STEERING A DOMESTIC VESSEL
(Extracts courtesy of A.N.T.A. publications, Ranger Hope © 2008 www.splashmaritime.com.au)
Master's instructions to watchkeepers
Manoeuvring difficulties of large vessels
Most vessels are steered with the aid of a rudder, which is rotated to the required angle by a steering mechanism but they will only be effective if water is crossing over them, i.e. if the vessel is moving or if the propeller is thrusting water across them.
The type and size of steering mechanism depends on the size and design of vessel.
Rudders
The National Standards for Commercial Vessels specifies the sizes and materials used for the compliant construction of rudders dependant on vessel class, size and operational zones. Two basic methods of mounting the rudder are shown in figure 1.
Figure 1 Rudders
Drawing (a) shows a mounting arrangement where the forces on, and the weight of the rudder are carried by the rudder stock. This means that the stock and bearings should be of adequate size and strength to withstand stresses under any weather condition.
Drawing (b) shows a pintle type rudder arrangement. The rudder is hung on pintles and has a further pintle in the extension to the keel.
The rudders are secured to the stock by flanged couplings and these need to carefully checked whenever the vessel is out of the water.
There is usually a gland around the rudder stock to prevent water entry into the hull. This needs to be checked regularly and possibly the packing or seals renewed when the vessel is in a safe situation (usually out of the water).
Next lets consider the mechanisms used to rotate the rudder, i.e. the steering gear. These can range from a simple wire and pulley system to a more complex electro-hydraulic one.
Wire and Pulley
A wire and pulley arrangement is shown in figure 2 which consists of a wire wound around a drum fitted to the wheel. The wire passes through a series of pulleys on the two sides which connect to the tiller or quadrant of the rudder mechanism. To avoid excessive strain and bending of the wire the pulley blocks should be as big as possible and positioned to avoid an excessive angle or be easily fouled. Buffer springs are provided on both port and starboard to prevent violent recoil of the steering wheel. All components should be inspected and greased or oiled as appropriate and the cable must be kept tight.
Figure 2 Wire and pulley steering gear
Chain and box steering gear
A wide variety of chain and box installations make use of automotive parts such as shafts, universal joints and truck steering boxes. These systems require periodic inspection and lubrication. The chain is liable to stretch and should be checked regularly. For this reason the chain length is usually adjustable.
Figure 3 Chain and box steering gear
Push-Pull Cable
A push-pull cable type steering arrangement is shown in figure 1.6. This arrangement is similar to that used on outboard motors. The length of the cable should not be too long or short as this can affect the tiller response. If the push-pull cable or rod seizes, there must be provision for releasing the push-pull rod from the tiller to operate the emergency steering.
Figure 4 Push-Pull Cable
Basic Hydraulic Systems
Hydraulic systems are common in vessels of 20 metres or more in length. These systems range from simple manual systems to electro-hydraulic ones. Figure 5 shows a simple manual system with a single steering station.
The system operates utilising the flow of hydraulic fluid under pressure to control the movement and position of the rudder. The system consists of a two way hydraulic pump, usually an internal gear pump, connected to the wheel. Two pipes lead from the pump to the hydraulic cylinder and ram, which in turn is connected to the tiller. The rotation of the wheel will force oil from the pump to one side of the ram thus rotating the rudder.
A major problem associated with any hydraulic system is air in the system therefore, most systems are built to be self purging of air.
The emergency steering will require the oil to be by-passed and a valve is placed between the two sides of the system for this purpose. To prevent hydraulic locking, this valve will need to be opened when the emergency system is to be operational. In addition, there should be a relief valve which spills the oil from one side to the other in the event of shock loading to the system.
Specific maintenance requirement for this system is to ensure that the oil level is adequate and that there are no leaks.
Figure 5 Basic Hydraulic Systems
Simple Telemotor
In most larger systems the signal from the steering wheel is transmitted to the steering gear by means of a telemotor. This not only ensures that the steering system is isolated to the steering flat, it also means that the steering system can be used even if the wheel and connections are damaged or become inoperative.
Figure 6 shows a steering system incorporating a telemotor. The latter consists of a transmitter in the wheelhouse and a receiver in the steering flat. The movement of the wheel activates an hydraulic piston in the transmitter. The fluid displaced by this piston is used to displace a similar piston in the receiver. This movement is used to control the main steering gear’s hydraulic pump, which in turn operates the steering gear and rudder. The receiver is usually spring loaded so that the steering wheel will easily return to the midships position.
