ESTABLISHING QUANTITY OF CARGO LOADED OR DISCHARGED
Methods of weighing bulk cargoes ashore, draft survey procedures, reasons for unexpected results
Shorebased methods of weighing
THE QUANTITY of cargo loaded aboard a bulk carrier can be measured in a variety of ways. The only method which directly involves the ship's personnel is the draft survey, but before considering this method in detail it is worth giving brief consideration to methods which may be used ashore. An understanding of these methods will help in assessing their reliability.
Electronic weighing of cargo on conveyor belt:
The most common method of weighing used at modern loading terminals is the belt scale. This device continuously weighs the material on a selected length of the loading conveyor belt and multiplies this instantaneous weight value by the belt speed. The signal thus obtained is at all times proportional to the rate of material flow on the belt. Some commercial belt scales rely on magneto-elastic load cells. These devices rely upon the fact that the magnetic characteristics of steel are affected by mechanical stress.
The accuracy of a belt scale depends largely on the design of the conveyor and the way it is maintained. Provided that the conveyor conforms to specified basic requirements for design and operation, an accuracy of better than +/- 1 per cent of nominal capacity within the flow rate range is claimed by one manufacturer. Others state that belt weigher systems are capable of achieving an accuracy of up to +/- 0.1 per cent of true weight for capacities of 10,000 tonnes/hour and can be relied upon for an accuracy of 0.5 per cent. Accuracy is likely to diminish to :t 1.0 per cent if the system is not used to capacity. Shipboard observers consider that inaccuracies rise on occasions as high as 10 per cent of true weight, presumably as a result of failure, or faulty calibration and maintenance.
Electronic weighing of cargo in grab:
Cargo being discharged or loaded by grab can be weighed whilst in the grab by an electronic system. A computerised system can then record and total the tonnage handled. A working accuracy of 0.1 per cent is claimed by the manufacturers, but this is dependent upon the crane being motionless and in windless conditions when the weight is recorded. In practice these conditions are rarely met.
One manufacturer of electronic weighing systems for cargo in grabs uses a load cell mounted on the hook block. The magneto elastic load cell is built into a load- bearing part of the lifting system. When there is no load on the load cell, the magnetic flux lines around the windings do not influence each other. When the load cell is subjected to a load, the flux lines will cut each other and a voltage proportional to the applied force is induced in the secondary winding. The trans- ducer which transmits the reading has low internal impedance and produces a powerful output signal, which is insensitive to interference, making the system reliable and accurate. The system is only suitable for use with grabs which are operated electro- hydraulically.
Weighing of trucks on weighbridge:
For accuracy this method depends upon all trucks passing over the weighbridge with the results being accurately recorded and upon the tare weight (i.e. , the unloaded weight) of each truck being accurately known. This is best achieved by weighing the unloaded vehicle on its return journey. Weighbridges have a potential accuracy of +/- 0.2 per cent. Most weighbridge systems can be indexed in the control house for any debris, water, ice or snow which may accumulate, but if the correct indexing is not applied (or is deliberately ignored or wrongly set) the weights recorded will be in error to a greater degree than would be expected by the manufacturers or the licencing authority.
Automatic bulk grain weighers:
These machines are suitable for weighing grain and free- flowing materials fed from elevators, conveyor belts, storage hoppers or silos. They are produced in various sizes and can record weights in cycles from 30kg up to 5 tonnes. They can deliver at rates of up to 1,000 tonnes per hour. When this machinery is correctly installed and maintained by the manufacturers and regularly inspected by a reliable local regulatory authority an accuracy of +/- 0.1 per cent is to be anticipated. Such a degree of accuracy is a general requirement within the grain trade. It should however be stressed that the degree of accuracy attained depends upon the regularity of inspection, servicing and maintenance. It is reported that some manufacturers are more realistic and suggest that operational accuracy is more like +/- 0.3 per cent.
