# Ship Motions

## Three linear motions

Surge (x axis): Lengthwise. Propulsion and drag act along this axis. Surfing is an example of surge caused by a wave.

Sway (y axis): Sideways. Generally a minor movement in a large vessel. Wave and wind loads could contribute to this motion.

Heave (z axis): Up and down. Wave motion causing the whole ship to rise and fall.

## Three rotational motions

Roll (Around x): The most significant stability criterion - capsize. Caused by waves, wind and can be induced by yaw.

Pitch (Around y): A rocking motion between bow and stern. Mostly wave induced.

Yaw (Around z): This is what the rudder is supposed to do. Rudder corrections for course keeping illustrate that wind and waves can cause yawing motions.

# Ship Geometry

Length Between Perpendiculars (LBP or L): The distance between forward and aft perpendiculars.

Beam (B): The breadth of the ship at the widest point. Molded beam is measured amidships or at the widest section from the inside surface of the shell plating.

Draft (T): The depth from waterline to the deepest part of the ship.

Depth (D). Total depth from bottom to the top watertight deck.
Depth = freeboard + draft.

Length Overall (LOA): The extreme length of the ship.

Length on Waterline (LWL): This is the length at the waterline in the ship's design loaded condition.

Freeboard (F): Distance between the waterline and the uppermost watertight deck.

Sheer: The rise of a deck - usually toward bow and stern. Sheer increases freeboard, and helps keep the vessel from shipping water in rough seas - particularly at the bow.

Camber: The convex upwards curve of a deck. Also called round up, round down, or round of beam, usually around one-fiftieth of the beam. Not all ships have cambered decks; ships with cambered weather decks and flat internal decks are not uncommon.

Tumblehome: Inward slope of hull sides above the waterline - the opposite of flare. Tumblehome was a usual feature in sailing ships and many ships built before 1940. Only seen on tugs and icebreaking vessels, sometimes used to reduce topside weight, and for reducing radar cross sections.

Flare: The outward curvature of the hull surface above the waterline, i.e., the opposite of tumblehome. Increases buoyancy when immersed. Flaring bows are often fitted to help keep the forward decks dry and to prevent "nose-diving" in head seas.

Deadrise: Rise of the bottom from baseline to molded breadth measureed amidships. Also called "rise of floor" or "rise of bottom". Full-bodied ships, such as cargo ships and tankers, have little or no deadrise, while fine-lined ships have much greater deadrise along with a large bilge radius.

Rake: Slope of profile lines - esp rake of stem (angle between the stem and vertical)

Cut-up: When a keel departs from a straight line at a sharp bend, or knuckle, the sloping portion is called a cut-up. High-speed combatants usually have a long cut-up aft (extending 13 to 17 percent of LWL) to enhance propeller performance and maneuverability.

# Ship Hydrostatics Parameters

Displacement Volume (V): The volume of the underwater hull at any given waterline.

Displacement (W): The weight of water of the displaced volume of the ship, which equals the weight of the ship and cargo.

Buoyancy: The upward push of water pressure, equal to the weight of the volume of water the ship displaces (W).

Reserve Buoyancy: The watertight volume between the waterline and the uppermost continuous watertight deck.

Moment of Inertia (I): Also called the Second Moment of Area unless specified otherwise. It is proportional to bending strength.

Tonnage: Cargo capacity of a merchant ship, measured by volume.
Trim: Longitudinal tilt. Stern draft - bow draft

List, Heel, and Roll: Angular transverse inclinations. List describes a static inclination such as list due to side damage. Heel describes a temporary inclination generally involving motion, such as wind or turning, while roll indicates periodic inclination from side to side such as wave action.

Center of Gravity (G). The center of all mass of the ship, acting vertically downward.

Center of Buoyancy (B). The geometric center of the submerged hull, acting vertically upward.

Metacenter (M). When the ship is inclined at small angles, the metacenter is the intersection of the buoyant force with the ship centreline. If the metacenter is above the center of gravity then the ship is stable.

Center of Flotation (F). The geometric center of the waterline plane, about which the ship trims fore-and-aft.

# Ship Terminologies/Glosseries

AFT: Toward the stern of the boat.

AGROUND: Touching or fast to the bottom.

AMIDSHIPS: In or toward the center of the boat.

BEAM SEA: Sea coming on the side of the ship.

BEARING: The direction of an object expressed either as a true bearing as shown on the chart, or as a bearing relative to the heading of the boat.

BILGE: The interior of the hull below the floor boards.

BOW: The forward part of a boat.

BROACH: The action of turning a vessel broadside to the waves.

BROADSIDE: Presenting the side of the ship

BRIDGE: The location from which a vessel is steered and its speed controlled. "Control Station" is really a more appropriate term for small craft.

BULKHEAD: A vertical partition separating compartments.

CAPSIZE: To turn over.

DISPLACEMENT: The weight of water displaced by a floating vessel, thus, a boat's weight.

DRAFT: The depth of water a boat draws.

FATHOM: Six feet.

FOLLOWING SEA: Sea coming on the stern.

GANGWAY: The area of a ship's side where people board and disembark.

HEADING: The direction in which a vessel's bow points at any given time.

HEADWAY: The forward motion of a boat. Opposite of sternway.

HEEL: Constant roll angle - such as caused by a side wind or turning of the vessel.

HELM: The wheel or tiller controlling the rudder.

HULL: The main body of a vessel.

KEEL: The centerline of a boat running fore and aft; the backbone of a vessel.

KNOT: A measure of speed equal to one nautical mile (6076 feet) per hour.

LEE: The side sheltered from the wind.

LEEWARD: The direction away from the wind. Opposite of windward.

LEEWAY: The sideways movement of the boat caused by either wind or current.

MARINE ENGINEERING: Propulsion and systems within the ship. (Pumps, power generation, air & water systems etc)

MIDSHIP: Approximately in the location equally distant from the bow and stern.

NAUTICAL MILE: One minute of latitude; approximately 6076 feet: about 1/8 longer than the statute mile of 5280 feet.

NAVAL ARCHITECTURE: Ship design: especially hull design, overall layout with attention to stability, seakeeping and strength.

PORT: The left side of a boat looking forward.

QUARTER: The sides of a boat aft of amidships.

QUARTERING SEA: Sea coming on a boat's quarter.

SEAWORTHY: A boat or a boat's gear able to meet the usual sea conditions.

SOUNDING: A measurement of the depth of water.

STARBOARD: The right side of a boat when looking forward.

STEM: The forward most part of the bow.

STERN: The after part of the boat.

WAKE: Moving waves, track or path that a boat leaves behind it, when moving across the waters.

WATERLINE: A line painted on a hull which shows the point to which a boat sinks when it is properly trimmed

WAY: Movement of a vessel through the water such as headway, sternway or leeway.

WINDWARD: Toward the direction from which the wind is coming.

YAW: To swing or steer off course, as when running with a quartering sea.

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