Figure 6 Simple Telemotor
Electro-Hydraulic
The electro-hydraulic system, shown in figure 7, has the advantage that the signal from the wheelhouse to steering flat is transmitted by electrical wires. Further, the system uses a uni-directional pump which is less complicated and cheaper than a bi-directional.
The pump supplies oil at a constant rate to a directional control valve, which is usually positioned in the steering flat.
The valve consists of three positions, and depending on the position, will supply oil to either side of the double acting ram. When in the neutral position, oil is locked in the ram, thus maintaining the given rudder angle, whilst the pump flow is circulated back to the tank. The valve is operated by solenoids controlled from the wheelhouse via the control box.
As with the previous system there is a by-pass and relief valve fitted between the left and right sides of the ram. Emergency steering can be carried out by operating the emergency steering lever located in the steering flat.
Figure 7 Electro-hydraulic system
Requirements for Steering Gears
Steering Gears are surveyed during a vessel’s annual and periodic surveys. The requirements for steering gears are laid down in NSCV. These are summarised below.
General Design
• All vessels except twin screw vessels and vessels where the normal means of steering is a hand tiller shall be fitted with two independent means of steering.
• The steering gear shall be of adequate strength to steer the vessel at maximum speed both ahead and astern.
• Rudder movement should be 35 degrees port and starboard.
• In vessels 12.5m and over the steering gear shall be capable of putting the rudder from 35 degrees on one side to 30 degrees on the other in 30 seconds at maximum speed.
• The steering gear shall be so designed and constructed to prevent violent recoil of the steering wheel.
• In hydraulic systems, changing over from primary to secondary systems should be able to be carried out easily and quickly.
• Power driven hydraulic systems shall be fitted with a relief valve to prevent mechanical damage.
• The rudder indicator shall move in the same direction and give a true indication of the rudder angle.
• If the emergency steering is remote from the steering/navigation position an adequate form of communication between these two positions shall be installed.
• Where necessary the steering gear will be fenced and have adequate guards to avoid injury to personnel.
As stated above if the tiller and hence the rudder is rotated by hand as in small vessels it is not required to have a back up or emergency system. However, in most modern vessels a mechanical means is employed to move the tiller, thus requiring an emergency back up. This may take the form of a hand tiller, which can be quickly and easily fitted to the top of the rudder stock. This emergency tiller must be kept in a place close to the steering flat and stock.
Different seamen will use different means to express the direction of buoys and lights from the vessel.
Often the direction of an object is be described as being seen over the port bow, or abeam to starboard or over the starboard quarter (as shown below), hence giving eight possible sectors of to view. Without the advantage of a sighting device a more refined description of directions can liken the all around view to a face of the clock, hence, straight ahead is at 12 o’clock, behind at 6 o’clock abeam to port at 3 o’clock etc. It can be seen here that the 360º of all around has been divided into twelve sectors of 30º.
Figure
8 Directions
Early seamen in classical antiquity found that there were eight seasonal winds whose different directions enabled them to make useful courses across the Mediterranean sea. These wind directions were incorporated as the cardinal points of the compass card as N, NE, E, SE, S, SW, W and NW. Our all around view is thus divided into eight sectors of 45º. Later, as ships could sail much finer courses, the card was further divided or “boxed” to form 32 sectors or “points of the compass” with each sector being 11.25º.
Figure 9 Compass direction
Hence, seamen in the days of sail might sight a trawler at four points off the port bow, a lighthouse at six points off the starboard bow or might alter course to steer their vessel a point closer into the wind.
Modern seamen are more likely to describe direction in relative angles from the ships heading (lubber line on the compass) or by sighting over the compass.
Relative direction
The relative direction can be described in two ways:
-
By degrees from 000ºRelative to 360ºRelative in a clockwise direction around the compass rose.
-
By degrees Green from G0º to G90º in an anticlockwise direction around the compass rose and
By degrees Red from R0º to R90º in a clockwise direction around the compass rose.
Figure 10 Relative direction
Relative direction is usually determined by sighting over a dummy compass (pelorus) aligned to the ships heading.
Figure 11 A pelorus
Compass direction
The compass direction is usually determined in small vessels by the use of a hand bearing compass that will give a Magnetic reading (000ºM to 360ºM).
However, in steel ships, no matter what their size, deviation will render hand bearing compass direction unreliable so the sighting device of an azimuth ring over the ships compass will be required. This will give True bearings if a Gyro Compass is in use (plus or minus the error of the Gyro) or Compass bearings that require conversion to True bearings, (000º T to 360ºT).
