Ship Handling Flashcards
Criteria that characterize a ship
- Hull: length, beam, draft, trim, block coefficient also called coefficient of fineness
- Propulsion: type of engine, power, propeller
- Rudder: type, surface area
- Special Equipment: transverse and azimuth thrusters
- Windward surfaces (longitudinal and transverse)
Density of water and air and effect on ship’s performance
Water is 850x more dense and 100x more viscous than air so it quickly has much more significant effect on ship’s performance
Rudder types
Flap (Becker) rudder: at extreme angles functions as thruster, over around 4 kts limit to 35 deg
Swiveling nozzle
Profiled rudder: increase lift by extending stall angle
Flow control rudder: 2x profiled rudders used to direct thrust of propeller that turns continually forward
Propeller action
Accelerates water particles as they pass through it and gives them rotary motion. The thrust which drives the ship comes from this acceleration.
Propeller main characteristics
Diameter Number and type of blades (typically 2-5)
Geometric pitch (angle of blades) FP or CPP
Propeller efficiency astern
Assumed to be low: 0.25 (25% of ahead) power
Turning effect of fixed pitch propeller
FPP in forward motion: drag force of blades pushes stern of vessel to starboard. Easily compensated with rudder
FPP in reverse motion: Deteriorating quality of water flow over blades increases drag and proximity of hull to discharge current (strongest on stbd) increases turning force and pushes vessel stern to port
Turning effect of variable pitch propeller
CPP in forward motion: similar to FPP and easy to control with helm
CPP in astern motion: Since the shaft direction does not reverse turn effect direction is the same as ahead I.E. for a RH prop, the stern always moves to starboard. The effects are reversed for a LH propeller Most ships with a single CPP shaft line are fitted with LH prop to gain the same turning effect astern as RH prop
Blade area ratio and skew
Blade area ratio: ratio between total blade surface and surface area of circle in which propeller lies. Typically 0.3-0.8
Skew: Eccentricity in tangent from straight blade. Higher skew is used to lessen drag effect of high blade area ratio.
Tunnel thruster efficiency
- Position of propeller with respect to G
- Speed of the ship: Thrust is greatest with ship practically stopped. At 4kts thruster has lost 50% of efficiency. Above this speed use helm and engine.
- Bow thruster is most effective moving astern.
Effect of thruster on ship’s motion
With ship stopped and thruster operating for example to starboard, the water sucked in on the starboard side is ejected to port. As the bow of the ship turns to starboard overpressure is created on the starboard bow causing some of the water flow to be accelerated forward along the port bow generating a low pressure zone. This movement of the water mass from one side of the ship to the other causes the ship to start making way forward.
Reference system linked to vessel
Dynamics
Reference system linked to Earth
Kinematics
Pivot point
Position on the vessel’s longitudinal axis, identified as not being subject to any transverse movement. The observer at this point will effectively see the vessel turn around his viewpoint.
Per Baudu: 1/4 length from bow/stern with headway/sternway. 1/3 length if turning
Added mass
Solid shapes have greater added mass than flowing forms. With equal power and displacement it will be more difficult for a crude carrier to make forward way than a container vessel. Similarly it will be easier to make a vessel with cylindrical shapes turn and drift than a vessel with straight shapes.
Inertia caused by a turn
When a vessel starts to turn with rudder it undergoes the effects of centrifugal inertia force, which draws it by slippage to the outside of the turning circle. During the turn, the vector of of inertia force at G is behind the heading of the vessel by about 15 degrees.
Force applied by wind on vessel
Apparent wind must be taken into account when maneuvering The instant center of windage is the point where the wind acts on the vessel’s superstructure The effect of wind is a force that causes drift and an effect that turns the vessel until it reaches a neutral position
Force applied by water on the vessel
When the vessel moves the water exerts a hydrodynamic force that counters the movement: this is hull resistance.