More on hogging

From: Mike W
Location: London
Email:
Remote Name: 209.239.5.136
Date: 07/20/03
Time: 10:09:51 PM

Comments

My apologies for flogging hogging as a topic. There appear to be two causes for two conditions: one cause is the loading pattern, and the other is ambient temperature. Imagine the ship's keel as a straight line (which it usually is in a long cargo-carrying hull). If the ends of the line curve downward, the hull is hogging. If the line ends are higher than the centre, the vessel is sagging or swaybacked. The extreme consequence is hull fracture. More usually, it alters the overall draught of the ship. Great care is taken during loading and unloading to minimize these deformations. Last year a Canadian ship suffered serious damage loading traprock at Bruce Mines. A properly loaded laker will hog in hot weather: the deck structures soak up solar heat, while the lower half of the hull is in cool water. The deck expands and gets longer than the keel. Hence the common if sometimes puzzling practice of wetting the deck with sprinklers. I have no data on how much this offsets the hot-deck/cool-keel tensions, but it is obviously worthwhile. Long hulls flex anyway. As we left Thunder Bay on the JOHN A FRANCE, we could look aft from the windows above the chart table and see wave-forms -- ripples, if you like -- moving bow-to-stern down the hull, subtly matching the two-to-three foot swells we were steaming through. I have no pictures of the phenomenon: we were shooting 16mm film, but it was too late into sunset to capture on the standard Eastman Kodak TV film of the era.