Ship Manoeuvring Principles and Pilotage

Question 1

Maneuvering Plan

Answer 1

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.

Question 2

Comparison by ship’s lengths

Answer 2

Ship’s length is main guide for comparing ships. Better than tonnage or any other measurement.

Question 3

First feature to note

Answer 3

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.

Question 4

A standard ship / old and modern designs

Answer 4

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.

Question 5

Reducing a ship’s speed to maneuver

Answer 5

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.

Question 6

Speed indicating instruments

Answer 6

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.

Question 7

Turning round

Answer 7

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.

Question 8

Transverse thrust effect

Answer 8

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.

Question 9

Turning a ship short round

Answer 9

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.

Question 10

Centre of Turn

Answer 10

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.

Question 11

The effect of forces on a ship and Center of Turn

Answer 11

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.

Question 12

Resistance to turning a ship

Answer 12

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.

Question 13

Sideways Slip

Answer 13

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.

Question 14

The effect of wind on a ship being maneuvered

Answer 14

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.

Question 15

Splitting the wind into two elements and definitions to support those elements

Answer 15

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.

Question 16

Drifting moment

Answer 16

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”.

Question 17

Wind force moment

Answer 17

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.

Question 18

Combining the effects of the drifting moments and wind force moments

Answer 18

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.

Question 19

Berthing a ship in calm conditions (little wind or tide to consider)

Answer 19

Plan the final approach to the berth heading for a point on the berth about one third of a ship length from forward, of the final berthed position, at a final speed so you can stop the ship in about half a ship’s length by going dead slow or slow astern. This speed will be determined by the type of ship and loaded condition.

Don’t try bringing the ship directly alongside the berth, but plan to bring the ship parallel to the berth and all stopped, just short of the berthing position and about 1/10 of the ship’s length off the berth. This will give you a sensible amount of room for safety should your maneuver not go exactly according to plan.

For much the same reason as giving yourself room for the maneuver not going exactly to plan, the author also suggests planning to bring the ship nearly to a stop about one ship’s length from the berth, prior to the final part of the maneuver approaching the berth.

When closing a berth in good visibility, in order to appreciate how to control the ship, it is necessary to monitor and compare the ship’s movement through the water with the progress the ship is making over the ground, by watching the ship’s speed and direction through the water against the relative movement of objects and other markers ashore.

Question 20

Use of indicating instruments when maneuvering

Answer 20

Some ships have various instruments, mainly working on the Doppler principle, to indicate ahead/astern movement of the ship and the amount of set and drift, normally indicated by the speed the bow/stern of the ship is making sideways. These are excellent aids, but visual assessment of relative movement of objects ashore is quicker and more accurate indication of direction and speed the ship is making over ground. When no objects are in sight these indicators are useful.

Question 21

Berthing in tidal waters

Answer 21

Easier in a tideway because you will be able to steer and control the ship while remaining in the same spot over ground. With a tide running it is easier to close the berth by canting the ship and allowing the tide to bring the ship as close to the berth as you want and eventually alongside. The further the ship is off the berth, the more it can be canted towards the berth, turning to reduce the angle as the ship closes the berth. It will also be necessary to reduce speed, otherwise the ship will move ahead of the berth.

If, for some exceptional reason, you have to berth a ship stern to tide, then it is necessary to mentally reverse the ship and think as though the bow is the stern and the stern is the bow.

Question 22

Unberthing

Answer 22

Most popular maneuver to hold forward spring and cant ship towards berth with lines, engines, or combination thereof. When the stern is sufficiently canted off, stop engines, and if clear to do so, let go everything. The ships CoT will move to a position relative to its trim and speed through water. Go dead slow astern if you want to minimize transverse thrust or full astern if the transverse thrust is going to help you clear the berth. As the ship picks up sternway, CoT will move aft, which will reduce the turning effect of the transverse thrust.

Alternatively the stern spring may be held and the bow canted off. The risk here is damage to propeller. You may be able to give a touch astern to help get the bow off a little bit more, but there is not much else to do here with the engines.

Unberthing with a tide running is much easier. As the tide gets between the berth and the ship use engines and rudder as necessary to move bodily off the berth.

Question 23

Berthing to buoys

Answer 23

Single buoy: While stemming the current and allowing for wind, steam up to the buoy until you judge that it is nearly abeam of the ship’s stem, either visually or by radar, comparing the distance to the ship’s stem with the distance to the buoy. Once a line is made fast, the ship can be allowed to lie to the buoy.

