What is a draft?
- It is the distance from the bottom of the keel to the waterline or rather the depth of the vessel under the water.
- The draft marks are painted on the ship as follows:
- Both port and starboard sides of the bow
- On both sides of midship on the shipside along with the loadline marks.
- On both sides of the stern as close to the after perpendicular of the ship.
Having seen where to find the draft markings on the ship a few definitions given below would be helpful:
Forward draft: The distance from the bottom of the keel to the waterline on the forward perpendicular, when the ship is upright.
Aft draft: The distance from the bottom of the keel to the waterline on the after perpendicular, when the ship is upright.
Midships draft: The distance from the bottom of the keel to the waterline at a position midway between the perpendiculars.
Arithmetical Mean draft (AMD): The arithmetical mean of the forward draft and after draft.
True Mean draft (TMD): The draft at the centre of floatation
Some related definitions:
Displacement: The amount of water displaced by the ship. It represents the total weight of the ship and is calculated as the:
Under water volume of the ship x density of water
Even keel: When the forward and aft draft are the same, the ship is said to be on an even keel. The ship’s hydrostatic data is calculated for this condition.
Freeboard: The distance from the waterline to the upper surface of the freeboard deck at the ship’s side. It can also be defined as the distance from the top of the deckline to the waterline.
Forward perpendicular (FP): A perpendicular drawn to the waterline at the point where the foreside of the stem meets the summer loadline or LWL (Load Water Line)
After perpendicular (AP): A perpendicular drawn to the waterline at the point where the aft side of the rudder post meets the summer loadline, where no rudder post is fitted it is taken as the centre line of the rudder stock.
Length between perpendicular (LBP): Distance from the forward perpendicular to the aft perpendicular.
Why do we need to take a draft?
- To find the displacement of the ship at any time.To calculate the cargo loaded or discharged.
- To adjust the loading and other consumables on board in such a fashion so as to arrive at a particular draft in a depth restricted port.
A well conducted draught survey of a large vessel should achieve accuracy to within 0.5%. The survey must be conducted meticulously taking into account all the prevailing circumstances.
Before we get further on and start reading the ship’s draft let us see learn about loadline marks and how they came into being.
- A ship is a floating object and for it to float it needs a certain amount of buoyancy.
- (We hope you all remember the Archemedis Principle)
- Any ship when loaded beyond its limit will reduce the buoyancy of the ship thereby rendering it unsafe.
- An overloaded ship is an unsafe ship as it does not then have sufficient reserve buoyancy.
- Hence to ensure that the ship is safe at all times a British Member of Parliament by the name of Samuel Plimsol introduced a regulation in the Merchant Shipping Act of 1876. This law requires all ships to have loadline marks on each side of the ship.
- Thus the loadline mark became known as the Plimsol Mark. This was to bring about shipboard safety for seamen against overloading for commercial gain by unscrupulous ship owners..
Loadline marks are normally marked amidships on the shipside on both port and starboard sides. Depending on the climatic condition and weather encountered, the oceans of the world are divided into 3 zones.
and tropical (T)
The present regulation dealing with loadlines is the LOADLINE REGULATION of 1966. All details of the loadline marks and how they are assigned are given in these rules. A shipowner can be fined heavily and sometimes the ship detained by shore authorities for non-compliance with these rules.
Some areas of the oceans also fall in the category of seasonal zones. This is a particular zone that is treated as summer zone during certain months and tropical or winter zones during the other months of the year.
(See zone chart on your ship. This chart will also be displayed on the bridge and in the cargo office.)
Some example of use for these loadline marks
When a ship is trading exclusively in a particular zone for for example the summer zone. The midships draft of the ship must never exceed the summer loadline during any stage of the voyage.
- If the ship on a particular voyage has to cross various zones during of the voyage then the Midships draft should never exceed the loadline mark for the current.
So if a ship enters a winter zone from a summer zone area the draft when entering the winter zone and throughout the time the ship is in the winter zone must not exceed the winter loadline.
Look at the loadline marks shown in this figure.
- Deckline: This line is marked amidship in lime with the uppermost continuous plating (weather deck) where it meets the sheer strake (the upper most strake of the ship’s side plate)
The deckline is 300 mm long and its upper edge is in line with the uppermost continuous watertight deck.
- Vertically below the deckline is a round circle whose outside diameter is 300 mm. This is called the loadline disc.
- The Plimsol line is the horizontal line which cuts through the loadline disc. On this line the initials of the assigning authority is marked, in this case the letters BV represent the Classification Society called Bureau Veritas.
- The Loadline markings are marked on the right side of the Plimsol marks when viewing the shipside on the starboard side and left side of the Plimsol mark when viewing the shipside on the port side.
