As Miraflores correct stated; it is the distance from keel of a vessel to the waterline. Or said in another way; how deep a vessel is under the waterline.
Wery important, as many ports has draught-restrictions. When loading in one port, we always have to obtain information of max.depth in discharging-port.
In the 70's and 80's many supertankers had to lighten the vessels to get a less draught, so that they could go alongside in many continental ports. Part of cargo was then transferred to "smaller" tankers of lets say 70000 tdw. in places like Lyme Bay. Then the supertankers had got a less draught, so that they could continue their voyage to final discharging port.
One must also be aware that a vessel is quite deeper in fresh-water than in sea (salt)-water, so when loading a vessel for max.draught in discharging-port, one must get information from agent about water-density in discharging-port.
Another thing affecting draught, is the hog/sag, which mean that if the vessel is completely flat on the sea, there may be difference between forward/aft-readings and the readings on the middle. Example: If we load all cargo in the middle of a vessel, she will be deeper on the middle than on the ends. This we call sag. If we load all cargo in the ends and leave middle empty, she will be deeper in the ends, and this we call hog. But this is never done, as a vessel then will break in two normally. Cargo has to be even stowed. Some years ago a large tanker broke in two in Rotterdam due to wrong discharging.
This was supposed to be a simple explanation, but I'm afraid it turned out to be a long lecture.
Anyway;
regards from Arnes