An azimuth ring is fitted over a ships compass
Figure 12 An azimuth ring
It is essential at all times to maintain a watch on the vessel, adequate to the prevailing circumstances and conditions. The IALA Buoyage system assists us in finding the correct channel and avoiding features and hazards. IALA is fully described here.
International Collision Regulations assists us in sharing the waters with other traffic, and is fully described here.
Together these two strategies are described as the Rules of the Road.
Rule 5 Look-out
Every vessel shall at all times maintain a proper look-out by sight as well as by hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision.
Furthermore that watch must be by a person qualified or knowledgeable enough to recognise and interpret the hazards as they appear.
The full collision regulations are presented in either of the recommended texts and you will need to have the full regulations. At first these may seem a bit daunting. However, you may find it easier to learn the rules if we consider broadly what each of the three sections in Part B are about, as follows:
Section 1
The rules in Section 1 always apply, regardless of the visibility.
Broadly, every vessel must keep a proper lookout, proceed at a safe speed, be able to determine if a risk of collision exists and know what action to take if the risk does exist.
Section 2
The rules in this section only apply when vessels can see one another (including night time).
It generally implies that when two vessels meet one has the right of way (called the ‘stand on’ vessel) and the other must give way.
Section 3
The rules in this section apply when vessels are not in sight of one another (in restricted visibility).
There is only one rule in this section and it generally implies that every vessel shall proceed with utmost caution and take avoiding action to keep clear of other boats. That is, no one has right of way if they can not see each other.
When learning the collision regulation remember that the rules in section 1 and 2 can go together and the rules in section 1 and 3 can go together, but the rules in section 2 and 3 are mutually exclusive because you can not be ‘in sight of one another’ and ‘not in sight of one another’ at the same time.
It is essential at all times to maintain a watch on the vessel, adequate to the prevailing circumstances and conditions.
The following are some of the factors that should be taken into account when determining the composition of the watch.
• at no time should the bridge be left unattended.
• weather conditions, visibility, daylight or darkness;
• proximity of navigational hazards;
• use and condition of navigational aids in use;
• whether the vessel is fitted with automatic pilot;
• any additional unusual demands that may be placed on the watch keeper by the operational activities of the vessel.
It is essential that the watchkeepers are well rested and not impaired by fatigue. The watchkeeper should not be under the influence of alcohol or narcotics so as to be able to maintain an efficient and competent watch. If the watchkeeper is not satisfied with the fitness of the relieving watchkeeper to take over the watch, the watch should not be handed over and other arrangements made with the master’s instructions as to the relieving watchkeeper.
Navigation
The intended voyage must be planned, taking into account all pertinent information and any course laid down shall be checked before the commencement of the voyage.
Frequent checking of the vessels position utilising all the available navigational aids, cross referencing the accuracy of one method with another, ensuring that the vessel follows the desired course.
The watchkeeper should also have a full understanding of the operation and limitations of all the safety and navigational equipment available.
The watchkeepers duty is primarily that of keeping watch and should not be required to carry out any additional duties that could interfere with the keeping of a safe navigational watch.
Navigational Equipment
The watchkeeper should have an good working knowledge of the navigational equipment at his disposal, taking into account the limitations, errors and idiosyncrasies of the equipment in use. The watchkeeper shall not hesitate to use the helm, engines or sound signalling appliances of the vessel.
Watchkeeping Duties and Responsibilities
A watchkeeper is not to leave the bridge unless he/she is properly relieved.
Regardless of the presence of the Master in the wheelhouse the watchkeeper continues to responsible for the safe navigation of the vessel till the Master expressly takes over the con of the vessel. If in any doubt as to the safety of the vessel the watchkeeper shall notify the Master immediately.
Taking Over the Watch
When taking over the watch the relieving watchkeeper shall satisfy him/herself of the vessel’s position, confirm its intended course/track and speed and note any dangers to navigation or alterations of course expected during the watch.
The hand over should include but not be limited to:
• standing orders and other special instructions of the master relating to navigation of the vessel
• position, course, speed and draught of the vessel
• prevailing and predicted tides, currents, weather, visibility and the effect of these factors upon course and speed
• navigational situation
• the operational condition of all navigational and safety equipment
• the errors of magnetic and gyro compasses
• the presence and movement of vessel in sight or known to be in the vicinity
• the conditions and hazards likely to be encountered during the watch
• machinery state
• cargo state
• state of auxiliary vessels/tenders
• operational activities
Master’s instructions to watchkeepers
The watchkeeper should inform the Master immediately in the following circumstances:
• if restricted visibility is encountered or expected.
