Timber Deck Cargoes and the Timber Load Lines

Timber is the only deck cargo for which the regulations grant a reduced freeboard and a separate set of stability criteria. It is also a cargo that gains weight during the voyage, because it absorbs water and collects ice, and that is why a timber ship has to be checked for the arrival condition as well as for the departure.

4.1 What a timber deck cargo earns, and what it costs

A properly stowed and secured timber deck cargo does two useful things. It closes the weather deck against boarding seas far more effectively than hatch covers alone, and, being made of a material lighter than water, it provides a reserve of buoyancy above the deck if the ship is heeled far enough for the stow to enter the water. The Load Line Convention recognises both by allowing a ship that meets a list of conditions to be assigned a second, smaller freeboard for timber deck cargo voyages, and the Intact Stability Code recognises them by offering an alternative and less demanding set of stability criteria.

It is worth being clear where the credit appears, because it does not appear where students usually look for it. The buoyancy of the timber is not added to the GZ curve. The GZ curve is calculated for the hull in the ordinary way, from the same cross curves used in every other chapter, with the deck cargo entering only as weight at its own height. The credit appears in two other places: in the reduced freeboard, and in the relaxed criteria, which are offered only for a stow of the kind the Code describes, one that fills the well between the superstructures, reaches across the full breadth of the ship, and is secured so that it stays put at large angles of heel. A candidate who also recalculates the KN values to include the deck stow has counted the credit twice.

Against that, timber absorbs water. A working allowance of about ten per cent of the weight of the deck cargo, added at the height of the deck stow, is the usual figure in stability calculations, and the ship’s own approved stability information will say what allowance she is to use. That is weight added at the highest point in the ship, and unlike fuel it cannot be burnt off. Add ice on the same surfaces in high latitudes, and take away the fuel and water consumed from the tanks, and the arrival condition can be a very different one from the departure condition.

4.2 The timber load lines

The MV Ninja booklet assigns ordinary freeboards only. Suppose for this chapter that she had also been assigned a timber summer freeboard of 3620 mm against her ordinary summer freeboard of 3920 mm, a reduction of 300 mm. Her timber summer draught would then be 9.900 m against an ordinary 9.600 m, at which the hydrostatic table gives 31518 t, and the extra deadweight is 1062 tonnes.

The seasonal marks follow the ordinary rules with one important difference. The winter timber allowance is one thirty sixth of the moulded summer timber draught, not the one forty eighth used for the ordinary winter mark. The tropical timber allowance is one forty eighth of the summer timber draught. The fresh water allowance is calculated in the usual way, but from the displacement and the TPC at the summer timber waterline (223 mm here, against the ordinary 216 mm), and the timber winter North Atlantic freeboard is the ordinary winter North Atlantic freeboard, which for a ship over 100 m in length is the ordinary winter freeboard. At the fresh water marks LF and LTF the ship floats in fresh water, so her displacement is the timber summer displacement (31518 t) and the timber tropical displacement (32248 t), not the salt water figure the hydrostatic table gives at those draughts.

4.3 What the freeboard has to be earned with

The reduced freeboard is not granted for carrying timber. It is granted for carrying timber in a particular way. In outline, the ship must have a forecastle of at least standard height and adequate length, and double bottom tanks amidships with satisfactory watertight subdivision; guard rails or bulwarks of at least one metre with efficient freeing arrangements; a safe walkway with rails, and access to the steering gear at all times. The stow itself must extend over the entire available length of the well, be as solid as practicable and as close to the ship’s side as the fittings allow, and be secured by lashings at intervals, each provided with a means of quick release. And the ship must satisfy the stability criteria throughout the voyage, allowing for water absorption and for ice.

The last condition is the one this chapter is really about. The others are checked before sailing and remain satisfied, provided the lashings and rails survive the weather. The stability condition changes throughout the voyage, and on a timber ship it changes for the worse.

4.4 The two sets of criteria

For a ship carrying a timber deck cargo stowed and secured as above, the Code offers an alternative to the general intact criteria of Chapter 15 of Volume Two.

