Seakeeping 475 As with so many other aspects of ship design a rigorous treatment of sea-keeping isvery complexand a number of simplifying assumptions are usually made. orinstance! the ship is usually regardedasresponding to the wavesas a rigid bodywhen assessing motions and wetness although its true nature as an elastic body mustbetakenintoaccount inastudyof structure. "nthesamewayit isinstructive!although not correct! to study initially the response of a ship to regular long-crestedwaves ignoring the interactions between motions! e.g. when the ship is heaving thedisturbing forces will generate a pitching motion. #his very simple approach is nowdealt with before considering coupled motions. Ship motions"t wasseenin $hapter 4! that afoating body has sixdegrees offreedom. #o completelydefnetheship motion it is necessary toconsider movements in allthesemodesasillustratedinig. %&.%.#hemotionsaredefnedasmovements of the centreof gravity of the ship and rotations about a set of orthogonal axes through the c.g.#hese are space axes moving with the mean forward speed of the ship but otherwisefxed in space. ig. %&.% Ship motions "t will be noted that roll and pitch are the dynamic e'uivalents of heel and trim.#ranslations along the x-and y-axis and rotation about the (-axis lead to no residualforce or moment! provided displacement remains constant! as the ship is in neutrale'uilibrium. or the other translation and rotations! movement ...............