Question 24

Relative speeds/Berthing a ship in tidal waters

Answer 24

If berthing a ship after stemming a tide by approaching the berth at a steady angle and by crossing the tide with with just enough speed so the tide can move the ship towards the berth on a steady bearing, then as you close the berth and swing to become parallel to it, you will have altered the ship’s head. Therefore the steady bearing of the berth will now start to change and in this case start moving astern, giving the illusion that the ship’s speed has increased, but of course it has not. What was a transverse component of engine speed when the ship was canted has now been shifted to an ahead component of engine speed with the ship stemming the tide head on. The bigger the angle at which you approach the berth, the greater this effect. To avoid going past the berth as you swing parallel to it, it will be necessary to go astern on the engines to reduce the ships speed as you close the berth.

Question 25

Berthing a ship using an anchor

Answer 25

An anchor may be used even on ships fitted with a bow thruster or with the assistance of a tug, but make sure the tug is well clear when you drop the anchor. Using an anchor on loaded ships up to 10,000 GT or empty ships up to 15,000 GT, provided the nature of the bottom is suitable (soft mud or sand) is pretty routine. Above that size, if there is a current running or a strong wind blowing, then the risk of breaking the anchor chain becomes significant and should not be considered unless there is no alternative like tugs to use. In which case, if you do decide to use an anchor or if you have to anchor in an emergency, then try and maneuver at minimum speed possible while dragging the anchor. If there is no current or wind, then with care, an anchor can be used safely on 30-40,000 GT ships. The main reason for using an anchor when berthing a ship, other than when in a restricted area, is to control the bow when it is being blown off by the wind.

Question 26

Mechanics of using an anchor to berth

Answer 26

At least 1 ship length from the berth (more is better), let go the offshore anchor and pay out 1.5 times the height of the anchor above the bottom for a start. In practice this means let go the anchor and hold it once it is on the bottom. If that is not enough to keep the ship's head steady, then pay out more until it is enough. You can check that there is enough weight on the anchor and that it is dragging by stopping the engines, if the CoT is observed to move towards the hawsepipe and the ship starts to loose way because of the weight of the anchor dragging, then it's about right. You can then use the engines to steer the ship without gaining too much headway and so control the ship against the effect of the wind. Avoid usng the shore side anchor as you would be more likely to damage the ship or anchor/chain. If not familiar with using an anchor at a particular berth, it would be better to let go the anchor further from the berth to give yourself more room to get the length of chain right.

Question 27

Tug length of tow line

Answer 27

When a tug is on a tow line which is too short, especially if the ship has a high freeboard, a steep tow line angle is created so that when the tug pulls on it, a lot of the tension on the tow line will be trying to pull the tug out of the water rather than towing the ship and could end up breaking the tow line.

Question 28

Transverse thrusters

Answer 28

Great assistance when maneuvering a ship, pushing the bow or stern sideways as required, without affecting the ship's STW at all. Good thrust relative to power Drawback is that they only supply maximum effect when stopped. As the ship starts to move through the water, their effect falls off rapidly and at about 3 knots cease to have any appreciable effect at all. If they are not deep enough in the water then their effects are diminished.

Question 29

Maneuvering in fog by radar

Answer 29

First concern is safety of the ship If radar is not fitted or in error, find a place to anchor and wait for fog to lift. If vessel has good radar, you may continue, providing there is no local harbor rule stopping you. What defines good radar? Good picture - define a buoy at 6 miles and within 1 cable on 0.75 mile range, heading marker is accurate - anything more than half a degree out should be treated with great caution. If considering closing a berth by radar: Do you have a good gyro stabilized radar, appropriate ship size/maneuverability, competent crew, density of the fog, amount of wind or current? If there is any doubt then dont! If decided safe to close berth by radar: Biggest problem is judging speed and set/drift most electronics lag (GPS) or are inaccurate below 1kt (Doppler). If you can see a light it will help with visual speed cues, so in poor visibility, arrange to have all berth lights switched on. Best way to establish speed in poor visibility is radar. VRM may be used from known stationary echo, so far as possible, right ahead or astern and on the smallest range possible. Adjust the VRM down by 0.017 nm (30 meters or 100 feet) or multiples thereof and time how long it takes the ship to move to bring the VRM back to the echo again. On the basis that 100 feet/min is 1 knot you can calculate the speed. This may be better delegated to another bridge team member so that you may focus on the maneuver.

