Ship powering, maneuvering, and seakeeping

Ship powering, maneuvering, and seakeeping

The three central areas of ship hydrodynamics. Basic concepts of powering, maneuvering, and seakeeping are critical to an understanding of high-speed craft.

Powering

The field of powering is divided into two related issues: resistance, the study of forces opposed to the ship's forward speed, and propulsion, the study of the generation of forces to overcome resistance.

A body moving through a fluid experiences a drag, that is, a force in the direction opposite to its movement. In the specific context of a ship's hull, this force is more often called resistance. Resistance arises from a number of physical phenomena, all of which vary with speed, but in different ways. These phenomena are influenced by the size, shape, and condition of the hull, and other parts of the ship. They include frictional resistance and form drag (often grouped together as viscous resistance), wavemaking resistance, and air resistance.

Many devices have been used to propel a ship. In approximate historical order they include paddles, oars, sails, draft animals (working on a canal towpath), paddlewheels, marine screw propellers, vertical-axis propellers, airscrews, and waterjets. A key distinction is whether or not propulsive forces are generated in the same body of fluid that accounts for the main sources of the ship's resistance, resulting in hull-propulsor interaction. See Propeller (marine craft)

Any propulsor can be understood as a power conversion device. Delivered power for a rotating propulsor is the product of torque times rotational speed. The useful power output from the system is the product of ship resistance times ship speed, termed effective power. The efficiency of this power conversion is often termed propulsive efficiency.

Maneuvering

Maneuvering (more generally, ship controllability) includes consideration of turning, course-keeping, acceleration, deceleration, and backing performance. The field of maneuvering has also come to include more specialized problems of ship handling, for example, the production of sideways motion for docking or undocking, turning in place, and position-keeping using auxiliary thrusters or steerable propulsion units. In the case of submarines, maneuvering also includes depth-change maneuvers, either independently or in combination with turning.

Seakeeping

The modern term “seakeeping” is used to describe all aspects of a ship's performance in waves, affected primarily by its motions in six degrees of freedom (see illustration). Seakeeping issues are diverse, including the motions, accelerations, and structural loads caused by waves. Some are related to the comfort of passengers and crew, some to the operation of ship systems, and others to ship and personnel safety. Typical issues include the incidence of motion sickness, cargo shifting, loss of deck cargo, hull bending moments due to waves, slamming (water impact loads on sections of the hull), added powering in waves, and the frequency and severity of water on deck.

In the past, initial powering, maneuvering, and seakeeping predictions for new designs depended almost entirely on design rules of thumb or, at best, applicable series or regression data from previous model tests, subsequently refined by additional model tests. With the increase in computing power available to the naval architect, computational fluid dynamic methods are now applied to some of these problems in various stages of the ship design process. See Computational fluid dynamics