- When the vessel is in a tropical zone and in fresh water of density 1,000 the ship may load up to the upper edge of mark ‘TF’ (Tropical Fresh)
- When in fresh water of density 1,000 and in tropical summer zone a ship may load up to the upper edge of the line marked “F” (Fresh)
- When in saltwater of density 1025 and in tropical zone the maximum the vessel can load is to the upper edge of the line marked “T” (Tropical)
- When the ship is in summer zone and in the sea water of density 1025 she must not load more than the upper edge of the line marked ‘S’. This is the SUMMER LOAD LINE of the vessel and it passes through the centre of the loadline Disc. The displacement when the ship is loaded to the line is known as the “SUMMER DISPLACEMENT”
- The ship can only load up to the upper edge of mark “W” when loading in winter zone in seawater density of 1025.
- The mark “WNA” is assigned to small ships less than 100 metres in length in the North Atlantic where the sea are so rough in winters. WNA stands for winter North Atlantic. Here smaller ships need more freeboard so they are not even allowed to load up to the ‘W’ mark. They can only load up to the upper edge of ‘WNA’ mark which is 50 mm lower than the ‘W’ mark.
Reading Draft Marks
when the markings are in metres, the main metre marks are marked as Arabic manuals followed by the letter M such as 8M, 9M, 10M or in letters such as VIII, IX, X . In between the metric marks are Arabic numeral markings such as 2, 4, 6 and 8 to indicate 20 cm, 40cm, 60 cm and 80 cm. The height of each mark is 10 cm and the distance between the mark in 10 cm.
If the water is just in line with the mark 10M or X the draft is 10.0 metres. If the water is in line with the top edge of 10 M, then the draft is 10.10 metres and so on. The diagram clearly explains the reading.
When the draft markings are in feet, for each mark (whether Arabic numerals or Roman letters) the height of the mark is 6″ and the distance between them is also 6″.
Measuring the draft in a swell
In heavy sea conditions there may be waves, swell, pitching and rolling to take into account.
There are two methods of taking a draft reading in such a conditions.
- Obtain a total of 12 mean readings.
- The highest and lowest means should be rejected.
- Average the remaining ten and this will give the most accurate reading possible.
- Reading is done using the wave cycle.
- Normally one wave out of every five wave will be absolutely calm.
- Read the draft at that particular wave a few times and average the same.
- This reading will give an accurate reading.
Some hints for draft reading
- Principle of draft surveys:
Weight of loaded ship – weight of an empty ship = weight of the cargo.
Apart from cargo there are other consumables on board ship like fresh water, fuel, stores and may be un-pumpable ballast which needs to be accounted for when calculating cargo by means of draft survey.
- When taking the initial and final drafts in a load or discharge port
Ensure that nothing should be moving when reading the drafts – no cargo transfers, ballast, stores, fuel transfers, hatch covers. Accuracy of reading may be affected if any of these operations is in progress when taking the draft. This may not be practical when taking regular AM and PM drafts as it may not be possible to stop cargo work every time you want to take a draft.
- Draft marks should be read from as close to the waterline as possible, in order to avoid errors due to parallax. Those read from a boat at rest with no wash are likely to be the most accurate.
- It is possible that at times the midship mark may not be clearly visible on the side the ship is alongside. Then the distance from the waterline to the deck is measured and this is deducted from the deck height which can be obtained from the ship’s plan.
- Take draft readings preferably at slack water as the ship may squat due to strong tide or current in shallow water and so the draft read during these times may not reflect the true, free floating draft.
- When taking a draft without a draft measuring instrument in a swell it is best to take the highest and lowest readings of a series of swells and then calculate the average, as described above.
- Some useful equipment to be kept handy when reading a draft is a hand lamp or touch with a powerful beam and a pair of Binoculars.
- Draft gauges if fitted on board should not be entirely relied upon. Visual draft readings should be used to calibrate draft gauge reading on a regular basis.
Draft Measuring Instrument
For reading drafts in swell, measuring instruments are available which some of the draft surveyors carry with them.
- The instrument consists of a very long, flexible open ended, weighted hose projecting downwards to below the waterline.
- The instrument is held directly in front of the draught marks and after allowing a few seconds for the tubes to fill with water, the mean water level is indicated by the float.
- The float may have a small rise and fall but this will be insignificant when compared with the wave motion.
- The device is difficult to use at the bow and stern.
- It should be held close to the draught marks under the overhang of the bow and the stern to avoid parallax.
- The device may be attached to the hull by a magnet.
These devices are usually used by surveyors when they needs to take a draft survey in heavy smell and sea conditions.
Several types of draft gauges are available – the most common one is the pneumatic type using compressed air. As the models vary we will not discuss details. These instruments are based on the same principle of the tank gauging system used on tankers.
Draft gauges should be calibrated on a regular basis by checking them against draft taken visually.