The design of a surface ship is said to follow a spiral This follows because a ship operator has to carry goods and other items both bulky and compact from one port to another in the most economical manner and the ship designer has to meet these demands via an iterative process that is best described as a spiral. The spiral starts with a preliminary estimate of the principal dimensions of the vessel There are two methods used to assess these dimensions they are either obtained from a survey of vessels performing a similar job or from a major statistical method of analysis of either a class of vessels or all vessels These principal dimensions are then used, with a sectional area curve, to produce a set of ship lines These lines then allow a preliminary estimate of the basic hydrostatic qualities of the ship to be calculated These are used to assess the overall statical stability and the carrying capacity at various draughts.

The spiral continues with preliminary arrangements of the accommodation of crew, passengers, and goods The details of this arrangement allow a calculation of overall centre of gravity to be compared with the hydrostatic value Alongside this calculation a structural sum is performed to make sure the designed ship satisfies a ship classification society rules.

After this stage a detailed weight and structural survey is performed The ship design starts an iterative process between the requirements and the actual design Finally the ship emerges from the spiral and is built.

The traditional design spiral often ignored two major dynamic facets of the performance of the ship, namely seakeeping and manoeuvring Of these dynamic properties of a ship, seakeeping has provided the most fruitful research of the past twenty years The results of this research are encompassed in a variety of computer programs that give reasonably good estimates of the seakeeping performance in random seas Thus for instance it is possible to design the freeboard line of a ship with some degree of accuracy from these programs using a probabilistic approach with respect to the shipping of green water on deck The most accurate programs are for the vertical plane responses heave and pitch. Theoretical methods and computer programs do exist for the lateral motions (roll, sway and yaw), but the results from a computer simulation are not as accurate as for the vertical plane responses They are, however, good enough to provide the ship designer with enough information to assess the performance of his design in whatever dynamic mode of operation is under scrutiny.

It is therefore a little surprising that manoeuvring simulation has not received the same accolade as seakeeping in the design spiral There are perhaps not so many theoretical techniques available as for seakeeping but they do exist. The traditional role for rnanoeuvring was to estimate the sue and shape of the rudder from semi-empirical formulae and then assess the ship's performance during sea trials using spiral tests and zigzag manoeuvres.

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