Ship handling involves the way a ship moves and the factors that affect its movements.
- Center of Mass (center of gravity). The imaginary point in a ship that is the center of the mass of the ship. It is the point about which the mass rotates.
- Pivot Point. The fore and aft location about which the ship pivots when her rudder is put over or when steering with engines. With the ship dead in the water, the ship pivots about a point 30 percent of the distance from the bow to the stern. When underway and proceeding ahead, the pivot point is abaft the bow about 15 to 20 percent of the length of the ship. The location of the pivot point must be considered in turning into and out of the wind.
- Axes. A ship moves in three-dimensions about its center of mass and along its 3-axes:
- Longitudinal Axis. An axis drawn through the center of the vessel from stern to bow, through the vessel’s center of mass, parallel to the waterline.
- Vertical Axis. An axis drawn vertically, through the vessel’s center of mass, perpendicular to the longitudinal axis, parallel to the waterline.
- Horizontal (lateral, transverse) Axis. An axis drawn from horizontally, through the vessel’s center of mass, perpendicular to the longitudinal axis, parallel to the waterline.
- Linear Motions
- Heave. The up and down motion along the vertical axis.
- Sway. The lateral (side-to-side) motion along the horizontal axis.
- Surge. The forward and backward motion along the longitudinal axis. The distance gained or lost while changing speed.
- Rotational Motions
- Pitch. Rotation about the horizontal axis. The bow and stern move up and down in opposite directions.
- Roll. Rotation about the longitudinal axis. The sides move up and down in opposite directions.
- Yaw. Rotation about the vertical axis. The bow and stern move side to side in opposite directions.
Inherent Factors that Affect Ship Handling
- Screw. The propeller used to drive all modern ships. A screw may have a various number of blades and pitch.
- Propeller Thrust. The force caused by the displacement of water along the propeller shaft to thrust the ship ahead as the ship moves in the direction of the low pressure area. The after face of the propeller blade creates a high pressure area.
- Side Force. The force that moves the stern of the ship in the direction of screw rotation. Side thrust produced by the screw's rotation through the water. It is noticeable at the stern of the ship. The twin-screw ships cancel side force created by the rotating the screws in opposite directions.
- Screw Wash. Turbulence produced by the screws turning against the water. While twisting or operating astern, it negatively effects a short radius turn by decreasing the efficiency of the rudder.
- Twist. On multiple screw ships, it is the effect of screws rotating in opposite directions.
- Pitch. The distance a propeller would travel in one revolution if water were a solid medium.
- Slip. The difference between the speed of a ship and its propeller.
- Cavitation. Cavities or bubbles around a propeller which are a result of the pressure on the lower and upper blade surfaces being unequal. It is caused by blade tip speed being excessive or by the vessel riding high in the water.
- Bow Thruster. A propeller on some ships that is in a fixed transverse tunnel at the bow. It is used as a maneuvering assistance device on during low speed operations.
- Rudder. A device used to control ship's course through the water. It is designed to produce the lateral forces used in the control of the ship's heading. The rudder force acts through the rudderstock, pushing the stern to starboard or port as the rudder is angled to the passing water. Angling the rudder to the flow of water creates a high-pressure force on the leading surface and a low-pressure force on the trailing side. This forces the stern in a direction opposite that in which the rudder is set.
- Freeboard, The vertical distance from the waterline to the weather deck, usually the main deck. It is determined by the ships mean draft.
- Sail Area. The area above the main deck, which in strong winds will complicate maneuvering and require compensating navigational corrections.
Environmental Factors that Influence Ship Handing
- Wind. Normally acts to force the ship bodily downwind. The force it exerts is proportional to the square of the velocity of the wind; the more freeboard and sail area, the greater the effect of the wind.
- Current. The movement of water exerts a force on the hull is similar to the force of wind against the superstructure, however, the force resulting is much larger for a given current velocity due to the density of water. Thus, current has more effect on the ship than wind does. Deep draft vessels are effected more by current the shallow draft vesels.
- Frictional Wake Current. Friction between the skin of the ship and the water. It is greatest at the stern along the waterline. It increases in shallow water or as speed increases.
- Flood Tide. The period when a tidal current is flowing landward.
- Ebb Tide. The period when the tidal current is flowing seaward.
- Slack Water. The period between the Ebb Tide and Flood Tide when the current is changing direction and no horizontal motion can be detected.
External Factors that Influence Ship Handing
- Mooring Lines. Assist in securing the ship alongside.
- Tug Boats. Assist in mooring and getting underway.
- Ship's Speed. If less than 5 knots, it produces a sluggish ship that is difficult to control. If greater than 5 knots, it produces a lively ship that requires shorter turning area, etc.
- Steerageway. Sufficient speed on to permit a vessel to be maneuver.
- Advance. The distance gained in the original direction prior to a course change. Advance will be a maximum when the ship has turned thru 90 degrees.
- Transfer. The distance gained at right angles to the original course when the ship has turned thru 90 degrees.
Engineering casualties that effect control of ship
- Loss of lube oil. Will stop and lock the shaft, causing ship to loss propulsion.
- Loss of vacuum. When vacuum level drops below normal, a RED alarm light in DCC will warn the Engineering Officer of the Watch (EOOW). If the vacuum drops to 21 inches, the ship's speed will be limited to 2/3, at 18 inches, the ship will be limited to 1/3, and at 15 inches the engines will be stopped, causing ship to loss propulsion.
- Low water in the boiler. Will limit speed and maneuverability, and could cause an excessive steam temperature which might warp the turbine blades causing the turbines to be shut down, causing ship to loss propulsion.
- High water in the boiler. Wet steam droplets could form that would destroy the steam turbine, causing ship to loss propulsion.
- Loss of generator. All power stops, emergency generator takes over providing enough power to sustain vitals until repairs are completed. Ship could still be controlled but its fighting capabilities would be limited.
- Loss of pitch control. Will limit attainable speed.
- Loss of steering control. Reduced control of ships heading.
- Jammed throttle. Loss of speed control and maneuverability.
- Jammed rudder. Loss of steering control.
- Hot bearing on main engine. Requires stopping of that shaft, resulting in loss of even thrust through the water and decreasing maneuverability and speed.
Navigation Lights a Ship Must Display Under Certain Conditions
- Underway. In general, a ship must show a masthead light, sidelights, a stern light, and a range light.
- In Port, Moored. In general, a ship must show a mast light, a jack staff light, a bow light, a stern light, and aircraft warning lights. Shipboard lights are constant and land-based lights are pulsating.
- At Anchor. In general, a ship must show two 32-point white lights, one forward and another aft stationed, with the aft light lower than the forward light.
- Engaged in Special Operations. In addition to normal underway lights, there are many required combinations of lights to be displayed when engaged in special operations, such as towing, underway replenishment, etc. For example, when engaged in underway replenishment operations, a ship must display a vertical Red-White-Red combination on the mast to warn other vessels.