Shorebased systems in general:
At best, all the foregoing methods can be only as accurate as the design of the weighing equipment allows. At worst, if the equipment is not regularly calibrated and if not all cargo is weighed, the results may bear little relationship to reality.
Cargo which drops on to the deck or into the dock from partly closed grabs or which leaks out of insecure trucks can form a significant percentage of the total, and it is worthwhile keeping a record of occasions when this is a problem. Even cargo which blows away from open grabs, trucks or stockpiles represents a loss of weight and should be noted. Cargo residues which remain aboard will also not be included in out turn weight if measured ashore. In addition, such residues (if substantial) present a real practical disposal problem for the ship's small staff, unless the ship returns uncleaned for a further load of the same commodity.
The operations departments of shipping companies with ships engaged on regular trades receive separate cargo figures from shippers, receivers and ship and can over a period of time build up a reliable picture of the accuracy of cargo measurements. The masters of such ships are often told by their operations department 'It is our experience that the shore weight in this port is always 1 per cent high', or some such figure, and this can be a useful point of reference.
There is no doubt that the operators in the loading or discharge port quickly obtain a feel for the accuracy of the measurements they obtain. They may choose to disregard the protests of a single ship, but if they are told by a succession of ships of different owners and nationalities that their cargo totals are too high or too low, they will begin to accept that there is truth in the allegation. Persuading them to pass this information to future ships attending the berth is another matter . Often informal questions to the loading foreman produce more accurate and reliable statements of known errors than do enquiries directed to management, who are reluctant to admit to the shortcomings of their system.
Shore measurement can be useful, but the importance of independent measurements made by the ship cannot be too strongly emphasised. Mistakes are sometimes made by operators ashore or instruments develop faults, and some spectacular and embarrassing errors in loaded quantity have resulted from failure by ship's staff to take their own accurate independent measurements. It cannot be emphasised too strongly that the ship must make regular draft checks during loading to avoid ending up loaded too deeply. Two final pours must be reserved at the end of loading for accurate trimming and to achieve the intended draft.
The ship's method of determining the amount of cargo loaded is by means of draft surveys taken before and after the loading is carried out. With the data so obtained the ship's displacement (the volume and therefore the weight of water displaced by the ship) before and after loading can be calculated. In simple terms the increase in displacement after loading, adjusted for any change in weights such as ballast, equals the weight of cargo loaded.
The draft survey may be the method of measurement specified in the charterparty for deciding the quantity of cargo carried, in which case one or several surveyors are likely to be employed to carry out the survey. When the charterparty specifies that shore measurement is to be used for deciding the quantity of cargo carried, the master will still be expected to calculate a ship's figure to provide a check. It is in his interests to do so and to ensure that the results are as accurate as possible. In special cases, the surveyor will have the benefit of equipment and instruments not found aboard the normal bulk carrier, but in most instances the ship's master or officer with careful attention to accuracy and procedure can obtain results quite as good as those of the surveyor .
Conditions for the commencement of survey:
1. Vessel afloat.
2. No cargo being worked.
3. No ballast, fuel, fresh water, etc., being pumped or run.
4. No hatch covers being opened or closed.
5. No spares or stores being shipped or landed. 6. All ballast tanks full or empty. 7. Ship upright.
8. Litde or no tide or current running. 9. Seas not unduly rough.
10. Temperature difference between sea water and ship's decks not excessive.
From the foregoing list, items 1 and 2 are essential, whilst items 3, 4 and 5 are equally important unless the ship is large and the tonnages involved are very small. For example, the loading, discharging or moving of stores or bunkers weighing no more than 5 tonnes would not be detectable aboard a Panamax-sized vessel but would have a measurable effect on the draft of a handy-sized vessel.
Subsequent calculations are simplified if ballast tanks are either empty or full (item 6). It is often suggested that ballast tanks should be pressed up and overflowed prior to survey, to demonstrate that they are full. If the ship possesses no trim corrections for the tank calibration tables, pressing up the tank may be the best option, but this method has its disadvantages. In some ports the overflow of ballast water on deck is prohibited. Even when overflowing is permitted it is possible for a tank to overflow without being completely filled. This problem occurs particularly with topside wing tanks, when the ship is heavily trimmed.
