although scaling and viscous effects limit trajectory predictor accuracy. Mathematical models must be carefully adjusted for variations, and usually separate model tests are required to obtain proper coefficients for use at different drafts or trims of the ship to provide accurate modeling. Properly modeling the level of dynamic course stability is very important in developing a model that will provide the proper vessel response to ship control actions.
The quickness and distance involved in stopping a ship is related primarily to its mass, astern power, machinery plant, and gearing. Steam turbines typically provide only 40 percent of their ahead thrust capability while astern. Diesel propulsion plants deliver about 80 percent of ahead power astern. The number of engine starts possible from the compressed air system is important for the direct-drive diesel. Once the air supply is expended, quick reversal of the engine is not possible.
Operation at acceptably slow speeds is also a function of the machinery plant. Direct-drive, low-speed diesels have minimum speeds. The trajectory during a stopping maneuver is very difficult to predict because of the complex flows associated with propeller reversal. Other aspects of slow-speed maneuvering that need to be realistically simulated include the use of tugs, thrusters, and anchors to control or impede the ship's movement.