Draft Survey

What determines the volume of cargo that a ship can carry?

The minimum depth of the navigation channel and a vessel’s squat (the hydrodynamic effect of lower pressure pulling the ship down as it moves) determines how much cargo a vessel sailing upstream to Montreal can carry.

Water levels

The depth of the river channel varies according to different elements, in particular climatic conditions. The Great Lakes supply water to the St. Lawrence; the more rain or snow in the Great Lakes region, the higher the water levels in the river. On the other hand, warm temperatures over an extended period of time cause average water levels in the river to decrease.

A shipping line that sails to Montreal must know in advance what the river’s water depth will be so that it can properly load its ship at the port of departure.

The reference point that is used is known as chart datum, which is an average of low water measured by the Canadian Hydrographic Service at numerous locations along the river. These references are indicated on nautical charts. Water can be one or two feet “above chart datum” in the Port of Montreal when there is a lot of water upstream. On the other hand, water can be one or two feet “below chart datum” during certain extremely dry periods.

The squat phenomenon

In addition to knowing the minimum depth in the channel, a vessel’s squat must be taken into consideration when sailing upstream to Montreal. The squat effect applies to all moving vessels with a low depth of water under the keel. The squat of a moving vessel varies from one ship to another. Shipping lines therefore include in their calculations a safety margin that can vary from three to four feet depending on the type of ship.

The safety margin, which takes into account the phenomenon of squat, is subtracted from the minimum water depth in the channel to provide the maximum authorized draft. This determines how much cargo a vessel can load.

Glace Ice
Courants Currents
Marge de manœuvre Safety margin
Limite du chenal Channel limit
Tolérance de surprofondeur Overdepth allowance

Marée Tide
Zéro des cartes Chart datum
Tirant d’eau Ship’s draft
Profondeur maintenue Maintained depth
Dégagement sous quille Underkeel clearance