Most accurate results are likely to be obtained if the topside wing tanks are filled to just below upper deck level at the sounding pipe, and if the tonnage of ballast is then calculated using the tank calibrations to take account of sounding and trim. Use of a correction can be avoided if the vessel is upright when the survey is undertaken (item 7).
Items 8, 9 and 10 are beyond the powers of ship's staff to influence unless the time of the survey can be delayed, but they should be remembered, and strong current, rough seas or great temperature differences should be recorded in the survey remarks.
A strong current can lead to squat, which will affect the accuracy of both mean draft and trim. Rough seas will make accurate draft readings difficult or impos- sible to obtain, and a ship with decks heated by the sun at a time when the underwater body is relatively cool will be distorted, which will introduce inaccuracies into the stability data. Fortunately extreme examples of these problems are rarely found in practice, so that accurate results can usually be obtained.
Full set of draft readings:
The first step in the survey is to obtain accurate readings of the six required drafts-namely, forward, amidships and aft on both port and starboard sides. An active person can obtain these readings easily with the help of a rope ladder on the outboard side if the vessel is small or medium sized, but for a large ship a launch or ship's boat is useful.
It is possible to obtain a good approximate reading of the drafts of a small ship from on deck, but this method will not be sufficiently accurate for the purposes of a draft survey because of the oblique angle at which the marks are then viewed. For illumination, a powerful torch or portable Aldis lamp may be required in conjunction with binoculars, as it may be necessary to read some drafts from a distance of 10 or 20 metres. Drafts are normally read to the nearest centimetre or half inch, any greater degree of accuracy being unrealistic.
When there is a substantial sea running, it is often easier to try to calculate the mid-point between the highest and lowest readings obtained over a period of several minutes or longer. Highest and lowest readings can be read from a weighted tape - for example, a sounding line-dangled overside close to the draft marks. It is useful to take the mean of readings obtained by several observers in these conditions.
Complete set of soundings:
Next, soundings must be taken of all spaces including compartments such as cofferdams, chain lockers and void spaces in addition to all bilge and ballast spaces. There have been plenty of instances in which compartments which the ships' personnel thought were empty were found to be full.
Additional soundings of 'empty' tanks should where possible be obtained later during the course of loading or discharge, when the vessel has a good trim. If this is done, the sounding is less affected by minor errors and the calibration tables can be used to obtain a tonnage of remaining ballast which is much sensitive than when the ship is even keel as she is likely to be when fully laden. A surveyor will often accept these ship's figures ifhe can see during the draft survey that the ship's approach is professional.
A full set of fuel tank soundings should also be obtained, but this requirement is often avoided by simply obtaining from the chief engineer the total quantity of fuel aboard. If the purpose of the survey is to measure the quantity of cargo aboard it does not greatly matter whether or not this fuel total is accurate. The constant (described below) will absorb any inaccuracies in the fuel total as an automatic consequence of the routine deadweight calculations. The total weights aboard ship will be unchanged.
It is important that the quantity of fuel consumed and loaded between the initial and final draft surveys is accurately known. In many cases the in-port consumption will be no more than two or three tonnes and no bunkers will be loaded, so an accurate figure for fuel consumption presents no problems. In cases when this is not so, a full set of fuel soundings must be obtained. A full set of all soundings, including fuel soundings, is also required when the purpose of the draft survey is to obtain an accurate value for the constant.
