Eng. 6002 Ship Structures 1 Hull Girder （Eng。6002船船体梁结构1）.pdf

Eng. 6002 Ship Structures 1 Hull Girder Response Analysis L E C T U R E 4 : T H E S H A P E O F O C E A N D E S I G N W A V E S , W A V E B E N D I N G M O M E N T S Overview  We can consider the wave forces on a ship to be quasi- static. This means that they can be treated as a succession of equilibrium states.  When a wave passes by a vessel the worst hogging moment occurs when the midbody is on the crest of a wave, and the bow and stern are in the troughs Overview  The worst sagging moment occurs when the midbody is on the trough, and the bow and stern are on crests  Furthermore, the highest bending moments occur when the wavelength approaches the vessel length Overview cont. The design wave for a vessel will therefore have a wavelength equal to the vessel length. The wave height (peak to trough) is generally assumed to be 1/20th of the wave length (any larger and the wave will break) Trochoidal Wave Profile The shape of an ocean wave is often depicted as a sine wave, but waves at sea can be better describaed as trochoidal. A trochoid can be defined as the curve traced out by a point on a circle as the circle is rolled along a line. Trochoidal Waves cont.  The discovery of the trochoidal shape came from the observation that particles in the water would execute a circular motion as a wave passed without significant net advance in their position.  The motion of the water is forward as the peak of the wave passes, but backward as the trough of the wave passes, arriving again at the same position when the next peak arrives. (Actually, experiments show a slight advance of the water with the waves, but that advance is small compared to the overall circular motion.) Source: /rotation.html