Draft Mark Measurement
All the ships have draft marks in the forward, aft and midship region. These marks are used to verify the draft of the ship in any loading condition. Usually modern ships have remote draft gauge to read the draft from the control rooms, but they are not always reliable, and is mainly used when the ship in sailing. In port or in dock, the draft mark reading (visual of using a manual draft gauge) is usually taken as the reliable reading.
This test is carried to confirm the marking are as per the actual measurements, ie., the draft level marking are corresponding to their vertical distance from the lowest point of the keel.
The first step is to get the lowest point of the keel.
In smaller shipyards a piano wire is drawn under the keel from forward to the aft end and the keel heights are measured from the wire and plotted. The curve normally follows an up-down path. The smallest height corresponds to the lowest point of the keel and this is taken as the keel line.
In modern shipyards (even in big ships) the keel is almost straight and hence the location near the draft mark is taken as the lowest point. The keel line is then the base line (for drawing purposes, also called moulded line) with an offset of the keel plate thickness.
Once the reference point in the keel line is taken, the draft marks are marked with respect to this point.
In small shipyards, the draft markings are checked with a vertical scale using a water-filled tube. One end of the tube is placed along the vertical scale and the other end close to the marking on the ship. With adequate length of the tube, water level can be matched along the vertical scale and the curved surface of the ship.
In modern shipyards, a laser coordinate equipment is used (see the photo on the right). This is a high precision instrument and very expensive (about $30,000).
Midship draft measurement
In this procedure the reference point is a target placed under the keel near the midship area. Usually the target is a point marked on a circle (like in archery target) and placed few centimeters below the keel (refer photo on the left). The exact measurement is taken and deducted to make sure that the keel is the zero point of the laser equipment. Now another target is placed at the lowest point of the draft mark, usually just above the bilge strake. The height is confirmed with the equipment.
In case of tankers the midship is vertically above the bilge strake and therefore a normal scale can be used from this lowest draft mark (refer photo on the right). However in case of container ships, LNG carriers and other ships where the midship is not vertical, we have to use the laser coordinate system for all marks (as described below).
Aft draft measurement
The reference point, here also is placed under the keel and the draft marks are marked with respect to his point. Since the aft body is curved through out the draft mark region, every ten centimeter is marked using the laser coordinate equipment and a horizontal line drawn with a leveller.
During the test, random draft marks are choosen and the marking line reference height with the keel is checked using the laser coordinate equipment.