The purpose of the set of soundings is to discover the volume and thereafter the weight of all liquids aboard the ship. It is good practice to note the reading on the rod/tape/line at the level of the sounding cap, too. Provided that the sounding to the cap is known or can be verified, a correct reading will show that the sounding pipe is not blocked - for example, by a broken link of a sounding rod or by mud. It will usually be found that similar tanks to port and starboard and along the length of the ship have soundings with the same total depth from the sounding cap. If any sounding gives unexpected or uncertain results it is necessary to recheck. If doubt remains and the compartment can be entered, this is often the quickest way of finding exactly what its contents are. Alternatively, it may be possible to sound or to ullage it through a different opening, such as an airpipe, to obtain a rough check on its contents.
Where soundings are taken with sounding rod or tape the signs of a suspect sounding are the failure of the rod to land cleanly on the striking plate, or a dirty rod without a definite waterline on it. The use of water finding paste will provide a clean waterline on a poor sounding line.
When a sounding is read from a gauge, the accuracy of the gauge should first be confirmed by ensuring that the reading is zero when the gauge is switched to 'null', or by obtaining the correct test readings when the gauge is switched to 'test' .Gauge readings are not considered sufficiently reliable for draft surveys and gauges should be used only as working instruments.
Density of water:
The final measurements which are required at the time of the survey are measurements of the density (in air, not in a vacuum) of the water in which the ship is floating and of any ballast water carried aboard ship. Unfortunately , accurate measurements of density are more difficult to obtain and to interpret than many seafarers will realise. If traditional routines are followed it is quite likely that unsuitable hydrometers will be used to measure incorrect water samples.
If the water density is measured as 1.025 when it is actually 1.020, the error in calculated tonnage of cargo aboard a laden 65,000 tonne deadweight (75,000 displacement) Panamax bulk carrier will be 317 mt. Even 1.024 instead of 1.025 will result in an error of 63 mt in the calculated figure. Clearly, it is important to obtain an accurate density. Unfortunately this is not easy, for reasons explained below.
Harbours are often filled with water of different densities as a result of a mixture of sea water and fresh water from a river and this condition can vary with the state of the tide. When water of different densities is present, it tends to form layers, with the most fresh (least dense) water on the surface. A sample of water taken from the surface is unlikely to be typical of water over the full depth of the vessel and water density may also vary between differ- ent positions along the length of the ship. For best results it is necessary to obtain a number of samples, from at least three positions on the offshore side of the vessel and from a number of different depths. Several patterns of sampling bottle are available.
The problem of obtaining reliable water samples can be a real one, particularly for big ships at deep draft, but this is an extreme case quoted to draw atten- tion to the problem. If, on the other hand, the port is wide open to the sea and if no rivers flow nearby, it is likely that the water density will be constant or nearly so over the full draft of the ship. In those circumstances a water sample taken from the surface will be adequate. Officers who are eager to build up an accur- ate set of measurements of the ship's constant will try to obtain careful measurements in places where the water is likely to be completely salt or completely fresh.
Density of ballast water:
In addition to the density of sea water it is also necessary to measure the density of any ballast water carried aboard at the time of the survey. It is quite possible for the density of water within a ship's ballast tanks to vary from tank to tank if the tanks were filled at different stages of the tide or at different points on a river passage, so samples should be obtained from a number of tanks if accurate results are required. To take an extreme case, if the ship's full ballast is assumed to be salt water when it is actually fresh water the resulting error in the calculated full cargo lifted would be about 1.0 per cent.
The next problem arises with the hydrometer supplied to the ship to measure density. A great variety of instruments using an assortment of scales, units and standard temperatures are supplied to ships. In addition to the fact that hydrometers are manufactured in various parts of the world where different units are used, part of this confusion arises because there is really a need for two different ship's hydrometers, each for a different task.
Most ships are (or should be) provided with a load- line hydrometer which measures specific gravity (also known as relative density). The specific gravity of fresh water is 1.000, and that of salt water is 1.025. This number has no units-it is the ratio of the density of the measured water with the density of fresh water . The loadline hydrometer is intended for use in the calculation of fresh water allowance. It enables the ship's officer to calculate how much the loadline can be submerged in fresh or brackish water and for that purpose it provides a direct ratio between water of different densities.