• if the traffic conditions or the movement of other vessels is causing concern.
• if difficulty is experienced in maintaining course.
• on failure to sight land, a navigational mark or to obtain soundings by the expected time
• if, unexpectedly, you sight land, a navigational mark or to obtain soundings.
• on the breakdown of engines, steering gear or any essential navigational equipment.
• if the radio equipment malfunctions.
• in heavy weather if in any doubt about the possibility of weather damage.
• if the vessel meets any hazard to navigation such as ice or derelicts.
• in any other situation in which he/she is in any doubt.
Despite informing the master in any of the above circumstances the watchkeeper must take immediate action if necessary to ensure the safety of the vessel, where the circumstances require.
Steering commands
To drive a large vessel the operations of steering, engine control and monitoring the electronic aids to navigation are often delegated to specialist crewmen. The master or the officer of the watch is directs this team in the process that is called “conning the vessel”. The consequences of misunderstanding an order from the person responsible for conning the vessel could be dire, so a formal dialogue is used to limit such risks.
The master issues an order. The order is repeated by the crewman. If that reply does not concur, the master corrects it. The crewman announces when the action is complete.
Engine controls
Master “ Slow ahead port, sailor”.
Sailor “ Aye, aye, slow ahead port, sir”.
The control setting should be left in this setting until further orders are received.
Master “ Full ahead starboard, sailor”.
Sailor “ Aye, aye, full ahead starboard, sir”.
The control setting should be left in this setting until further orders are received.
Master “ Ahead on both 800 revolutions, sailor”.
Sailor “ Aye, aye, ahead on both 800 revolutions, sir”. And when both engine have slowed.
Sailor “ Now steady on ahead both 800 revolutions, sir”.
The control setting should be left in this setting until further orders are received.
Master “ Slow astern, starboard, sailor”.
Sailor “ Aye, aye, Slow astern, port, sir”.
Master “ Listen again, sailor, slow astern starboard”.
Sailor “ Aye, aye, slow astern, starboard, sir”.
The control setting should be left in this setting until further orders are received.
Helm controls
The directional orders for steering may be:
a. by rudder - to the displayed angle on the rudder indicator.
b. by compass - to the required course.
c. visual - to steer for a distant feature or object.
Rudder indicator
Master “ Steer 10º to port, sailor”.
Sailor “ Aye, aye, going 10º to port, sir”. And when the rudder indicator shows 10º to port.
Sailor “ You have 10º to port, sir”.
The wheel should be left in this setting until further orders are received.
Master “ Hard to starboard, sailor”.
Sailor “ Aye, aye, going hard to starboard, sir”. And when the rudder indicator shows 35º to starboard.
Sailor “ You have 35º starboard helm, sir”.
The wheel should be left in this setting until further orders are received.
Master “ Ease to amidship, sailor”.
Sailor “ Aye, aye, easing to amidship, sir”. And when the rudder indicator shows 0º to starboard.
Sailor “Helm amidship, sir”.
The wheel should be left in this setting until further orders are received.
Compass
Master “ Steer 220º, sailor”.
Sailor “ Aye, aye, going 220º, sir”. And when the compass shows 220º.
Sailor “ Steering 220º, sir”.
The vessel should be kept on this course until further orders are received.
Visual
Master “ Maintain a ships width off the bank, sailor”.
Sailor “ Aye, aye, a ships width off the bank, sir”.
The vessel should be steered to keep a ships width distant from the bank until further orders are received.
Combined
Master “ Ease to amidships, sailor”.
Sailor “Helm amidship, sir”.
Master “ Steady by compass, sailor”.
Sailor “ Aye, aye, steady by compass, sir”.
The vessel should be kept on this course that the compass now reads until further orders are received.
Master “ Ease to amidships, sailor”.
Sailor “Helm amidship, sir”.
Master “ Steady as she goes, sailor”.
Sailor “ Aye, aye, steady as she goes, sir”.
The vessel should steered for the object now sighted dead ahead until further orders are received.
While a smaller vessel with fewer crew may be much less formal, the consequences of misunderstanding are no less severe. Similar principles must be adopted especially when getting it right first time counts.
An automatic pilot - a device that will automatically steer the vessel on a pre-determined compass course -is standard equipment in most ships today, and most fishing skippers will readily admit that such an instrument is worth two extra men in the crew.
