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.
Turning process consists of 3 phases
- Maneuvering phase: initial period when the helm is moved
- Rotational phase: the ship begins its turn by drifting
- Turning phase: When the ships heading has moved through 90 degrees, the turning then becomes uniform. Hydrodynamic lift is stabilized with drift and ROT is constant.
Trajectory of center of gravity G imposed by vessel handler (2 questions)
- What movements will be as a result of the actions I am implementing?
- How will the vessel respond to the actions I am implementing?
Influence of speed on turning
- The speed at which a turn begins has little influence on the diameter of the turn.
- At running speed a ship loses about 25% of its velocity after a heading variation of 60º. The speed then stabilizes at about 60% of the initial speed with a helm angle of 35º
- Turning is the easiest and most efficient way of arresting the ship’s forward movement.
- A variation of speed during turning modifies the diameter of the turn. Increase speed, reduce diameter. Decrease speed, increase diameter.
The phenomenon of skidding
Due to lift of rudder (transverse component of pressure force) and centrifugal force, the ship drifts and the rear “skids” to the outside of the turn.
This effect must be anticipated and controlled in confined waters so the ship follows the planned trajectory without coming dangerously close to shallow waters.
The turn can be stopped quickly and the ship brought back onto a straight course by putting the helm over at double the angle used to start the turn and if needed temporarily increasing engine speed as well to improve efficiency of the rudder and cancel the drift.
Influence of helm angle on turning
Unlike speed the effect of helm angle entering a turn is a determining factor in radius/rate of turn. The smaller the helm angle, the greater the diameter of the turn will be.
When turning at a given speed, changing the helm angle modifies the radius of the turn. Radius is reduced by increasing helm angle and increased by decreasing helm angle.
In practice, increasing the helm angle is often the quickest and most effective way of speeding up the turn.
Influence of wind and current on turning
Wind: Causes the turning circle to flatten and the tactical diameter to increase in size due to the chainging effects of the wind on the ship’s windage during the stages of the turn.
Current: In deep water the turning circle will retain its original shape but shift over ground during the time taken to complete in proportion to the speed of the current. In shallow water the effects can be more difficult to anticipate due to hull resistances.
Influences on turning due to ship’s characteristics
- Shape of hull: rounded shapes = tighter turn
- L/B ratio and block coefficient: wide, bulky ship turns more easily
- Engine power and propeller capacity to accelerate water flow around the rudder
- Rudder surface area/drift plane of ship surface area ratio
- Trim of the ship
- Stability
Influence of the ship’s trim on turning
- Positive trim (by the stern) turns easily begins turn under rudder effect. Reversing the helm easily stops the turn.
- Negative trim (by bow) improves hull capacity to counter drift. Although radius of turn is reduced, the rudder’s turning capacity may no longer be effective engough to stop turning inertia. It is therefore preferable to avoid negative trim.
Good turning capacity
Average directional stability
- L/B ratio large
- Cb large
- Ship with bulb
- Negative trim
Less good turning capacity
Good directional stability
- L/B ratio small
- Cb small
- Ship without bulb
- Positive trim
The squat effect
When the ship gets underway, the pressure of the water supporting its weight is reduced as the speed of the water flow along the hull increases (Bernoulli). This leads to sinkage and possible alteration of trim of the ship.
In the shallow water of the harbor environment, the confinement of the hull increases squat.
Squat phenomena will be at a maximum for a depth/draft ratio close to 1 and negligible for a ratio close to 5. Squat effects become significant at a ratio under 1.5.
Reducing squat effect
In practice it is sensible to reduce the squat phenomenon by keeping the ship’s speed down as soon as the first signs appear (waves, vibration, etc.) When crossing or overtaking another vessel, it is also important to preserve a minimum distance between the vessels equal to the width of the larger of the two, and of at least 30 meters. When circumstances allow this safety distance must also be maintained between the vessels and the banks when passing through a channel.