Surprising as this statement will be to many, a load-line hydrometer is not suitable for the calculation of displacement unless a correction is applied. For an accurate calculation of the weight of water displaced by the vessel it is necessary to know the apparent density in air of the sea water, in kilograms/litre or equivalent units. Draft survey hydrometers made of glass and calibrated in these units have been available for some years but their use is not yet widespread. Their read- ings extend from 0. 990 kg/l to 1 .040 kg/l .
Many ships carry a loadline hydrometer and few carry a draft survey hydrometer, so it is helpful to know that a reasonably accurate conversion of the reading taken from a loadline hydrometer can be made. If the hydrometer is marked with graduations ofg/mgat 15Cor Spec. Grav. 15C/4Cthen 0.0011 must be deducted for the reading obtained. If the hydrometer is marked with graduations of Spec. Grav. 60F/60F then 0.0020 must be deducted from the reading obtained.
Hydrometer graduations Correction to apply Reading obtained Corrected reading
g/mg at 15C -0.0011 1.023 1.0219
Spec. Grav. 15C/4C -0.0011 1.023 1.0219
Spec. Grav. 60F/60F -0.0020 1.023 1.021
The corrected reading should be used for the calculation of displacement.
Calculation of the displacement:
On completion of the readings and observations described above the calculation of displacement can be undertaken.
Described below is the purpose of each step in the calculations.
Correction of the draft readings
The ship's stability data will have been compiled using drafts at standard positions-namely, at the ship' s forward and after perpendiculars and at the ship's mid- length between the perpendiculars. The ship's draft markings are not usually placed at the perpendiculars. If they do not coincide there will be a discrepancy between the draft as read, and the draft at the perpendicular, except when the ship is at an even keel. The corrections, which may be obtained from the ship's tables or from formulae must be subtracted from the observed reading to obtain the corrected reading when the ship is trimmed by the stern and the draft markings are located abaft the perpendicular. Formulae should be used in preference to the ship's tables to avoid the risk of errors in interpolation.
Correction for hull deformation: calculation of mean of mean, or quarter mean, draft
If a ship's hull is completely undeformed (i.e., not distorted) the midships draft will equal the mean of the forward and after drafts. In practice, this is rarely the case. The ship is normally either hogged (with the midships draft less than the mean of the forward and after drafts) or sagged (with the midships draft greater than the mean of the drafts at the extremities). A ship which is hogged will displace more than an undeformed ship at the same midships draft, whilst a ship which is sagged will displace less. The purpose of this correction is to take account of this fact.
The formula for mean of mean draft (or quarter mean draft - an alternative name for the same calculation) assumes that a ship deforms in a parabolic fashion. This is not strictly correct, but is accepted as being a suffi- ciently good approximation for practical purposes and is used almost universally. The formula, when evaluated, gives a corrected mean draft with a value which takes account of hull deformation.
(Alternative methods for correcting for hull deformation are: [a] use of ship's approved table of corrections: [b] integration of the transverse section areas representing the immersed portion of the hull as actually trimmed and deflected, using approved parabolic coefficients: and [c] correction related to the waterplane area with use of approved hog and sag correction factors. Whilst each of these methods is valid, and is described in the UN code none is widely used in practice. )
Displacement (in tonnes)
Displacement is obtained from the ship's hydrostatic tables (stability data) entered with the corrected mean draft. This requires further corrections, described below.
First correctionfor ship trimmed (layer correction)
This correction is necessary because the longitudinal centre of flotation (LCF) about which the ship trims is not a fixed point and does not normally coincide with the ship's mid-length, the point for which the corrected mean draft is calculated. The correction is positive when the ship is trimmed by the stem, and the LCF is abaft midships. (For a loaded Panamax vessel - displacement 75,000 tonnes - with a trim of 1 metre the correction has a value of about 65 tonnes)
Second correction for ship trimmed (form correction)
This correction is necessary when the trim is large to take account of inaccuracies which arise in the layer correction in these circumstances. This correction is always positive. (For a loaded Panamax vessel - displacement 75,000 tonnes-with a trim of 1 metre the correction will have a value of about 12 tonnes. If the trim is 2 metres the correction will be about 50 tonnes.)
