The Stability Information Book and Approved Loading Instruments

Every calculation in the first two volumes assumed that a table was open on the desk. This chapter is about that table: where it comes from, what it must contain, who approves it, and what the officer is required to do with the computer that has largely replaced it.

1.1 Why the book exists at all

A ship is a machine whose safety depends on how it is loaded, and the person loading it is not the person who designed it. The naval architect knows the hull form, the position of the metacentre at every draught and the righting levers at every angle. The chief officer knows the cargo, the tanks and the sailing time. The stability information book is the document that carries the first person’s knowledge to the second, in a form that can be used at two in the morning with a torch.

That is why the requirement is a legal one rather than a matter of good practice. Under SOLAS chapter II 1, the master must be supplied with stability information sufficient to obtain accurate guidance on the stability of the ship under the conditions in which she is likely to be operated, and that information must be in a language the officers understand. The 1966 Load Line Convention carries a parallel requirement, and the 2008 Intact Stability Code sets out in detail what the information must contain. The book is approved by the flag Administration, or by a recognised organisation acting for it, and it is approved for one particular ship. A sister ship gets her own book, because her own inclining experiment will have given a slightly different light ship weight and centre of gravity.

Approval matters more than it may appear. When a surveyor, a port State control officer or a court asks whether a ship sailed in a legal condition, the question is settled by comparing the loaded condition against the approved book. Not against a spreadsheet, not against a printout from an unapproved program, and not against what an experienced master believed to be safe. The book is the standard.

1.2 The seven parts of the book

Each part answers a different question, and Volumes One and Two have already used every one of them. General particulars and plans identify the compartment and fix the openings that decide the angle of flooding. The hydrostatic particulars supply displacement, TPC, MCTC, KM, KB, LCB and LCF against draught, which is where every trim problem and every draught survey began. The cross curves give KN, the righting lever for an assumed KG of zero, so that one table serves every condition through GZ = KN − KG sin θ. The tank tables give capacity, centres and the transverse moment of inertia of each free surface, without which the fluid KG cannot be found. The limiting KG data give the single line the answer is compared against. The standard loading conditions demonstrate that she can be worked legally. And the instructions to the master carry the sequencing, the slack tank restrictions and the checks required.

1.3 The standard loading conditions

Almost every book carries a set of fully worked conditions: light ship, a normal ballast departure and arrival, and a homogeneous full load departure and arrival. Cadets often treat these as examples to copy. They are not. They are evidence, offered to the Administration, that the ship can be operated across her intended range of service while meeting every applicable criterion. Departure and arrival are both shown because you cannot tell in advance which is the worse. On a ship that burns fuel from double bottom tanks the arrival condition, with the low weight gone and the tanks slack, is usually the critical one. MV Ninja carries her heavy fuel oil in wing tanks at Kg 12.65 m, above her loaded centre of gravity, so burning it lowers G and for her the departure condition is the tighter of the two. The book must show both, because the answer depends on where the fuel is.

1.4 Worked example 1.1: the homogeneous full load departure

MV Ninja is to load a homogeneous cargo stowing at 1.30 cubic metres per tonne, filling the grain capacity of all five holds. On departure she carries 509 t of heavy fuel oil of RD 0.950 in the four No.1 and No.2 heavy fuel oil tanks (Kg 12.65 m), filled in proportion to their capacities and all slack; 35 t of diesel oil of RD 0.850 in the diesel oil tank (Kg 11.45 m); and 165 t of fresh water shared between the two fresh water tanks (combined Kg 11.86 m), both slack. Light ship is 4950 t at Kg 8.86 m. The free surface moment of each slack tank is its tabulated inertia times the density of its contents: 2 × 147 × 0.950 = 279.3, 2 × 139 × 0.950 = 264.1, 28 × 0.850 = 23.8 and (145 + 103) × 1.000 = 248.0 tonne metres, 815.2 in all. Prepare the departure condition and show that she complies.

Weight is capacity divided by the stowage factor, so No.2 hold takes 6900 / 1.30 = 5308 t to the nearest tonne (cargo is carried to the tonne, tank weights to 0.1 t and moments to 0.1 t m, each result rounded as it is written and carried forward). The heavy fuel oil splits 369.6 : 349.6 by capacity, 261.6 t to the No.1 pair and 247.4 t to the No.2 pair. Weight times Kg gives the vertical moment on each line. The free surface moments are kept in their own column, because they are added to the total moment but contribute no weight at all. That distinction is the whole of the free surface correction.

The displacement of 30456.0 t is her summer displacement, so she is exactly on her marks at a draught of 9.60 m. Entering the hydrostatic table there gives KM = 10.330 m, so the fluid GM is 10.330 − 8.113 = 2.217 m. Entering the maximum KG table at 30456 t and interpolating between the 30000 t and 30500 t rows (9.753 and 9.631 m, fraction 0.912) gives a maximum permissible fluid KG of 9.642 m, so she complies with 1.529 m in hand. Moments about the after perpendicular give LCG 77.089 m against LCB 76.90 m, so she trims 30456 × 0.189 / 405.7 = 14.2 cm by the head: 9.673 m forward and 9.531 m aft.

Two things are worth noticing. The free surface correction is only 0.027 m, because 815.2 tonne metres divided by 30456 t is very little; the same slack tanks cost more than twice as much in the ballast condition, where the displacement is less than half of this. And a GM of 2.217 m is large: she is a stiff ship here, which Chapter 14 of Volume Two showed will give her a short, violent roll. Compliance and comfort are not the same thing.

