In solid mechanics, structural engineering, and shipbuilding, hogging and sagging describe the shape that a beam or similar long object will deform into when loading is applied. Hogging describes a beam that curves upwards in the middle, and sagging describes a beam that curves downwards.
A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list, to provide a more even load distribution along the hull to reduce structural hogging or sagging stresses, or to increase draft, as in a semi-submersible vessel or platform, or a SWATH, to improve seakeeping. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels, and makes it easy for the crew to reduce a vessel's draft when it enters shallower water, by temporarily pumping out ballast. Airships use ballast tanks mainly to control buoyancy and correct trim.
Carvel built or carvel planking is a method of boat building in which hull planks are laid edge to edge and fastened to a robust frame, thereby forming a smooth surface. Traditionally the planks are neither attached to, nor slotted into, each other, having only a caulking sealant between the planks to keep water out. Modern carvel builders may attach the planks to each other with glues and fixings. It is a "frame first" method of hull construction, where the shape is determined by the framework onto which the planks are fixed. This is in contrast to "plank first" or "shell first" methods, where the outer skin of the hull is made and then reinforced by the insertion of timbers that are fitted to that shape. The most common modern "plank first" method is clinker construction; in the classical period "plank first" involved joining the edges of planks with mortise and tenon joints within the thickness of the timbers, superficially giving the smooth-hull appearance of carvel construction, but achieved by entirely different means.
Compared to clinker-built hulls, carvel construction allowed larger ships to be built. This is because the fastenings of a clinker hull took all the hogging and sagging forces imposed by the ship moving through large waves. In carvel construction, these forces are also taken by the edge-to-edge contact of the hull planks.
Structural steel is steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, chemical composition, mechanical properties such as strengths, storage practices, etc., are regulated by standards in most industrialized countries.
Structural steel shapes, such as I-beams, have high second moments of area, so can support a high load without excessive sagging.