Trim correction by ship's tables
It is quite likely that the ship's trim correction tables will be based upon the first correction, but will not take account of the second correction. This can be readily checked by working some examples by tables and by the formulae provided, to see whether the tables give a correction which equals the first correction or the sum of the two trim corrections. In addition, the use of tables permits errors of interpolation and the tables themselves may contain errors. It is advisable to calculate the trim corrections by formulae and to avoid the use of the tables except as a check.
Correction for ship listed
A ship when listed experiences a reduction in mean draft since the effect of the list is to increase her waterplane area and displacement, and to cause her to rise in the water. Therefore the correction is always positive to reflect the greater displacement which corresponds to the deeper draft when the ship is upright. (For a loaded Panamax vessel - displacement 75,000 tonnes-with a list of about 3 degrees the correction has a value of about 15 tonnes.)
When the ship is floating in water of a different density to that assumed in the ship's hydrostatic tables the displace ment must be corrected for density. When the water is less dense (e.g., fresh water) the ship's displacement at any draft will be less. The hydrostatic tables for most ships are compiled for a density of 1.0250 mt/m3 ( equivalent to an SG of 1.0250), but occasionally other values such as 1.000 (fresh water) or 1.02518 ( = 35 ft3/long ton) are found.
Completion of the foregoing corrections provides the ship's true or actual displacement. When all known weights have been deducted from the displacement, the weight remaining ,will be that of an unknown quantity - for example, the cargo when the ship is loaded, or the constant when the ship is in ballast. Calculation of the constant normally precedes calculation of the cargo loaded.
Calculation of the constant:
The 'constant' is the name commonly given to the unidentified weights and inaccuracies which remain when all listed weights have been deducted from the true displacement. It is called the constant because when it has been calculated with the ship empty of cargo it is assumed to be unchanged (i.e. , constant) for the purposes of the calculations when the ship is loaded. Some people find it confusing that the constant is found to vary from voyage to voyage. This probably arises from differ- ences in quantities in engineroom tanks and bilges, changes in the tonnage of stores and spares carried, and minor inaccuracies in the draft surveys.
No opportunity should be lost to recalculate the ship's constant as accurately as possible, and to main- tain a full record of values obtained. If the ship's records are carefully and fully maintained they are more likely to be accepted and used by a surveyor when something goes inexplicably wrong with a draft survey and reobserving is no longer possible. A recom- mendation of the UN draft survey code6° is that the ship should keep a record of all constants on a Light Ship Correction Certificate which should show date, place , constant in metric tonnes and signatures of chief mate and surveyor, with the surveyor's stamp. If this recommendation is followed, it will be important to ensure that the calculation is consistent and that the same items are always deducted from the displacement.
In practice, the weights which are separately item- ised vary from ship to ship, surveyor to surveyor, owner to owner and time to time. Usually the itemised weights will include fuel oil, diesel oil, fresh water and ballast water. Sometimes stores and spares will also be separately listed, but they may alternatively be included in the constant. The ship's light weight (her weight as built before being stored or bunkered) will always be a separate item. Luboils may be a separate item, and other items such as ER water (water for engineroom purposes) can be itemised separately if convenient.
If the constant is to be calculated simply as part of the process of calculating the tonnage of cargo aboard, it will only be necessary to make accurate calculations of the ballast which will be discharged whilst the cargo is being loaded. Any inaccuracies in the values given to the other weights will be absorbed into the constant. If the constant is to be calculated as accurately as possible, then all the other weights must be calculated with care.
The soundings must be used with the ship's calibra- tion tables to find the tonnage of water or fuel in each tank. Corrections must be applied to take account of trim. If no corrections are included in the tables themselves, it will be necessary to use a formula to correct the sounding to an even keel value.
