Load Line Regulations

INTRODUCTION

The Load line regulations have a long history of over 140 years. They were introduced in the British parliament by Lord Plimsoll, in the year 1854 and finally passed in 1876 as the first merchant shipping Act. As per this Act all Merchant ships were required to have a Load line marked on the ship side at the mid-ship length. This Load line mark has a circle with a diametrical line cut across the circle . This mark is also known as plimsoll mark and ships were allowed to load only till this line reached the waterline, thus preventing overloading of the ship. The rule adopted to determine this line was by referring to a table of Freeboards against length of ship and from it determine the allowable Freeboard. This Rule is still in force even in India for assigning Load line to sailing vessels.

With the advent of steel ships and powered propulsion, the very simple Load line regulations has slowly developed into a more elaborate set of rules covering all known aspects of flooding and damage control. The 1930 Load line regulations came into force after the development of steel ships powered by steam or I.C. engines . At That time tankers were also in vogue . The rules therefore catered to dry cargo ships as well as tankers and the concept of two separate tables of Freeboards based on length of ship was in existence then as it is now. Most of the rules now in force were also in force then.

The 1966 Load Line Convention happens to be the first International Load Line Convention after IMO (IMCO) came into existence. The Convention was held from third March to fifth April 1966 . The L/L Regulations being the outcome of this Convention was adopted on fifth April 1966. The Regulations came into force six months after it was officially adopted ie on fifth October 1966.

The special provisions in these regulations is the official recognition given to steel hatch covers and an incentive to adopt steel hatch covers instead of wooden hatch covers. By favouring ships with steel hatch covers by a reduction in summer Freeboard., thereby increasing their dead weight capacity . Hence in these regulations there are actually three sets of basing summer Freeboards. Table A for liquid bulk carriers (tankers) , Table B for dry cargo carriers with steel hatch covers and a third Table showing the increase in Freeboard from Table B for ships fitted with wooden Hatch covers.

Another important feature in these regulations is the assignment of timber Load line to ships specially equipped to carry timber as deck cargo..Considering that timber is a buoyant cargo special regulations have been provided for carriage of timber deck cargo and conditions for its assignment. These are stated in chapter 4 of the Regulations.

The various chapters of these Regulations and their contents are shown in the following table:

OBJECTIVES OF ASSIGNING A LOAD LINE WITH FREEBOARDS

The goal objectives for assigning a load line to ships are twofold :

• To provide adequate reserve buoyancy against accidental flooding of any water-tight compartment in the
• To give protection to the crew on board for their safe movement within the

These goal objectives are realised by the following functional objectives, which are:

• Weather tight integrity of the freeboard or main deck
• Providing sufficient subdivision within the hull so that reserve buoyancy is sufficiently
• To provide adequate strength to the
• To provide comfortable and safe accommodation for the crew and facilities for their safe movement within the ship and to land ashore during port stay.

DIFFERENT FREEBOARD TABLES

The regulations provide primarily two separate tables, one for dry cargo vessels and another for liquid cargo vessels. From the tables one can observe that the basic freeboard for a tanker is considerably less than the freeboard of a dry cargo vessel of same dimensions. The reasons are as follows:

• Weather-tight integrity of freeboard deck : – The tanker has small openings for cargo ullaging and tank entry on the freeboard deck, which can be efficiently maintained in tight condition as compared to the dry cargo carrier which has large hatch covers for the holds which are rather difficult to maintain in a weather tight
• Cargo space subdivision :- Considering two equally dimensioned ships , one a tanker and the other a dry cargo vessel, the tanker has its cargo space divided into 3*5=15 tanks by transverse and longitudinal bulkheads, whereas the dry cargo vessel’s cargo space is divided into 5 holds by transverse bulkheads. The reserve buoyancy potential for the tanker becomes superior to the dry cargo vessel’s reserve
• Hull strength :- The tanker hull strength is superior to the dry cargo ship strength of same The longitudinal bulkheads and Isherwood framing system improve considerably the resistance to overcome large bending moments the tanker is subjected to. The large web frames within the tanker assist the tanker to resist the large shear forces the tanker is subjected to. This aspect makes the tanker more stronger than the dry cargo ship of equal dimensions.
• Permeability ;- In a tanker because of the liquid nature of the cargo which fills up the cargo space to its full level, the permeability of sea water entering the cargo space due to bilging is very negligible as compared to a dry cargo vessel whose permeability of the cargo space is about 60% on an average

Taking into account all the above factors all of which are in favour of the tanker, the regulations provide for a considerable difference in the freeboards of these vessels.

SUMMARY OF THE REGULATIONS

1Strength of hullShips built and maintained in accordance with class regulations are considered to have adequate strength.10Information to be supplied to the masterThe need to provide a ship specific approved stability booklet with relevant damage stability calculations for masters use. Where D is less than L/15 no reduction in freeboard.

