Chapter 28 Load lines and freeboard marks The link Freeboard and stability curves are inextricably linked. With an increase in the freeboard: Righting levers (GZ) are increased. GMT increases. Range of stability increases. Deck edge immerses later at greater angle of heel. Dynamical stability increases. Displacement decreases. KB decreases. Overall, both the stability and the safety of the vessel are improved. Historical note In 1876, Samuel Plimsoll introduced a law into Parliament that meant that ships were assigned certain freeboard markings above which, in particular conditions, they were not allowed to load beyond. Prior to this law a great many ships were lost at sea mainly due to overloading. In 1930 and in 1966, international conferences modified and expanded these statutory regulations dealing with the safety of ships. These regula- tions have been further improved over the years by conference meetings every 3 or 4 years up to the present day. One such organisation was the Safety of Life at Sea organisation (SOLAS). In recent years, the IMO has become another important maritime regulatory body. Definitions Type ‘A’ vessel: A ship that is designed to carry only liquid cargoes in bulk, and in which cargo tanks have only small access openings, closed by water- tight gasketed covers of steel or equivalent material. The exposed deck must be one of high integrity. It must have a high degree of safety against flooding, resulting from the low permeability of loaded cargo spaces and the degree of bulkhead subdiv- ision usually provided. Load lines and freeboard marks 249 Type ‘B’ vessels: All ships that do not fall under the provisions for Type ‘A’ vessels. For these ships it may be based on: ● The vertical extent of damage is equal to the depth of the ship. ● The penetration of damage is not more than 1/5 of the breadth moulded (B). ● No main transverse bulkhead is damaged. ● Ship’s KG is assessed for homogeneous loading of cargo holds, and for 50% of the designed capacity of consumable fluids and stores, etc. Type (B-60) vessels: The vessel must have an LBP of between 100 and 150 m. It must survive the flooding of any single compartment (excluding the machinery space). If greater than 150 m LBP, the machinery space must be considered as a floodable compartment. A typical ship type for a Type (B-60) vessel is a bulk carrier. Type (B-100) vessels: The vessel must have an LBP of between 100 and 150 m. It must survive the flooding of any two adjacent fore and aft com- partments (excluding the machinery space). If greater than 150 m LBP, the machinery space must be considered as a floodable compartment. Such a vessel may be classified as a Type ‘A’ vessel. The minimum DfT tabular freeboard values are shown in graphical form on Figure 28.1. Freeboards of oil tankers and general cargo ships Oil tankers are permitted to have more Summer freeboard than general cargo ships with a similar LBP. They are considered to be safer ships for the following reasons: 1. They have much smaller deck openings in the main deck. 2. They have greater subdivision, by the additional longitudinal and trans- verse bulkheads. 3. Their cargo oil has greater buoyancy than grain cargo. 4. They have more pumps to quickly control ingress of water after a bilging incident. 5. Cargo oil has a permeability of about 5% whilst grain cargo has a permea- bility of 60% to 65%. The lower permeability will instantly allow less ingress of water following a bilging incident. 6. Oil tankers will have greater GM values. This is particularly true for mod- ern double-skin tankers and wide shallow draft tankers. Tabulated freeboard values The procedure When the freeboard for vessel is being assigned, the procedure is to compare a basic Department for Transport (DfT) ‘standard ship’ with the ‘new design’ about to enter service. Figure 28.2 shows profiles of these two vessels. 