When using the calibration tables, ship's staff are well advised to look critically at them since they occasionally contain obvious errors. It has been known for similar double-bottom tanks to be provided with the same calibrations, despite the fact that in one tank the sounding pipe ran vertically down a bulkhead amidships, whilst in the other it followed the ship's side and sloped at an angle of 45? in way of the turn of the bilge. The procedure mentioned earlier of taking a reading of the sounding line at the level of the sounding cap would lead to the detection of this discrepancy.
The volume of water or fuel in each tank must be multiplied by the density (NE: the apparent density in air) of the liquid as measured to obtain the tonnage contained therein, and readers are reminded of the importance of accurate measurement of the density.
The ship' s light weight is obtained from her stability information. It changes only if the ship's structure is modified. There is no easy way of measuring the weight of stores and spares carried aboard ship. This will vary with the size of ship and the nature of her trade, and will tend to increase as the ship grows older . An estimated figure will be used for this item, a figure probably based upon that used in past voyages.
Once all these weights have been calculated or estimated, and deducted from the true displacement, the weight which remains is the constant, a figure which can be expected to range from 30 or 40 tonnes aboard a relatively new mini-bulker of 3,000 tonnes deadweight to 300-400 tonnes for a Panamax bulk carrier. Actual values will vary substantially for individual ships, depending upon many factors.
The constant will probably include the accumulation of paint on the ship's structure, the build up oJ mud in the ballast tanks, the increasing weight of stores, spares and equipment carried, the water in the engineroom bilges and the fuel in the engineroom settling and service tanks. It may also include ballast residues and luboil. The constant will also, because of the manner in which it is calculated, inevitably include tonnages to match any deliberate or accidental overestimates or underestimates in the itemised weights. U the figure for fuel bunkers is 50 tonnes too low, the constant will be 50 tonnes higher than it would otherwise be.
Calculation of the cargo loaded:
Once the constant has been calculated, it is possible to list all the weights aboard the vessel on completion of the loading of her cargo. Some of the weight totals will have to be amended from those used at the start oJ loading. Almost all of the ballast will have been discharged and some fresh water and fuel will probabry been consumed. Additional bunkers may have been loaded.
A second draft survey is undertaken and a new true displacement will be calculated. All the itemised weights, correctly updated, will be deducted from the displacement. The light ship weight, and the constant will be deducted. The tonnage remaining is the cargo tonnage by draft survey, often known as the ship's figure.
Possible sources of error
Occasionally it will be found that the results obtained from a draft survey are unexpected. The constant may be found to be much larger than the normal for that ship, or a negative constant may be calculated. The ship's figure for the tonnage of cargo lifted may differ from the shore figure by an unusually large amount. If the ship's officer and surveyor work independently, but compare figures at each stage of the calculation, then calculation errors are minimised. Since the discrepancy may be the result of a mistake in the draft readings or soundings, these should be rechecked, if still possible.
If the result remains unchanged it will be necessary to look further for an explanation. All the information used in the calculations must be studied to assess its reliability. Where possible data should be rechecked by a different method. It is useful to consider whether the discrepancy has occurred once only, or occurs every voyage. If it occurs every voyage, then it arises from data which are used every voyage. If it occurs once only, then it is more likely to be caused by some- thing specific to that voyage.
Always investigate any substantial changes in the calculated value of the ship's constant. Accurate and reliable draft surveys are more difficult to achieve when a vessel has a large stern trim, such as may occur when a vessel has been partly deballasted to permit a quick loading. The master should hesitate to berth his vessel with an excessive trim ifhe knows this will make the draft survey less reliable and should use whatever means are available to persuade the terminal manager to accept his vessel with more ballast. The interests of ship and terminal do not always coincide, and the master should ensure that an accurate draft survey is made, which will enable him to produce the correct amount of cargo at the discharge port.
