Draft survey by using Archimedes' principle of buoyancy 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 draft reading is greatly affected by the working environment, such as the weather and sea condition at the time of work. In other words, the waves and swell on the water change depending on the time and place, there is personal variation in visual reading, and the draft mark reading value also 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.
Currently, the draft survey method used on site is carried out as follows.
There are a total of 6 draft marks (each on the port side and starboard side of the bow, center and stern) on a vessel.
Usually, vessels are berthed at the wharf of shore side, so in that case, for the shore side, we read the three draft marks at the bow, stern and center from the berth.
On the other hand, for the opposite side of the shore side (hereafter referred to as the sea side), it is common to only read the center draft mark, and then determine the readings of the bow and stern draft marks on the sea side by calculating the left and right inclination value according to each width and adding or deducting the inclination figure to the reading value of shore side.
And, when it comes to reading the center draft mark on the sea side, one of the following three methods is generally used:
@ Method using a manometer
A Method using a Jacobs ladder
B Method using a draft boat
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 other words, since there are variable factors in the actual working environment of draft surveys, we believe it is important to improve them in a scientific and rational way and obtain stable and accurate draft readings. In order to do that, when we read the draft marks of a vessel, it is necessary to standardize it to scientific and rational method.
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, 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, wind 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. By using KENDRAFT, the reading of the center draft mark can be done scientifically and rationally, and then the accuracy of draft surveys could be improved.
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 waves and swell, 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 float reflected in the mirror is located, you can get an accurate draft mark without parallax.
@ 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.