250 Ship Stability for Masters and Mates Tabular freeboard for Type ‘A’ Type ‘B’ and Type (B-60) ships (5.303, 365 m) 5.160 5.00 Type ‘B’ ships 4.630 4.50 4.00 4.018 40% 60% 3.50 3.264 3.262 (3.433, 365 m) (B-60) Ships 3.00 3.012 3.406 2.612 2.50 2.315 Type ‘A’ ships 2.00 1.968 Tabular freeboard in mtrs 1.50 1.271 1.135 1.00 (0.587, 61 m) For all types 0.50 0.443 0 0 50 100 150 200 250 300 350 Freeboard length (LF) in mtrs Fig. 28.1 Tabular freeboard for Type ‘A’,Type ‘B’ and Type (B-60) Ships. Both vessels will have the same freeboard length (LF): f1 ϭ the basic or tabular freeboard f2 ϭ the final assigned freeboard for the new design Differences in hull form and structure are considered and compared with these two vessels. If the new design has hull form and structures that would Load lines and freeboard marks 251 Tabular freeboard f1 Freeboard Deck W L Draft Mld AP FP Freeboard f2 W L Draft Mld AP FP Fig. 28.2 (a) Basic DfT standard design and (b) New design being considered. increase the danger of operation, then the tabular freeboard is increased by pre-arranged regulations and formulae. The DfT Tabular freeboard value, based on the freeboard length value, is adjusted or modified for the following six characteristics: 1. Depth D. 2. Block coefficient Cb. 3. Bow height. 4. Length and height of superstructures. 5. Freeboard deck sheer. 6. Structural strength of the new design. For a standard ship, Depth ϭ LF/15. For a standard ship, Cb ϭ 0.680. Standard camber is assumed to be parabolic and equal to B.Mld/50. Freeboard deck sheer is assumed to be parabolic with the sheer forward being twice the deck sheer aft. Standard sheer aftϭϩϫ (L/3 10) 25 mm Standard shheer forwardϭϩϫ (L/3 10) 50 mm Note of caution: for the sheer formulae, L is a ship’s LBP in metres (not LF). 252 Ship Stability for Masters and Mates A new vessel can be built to a structural strength of the Lloyds ϩ100A1 standard. If the vessel is indeed built to this classification, then the modifi- cation (for strength) to the tabular freeboard is zero. Whenever the new design has a characteristic that is less safe than the standard DfT standard vessel, the tabular freeboard value will be increased accordingly. Figure 28.3 shows the bow height measurement to be considered. It is to be measured at the FP, to the uppermost deck exposed to the weather. Forecastle DK Upper deck SLWL The bow height. This height is measured to the uppermost deck exposed to the weather Draft moulded Base FP line Fig. 28.3 The bow height, as per DfT regulations. The final assigned statutory freeboard is always measured from the Summer load water line (SLWL) to the top of the freeboard deck’s stringer plate at amidships. The stringer plate is the outermost line of deck plating. It is the line of deck plating connected to the sheerstrake or gunwhale plate. The corrections in detail Depth correction If Depth D exceeds LF/15, the freeboard is to be increased. If this is so then the correction ϭ (D Ϫ LF/15) ϫ R where R ϭ LF/0.48 if LF is less than 120 m R ϭ 250 if LF is 120 m and above If LF is 155 m and the Depth D is 11.518 m, then: Depth correction ϭ (11.518 Ϫ 155/15) ϫ 250 ϭϩ296 mm If Depth D is less then LF/15, then no reduction is to be made. Load lines and freeboard marks 253 Cb correction If the Cb is greater than the standard 0.680, then the freeboard is to be increased by the following: Correctionϭϩ{. (Cb 0.680)/1 360} ϫ Tabular freeb ooard figure If the ship’s Cb is 0.830 and the Tabular freeboard figure is 2.048 m, then: Cb correction is {(0.830 ϩ 0.680)/1.360} ϫ 2.048 ϭ 2.274 m Hence addition for actual Cb value ϭ 2.274 Ϫ 2.028 ϭϩ0.226 m or ϩ226 mm Bow height correction If the bow height on the actual vessel is less than the standard bow height, then the freeboard must be increased. If the bow height on the actual vessel is greater than the standard bow height, then there is no correction to be made to the freeboard. The minimum bow height (mBH) for ships is as follows: Ͻ ϭ If LF is 250 m, then mBH 56L{1 Ϫ L/500} ϫ 1.36/(Cb ϩ 0.680) mm. If LF ϭ 250 m or is Ͼ250 m, then mBH ϭ 7000 ϫ 1.36/(Cb ϩ 0.680) mm. Worked example 1 An oil tanker is 155 m freeboard length with an actual bow height of 6.894 m and a Cb of 0.830. Does the bow height meet with the minimum statutory requirements? Ͻ ϭ Ship is 250 m so mBH 56 L{1 Ϫ L/500} ϫ 1.36/(Cb ϩ 0.680) mBH ϭ 56 ϫ155 ϫ {1 Ϫ 155/500} ϫ 1.36/(0.830 ϩ 0.680) Hence minimum bow height ϭ 5394 mm or 5.394 m. The actual bow height is 6.894 m so it is 1.50 m above the minimum statu- tory limit. No correction to the tabular freeboard is therefore necessary.