Wind and current are usually associated as both being forces not under control of the shiphandler. The two forces have, however, a different effect on the ship because of the difference in nature of the two. When the ship is affected by wind alone and moves through the water, the hull meets underwater resistance. When, on the other hand, the ship’s motion originates from current, there is practically no resistance of the above-water area to air. As water is eight hundred times denser than sea level atmosphere, current must, by nature, have considerably stronger effect than wind, especially on loaded ships.
Current has a direct effect on the underwater part of the ship and an indirect effect expressed in momentum after the ship alters course or comes out of a current, when the ship will carry momentum in the direction of the current that the ship was previously subjected to.
Effect of Wind and Current
Whereas the effect of wind on the ship has to be considered with respect to the pivot point, current affects a freely moving ship as a whole and consequently its effect is on the center of gravity. However, when we try to keep the ship stationary relative to the ground, we must arrest the ship’s movement and let the ship make speed through the water contrary to the current, in which case the ship meets underwater resistance.
All freely moving ships, not being subjected to wind and dead in the water, have the same speed as the current, whether the ships are big or small, loaded or light. Ships not freely moving, as ships at anchor or moored, are subjected to pressure exerted by the current, pressure which is directly proportionate to the exposed underwater area and to the square of the current velocity.
In a strong tide we see that ships at anchor, or moored to a single point, are heading into the tide; when it is nearly slack water ballasted ships will be more affected by wind while the loaded tankers still remain heading into the tide. When we approach the monobuoy with a bal lasted tanker in wind and tide condition, the direction of the loaded tankers, moored on single points nearby, gives us an indication of the direction of the current.
However, the heading of the ballasted ship, after having been tied up to the buoy, may be quite different from the heading of the loaded ship (fig. 31).