The timber set is shorter but it is not simply weaker. It drops the shape tests around the thirty degree region and replaces them with a direct requirement on the height of the peak, which is a sensible thing to ask of a ship whose deck cargo will start to give buoyancy at large angles. The metacentric height of 0.10 m is required at all times during the voyage, after the free surfaces, the absorbed water and any ice have been allowed for, not only on departure (Part A, 3.3.2.3; the 1993 Code asked for 0.10 m on departure and a positive GM thereafter). The wind and rolling criterion of Chapter 2 still applies, with the 16 degree limit on the steady wind heel but without the deck edge limit.

4.5 Worked example 4.1: the departure condition

MV Ninja loads packaged sawn timber in all five holds at 1.75 cubic metres per tonne. Packaged timber is a bale cargo and fills the bale capacity of a hold, 30672 cubic metres in all, giving 17526.9 t with each parcel at the centroid of its hold (KG 7.807 m). On deck she carries a stow 4.75 m high over the 120 metres of the five hatches and 22 metres of breadth between the uprights, at 1.90 cubic metres per tonne, which is 6600.0 t with its centre of gravity half way up the stow at Kg 15.875 m. Her fuel, diesel oil, fresh water and stores come to 742.8 t from the tank table of the booklet, with the slack tanks (No. 2 HFO port and starboard, diesel oil, fresh water starboard) giving a free surface moment of 390.9 tonne metres, each as i multiplied by the relative density of the contents.

The displacement comes to 29819.7 t at a draught of 9.419 m: 18 centimetres inside her ordinary summer marks and 48 centimetres inside her timber marks. The solid KG is 9.883 m, the free surface correction 0.013 m, so the fluid KG is 9.896 m; KM is 10.328 m and the fluid GM 0.432 m. The maximum KG table of the booklet allows 9.845 m at this displacement, so she is 51 mm over the limit set by the general criteria.

The righting levers at the tabulated angles (KN interpolated between the 29000 and 30500 t rows, less KG sinθ) are 0.039, 0.089, 0.114, 0.256, 0.250 and 0.079 m at 5, 10, 12, 20, 30 and 40 degrees. Against the general criteria she fails: the area from thirty to forty degrees is 0.029 against 0.030, and the maximum righting lever occurs at 20 degrees against a requirement of twenty five. Against the timber criteria she complies: area to forty degrees 0.113 against 0.080, maximum GZ 0.256 m against 0.25, fluid GM 0.432 m against 0.10.

4.6 Worked example 4.2: the voyage

Four things happen between departure and arrival, and only one of them is in the officer’s favour: the deck cargo absorbs 660.0 t of water (ten per cent of its weight) at Kg 15.875 m; ice adds 103.8 t at Kg 17.39 m (79.2 t on the top of the stow at 30 kg per square metre and 24.6 t on the two sides at 7.5 kg per square metre); 543.8 t of fuel and water is burnt, from tanks which on MV Ninja lie high in the ship (11.45 to 12.65 m above the base, so that burning them lowers G a little); and the free surface moment rises from 390.9 to 815.2 tonne metres as every tank goes slack.

The displacement rises, from 29819.7 to 30039.7 t, because the water taken up by the deck cargo and the ice together outweigh what has been burnt. She arrives deeper than she sailed, at 9.482 m, though still 12 centimetres inside her ordinary summer marks and 42 centimetres inside her timber marks.

What is not visible is the shape of the righting lever curve. The fluid KG has risen from 9.896 to 10.021 m, a rise of 125 millimetres, and the GM has fallen from 0.432 to 0.308 m. Neither figure looks alarming. But the maximum righting lever has fallen from 0.256 to 0.210 m, below the 0.25 the timber criteria require, and the area to forty degrees from 0.113 to 0.078 metre radians, below the 0.080 required. She complied on departure and does not comply on arrival.

4.7 Worked example 4.3: how much can she actually carry?

The right question at the loading berth is not how much deck cargo will pass the criteria today, but how much will still pass them on arrival.

