【大副业务】干了这么多年船,既然还不会使用密度计？
A dry-bulk carrier under Hanghai 52 operation was recently involved in an over-draft incident. While the vessel was loading at the anchorage of Samarinda, Indonesia, a draft survey was carried out during moderate to rough swells of 1.0 to 1.5 meters. The vessel then arrived at Kawasaki, Japan, at a draft of 12.10 fwd /12.18 aft against the draft limitation of 11.99 meters at this berth.
At the final draft survey in Samarinda, the vessel draft was read as 11.90 fwd / 12.08 mid / 12.28 aft at a sea density of 1.020, and the master was convinced that the vessel’s draft would be 11.99 meters by the time of the vessel arrived at Kawasaki because of the difference in sea density at Samarinda (1.0200) and Kawasaki (1.0250), and bunker & F.W. consumption.
However, the arrival draft at Kawasaki was 12.10 fwd / 12.18 aft and the sea density is 1.020; the vessel was 19 centimeters over draft. The reason for this trouble was (1) a misreading of the draft at Samarinda, and (2) the vessel did not use the correct figure for sea density when the vessel calculated the maximum amount of cargo the ship can carry.
We recommend that you perform all items mentioned below to prevent a similar accident.
(1) At the time calculate the maximum amount of cargo the ship can carry, correct figure of sea density should be used both the loading and discharging port. If the vessel don’t have any reliable data, should confirm the figures with the local agent or an operator.
(2) The value of a vessel’s 'sag/hog’ (deflection) should be taking into account when judging maximum draft.
(3) The differences of the seawater specific gravity because of hydrometer differences should be well examined. (The hydrometer using is different by each country.)
(4) Light up the vessel when entering and departing port.
(5) Pay special attention for reading draft while ship is in a rough weather conditions.
Remarks
In addition to the above recommendation, we remind you to give special attention to use of the marine hydrometers. In particular, there are many seafarers who do not know how to read the vessel’s hydrometer and thus cannot use it.
Usage of Hydrometer
a) Water sampling
Harbors are often filled with water of different densities as a result of a mixture of seawater and fresh water from rivers, 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 different 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 types of sampling bottles are available.
The problem of obtaining reliable water samples can be a real one, particularly for big ships at deep drafts, but this is an extreme case quoted to draw attention 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 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 accurate 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.
b) Density measurement
Most ships are (or should be) provided with a loadline hydrometer that 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. A draught survey determines the displacement of a ship in air, using a draught survey hydrometer, and hence the weight of any cargo on board. The hydrometer measures the apparent density of seawater in kilograms per liter in air and is calibrated to do this. This hydrometer must not be used for loadline purposes unless a correction is applied to any reading that is taken.
If a loadline hydrometer and a draught survey hydrometer are placed in the same water sample the loadline hydrometer will read 0.002 higher on its scale. The air buoyancy correction that is required to convert apparent density to relative density accounts for 0.001. The correction needed to convert density in kg/l to specific gravity accounts for the remaining 0.001. The 0.002 difference will be the same over the full range of densities from 0.990 to 1.040 kg /l.
Two types of marine hydrometers are commonly found in the shipping industry. They are used to measure related properties but have different uses. The purpose of this marine notice is to highlight the correct usage of each type. Ships’ officers, marine surveyors, draught surveyors and other persons involved in the loading of ships should be familiar with the correct usages so that no confusion arises in relation to overloading, stability calculations or draught surveys.
Load Line Hydrometers
Load line hydrometers are used to determine the displacement of a vessel at a given waterline and enable compliance with the requirements of the International Convention on Load Lines 1966. The Convention permits a vessel to load to the appropriate load line mark with a dock water allowance correction to compensate for the relative density of the water in which the vessel is floating. The Convention uses density in vacuo (i.e. mass per unit volume). Load line hydrometers measure the relative density of a seawater sample at a standard temperature (T1) against pure water at a standard temperature (T2). Temperatures T1/T2 are usually 15º/15ºC or 60º/60ºF. In extreme cases of variation between the seawater sample temperature and the standard sample temperature a small temperature correction should be applied to compensate for the slight expansion or contraction of the hydrometer.
Draught Survey Hydrometers
Draught survey hydrometers are used to determine the apparent weight (i.e. the weight in air) of the vessel and from this the commercially accepted weight or apparent density of the cargo on board. These hydrometers are also calibrated at standard temperatures but no temperature correction is required. A small error is introduced if the hydrometer is not at its standard temperature but this is compensated for by a change in volume of the ship. This change is due to the same temperature difference but the cubical expansion coefficient for steel expansion is of the opposite sign to that for glass expansion and the two differences tend to cancel each other out. These hydrometers should not be used for load line purposes.
Relationship between hydrometers
The displacement and apparent weight of the vessel are related, as are the relative and apparent densities of the water the vessel is floating in. The difference between the relative density or specific gravity as determined by the load line hydrometer and the apparent density as determined by the draught survey hydrometer is termed the “air buoyancy correction” and corresponds to 0.002 for all usual ranges of marine related work. The correction is always added to the draught survey hydrometer reading to obtain the load line hydrometer reading ignoring any temperature corrections.
Example
The density of pure water at 15ºC is 0.9991kg/l. A seawater relative density reading of 1.025 on the load line hydrometer means that the seawater has an actual density of 1.025 x 0.9991kg/l = 1.0241kg/l. A one litre sample of seawater of this density will weigh 1.0241 kg in a vacuum.
Commercial weights are measured in air not in a vacuum and in air one litre of seawater will have a buoyancy force of 0.0011 kg. The commercial weight is therefore smaller than the weight in a vacuum by this amount. The one litre sample will weigh 1.0241 – 0.0011 = 1.023kg in air.
For the sample of seawater the load line hydrometer shows a relative density of 1.025 and the draught survey hydrometer shows an actual density of 1.023kg. Both are correct.
Most marine hydrometers have markings that indicate their type. Load line hydrometers are usually marked with the notation RD or Sp. Gr and the two standard temperatures, T1/ T2. Draught survey hydrometers are usually marked with the units kg/l and temperature T. Like all testing equipment hydrometers should be regularly calibrated.
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