Bilge keels, or as their older name of rolling chocks implies, are fitted io ships to influence the extent of rolling, through the form of the midship section. For this purpose the surface friction of the water on the ship's side has a considerable effect, writes a naval architect in the Liverpool -Journal of Commerce. The action is that of a fin moving flatwise through the water, and the maximum effort being when they are moving square to their surface, This is when the plane in which they lie passes through the axis of rotation, or more simply, when the trace of the plane in the body plan passes through the instantaneous centre of rotation of the ship. Unfortunately, this point is not a fixed one, even when rolling. in still water. obviously a very much more difficult point to determine when a ship is in a seaway.
There are differences of opinion as to the instantaneous position of this point at any given angle of roll. ' The. One most generally accepted fix if at the intersection of a horizontal line through the centre of gravity, of the ship with the normal to the curve of flotation. This is a curve touching the waterlines at all angles of heel—at a point of contact with the waterline corresponding to the angle. This theory would place it for small angles of heel at about the common.centre of gravity of the ship and her cargo.
A common error is that a ship will rotate about her. metacentre, but as already mentioned, all the accepted theories agree that the point {or small angles of heel is at, or near, ihe common centre of gravity.
Text read from the printed page and cleaned; old type leaves the occasional error.