Barass Formula simplified
open water (depth is reduced but not width of channel)
e (squat in m) = Cb • V2 / 100
confined water (depth and width of channel are reduced, blockage factor between 0.06 and 0.30)
e = 2 • Cb • V2 / 100
Reflection from bank
In practice, the bank suction and the bank cushion effect combine and are normally linked using the term bank reflection.
Interactions between ships
Passing: Each ship experiences lateral effects created by the pressure zones that surround them. The ships experience these effects proportional to the relative approach speed and sizes of the ships. These hydrodynamic pressure zones can be felt up to about one ship’s width on either side of the hull. The ship that has the lower tonnage experiences the greater effect.
Overtaking: During overtaking the hydrodynamic phenomena generated is typically greater and will last longer than when passing. The ships therefore have to reduce speed beforehand.
It should be noted that the blockage factor of ships passing or overtaking increases for the same channel configuration. This means that the squat of the two ships will be greater than if they were alone. It is only possible, therefore, to overtake in a river/narrow channel at slow speed, in the widest part of the channel, in order to limit interaction effects.
Maneuvering in current
The shiphandler may:
- Angle the ship into the current, creating a transverse movement that is very helpful when coming alongside or moving off.
- When turning, utilize the pivoting moments generated by the difference in speed of the current between slack water zones and the current flow itself.
Anchor areas of use
- Commercial reasons (mooring off, loading/unloading goods)
- Seeking shelter (poor weather conditions, repairs)
- Emergency mooring (damage when in shallow water or in a channel)
- For maneuvering (tying up bow and stern to dockside, coming alongside the dock and moving off)
Anchor holding
To improve the ship’s holding at anchor, the anchor shank should not lift off the ground. This is accomplished by paying out enough chain for prevailing sea conditions.
Different anchoring methods
Mooring on one anchor: Easiest and most commonly used method. Safe for use in bad weather
Mooring on two anchors: Used when the ship needs to reduce turning radius and sway considerably. Not suitable for bad weather.
Mooring in bad weather: A second anchor can be “backed” to windward, 20°-30° from the first anchor rode with 1-2 shots difference between the first rode length. Alternatively, the second anchor may be dropped vertically to reinforce the holding.
Mooring with head and stern anchors: Used when the ship needs to stay at the same heading. Not suitable for bad weather
Emergency anchoring: If the ship is loaded, it first has to be slowed down by dragging the anchor (1-2 shots depending on water depth)
Stopping Maneuvers
- Inertia: slowing down and stopping without power
- With the propeller running in reverse
- Turning alone
- Zigzag movement, combining turning and engine in reverse
- Dropping anchors to act as drag
- Using external means (tugboats, mooring lines)
Conditions affecting the ship’s stopping distance
- During deceleration the wave stream created by the ship creates a propulsive force linked to the previous movement and mass of water. This force increases the ship’s inertia 10%.
- Confined or shallow waters (depth less than 4x draft) increase hull resistance but reduce the efficiency of the propeller in reverse. In the final phase of the stopping maneuver, prop walk effect is greater.
- Wind and waves modify trajectory and help to increase hull resistances and generally stopping distance (except following wind or sea).
Stopping with single-shaft, fixed-pitch propeller
- Most common propulsion, gives maximum power astern.
- Time needed to put engine astern about one minute.
- Most important factor is speed of ship when put astern. Speed less than 7 knots, reversing of the engine is reliable and gives its full power very quickly. Not the case at full ahead or sea speed. The greater the ship’s velocity the greater the propeller drive power working against starting the engine in reverse - therefore best to reduce speed to 7 knots or less before starting engine astern. In addition propeller efficiency is reduced with significant headway on because it is trying to accelerate a mass of water forward whose initial speed is toward the stern.
Stopping with variable pitch propeller
- Able to move from full ahead to full astern quickly while engine rotation continues in same direction. If this transition is applied too abruptly, the load on the engine becomes significant and the propeller develops cavitation therefore limiting thrust. Better to reduce speed before going astern.
- The flexibility achieved by quickly reversing the blades is at a cost of less efficiency astern compared to fixed pitch (often around 50% of available power in forward motion).
- Ships with twin shafts can keep a straight heading when running engine full astern.
