(Photo: Laviolette bridge, in Trois-Rivières)
We know what a water draft is, it’s the height of the part of a vessel that is submerged, in other words, under water.
Logically, the air draft would be, you guessed it, the height of the part of the ship that is emerged, in other words, out of the water. To put it another way, the part we can see when a ship is in the water.
Why is this information important for vessels sailing on the St. Lawrence River? Quite simply, because bridges and electrical wires cross the river. Vessels cannot exceed a certain clearance height, or else they’ll hit them. The lowest insurmountable obstacle on the river is Laviolette Bridge in Trois-Rivières, which has only 52 metres of clearance under its structure. Consequently, vessels with a draft air higher than 52 metres can’t go any further upriver.
The air draft is vital data, especially for cruise ships. While they are usually lighter than merchant ships and therefore have a shorter water draft, they are built higher. The many decks intended for passenger activities make their air draft mount up fast. A cruise ship such as Cunard’s Queen Mary 2 has 15 decks and a 72-metre air draft. That’s with only a 10-metre water draft, whereas a large container ship can reach 15 or 17 metres.
Some power lines that cross the river are lower in height than the Laviolette Bridge, in Trois-Rivières West and Longue-Pointe, in Montreal’s east end. Hydro-Québec is getting ready to raise then to 52 metres by fall 2014, which will allow large cruise ships to reach the Port of Montreal.