A good auto-pilot will steer a better course than any helmsman, provided the controls are properly set, and this aid has been claimed to save as much as 20% in fuel on a long passage -as well as improving the standard of navigation.
The auto-pilot consists of:
* a course sensor built into the compass, which can be set to any pre-determined course;
* a panel with up to four settings, depending on the sophistication of the equipment;
* a means of transmitting rudder instructions to the steering or, in a small vessel, a special steering motor which is activated whenever the vessel deviates from that pre set course;
* a wheelhouse rudder indicator (as distinct from a helm indicator) which shows the position of the rudder at any given moment;
* a clutch/de-clutch device -either mechanical or electrical, to engage or release the pilot.
All auto pilots will be different but they all have some or all of these features.
Method of operation
(a) Engaging
1. The ship is manually steered to the desired course.
2. The course selection is set to the required course and the unit engaged.
3. From this point on it is now steering automatically and you should check the actual course being steered by compass and make any small correction necessary moving the selector a small amount in the required direction.
(b) Altering Course
Small alterations of course can usually be made by moving the course selector to the new course.
However, as the vessel will only swing on the amount rudder indicated by the auto setting, it is nearly always better to disengage the pilot, bring the ship manual to her new course and re-engage the pilot, unless the alteration is only a few degrees.
Settings
Good as it is, an auto pilot cannot think for itself but responds totally to a deviation of the ship's head from the set compass course.
Different sea conditions and changes in handling character of the vessel, require differing steering responses -amount rudder used, delay in applying the helm, bias in one direction or the other and, in large vessels, the tendency to continue turning after the helm is centred in the midships position
Different makes of auto pilot give differing names for the settings and those lists are the most common.
Figure 13 Auto pilot
1. Ratio, Rudder or Response
This determines the amount of rudder (or helm) the pilot will use to maintain course and equates the number of spokes of wheel a helmsman would use in the existing conditions.
For example, in a calm sea, very little rudder is needed hold the ship on a straight course.
In a following or quartering sea, she may need as much 10 to 15 degrees of rudder to bring her back each time
she starts to pay off.
The same applies when using the auto pilot for shooting away trawl gear in twin boom trawlers. The sideways drag from unbalanced veering of the warp must be compensated by at least 100 of rudder if the ship is to be held to a straight course and bogging of the gear avoided. You will need to experiment with your own particular equipment to find what setting relates to a particular rudder angle.
2. Sensitivity or Weather
This is a delay setting which allows the vessel to veer up to 5° off course before the pilot starts to apply compensating wheel.
It is particularly useful in a beam sea where severe rolling often gives a 'tilt error' to the compass. The vessel is probably holding a true course but the pilot does not know this and immediately applies opposite wheel.
Unless this delay setting is used. the steering engine will be 'working overtime' and in fact. tending to steer a zig- zag course.
For normal sea conditions. a setting between 2° and 3° is fairly satisfactory; in a following quartering sea or when shooting away gear -times when you need instant response - make sure it is set to zero.
3. Trim, Standing Helm, Bias or Weather Helm.
This setting applies a bias to compensate the type of continuous helm correction a helmsman would have to make if steaming on one engine in a twin-screw vessel or where trawling gear was pulling slightly heavier on one side than the other; or in sailing vessels carrying weather helm on the wind.
If your pilot does not have this adjustment you can achieve the same result by setting your course indicator
the appropriate amount to one side until the ship's head by compass is holding steady on the required course.
4. Counter Rudder
This is a setting that only has any real application in a large vessel that requires opposite helm to prevent her swinging past the course when the helm is centred.
It can be set to apply automatically the correct amount of counter helm as the ship comes back on course.
It is essential to understand all these settings if best use is to be made of the equipment.
Limitations and precautions in use
Limitations of autopilots
Auto-pilots should not be used in any situation where emergency course alterations may be called for, such as navigating in fog or in close company with other vessels.
It is also unwise to use the auto-pilot in restricted channels and waters very close to navigational dangers. Such situations often call for large amounts of wheel in a hurry, and the pilot may not be programmed to cope with this. A good example is a channel subject to strong tidal influence where overfalls and rips can swing a ship suddenly off course.
Excessive weather conditions are often the occasion when one should revert to hand steering. Remember, the auto-pilot only reacts to the ship deviating from her course. The equipment cannot anticipate that extra large and breaking sea on the bow that may necessitate easing the ship's head up to take it right ahead. It cannot see the big sea coming on the quarter which may mean you should square away and run directly with it to avoid a broaching situation.