Explanations

1. Definitions

   1. Superstructure:- A superstructure is a decked structure on the freeboard deck extending from side to side of the ship or with the side plating not being inboard of the shell plating more than 4 percent of the breadth (B). A raised quarter deck is regarded as a superstructure.In common parlance superstructure means any construction above the main deck.. This however is not correct and what is stated above is the correct meaning of superstructure with reference to ship construction.
   2. Water- tight :- Means prevention of leakage of water from a closed opening when the closure is subjected to a predetermined head of water. Example—water-tight door in engine room which is subjected to a head of sea water up to main deck level.
   3. Weather-tight:- Means prevention of leakage of water from a closed opening when the closure is subjected to a strong spray of water, such as a swell hitting a weather tight door. Water –tight condition is more stronger than Weather tight condition.

2. B-60 and B-100 SHIPS

   B-60 :- Dry cargo ships of over 100 metres in length, when loaded up to summer draught, will remain afloat in a satisfactory condition of equilibrium after flooding of any single damaged compartment at an assumed permeability of 0,95 excluding the machinery space. In such ships the freeboards shall be reduced to not more than 60% of the difference between table A and table B for the same length of ship.

   B-100:- In dry cargo ships of over 225 metres in length, the machinery space will be treated as a floodable compartment with an assumed permeability of 0,85. The ship will remain afloat in a satisfactory condition of equilibrium after flooding of any single damaged compartment with assumed permeability 0,95 and the machinery space with assumed permeability of 0.85.In such ships the freeboards shall be reduced to not more than 100% of the difference between table A and table B for the same length of the ship.Limits of status and stability in damaged condition:- The following limits may be regarded as satisfactory:

   • The final water line after flooding is below the lower edge of any opening through which progressive flooding may take place
   • The maximum angle of heel due to unsymmetrical flooding is of the order of 15 degrees.
   • The metacentric height in the flooded condition is

3. Calculation of deck sheer

   The deck sheer, which is the upward curvature of the deck plating starting from mid-ship to the ends forward and aft is provided to restrict the water entering the deck when the ship pitches in a heavy sea. The standard sheer profile is specified under regulation 38. Standard sheer profile stipulates that the forward sheer is twice the aft sheer and this is provided to prevent shipping water on deck as the ship moves forward in a rough sea.

   The standard sheer profile for after half and forward half of the Freeboard deck is given in two tables as under: L is length of ship.

   After Half

   Station	Ordinate (in mm)	Factor
   After perpendicular	25(L/3 +10)	1
   1/6 L from AP	11.1(L/3+10)	3
   1/3L from AP	2.5 (L/3 +10)	3
   Amidships	0	1

   Forward Half

   Station	Ordinate (in mm)	Factor
   Amidships	0	1
   1/3L from FP	5.6 (L/3 +10)	3
   1/6 L from FP	22.2(L/3 +10)	3
   Forward perpendicular	50(L/3 +10)	1

    

   The profile area of the two halves in terms of length is given as follows ; Forward half = 44’47 L +1334 mm².

   Aft half = 22.23 L + 667 mm²

   Consider a ship of length 200 metres. As per above calculation the standard forward half sheer area will be = 44.47*200 +1334 =9561 mm²

   The Aft half standard sheer area will be = 22.23 *200 + 667 =5113mm².

   Reg 38.9 states as follows “Where the sheer profile differs from the standard, the four ordinates of each profile in the forward or after half shall be multiplied by the appropriate factors given in the table of ordinates. The difference between the sums of the respective products and those of the standard divided by 8 measures the deficiency or excess of sheer in the forward or after half. The arithmetical mean of the excess or deficiency in the forward and after halves measures the excess or deficiency of sheer

   Forward Half

   Station	Ordinate (in mm)	Factor
   Amidships	0	1
   1/3 L from FP	200	3
   1/6 L from FP	400	3
   Forward perpendicular	600	1

   After Half

   Station	Ordinate (in mm)	Factor
   Amidships	0	1
   1/3L from AP	100	3
   1/6L from AP	200	3
   After perpendicular	300	1

    

   Forward sheer area = 2400mm² , After sheer area =1200mm²

   Applying reg 38.9 we have:

   Deficiency Forward sheer = frac{9561-2400}{8} = frac{7161}{8} =895.125 mm

   Deficiency after sheer = frac{5113-1200}{8} = frac{3913}{8} = 489.125 mm

   The average deficiency =frac{(895.125+489.125)}{2}= 692.125 mm

   Therefore 692.125 mm will be added to the tabular freeboard

   Reg 38.14 : Addition for deficiency in sheer :- Where the sheer is less than the standard, the correction for deficiency in sheer shall be added to the Freeboard.

   Reg 38.12 Where sheer credit is given for a poop or forecastle, the following formula shall be used;

   S=y/3* L’/L

   Where s=sheer credit to be deducted from the deficiency or added to the excess of sheer

   Y=difference between actual and standard height of superstructure at the end of sheer.

   L’= mean enclosed length of poop or forecastle up to a maximum length of

   0.5L and L=length of ship (LBP)

4.  Minimum Bow height

   The bow height is defined as the vertical distance at the forward perpendicular between the waterline corresponding to the assigned summer freeboard and the designed trim and the top of the exposed deck at side shall not be less than;
   For ships below 250 metres in length,

   frac{56 ast L (frac{1-L}{500})ast 1.36}{C_b + 0.68} millimetres.

   For ships of 250metres and above in length,

   frac{7000times 1.36}{C_b + 0.68} millimetres.

   Where L is the length of the ship in metres and C_B is the block coefficient which is to be taken not less than 0.68