1.5 The approved loading instrument

On almost every ship in service that calculation is done by a computer. SOLAS chapter XII, regulation 11, requires bulk carriers of 150 metres in length and upwards to carry an approved loading instrument giving the hull girder shear forces and bending moments, and bulk carriers under that length built on or after 1 July 2006 to carry one giving intact stability information; stability software is in practice fitted very widely elsewhere. The 2008 Intact Stability Code classifies onboard stability software by what it calculates: Type 1 handles intact stability only; Type 2 adds damage stability by comparing the condition against a limiting curve worked out ashore; Type 3 calculates each pre programmed damage case directly on board; and Type 4 calculates the damage stability of the actual loading condition for an actual flooding case. A loading instrument fitted to meet the bulk carrier requirement must also compute hull girder shear force and bending moment, which is the subject of Chapter 10.

Approval covers three things: the program, the ship specific data loaded into it, and a set of test conditions with the correct answers attached. It covers nothing at all about the numbers an officer types in on a given morning. That is the gap the rest of this chapter is about.

1.6 Worked example 1.2: checking the instrument

For the departure condition above, the loading instrument prints a displacement of 30461 t, a fluid KG of 8.11 m, a fluid GM of 2.22 m, and draughts of 9.68 m forward and 9.54 m aft. The displacement differs from the hand figure by +5 t, which is 0.016 per cent, or 0.14 cm of draught at a TPC of 35.28. The fluid KG and GM differ by 0.003 m, no more than the instrument’s rounding to two places. The draughts differ from the hand figures of 9.673 and 9.531 m by 0.7 and 0.9 cm, and the instrument’s mean draught of 9.61 m is within a centimetre of the hand figure of 9.60 m. Every difference is inside the tolerance normally applied, and the instrument is confirmed for this condition.

1.7 Worked example 1.3: the error the GM will not show

Now suppose the officer keys the tonnage of No.1 hold into the line for No.4 hold. No.4 should hold 5308 t and is entered as 4503 t, so 805 t of cargo is left out of the calculation while remaining on board.

Because the differences are millimetres, the working carries a fourth figure. The fluid KG shown is (246256.7 − 805 × 7.70 + 815.2) / 29651 = 8.1236 m against the true 8.1124 m, and with KM 10.3286 m at the new draught the fluid GM shown is 2.2050 m against the true 2.2176 m: the KG is shown 0.011 m too high and the GM 0.013 m too low. Nothing on the stability page looks wrong, because the missing cargo sat at Kg 7.70 m, close to the ship’s own centre of gravity, so removing it barely moves G. The displacement, however, falls to 29651 t, and the predicted true mean draught falls to 9.372 m against 9.60 m at the marks: a shortfall of 22.8 centimetres that no officer reading the marks would miss.

1.8 When the instrument fails

The requirement that the ship carry an approved stability book in hard copy is not nostalgia. A loading instrument can fail, and when it does the ship must still be worked. The regulations treat the instrument as an aid: it never replaces the book, and the master remains responsible for the condition of the ship whatever the printout says.

In practice this means three things. The book must be on board and current. The officers must be able to use it, which means the hand calculation above must be a routine skill rather than an examination memory. And the standard conditions must be understood well enough that an officer can interpolate between them and know when he has wandered outside their range. A ship loaded with a cargo that stows very differently from the homogeneous case, or with holds deliberately left empty, is outside the range those conditions demonstrate. That does not make her illegal, but it does mean the calculation must be done in full.

Test yourself

Questions

1. State the seven principal parts of an approved stability information book and give, for each, one calculation from Volume One or Volume Two that could not be done without it.
2. MV Ninja loads a homogeneous cargo stowing at 1.45 cubic metres per tonne, filling all five holds, with the same consumables as worked example 1.1. Calculate her displacement, fluid KG and fluid GM, and state whether she complies.
3. Explain why the free surface correction in worked example 1.1 is only 0.027 m, and state the condition of loading in which the same seven slack tanks would matter most.
4. A loading instrument prints a displacement of 27400 t and a mean draught of 8.72 m for MV Ninja. Using the hydrostatic table, decide whether these two figures are consistent with one another, and state what you would do if they were not.
5. In the departure condition of worked example 1.1 the chief officer keys 4750 t into No.4 hold, which actually contains 5308 t at Kg 7.70 m. Calculate the resulting error in displacement, in fluid KG and in predicted true mean draught, and state which of the three would first alert a careful officer.
6. Distinguish between Type 1, Type 2 and Type 3 stability software, and state which type would be required on a ship whose damage stability is demonstrated by a limiting KG curve prepared ashore.
7. A ship is to load a part cargo leaving No.3 hold empty. Explain why the standard loading conditions in the book cannot be used directly, and set out the steps you would take instead.
8. Explain, with reference to the arrival condition, why a book that showed only departure conditions would be inadequate.
9. The master is told the loading instrument has failed and cannot be repaired before sailing. State what must be available on board for the ship to sail lawfully, and what the officers must be able to do.
10. MV Ninja is at a displacement of 24000 t. Using the maximum KG table, state her permissible fluid KG, and calculate the free surface moment that would just bring a solid KG of 10.25 m up to that limit.

Looking ahead

This chapter established the document that everything in Volume Three will add pages to. The book as it stands proves compliance with the intact criteria of the 2008 Code, worked in still water with the ship upright. Chapter 2 introduces the criterion that does not assume still water: the severe wind and rolling criterion, in which a beam wind heels the ship while she is already rolling to windward, and the areas under the righting lever curve are compared on either side of the resulting angle. It is the first criterion in this volume that uses the wind heeling data in section 7 of the booklet, and the first that asks what the sea is doing rather than only what the cargo is doing.