire consequences for the ship and its passengers.

Why Is the Draft of a Ship Such an Important Measurement?

The main reason a ship’s draft is so important is that it significantly impacts the ship’s overall stability, maneuverability, and ability to navigate shallow waters.

The Relevance of Draft for the Stability of a Ship

In simple terms, a ship with a deep draft, meaning a significant distance between the waterline and the bottom of the hull, will have a better chance of remaining upright in challenging sea conditions. This is because a larger draft could lower the ship’s center of gravity in the water.

That said, the ship’s draft is only one factor determining how stable a vessel will be at sea. Other factors can include how evenly the weight is distributed, the ship’s buoyancy rating, as well as the conditions of the water itself.

Importance of Draft for the Maneuverability of a Ship

The deeper the ship’s draft, the more difficult it is for the ship to change directions and even increase its speed. Vessels with a deeper draft have a much larger turning radius, which can restrict the ship’s ability to navigate narrow and confined waterways, such as certain ports, bays, and channels. 

This is why smaller cruise ships, like those chartered for river cruises, tend to have very shallow drafts.

The Impact of Draft on the Ship’s Ability to Traverse Shallow Waters

While this might be self-explanatory, it is an essential consideration for those charter routes for cruise ships and other vessels. 

Ships with deeper drafts are far more restricted when navigating shallow waters. As you would expect, they run the risk of running aground, meaning the hull could scrape the seabed or ocean floor.

Not only could this strand the ship and make it impossible to continue the voyage, but it could also cause significant damage to the hull and even the ship’s propellers.

On the other hand, ships with shallow drafts can access shallow waters. For the cruise industry, this means these lower draft ships can access shallow water ports and even navigate shallow bodies of water.

The importance of draft depth for navigation purposes explains why many shipping vessels and barges are designed with almost perfectly flat hulls.

Since these types of ships need to access ports and shipping yards, they are designed with extremely shallow drafts, but this also means they need to be very wide, which reduces their top speed and makes for a much more steady and comfortable ride.

How Deep Is the Draft of a Typical Cruise Ship?

The draft of a cruise ship will depend on the physical size of the vessel and the purpose it was designed for. 

Smaller cruise ships that were designed specifically for river cruises have relatively shallow drafts, while those that are designed to traverse the rough waters of the ocean and large seas, like the Mediterranean, are required to have much deeper drafts for stability purposes.

That said, most modern-day cruise ships designed to traverse open water have a draft depth measuring between 25 to 30 feet, or roughly 8 to 9 meters. 

For perspective, some of the largest cruise ships ever made, like Royal Caribbean’s Oasis-Class cruise ships, have a draft depth of nearly 32 feet! Cruise ships with deeper drafts were typically designed this way to handle rougher seas, longer voyages, and maintain faster cruising speeds.

While each ship will have its own unique design and draft depth, a general rule for most cruise ships is that the draft represents roughly 10% of the ship’s total height. Compared to other types of ships comparable in size to cruise ships, this draft-to-height ratio is considered fairly low. 

While having a deep draft is great for stability, which can make for a far more comfortable experience for passengers, it also means that they are restricted in the waters they can traverse. Shallow water coastlines and ports are simply off-limits, as the risk of getting stuck or damaging the ship’s hull is too great.

Can the Weight of a Ship Impact Its Draft?

You might be wondering if a heavier ship would have a deeper draft. As it turns out, draft depth can be impacted by the weight of the ship as well as the total weight of what it is carrying, including cargo, passengers, fuel, supplies, and in the case of cruise ships, amenities like swimming pools, restaurants, and so much more. 

Simply put, as a ship takes on more weight, more of the vessel becomes submerged, which increases the draft. This is why captains and other officers must be able to accurately calculate the weight of their ship and everything that has been brought on board. This is particularly relevant if a course has been plotted that would involve navigating shallow waters or even entering a port with a relatively low depth.

So, if you have ever wondered why certain cruise lines encourage you to limit the weight of your luggage, now you have your answer. While passenger bags may not significantly impact the ship’s weight, when it comes to the safety of the crew and passengers, the ship’s total weight is monitored and calculated with precision.

What Happens if a Cruise Ship’s Draft Is Too Deep for a Port?

If a cruise ship’s draft is too deep for a particular port, there are basically two options for the captain and navigation crew. They can either move on to another port with deeper water or lie offshore and call for ferries to take passengers and crew to shore.

