There are many different types of dry bulk cargoes in the world, including iron ore, coal, grain, salt, fertilizer, wood chips, and cement.
Total quantity of the dry bulk cargo transported at sea by vessels achieves over five billion metric tons per year (2024), and most of the trading quantity is determined by a vesselfs draft survey.
Draft survey is an internationally recognized method for determining the quantity of large dry bulk cargoes. This method involves visually reading the draft mark, which is located above the waterline of a large vessel floating on the water, so the accuracy of the reading is greatly affected by the environment at the time of work. In other words, the wind and waves on the water change depending on the time and place, there is personal variation in visual reading, and the draft mark reading value differs due to parallax caused by the difference in height between the wharf and the water surface. Therefore, the draft mark readings vary depending on the time and place, resulting in a large fluctuation in the draft survey quantity. Below are some specific proposals to improve the fluctuation and accuracy of draft survey.
First, we explain the current status of draft mark reading that are actually being conducted in the field.
As you know, a vessel has a total of six draft marks on the port and starboard shell plates at the bow, stern and center.
Usually, vessels are berthed at the wharf, so in that case, for the shore side, we read the three draft marks at the bow, stern, and center.
On the other hand, for the opposite side of the shore (hereafter referred to as the sea side), it is common to only read the central draft mark, and then determine the readings of the bow and stern draft marks by calculating the left and right inclination values according to the each width.
The main reason for only reading the center draft mark on the sea side is that the final calculated average draft reading figure is that the 75% is based on the center draft mark reading, and the remaining 25% is based on the bow and stern readings, with 12.5% each.
Therefore, in practice, only the center draft mark is read on the sea side in most cases.
Next, regarding the method for reading the center draft mark on the seaside, three methods are generally used.
@ @Method using a manometer
A @Method using a Jacobs ladder
B @Method using a draft boat
@ The manometer method involves running a tube filled with liquid on the port and starboard sides of the deck in the center of the vessel to measure the heel of the hull. The reading of the center draft mark on the sea side is calculated by adding or subtracting the port and starboard inclination values measured with a manometer to the reading of the center draft mark on the shore side. Therefore, the reading value on the shore side is very important and if there is an error in it, that error will be directly reflected in the reading value on the sea side.
A The Jacobs ladder method involves fixing a rope ladder to the handrail near the center draft mark on the sea side and hanging it down toward the sea surface, then climbing down the ladder to near the sea surface and reading the center draft mark on the sea side. The reading are taken while hanging from a rope ladder, which can be dangerous and prone to inconsistent readings.
B The draft boat method involves boarding a small boat and reading the draft marks on the seaside, so in addition to the center draft mark on the seaside, you can also read the bow and stern draft marks.@However, this method is becoming less common in Japan recently due to the risk of boats swaying and becoming dangerous when the weather worsens, as well as the time and cost involved in taking readings.
Most of the draft surveys currently conducted in the world relies on one of the three methods mentioned above.
Either method will result in various fluctuations as mentioned at the beginning of this article when reading the draft marks. Unless these fluctuations are corrected, the accuracy of the draft mark reading values cannot be improved.
In this situation, by using the draft reading device (KENDRAFT) below when reading the center draft mark, it is possible to improve the fluctuations and bring it as close to zero as possible.
Principle of KENDRAFT is very simple.
If water is taken into the pipe from a depth of about 5 meters below the water surface at the same atmospheric pressure, the water surface inside the pipe will stop at the same height as the water surface outside the pipe.
The water surface outside the pipe rises and falls due to wind and waves, but the water surface inside the pipe hardly moves because it takes in stationary water from a depth of about 5 meters.
Then, if you put a float in the pipe, the float will stop on the stationary water surface which is not affected by the wind and waves, and if you look at the draft mark where the float is stopped with a mirror, you can read the draft mark corresponding to the correct water surface.
In addition, the angle of the mirror is set at 45, so the draft mark where the float is located can be reflected horizontally on the mirror from a close distance.
If you read the draft mark where the center of the float reflected in the mirror is located, you can get an accurate draft mark without parallax.
When sea conditions are poor, the effects of wind and waves become greater, and it is not uncommon for the reading of draft mark to change by more than 2 centimeters
For example, in the case of a Handymax-type vessel (typically with a deadweight of 52,000 to 58,000 tons), a 2-centimeter difference can lead to a difference of about 100 tons. In such a situation, by using KENDRAFT, it is possible to keep the difference within 1 centimeter.
In conclusion, there are various fluctuating factors in the working environment where draft surveys are conducted, so it can be said that it is difficult to obtain stable and highly accurate readings of the draft marks unless those factors are fundamentally improved.
In order to obtain highly accurate readings consistently in any working environment, it is essential to introduce a scientific and standard method for reading draft marks.
In order to do so, we conclude that using KENDRAFT is a logical method suitable for the actual conditions on field.
The advantage of draft survey is that it can be calculated the large amount of the cargo in a short time by reading the draft marks of the vessel floating on the water surface by using Archimedes' principle of buoyancy, and furthermore is that we can get the high accuracy if the weather and sea conditions are good at that time. However, in the actual weather and sea conditions at sea, winds and waves are constantly changing, and as a result, the reading value of the draft mark fluctuates depending on the time and place. In addition, readings fluctuate due to differences in individual reading techniques and parallax due to vessel berthing conditions. This draft reading device, KENDRAFT, was developed to reduce the above various fluctuations.
@ It's possible to create a stationary water surface in the pipe that is not affected by wind
@ and waves.
A Furthermore, you can create the same condition as you are looking at the draft mark
@ horizontally from the water surface without parallax.
Click here for more details of KENDRAFT.
Click here for more information about the benefits of introducing a consistent survey from loading to unloading.
In marine logistics, we conduct various marine survey operations from a neutral and fair standpoint.