Examples Vessel aground:
A Panamax vessel was completing loading a cargo of iron ore in a West African port at 0300 hours with the final trimming pour being loaded in a forward hold. The chief mate was on the quay watching the forward draft. While loadihg continued the forward draft stopped increasing. Loading continued until the tonnage calculated for the trimming pour had been loaded. The final draft was found to be less than expected, with a trim by the stern. The explanation for this was not immediately realised and it was thought that there had been an error in the cal- culation of the trimming pour. Since departure from the port was governed by a limiting draft of 45 ft, ballast water was put into the forepeak tank to bring the vessel to an even keel, but the trim by the stern persisted. Finally, it was realised that the vessel must be aground forward and the ballast was pumped out. Fortunately for those involved the vessel refloated at high water, although the tidal range was small. When afloat she was found to be a little by the head and listed a little to starboard.
Grounding in the berth is a possibility where the bottom is mud or sand and has to be dredged, particularly if the port authority is inefficient. Partial grounding is also liable to occur on a shallow patch which may occur close to the quay, where cargo may have been spilt. If the vessel is aground on completion of loading the final draft and trim will be wrong. If the vessel grounds at an earlier stage - for example, at the time when the tonnages for the trimming pour are calcu- lated-then the final draft and trim will be accurate if the vessel is afloat by; that time, but they will not be the draft and trim intended.
Inaccurate draft marks:
Following drydocking on a mini-bulker, the ship's figure for tonnage lifted was lower than expected for several voyages in succession. The draft marks were carefully measured from a dinghy and it was found that the upper port after draft marks (painted on the sloping surface of the stern) were several inches in error. The lower marks, cut into the stempost, were easier to see and to verify and were used as a datum. The ship was ten years old, the hull was rusty and thickly coated with paint in the vicinity of the draft marks, and the original lines cut into the hull to mark the upper drafts were almost invisible.
Raised draft marks are unlikely to be wrong and inaccuracies in draft marks painted on the vertical or near vertical surfaces amidships and at the bows can easily be noticed. The after draft marks painted on a sloping surface are the only ones which are difficult to verify. Suspicions should be aroused if the draft readings suggest that the ship is twisted between midships and the stern. If the midship drafts are equal, port and starboard, but the after drafts suggest a list to one side, or vice versa, it is possible that the draft marks are incorrectly marked.
Deck line not placed at deck level:
Where a ship has no draft marks amidships, the mid ship drafts are obtained by measuring the freeboard from the water- line to the top of the loadline, or deck line, with a steel sounding tape or tape measure. When the draft is light it is often easier to measure from the deck line, with one person descending to water level whilst the other takes the reading at deck level. Freeboard is normally converted to draft by adding the deepest summer draft to the summer freeboard, and then subtracting the measured freeboard. In some cases this will give a false draft.
On some ships - for example, those with a rounded deck edge - the deck line is likely to be located on the vertical ship's side, at a distance (d) below the free- board deck. Distance d will be stated on the loadline certificate and in the ship's plans. In this case the mid- ships draft is obtained by subtracting the measured freeboard, plus d, from the sum of the deepest summer draft and the summer freeboard.
Ballast retained by mistake:
Ballast has on occasion been retained by mistake aboard a ship loading a deadweight cargo. This has occurred as a result of a misunderstanding at change of watch or as the result of a tank being refllled or part filled by mistake through a leaking ballast line or an open valve. If no draft surveyor is in attendance, and if ship's staff do not follow sound procedures, such an error may not be detected.
The soundings of all ballast tanks should be rechecked before the final trimming pours are calcu- lated and loaded. If this procedure is followed without fail any remaining ballast will be detected and can be discharged before completion of loading.
When completing the discharge of ballast from a compartment the valve should be closed before the pump is stopped. If these operations are carried out in the reverse order water will gravitate back through the stopped pump and past the valvet until such time as the latter is fully closed.
When there is any possibility of a misunderstanding of orders about ballasting all orders should be given in writing and acknowledged with a timed signature.