Ship Manoeuvring Principles and Pilotage Flashcards
Maneuvering Plan
Is is essential before starting any maneuver to have in your head a plan of how you plan to maneuver the ship, taking into account wind, tide, state of the ship’s trim, draft and freeboard, the ship’s equipment and maneuvering aids etc, including tugs.
Pilot must acquaint the ship’s captain of his plan so any differences can be resolved.
Any special circumstances may need to be discussed with working parties fore and aft.
The plan needs to be flexible with alternatives in mind.
Comparison by ship’s lengths
Ship’s length is main guide for comparing ships. Better than tonnage or any other measurement.
First feature to note
On any ship, the first feature to note is how deeply loaded it is. A loaded ship is many times heavier than a light ship but the engine power remains the same. Loaded condition also dictates the amount of resistance there is to the effect of the wind.
A standard ship / old and modern designs
The standard ship under discussion (unless indicated otherwise) is a normal average ship, has a loaded draft about 1/2 its beam (old styled) or a draft of 1/3 its beam (modern ship) and a length of 6-7 times its beam with engines and superstructure aft.
The old styled ship will normally maintain its speed better in heavy weather, whereas the modern more beamy and not so deep ship is reckoned to be more economical.
Reducing a ship’s speed to maneuver
Rule of thumb guides:
1st rule: When steaming full ahead, fully loaded, even keel, slack water, calm weather - start reducing the ship’s speed a minimum of at least 20 times the ship’s length from the position which you require to start maneuvering the ship. Maneuvering means the speed at which you can safely go astern on the engines - normally about 5 knots fully laden. You need 5 more ship lengths to bring the ship to a stop. If the ship is empty, she will slow down quicker, maybe 10 ships lengths and it will be easier to put the engine astern. Reduce speed in stages (half, slow, dead slow), do not make big or quick engine reductions as this will reduce the thrust from the propeller onto the rudder and it is possible to loose control of the ship, particularly if there is little UKC and especially with CPP.
Speed indicating instruments
When using instruments to help you judge your speed as you approach a berth, keep in mind: 1 knot = 100 feet (30 meters) per minute or 0.017 NM per minute using the VRM on a radar.
A waring regarding instruments (not including radars): When maneuvering a ship with alterations happening quickly, they tend to trail a long way behind what the ship is doing and the indications they give for speed and set/drift are not reliable, especially regarding GPS where the speed is averaged over a given length of time.
Turning round
Rule of thumb guide:
2nd rule: With a fully loaded ship on an even keel in calm weather, making no more than 5 knots ahead, if you put the rudder hard over to port or starboard (35° on standard rudder) and steam at engine speed of about 5 knots (normally slow ahead) you can expect the ship to turn round 180° in less than 4 ship’s lengths. A light ship will turn around in less room. Check to ensure you get 35° rudder, on some older suspect ships it is common practice to use less for hard over.
When the ship is stopped or nearly so, then an engine speed of 5 knots is about the best speed to use to turn an average single screwed fully loaded ship on even keel around in the smallest area. A fast fine lined, high powered ship may only need dead slow to make the tightest turn.
If you want to start the turn quickly, start with a kick of half ahead on hard over rudder to start the ship swinging, then reduce to slow or dead slow as required.
If the ship is making sternway through the water, put the rudder hard over, then to make sure the thrust of water from the propeller reaches the rudder, go half or even full ahead on the engines. As the ship reduces her sternway, reduce the engine speed until the ship is stopped in the water. If you wish to continue the turn, continue going slow or dead slow on the engines as necessary.
Transverse thrust effect
The element of the propeller turning in the water which paddles the stern of the ship sideways but does not contribute to propelling the ship ahead or astern.
When going astern the effect is far greater.
With a normal RH propeller it will turn the ships head to port going ahead and to starboard when going astern.
If going astern and the ships head does not swing in the direction expected from transverse thrust, then it is almost certainly due to the effect of wind or current. If the circumstances require and allow, an increase in astern power could be sufficient to override the wind/current so the ship will swing in the direction expected due to transverse thrust.
To maximize transverse thrust effect, use maximum engine power (full ahead or astern) to create confused water and reduce the grip of the propeller in the water. Use short bursts if not wanting to gather speed.
To minimize the transverse thrust effect, build up the engine speed slowly in stages so it can gain maximum grip on the water.
Turning a ship short round
Hard over rudder, ahead on engine, as you run out of room ahead, stop engine and go astern on engine and transverse thrust will help continue the turn. As headway is lost, the rudder may be placed amidships and even put over the opposite direction if significant sternway develops. When running out of room astern, the engine is again stopped and put ahead, with rudder hard over to continue turning the ship short round.
