A method for calculating the surface vector of at least one vessel progressing under engine at cruising speed. When the vessel is in a defined position, the parameters of the vessel, including its position, its heading, its speed over ground and its course over ground, are obtained. Wind and/or current measurements in the proximity of the vessel are obtained using a device external to the vessel. The drift vector is determined from wind and/or current measurements. The over-ground vector is determined from the parameters selected from the position, the speed over ground and the course over ground of the vessel. The magnitude and the direction of the surface vector of the vessel is calculated.