Typically, cruises will be chartered so that the ship only visits ports it can enter, but some cruise ships will drop the anchor offshore, then tender passengers to shore for excursions. Tendering is the term used to describe this particular activity, but it simply means to ferry passengers to shore using any form of water shuttle.

What Is Kept in the Draft Section of a Cruise Ship?

The area of a cruise ship that remains beneath the water, which you might deem the draft section, is known as the hull or simply the bottom portion of the ship. This area can be used to store vital equipment for the ship’s ability to navigate the water safely and efficiently.

The Propulsion System and Engine Room

The most essential pieces of equipment that are kept in this portion of the ship are related to its propulsion system. This includes the ship’s primary engines and propeller systems.

A cruise ship’s propulsion system and engine room are vital, as this equipment and machinery allow the cruise ship to maintain its movement, speed, and direction. We’ve got a more dedicated article on what a cruise ship engine room looks like, which you can read here.

The Ballast System

You will also find the ship’s ballast system in the section of the ship that remains below the waterline. This system is used to adjust the weight distribution of the ship, which is critical for its overall stability. The ballast system uses massive tanks filled with seawater and emptied as required to keep the ship stable. 

This is particularly important if the ship encounters rough seas, as it helps limit the swaying felt in the upper decks. Ballast tanks also have an essential role in determining the depth of the ship’s draft.

When the ballast tanks are emptied, the ship’s weight decreases, reducing the depth of the ship’s draft. This can be important if the ship is forced to enter shallow waters.

Additional Equipment and Systems

Depending on the cruise ship, you can also find other mechanical systems in the ship’s hull, such as backup generators, safety pumps, and refrigeration units.

This massive area of the ship can also be used to store the ship’s fuel tanks, extra equipment, and various supplies that might be needed for a long voyage.

One of the most interesting features of a cruise ship’s hull is the fact that it will be compartmentalized for the overall safety of the vessel. If the hull is damaged and water leaks into the ship, areas can be sealed off.

This can prevent the ship from sinking, so the ship can be stabilized, and the crew and passengers can be evacuated if needed without using lifeboats.

How Is the Draft of a Ship Measured?

Although cruise ships are designed to have a specific draft, measurements can be tracked through draft markers located on the ship’s sides. These simple depth markers allow the crew and other observers to have live updates on how deep the ship’s draft is at any given moment. 

The next time you board or disembark a cruise ship, look at the front of the ship. You should see a draft measure line just above the waterline. These can be found on the ship’s port and starboard sides, so it should not matter which side you are on.

Modern-day cruise ships are also equipped with numerous depth sensors, which provide the ship’s navigation crew and engine room with regular updates on the ship’s draft, balance, stability, and more.

What Is the Keel of a Cruise Ship?

The keel of a ship is the central beam that runs along the very bottom of the ship. The keel runs from the bow to the stern and provides critical structural support to the entire ship.

During construction, the keel is the first part of the ship’s hull that is welded together, so it almost acts as a sort of scaffolding for the rest of the vessel. From the keel, other structural components of the ship will be attached, including the ship’s frame (ribs), and the outer plating that composes the finished ship’s outer barriers.

Since the keel runs through the deepest part of the ship, it is often seen as the reference point for measuring the draft of a ship. If the ship was to enter waters that were too shallow, it would be the keel of the ship that would most likely scrape the seafloor. 

What Is Meant by the Lowest Point of a Cruise Ship?

As the name implies, the lowest point of a ship is a phrase used to describe the part of the ship that sits the deepest in the water while the ship is floating. Typically, the ship’s keel will also be its lowest point.

Read Also: What’s on the Bottom of a Cruise Ship?

Knowing the lowest point of a ship’s hull and how low it actually sits is incredibly important for calculating a ship’s draft. It is also an essential piece of information for assessing how stable and maneuverable a ship will be.

Final Words

While it is often overlooked, a cruise ship’s draft (or draught of a ship) is crucial for its ability to navigate certain waters. In addition to determining where the ship can go and which ports it can visit, its draft dramatically affects its overall maneuverability and stability.

Draft also plays a role in deciding a ship’s cargo capacity, as ships with deep drafts have extra space to store equipment and supplies. Finally, a cruise ship’s draft is critical for safety. Without careful calculations to ensure that a vessel can navigate certain waters, there would be a constant risk to the safety of the crew and passengers.

Now that you know more about the draft of a ship, you can better understand why cruise ships look the way they do. Are you interested in seeing the bottom portion of a ship? Check out Cruise Ships Out of Water – How Does it Look?