The vessel design generally consists of 3 parties. Yourself and your representative, be they a Project Manager, Broker, Shipyard or who ever you may choose. You have an interior designer who helps to lay out the interior gereral arrangement (GA) along the style you wish, and a hull designer (naval architect) who designs the hull form and characteristics so it performs as it is supposed to through the water. Once a GA is near completion the naval architect can use these dimensions to finalise the design of the hull and make sure everything will fit within the hull form. Both designers will work together to make sure of this. Once you are happy with the layout of the vessel, the next decision to make is whether or not to initiate a tank test program.
Whether or not you should perform a tank test depends on whether or not the shipyard you intend on using has built this design of hull before, or at least one very close to it. In the production and semi production shipyards they have generally completed tests for their range of hulls, and they just use a slightly different interior in the same hulls depending on their clients wishes. For full custom shipyards, their hulls are normally specifically designed for one client in mind, and so very often it is advisable to use a tank test facility. One or more scale models of the hull are built, either by the facility or a model building contractor, from 3D drawings supplied by the naval architect. The model/s are then placed in tanks and towed at various speeds, sea states, sea angles, and stationary in a sea way. This produces a lot of information regarding the hull resistance, it’s roll characteristics, sea handling capability and the way the hull behaves at different speeds and in different sea states. Any design alterations needed can then be completed by the naval architect. The hull resistance can be used to predict a kilowatt power rating needed to push the vessel at the required speed. With a few more calculations, which take into account propellor efficiency, running gear drag, gearbox efficiency etc, we can arrive at a per engine hoursepower rating requirement. Once the hull has been proven the construction process can begin.