2.6 Other resistance components

Drag forces on appendages have to be considered. Examples of appendages for a hydrofoil vessel are inclined propeller shafts, rudders not included in the foil system, propeller nacelles, and waterjet intakes (pods). Similar appendages may also be used for other high-speed vessels. Empirical formulas may be found in van Walree (1999) and Hoerner (1965).

The skirt of an SES will cause a resistance component. The skirt may be considered a high-aspect ratio planing surface. The forward jet flow that is generated will cause a drag force, as described in section 9.2.4 for a 2D rigid planing surface. However, a skirt is in reality flexible and vibrating, even in calm water conditions. The percentage of additional resistance related to the flexible vibrations is hard to estimate. The viscous resistance on the skirt can be approximated by considering it as a flat plate.

A planing vessel will have an important resistance component associated with the hydrodynamic pressure causing lift force and trim moment on the vessel. This is considered in more detail in section 9.2.

All the previous resistance components refer to a ship with constant speed on a straight course in calm water. Added resistance that is caused by waves, wind and ship maneuvering should also be considered. Added resistance in waves is often misunderstood and believed to be wave resistance. The added resistance R _AW in waves is a consequence of interaction between incident waves and the ship. It is particularly large...