Question 30

Anchoring a ship

Answer 30

To bring a ship to anchor: Position the ship over the spot where you have decided to let go the anchor, all stopped, and stemming the direction of any flow of water before you let go the anchor. If the visibility is good enough to be able to used fixed objects to establish position and the ship's movement over ground visually, then the job is simple enough In poor visibility or open sea, with no fixed objects to use stem the flow of water at the speed of the flow as best you can. Let go the anchor from the waters edge or depending on water depth, lower to near the bottom, then let go. Pay out enough chain to be sure that the anchor is on the bottom and holding, let everything settle so the ship is lying at anchor comfortably, if necessary, pay out more chain. If anchoring in a restricted anchorage, select a position in the anchorage with enough room to swing without the risk of colliding with another ship

Question 31

Anchoring a ship using radar

Answer 31

Use north up relative display Select a suitable isolated target on the radar screen as near to the anchorage position as possible. Set the cursor on the bearing and distance of the selected target from the position at which you want to let go the anchor and with a chinagraph pencil draw a circle (radius = distance between radar scanner and anchor) centered on the position of the cursor. Alternatively, set the VRM to the above radius and in your mind visually transpose the circle made by the VRM over to the cursor and use that to avoid drawing on the radar screen. Now you have to maneuver the ship to put the point on the edge of the circle making a bearing from the center of the circle that corresponds to the ship's head, on the selected target. You will then be in the correct position to let go the anchor.

Question 32

How much anchor chain to use

Answer 32

Depends very much on the holding ability of the bottom, the sea, swell, and wind conditions. With good holding ground such as mud, clay, and sharp hard sand, less chain is needed than with stone and smooth, soft sand. Anchoring in a good sheltered anchorage, with a UKC at chart datum no more than the ship's loaded draft: LOA up to 100m - 3 shackles, up to 150m - 4 shackles, and over 150, - 5 shackles should be enough to hold any normal standard ship.

Question 33

Passage planning

Answer 33

The reason for having a passage plan (written or mental notes) is so that when compared with the ship's progress you will immediately become aware of wether you are on schedule and can complete your passage safely and on time. Simplest passage plan is working out an ETA in an area without tidal streams, followed by making allowances for tidal current, then allowing for depths of water and tidal heights to ensure sufficient UKC. Passage plan can be headed with all the particulars of the ship - name, date, size, draft, trim and anything else you might want to note, like maneuvering speeds, berthing aids, etc.

Question 34

Correcting for set and drift across narrow channels

Answer 34

Make larger alterations than necessary to just balance for set and drift, so that you not only maintain your required course, but get back on track. Should you still not have enough correction for set and drift after making a course alteration then make the next alteration larger than you think is necessary since it is essential to get back on track, otherwise you may never catch up before it is too late. Ship's speed is factor in allowing for set and drift. Changing ship's speed will change the amount of allowance needed for set and drift.

Question 35

Overtaking another ship

Answer 35

As a general rule, in a narrow channel, overtake another ship on its port side. If you are overtaking on the ship's starboard side, you are restricting its ability to take avoiding action by altering course to starboard for another ship, leaving them only the option of slowing down. If the overtaken ship is deep draft and needs to stay in the middle, you should overtake on its starboard side. It is perfectly seamanlike to overtake a ship on its starboard side if there is plenty of sea room between you and the starboard side of the overtaken ship so it may alter course to starboard if it wants to. It therefore follows that small, shallow drafted and slow ships, should if possible, keep well to the starboard side of a channel, leaving plenty of room for other ships to overtake on its port side. To calculate when and where you expect to overtake another ship, you need to know the rate at which you are catching up to that ship. The VRM on the radar can be utilized to measure the reduction in distance over a time period, allowing calculation of catching up speed and therefore calculation of where you will overtake the other ship.

Question 36

Identifying ships with the VHF

Answer 36

When making your initial call (to ensure no discrepancy in identity): After giving your ship's name, you must at the very least also give the ship's position ans the direction it is heading. Then state the ship that you wish to speak to, such as: "This is the North Peak, inbound, passing the blacksand buoy, calling the southern most of the two outbound vessels approaching the no. 2 fairway buoy. " Assuming you get a correct answer, you should be able to complete your message without any further problems, however if the answer is not conclusive, such as: "Yes, good evening and what can I do for you?" you will then have to request a correct identification until you are sure you have established contact with the ship you require.

Question 37

Navigating ships with bridge forward

Answer 37

If not careful, it is easy to pass buoys and other marks closer than intended to. Due to different perspective than the average aft house ship.

Question 38

Taking the ship's way off

Answer 38

Rudder cycling: If necessary to slow a ship down quickly from steaming at half or even full speed and the channel or traffic conditions preclude from leaving intended track, then reduce engine speed or stop engines (depending on STW). Put the rudder to port as far as you can, say 20° (again depending on STW and severity of situation). Once the ship has swung 10° to port, amidships the rudder, and then put the rudder as far over to starboard as you can, lets say 25° and when the ship has swung 10° to starboard of its original course, amidships the rudder again, then hard over to port and then hard to starboard until the ship is slowed down to a comfortable speed. If the ship is going more than 5 knots ahead through the water, this will reduce the ship's speed much quicker than going astern on the engines, assuming the engines could be put astern, which if the ship is loaded is highly unlikely. In emergency with no alternative to a serious accident because of an engine or steering gear failure, one or both anchors could be let go and held as soon as they were on the bottom so that they will drag and take way off the ship.

