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.
Hull Resistance
= wave and pressure resistances + viscous resistance
Wave Resistance
As the vessel moves forward, it distorts the “free” surface area of the water. It “pulls” the water, forming lines of waves. The bigger the waves, the greater the resistance the water exerts on the hull.
If speed increases, wave resistance increases.
To reduce the size of the bow wave and therefore wave resistance, bulbous bows are used.
Pressure Resistance
A moving vessel disturbs the mass of water around it in a zone 1 - 2x its width and about 1.5x deeper than its draft.
Pressure forces exerted over the whole hull are hydrostatic pressure (immersion) + hydrodynamic pressure (proportional to speed).
Owing to the flow of water currents over the hull, distribution of these pressures over the hull are:
- Overpressure zones at the bow and stern (greater at bow)
- A relative low pressure zone at the center
Viscous (Frictional) Resistance
Considering the liquid envelope surrounding the hull:
The water molecules in contact with the hull are drawn along at the same speed as the vessel. Further out, at some distance from the hull, the water is stationary. Between these two extremes is the boundary layer where the water particles generate friction forces caused by the difference in speed of water particles. There are three flow zones from forward to aft in the boundary layer:
- Laminar at the bow of the vessel
- Turbulent midship
- Increased turbulence astern (vortices)
Frictional resistance depends on the quality/fouling of the hull and its capacity to limit eddies and on the speed of the vessel which encourages instability in the flow. At a certain speed the boundary layer deepens.
Resistance to oblique motion
A vessel’s motion is said to be oblique when the flow of water along either side of its hull is not symetrical. This is typically due to the following actions:
- wind causing drift or current in confined waters
- turning
- external forces (tug boat, etc.)
In these situations, one side of the vessel “presses” on the water and on the other side vortices form. The water reacts by exerting a force on the vessel (resistance to oblique motion) whose moment with respect to G causes a typical turning tendency to appear.
Bernoulli Law - Conservation of Energy
Bernoulli’s Law states that along a stream tube, total pressure, flow rate, and total energy remain the same.
The ship’s hull is a virtual stream tube on each side. It is obvious that what enters the tube also leaves it, neither more nor less.
This means that if the cross sectional area of the tube decreases, the velocity of the water streams must increase equal to the inverse ratio of the sections.
Starting at the bow, as the hull broadens, the velocity of the flow increases and static pressure diminishes. Towards the stern, the flow slows and the static pressure increases again. This phenomona is amplified in narrow channels where the ships sides and channel sides form a physical stream tube.