On its journey to the port of destination a ship on the ocean experiences operational conditions which are significantly different from those typically assumed during design. Environmental forces, wind and specifically waves, add to the resistance in calm water and hence require more power output from the engine to propel the ship at a given speed.
During “real life” operation a ship experiences
Speed loss in extreme seas, which is more substantial for low-powered vessels, will indirectly affect a ship’s propensity for unsafe dynamic behaviour. There are several contributions to the involuntary speed loss in severe seas. The principal one is the wave-added resistance; another is the change of the propeller operation for a ship in waves, due to a modified propeller inflow; and in certain cases, the larger propeller loading might result in a decreased delivered engine power, depending on the engine’s characteristics. These aspects will be considered in the current investigation, and have rarely been addressed before.
In the emerging “low power regime”, the usual service (sea) margin may not be adequate for sustaining a minimum “safe” service speed. As a matter of fact, added resistance in waves is an important factor in determining the safety margin of a “reduced power” vessel. Moreover, arrival times will be extended as a relatively stronger effect by the weather is likely to produce a lower than desired mean sustained speed. Reduction of speed also incurs increased time of exposure to “severe” sea conditions and the risk associated with a journey will be increased too. As a matter of fact, it is very essential to develop capability for the quantification of added resistance in various wave environments.