Precautions in use of autopilots
Theoretically, the auto-pilot allows the single bridge watchkeeper in a small vessel to give his full attention to lookout and navigation duties and, in a working trawler, to attend to his fishing supervisory duties.
Too often, unfortunately, it allows the man on watch to get so totally absorbed in other pursuits that he neglects to keep a proper lookout and, on many occasions, to fall asleep.
Apart from the obvious risk of collision, there is always the chance that the pilot will fail mechanically. Some instruments have an alarm to call attention to this but, in most small ship installations the only safeguard is a regular check on the compass course being maintained.
Manoeuvring difficulties of large vessels
Larger vessels due to their size, hull form and power are not as manoeuvrable as smaller vessels. Fighting the wheel to keep a vessel dead on the lubber line in a following sea will be less effective than allowing it to yaw (swing) within its natural motion.Stopping distances are increased by the huge momentum of a large vessel. Turning circles are large and response to the helm relatively slow, all these factors make it harder for a larger vessel to make swift and nimble manoeuvres as can be made by most small vessels. In light of the above the Master on a small vessel must bear in mind these constraints on a larger vessel before impeding it’s path or passing so close so as to not allow any margin for error or the manoeuvring characteristics of the larger vessel.
Turning factors
Many operational factors will affect the vessel’s ability to turn. Therefore, the rate of turn and turning circle will be influenced by these factors:
Trimmed by the stern
• pivot point moves aft
• increases turning circle diameter
• will steer well
• develop maximum power
Trimmed by the head
• pivot point moves forward
• smaller turning circle
• difficult to turn
• does not develop full power
Displacement loaded
• slow to answer helm
• larger turning circle
• turning circle normally not affected by speed
• slow in gathering way
Light
• responsive to helm
• smaller turning circle for same speed if loaded
• turning circle will increase with speed
• wind affected
• responsive to engine movements
List
• turns readily to high side
• smaller turning circle to high side
• twin screw - low screw more effective
Available depth of water in relation to shallow water effect.
The fixed turning factors include: the vessel’s propulsion arrangement, the type of rudder and hull design.
Turning
The rate of turn and the turning circle is affected by the amount of rudder applied.
Small rudder angles will result in large turning circles with minimum loss of speed.
Large rudder angles may reduce the turning circle, develop excessive drag and therefore reduce speed.
Most vessels use 15° - 20° of rudder as standard wheel for course alterations. Whilst manoeuvring, greater angles are applied. Standard wheel is therefore, the most effective amount of rudder to use without developing excessive drag and reduction of speed.
Turning circle
Every Master needs to appreciate the turning ability of their vessel, i.e. the distance it takes to turn and the time it takes to complete the manoeuvre. The turning circle represents the vessel’s path through 360° whilst keeping the engine revolutions and rudder angle constant throughout the turn. The turning circle is drawn from data obtained from turning trials.
Figure 14 Tactical circle
Definitions
Advance: distance measured on the original course until the turn is completed.
Transfer: the distance measured at right angles off the original course until on the new course.
Tactical diameter: greatest diameter measured from the original course when the vessel is 180° into the turn.
Final diameter: the internal diameter of the complete turning circle.
Drift angle: the angle between the fore and aft centre line point perpendicular to the centre of the turning circle on the vessel’s path.
Turning data sheet
When undergoing turning trials the data sheet is used to compile the turning
characteristics of the vessel.
Turning trials should be conducted in relative calm weather, negligible tidal, stream current and where the vessel will not be subject to shallow water effect. When the first vessel of a particular design is built this data is used to prove that the design and propulsion system meet the designer’s specifications. The example turning data is from a small coastal trading vessel:
LOA 35 metres
Displacement 130 tonnes
Twin screw
Twin rudder
Examine the data for a turn of 90°. The vessel advances 199 metres, transfers 109 metres and takes 57 seconds to complete.
Practical appreciation
The Master can identify specific handling features from the turning data and use these to advantage; e.g.
• will turn quicker to port
• have a smaller turning circle to port
Loss of speed when turning
If the rudder angle and revolutions are kept constant a vessel altering course will experience a reduction in speed.
Example: a displacement hull vessel.
Degrees of alteration speed reduction
0-20° minimal
20-90° reduction 1/3 of initial speed
>90° generally remains steady at 1/3
Obviously different hull designs, propulsion systems, rudder angle and speed will vary the results illustrated in the example.