The largest stow that still complies on arrival is 4.50 metres, and it does so with 3 mm of maximum righting lever to spare. A prudent departure is a stow of 4.40 metres, 6113.7 t of deck cargo against the 6600.0 t of worked example 4.1, which arrives with a maximum lever of 0.285 m and 35 mm in hand. To arrive in compliance, therefore, 486.3 tonnes of timber, about seven per cent of the deck cargo and 350 mm of stow, has to be left on the quay.

4.8 What happens if she loads right to the marks

The timber summer mark is at 9.900 m and 31518 t. With the holds full and the same consumables, reaching it needs 8295.2 t of deck cargo, a stow of 5.97 metres, and the table above has already shown that a stow of 5 metres fails both sets of criteria on departure: on MV Ninja the load line is not the binding limit, because she runs out of righting lever before she runs out of freeboard. Suppose a master loaded to the mark regardless, and consider only the draught. She sails at 31514.9 t and 9.899 m, on the mark within a millimetre. On passage she absorbs 829.5 t, collects 105.6 t of ice and burns 543.8 t, and arrives at 31906.2 t and a draught of 10.010 m: 11 centimetres over her timber summer mark and 41 centimetres over her ordinary one. The load line offence is committed at sea, not at the berth, by a cargo that has been gaining weight throughout the passage.

There is a second and less obvious consequence. The cross curves of the booklet end at 30500 t. At 31514.9 t on departure she is already 1015 tonnes beyond their last row, and at 31906.2 t on arrival 1406 tonnes beyond it. Her righting lever curve cannot be constructed from the approved book at all, so her stability cannot be checked even by an officer who wants to check it. A ship actually assigned timber freeboards must carry cross curves and hydrostatic data that run at least to her timber tropical displacement. For this ship, exceeding the marks and exceeding the range of the cross curves are the same event.

Test yourself

Questions

1. Explain where the buoyancy of a timber deck cargo is credited in the regulations, and state why it is wrong to add it to the cross curves.
2. MV Ninja is assigned a timber summer freeboard of 3620 mm. Calculate her timber winter and timber tropical draughts, and state the rule used for each.
3. List six of the conditions of assignment that must be satisfied before the timber freeboard may be used, and state which of them has to remain satisfied throughout the voyage.
4. Set out the three stability criteria for a ship carrying a timber deck cargo, and state for each one whether it is more or less demanding than the corresponding general intact criterion.
5. A timber deck cargo of 6600.0 t is stowed with its centre of gravity at Kg 15.875 m on a ship of 29819.7 t displacement whose fluid KG is 9.896 m. Calculate the rise in KG caused by an absorption of ten per cent of the cargo’s weight.
6. Calculate the weight of ice accreted on a deck stow 120 m long, 22 m wide and 4.50 m high, using 30 kg per square metre on the top of the stow and 7.5 kg per square metre on the projected lateral area of each side, taking the bare ship profile as 1086.2 square metres.
7. In worked example 4.2 the fluid GM fell by only 124 mm and remained three times the minimum, yet the ship arrived in breach of the criteria. Explain why, and state which properties of the righting lever curve were responsible.
8. A master proposes to load his timber deck cargo until the timber summer mark is exactly at the waterline. Give two separate reasons why this is imprudent.
9. Explain what is meant by the statement that, for MV Ninja with timber freeboards, exceeding the marks and exceeding the range of the cross curves are the same event.
10. A ship carrying timber on deck loses two lashings and part of her walkway rails in heavy weather. State what this does to her entitlement to the timber freeboard, and what the master should do.

Looking ahead

Chapters 2, 3 and 4 each drew a heeling lever across a righting lever curve, and each time the curve itself was taken from the approved cross curves of an intact hull. That assumption now goes. Chapter 5 opens the damage stability half of this volume with the deterministic approach: a compartment is opened to the sea, and the ship must be shown to survive it. Floodable length, permeability, and the two ways of accounting for a flooded space, as added weight or as lost buoyancy, replace the heeling levers of the last three chapters. The righting lever curve remains the judge, but the ship it belongs to is no longer the one in the booklet.