Centre of Turn
Otherwise know as the pivot point, is the point around which the ship will rotate as a result of a turning force.
Knowing where the CoT of the ship is is the crux of knowing how a ship will react to any force on it.
Position of CoT:
Initial (stopped) - at the center of the underwater block section of the ship. If the ship is on even keel this will typically be amidships on the ship’s centerline. If trimmed by stern CoT will move further aft. If trimmed by bow, CoT will move further forward.
Moving through the water: Does not appear to move if 1 knot or less. As a practical guide, at 3 knots ahead/astern on even keel, the CoT will move to a point about 1/3 from bow/stern.
The effect of forces on a ship and Center of Turn
The position of the CoT is particularly significant when considering the effect on forces working on a ship when being maneuvered. The forces are:
(a) The ships rudder and propeller: Will only effect the position of ther ship’s CoT insofar as it causes any change in the ship’s speed.
(b) Transverse thrust: Will not alter the position of the ship’s CoT.
(c) Thrusters do not alter the position of the ship’s CoT.
(d) Weight on a mooring line or anchor chain: Will alter the position of the ship’s CoT.
(e) Tugs towing or pushing: Will alter the position of the ship’s CoT.
(f) Wind blowing on the ship and the underwater resistance to wind blowing on the ship are equal and opposite forces that cancel eachother out and therefore do not change the position of the ship’s CoT.
Any outside force exerted on the ship at a specific point (mooring line, anchor chain, tugs, landing on fender, etc) will alter the position of the ship’s CoT, causing it to move toward the point where the force is being exerted.
The movement of the ship’s CoT towards the point a force is exerted is relative to the magnitude of the force in relation to the inertia of the ship. As the magnitude of the force approaches the inertia of the ship, the CoT will virtually be at the point the force is applied.
Resistance to turning a ship
When a ship is stopped in the water on an even keel with the CoT amidships, the resistance to turning will least because it is balanced fore and aft, making the amount of water that needs to be transferred from one side of the bow and stern to the other about the same during a turn. If the ship is moving ahead then the CoT will move ahead, that will mean if the ship is being turned, there will need to be more water moved from one side of the ship to the other aft than forward, so that will use more power to turn the ship.
Sideways Slip
Is when a ship’s stern is pushed sideways when a ship is being turned by the effect of thrust of water from the propeller on the rudder.
The effect of wind on a ship being maneuvered
More complicated and difficult to assess than other forces working on a ship. Its effect is not normally significant up to a force 3 to 4 (7-16 knots) wind.
Splitting the wind into two elements and definitions to support those elements
Drifting moment and movement element: Consists of the moment between the point of balance of the wind force and the point of balance of the underwater resistance, which is mainly determined by the ship’s trim combined with the leeward movement of the ship.
Wind force moment: Cosists of the moment between the point of balance of the wind force and the ship’s center of turn.
Definitions for studying the drifting moment and movement element:
Drifting movement - movement of the ship to leeward.
Wind force point - point where the force of wind blowing on the ship’s windage is centered. This will not change as long as the wind remains from the same relative direction.
Wind resistance point - point on the leeward side of the underwater section of the ship where resistance (to wind) moments balance
Drifting moment lever - the distance between the wind force point and wind resistance point, measured at right angles to the direction of the wind.
Drifting moment
The product of multiplying the “drifting moment lever” by the force of the wind. It is the moment that turns a ship, when drifting, onto the heading with the wind force point and wind resistance point in line with the direction of the wind, onto the “drifting heading”.
Wind force moment
Determined by the position of the wind force point relative to the position of the ship’s center of turn. The resultant of multiplying the wind force lever by the wind’s force.
Combining the effects of the drifting moments and wind force moments
Normally one effect will be dominant but this can change with just an alteration of the ship’s head or speed, so when maneuvering a ship it needs to be safely monitored.
Before maneuvering a ship when sailing, starting from a position with the ship stopped. It will be necessary to assess the relative positions of the relative positions of the wind force point, wind resistance point, and the position of the ship’s initial center of turn, then which way the wind force will try and turn the ship. If they are inline with the wind direction, then the ship will be on its drifting heading. If they are not, then it is necessary to assess which direction the drifting moment will turn the ship’s head and which way the wind force moment will turn the ship’s head. If they are in opposite directions then which will be the most powerful force.
After assessing the how the ship will react when stopped in the water, now you must assess how the ship will react when it starts to move through the water. There will be two main elements to watch, the change in direction and force of the wind due to ship’s heading and speed changes, and any change in the ships CoT due to moving ahead or astern.