What Is Vessel Draft?

In the simplest terms, the draft of a ship or boat is the distance between the surface of the water and the lowest point of the vessel. The measurement should be made as close to vertical as possible.

The lowest point on a boat is found at different places in different designs and hull shapes. Sailing vessels have some of the deepest keels and large boats may need water as deep as some small ships. For example, tugboats have much of their mass below the waterline. This helps them push and tow ships without making the ride uncomfortable for the crew.

Shallow draft vessels are built for getting to favorite beaches and fishing areas. Many militaries have developed hovercraft or shallow draft ships to protect coastlines and deliver troops and supplies directly to shore.

Taking on heavy loads of passengers, equipment, or fuel can change your draft significantly. Observing the normal waterline can give you some idea of your change in draft. If you want to know how much load you can carry safely, look for a certification placard for passengers. If you want to carry supplies or equipment then you can use the concept of displacement to make a close estimate of a loaded draft.

When the majority of people are wondering about the draft of their boat it’s usually because they want to know if an area is too shallow. Avoiding contact between underwater objects is important so every person driving the boat needs to be aware of the draft of the vessel.

Depth markings usually don’t appear on navigation buoys, but sometimes special areas like anchorages are marked so scope ratio can be estimated before anchoring. Depth markings are more common along wharves and seawalls especially if there is a fuel dock or crane facility. A variety of vessels visit these kinds of facilities so knowing the depth is important.

In general, it is a good idea to carry and understand a chart or survey map even on an inland body of water. Don’t ever assume that a shallow area or sub-surface obstruction will be marked as a hazard.

In tidal areas and on rivers knowing the depth is more difficult since the tides can be tens of feet in some places. Observing the high and low water marks on nearby structures is the best way to know depths unless you have a chart.

Depth sounders are good secondary devices but they are limited since they need to be set up near the bow to allow any warning. Often these small sonar devices are mounted just in front of the rudder or at the stern. The idea is to protect the rudder which usually is lower than the keel.

A forward mounted depth sounder can be added to allow some warning when the boat is moving into shallow water.

It’s important to understand the limitations of the different vessels operating in your traffic area since all ships and large boats take a long time to stop. Understanding maritime traffic lanes and observing traffic is one of the first things any good pilot must learn.