What is a Ship’s Air Draft and How to Best Calculate It

A ship’s air draft, sometimes called air draught, is an important measurement to consider when planning a voyage. It is one of the factors seafarers check if their vessel can safely navigate within harbors where suspended obstacles are present.

While the regular draft and under keel clearance (UKC) are of primary concern, air drafts can also be a significant limitation, especially in ports where overhead clearances are required.

What is an Air Draft?

A vessel’s air draft is the distance between the waterline and the highest point of the ship. Knowing its value helps determine whether a ship can safely pass under bridges, power lines, and other overhead obstacles.

While the “deep draft” measures the distance from the surface of the water to the keel of the vessel, the air draft is measured from the same point (water surface) to the topmost part and is expressed as a height.

If you’re familiar with “vertical clearances” in parking spaces or even tunnels during one of your road trips, knowing the air drafts serves a similar purpose.

They provide seafarers and other interested parties with information on whether the ship can safely pass under those obstructions without hitting them.

This may look common sense but ships actually hit those obstacles, especially when passing bridges due to miscalculations. You don’t want your vessel to be in the headlines, do you?

Here’s something interesting:

Air draft can be useful not only for bridges, power lines, and other fixed obstructions but also for airplanes. 

My friend, also a seafarer, was sailing on an offshore tugboat. They were towing a colossal oil platform going to Europe from Japan.

Upon passing Singapore Strait, Changi Airport had to postpone the airplanes’ landing schedules by 1 hour to let the tugboat pass with her tow having a 100-meter air draft!

How to Best Calculate the Air Draft

Finding the air draft of a vessel can be straightforward using a simple formula. You don’t need long pages of crazy calculations, haversines, and tabulations, like finding the gyro error.

There are only two methods, and I will share them with you here.

When the ship is at Even Keel

Calculating the air draft is the easiest when you’re ship is on an even keel, meaning the forward and aft drafts are the same.

Here’s the formula for that:

Air Draft = Height of the Vessel – Aft Draft

or

AD = HoV – Da

You should be able to find the height of the vessel in the Ship’s Particulars, which is readily available inside the wheelhouse. It’s there because the officer in charge of finding the air draft is the Second Mate.

Meanwhile, you can get your draft figures from the Chief Mate.

Example:

If your ship has a forward draft of 10.3 meters and an aft draft of 10.3 meters, calculate your air draft if the height of your ship is 42.5 meters.

Solution:

AD = HoV – Da

AD = 42.5 meters – 10.3 meters

AD = 32.2 meters

When the ship is NOT at Even Keel

This one is also easy because we will use the same formula. 

The only difference is that we will apply an Air Draft Correction or ADCorr because the ship’s radar mast, which is usually the highest part of the ship, is not located in the same place as the aft perpendiculars.

Let’s get down with a sample problem.

Example:

Your ship has a forward draft of 10.3 meters and an aft draft of 12.9 meters. Your ship’s LBP or Length Between Perpendiculars is 178 meters, and the distance between the Aft Perpendiculars to the relevant point is 20.3 meters. Calculate your AD if the height of your ship is 42.5 meters.

Let’s arrange the given number first.

Df = 10.3 meters (Draft forward)

Da = 12.9 meters (Draft Aft)

LBP = 178 meters (Length Between Perpendiculars)

Ap to Rp = 20.3 meters (Aft Perpendiculars to the Relevant Point)

HoV = 42.5 meters (Height of the Vessel)

Note:

In the “Aft Perpendiculars to the Relevant Point”, the relevant point we’re talking about here is the point in the hull where the highest part of the ship stands. Refer to the image below.

Solution 1:

AD = (HoV – Da) + ADCorr

ADCorr = (Trim x Ap to Rp)/ LBP

ADCorr = (2.6 meters x 20.3)/ 178 meters

ADCorr = (2.6 meters x 20.3)/ 178 meters

ADCorr = 0.3 meters

AD = (HoV – Da) + ADCorr

AD = (42.5 meters – 12.9 meters) + 0.3 meters

AD = 29.9 meters

In this solution, you’ll notice that the Correction is in addition operation. This is because the relevant point is higher than the aft draft due to the ship being trimmed by the stern.

Solution 2:

Alternatively, we can also use this one-liner formula using the example above if you prefer to make it neat.

AD = HoV – [ Da – ( Ap to Rp x Trim / LBP)]

AD = 42.5 meters – [ 12.9 meters – ( 20.3 meters x 2.6 meters / 178 meters)]

AD = 29.9 meters

Now that you have your air draft, you can use this information to check the clearance between the highest point of your ship and any overhead obstruction when passing bridges, pylons, cables, etc.

May the winds be in your favor.