Question 39

Tug and tow

Answer 39

Consider length of tow line: Outbound - start short and lengthen as appropriate. Inbound from sea - will need to be shortened at some point before arriving at berth. When sea room is constrained in restricted waters, tow line must be short enough to control the tow from yawing too much to stay in good water and avoid collision, hitting a buoy, or running aground. Any time there is no weight on the tow line the tow's CoT will be at its free running position, depending on its speed, draft, and trim. Whenever there is any weight on the towline, the tow's CoT will move towards the position where the towing line leaves the tow. In theory, it is a matter of proportion between the tow and tug as to how much it moves. When wind force acting on the tow exceeds 1/4 of the tug's power (exceptional situation), then the tow's CoT moves to a point between whert the tow line leaves the tow and the position of the tow's free running CoT. If tow is barge with squarew shaped bow a bridle should be used. A ship shaped tow should use a tow line leading from the tow's stem.

Question 40

Tidal streams

Answer 40

Spring tides bigger than neap tides therefore corresponding tidal streams are much faster with spring tides. Spring tides are faster on rivers with sharp bends, making the tidal streams, counter currents, and areas of slack water more pronounced Rule of thumb: Spring tides start about 2 days before a new or full moon and last until about 5 days after a new or full moon. The time of HW alters about 30-45 minutes later per day during spring tides and 45-75 minutes later per day during neap tides. Tides also tend to alternate , each day one big tide and one lesser tide and each month there will be one big spring tide and one lesser spring tide.

Question 41

Squat

Answer 41

Basic principal that causes squat is that as ships gather speed, the water close to the ship's hull has to move faster than the surrounding water to get out of the way of the ship and due to the Bernoulli Effect, is less dense. This causes the ship to sink down in the water which is known as squat. The greater the ship's speed, the faster moving water therefore, it will be less dense and the ship will squat more. Factors governing how much a ship squats: lines of the ship's underwater hull, draft, speed, UKC, and block coefficient (bigger block coefficient squats more). Depending on UKC and speed, a vessel may squat to the point of grounding. As UKC reduces control of vessel steering may be lost. When steaming too fast in shallow water - 4 indications that ship is squatting and there is insufficient UKC: 1. The ship starts sheering off course unexpectedly 2. Increased vibrations from the propeller. 3. Stern waves getting steeper and the tops of the waves breaking. 4. A reduction in the propeller RPM, with the consequent reduction in speed. Whenver aware of any one of these indications, an immediate reduction in engine speed is required.

Question 42

Passing ships interaction

Answer 42

When two ships are passing close to each other, the pressure waves found ahead of the bow meet, causing interaction that forces the ships bows AWAY from each other. After the pressure waves have passed the two ships 'faster moving water' meet and join, causing an interaction that draws the two ships bows TOWARDS each other. The first indication you will get from this interaction between two ships passing will be through the steering, when the ships bow turns away from the other ship. You must react immediately by putting on rudder to turn the ship away from the other ship and by reducing your engine speed, but not too much, so that you keep enough thrust from the propeller to allow the rudder to be fully effective to steer away from the other ship. When the two ships are abeam of each other the wheel can be brought amidships. When your bow starts to clear the other ship's stern, apply rudder toward the other ship and increase speed to counter the effect of the two ships sterns being drawn together.

Question 43

Overtaking ships interaction

Answer 43

If your ship starts interacting with a ship you are overtaking, immediately make a big reduction in your ship's speed and put the wheel of your ship hard over to turn away from the ship you are overtaking. The overtaken ship may decide to reduce speed and/or it may turn its rudder towards your ship to counteract the effect of its stern being drawn toward your ship or the overtaken ship may prefer to allow the ship to swing away.

Question 44

Reaction between the ship and the bottom

Answer 44

Authors experience is that when a ship is only moving slowly over the ground (5-7 knots depending on size and shape of ship) when it has little UKC, the water between the ship and the bottom forms a cushion which has the effect of pushing the ship away from shallow water or steep banks towards deeper water or the center of the channel. On the other hand, if the ship is going faster, then the lower density of the 'faster moving water' close to the ship's hull extends further than when the ship is moving slowly and the effect will be the ship being drawn towards the shallows.

Question 45

Draw off

Answer 45

When a ship moves through the water, it develops waves ahead of it and to a lesser extent leaves waves astern. Normally not noticable out in the open but more significant in restricted waters. If the ship is moving along a canal where the cross section of the ship may be a quarter or more of the canal cross section, then the waves grow in size and significance and travel up to a mile or more ahead of the ship.

Question 46

Drag

Answer 46

The accumulation of all the different forces acting on a ship in shallow water, which cause it to lose speed.