By ShipCalculators.com · Published 14 June 2026

MARPOL Annex I Reg.24: Damage Assumptions

MARPOL Annex I Regulation 24 sets the assumed extent of hypothetical side and bottom damage, a parallelepiped breach, that an oil tanker delivered before 1 January 2010 must survive in the Regulation 25 hypothetical oil outflow and Regulation 26 tank size calculations. Side damage assumes a longitudinal extent of $\tfrac{1}{3}L^{2/3}$ or 14.5 m, a transverse extent of $B/5$ or 11.5 m, and a vertical extent from the baseline up without limit. Bottom damage is split between the forward 0.3L and the rest of the hull, with a vertical penetration of $B/15$ or 6 m. The regulation fixes input geometry, not a discharge rule.

Contents

MARPOL Annex I Regulation 24 carries the title “Damage assumptions.” It does one job: it fixes the size and shape of the hypothetical hull breach that an oil tanker’s cargo arrangement has to survive on paper. The regulation defines a rectangular box, a parallelepiped, on the side and on the bottom of the ship, and it gives the longitudinal, transverse, and vertical dimensions of that box in terms of the ship’s length and breadth. Nothing in Regulation 24 says how much oil may spill. It says only how big the assumed hole is. The outflow that follows from sliding that box along the hull is computed in Regulation 25, and the cap on how that outflow may be distributed across cargo tanks comes from Regulation 26. Read Regulation 24 as the geometry input to a two-stage compliance calculation, never as a performance limit in its own right.

The regulation sits in the revised Annex I adopted by Resolution MEPC.117(52) on 15 October 2004, which entered into force on 1 January 2007 and renumbered the whole Annex. The damage-assumption content lives in Chapter 4, Requirements for the cargo area of oil tankers, Part A, Construction. Before the 2004 renumbering the same logic sat under different numbers across earlier editions, which is why older drawings and stability books reference a different regulation number for what is now Regulation 24. The dimensional values themselves did not change in the renumbering; the box has been the same box since the deterministic outflow scheme was written into Annex I.

What Regulation 24 is for

The opening line of Regulation 24 is precise about scope. It reads, “For the purpose of calculating hypothetical oil outflow from oil tankers in accordance with regulations 25 and 26, three dimensions of the extent of damage of a parallelepiped on the side and bottom of the ship are assumed.” That sentence ties Regulation 24 to two downstream calculations and to nothing else. The three dimensions are longitudinal (along the ship), transverse (athwartships, inboard from the shell), and vertical (up from the keel). Together they bound a rectangular volume of assumed structural loss.

The deterministic outflow scheme works in three steps, and Regulation 24 is the first. Step one, Regulation 24, sets the box dimensions. Step two, Regulation 25, slides that box to every conceivable location along the hull, works out which cargo and ballast compartments the box intersects at each location, and computes the volume of oil that would leave through the breach. Step three, Regulation 26, takes the worst outflow the box can produce and tests it against the permitted maximum hypothetical outflow, which in turn limits how large any single cargo tank may be. So the size of the cargo tanks a designer can draw is governed, ultimately, by a box whose dimensions Regulation 24 hands over.

This is why naval architects treat Regulation 24 as a constraint on tank subdivision rather than as a pollution rule. A wider transverse damage extent reaches deeper into the cargo block and breaches more wing tanks. A taller vertical bottom penetration opens the cargo tanks above a double bottom. Every dimension in the box is therefore a lever on the tank plan. The designer cannot change the box; the box is fixed by L and B. What the designer can change is the subdivision so that, wherever the fixed box lands, the outflow stays under the Regulation 26 cap.

The parallelepiped is a deliberate simplification. Real grounding and collision damage is irregular, three-dimensional, and rarely a clean rectangle. The regulation trades that realism for a reproducible, auditable shape that any surveyor or class plan-approval engineer can apply identically to the same ship. Two independent offices computing the Regulation 25 outflow for the same hull form should arrive at the same answer, because the input box is not a matter of judgment. That reproducibility is the point of writing the extents as formulas in L and B rather than as case-by-case assessments.

The freeboard length and breadth that drive the box

Every dimension in Regulation 24 is a function of two ship particulars, L and B, and both are defined terms in Annex I, not the figures a layperson might reach for. Getting them wrong shifts the whole box.

L is the freeboard length from Regulation 1.19. It is 96 percent of the total length on a waterline at 85 percent of the least molded depth measured from the top of the keel, or the length from the fore side of the stem to the axis of the rudder stock on that waterline if that length is greater. It is the same length the International Convention on Load Lines uses. It is not the length between perpendiculars and it is not the registered length, though for many hull forms the freeboard length sits close to LBP. A surveyor checking a Regulation 24 box starts by confirming which L the original calculation used, because a 2 percent error in L feeds through $L^{2/3}$ and $L/10$ into both the side and bottom boxes.

B is the maximum molded breadth of the ship measured amidships, to the molded line of the frame in a metal-shelled ship and to the outer surface of the hull in a ship shelled in any other material. It is the beam at the widest section, not the beam at the waterline. B drives the transverse damage extents $B/5$ and $B/6$ and the vertical bottom penetration $B/15$ . Because B sets three of the box’s dimensions, a beamy tanker takes a deeper transverse bite and a deeper bottom penetration than a slender one of the same length.

Both definitions matter when reconciling an old stability book against a class drawing, because a yard that used LBP in place of the freeboard length, or the waterline beam in place of the molded amidships beam, produces a box that looks right but sits a few percent off. The damage box is only as trustworthy as the L and B fed into it.

Side damage assumptions

For side damage Regulation 24.1.1 gives three extents. Each is written as a formula capped by a fixed metric value, and the rule that governs is “whichever is less.” The box is a rectangular volume against the ship’s side.

The longitudinal extent $l_c$ is

l_c = \min\left(\tfrac{1}{3}L^{2/3},\ 14.5\ \text{m}\right).

The $\tfrac{1}{3}L^{2/3}$ term grows with the cube-root-squared of length, so it rises slowly. For the formula term to reach the 14.5 m cap, $L^{2/3}$ must reach 43.5, which needs $L$ near 287 m. Below that length the formula governs; above it the 14.5 m cap takes over. Most tankers short of a large VLCC therefore use the formula value, and the box length grows gently with ship length rather than running away on the largest hulls.

The transverse extent $t_c$ is

t_c = \min\left(\tfrac{B}{5},\ 11.5\ \text{m}\right),

measured inboard from the ship’s side at right angles to the centerline at the level corresponding to the assigned summer freeboard. The reference level matters: the transverse penetration is taken at the summer load line, not at the keel or the deck. The $B/5$ term reaches the 11.5 m cap only when $B$ reaches 57.5 m, which is beyond the beam of almost every oil tanker afloat, so in practice the side transverse extent is $B/5$ for the entire fleet. A 32 m beam tanker takes a 6.4 m transverse bite; a 60 m beam ultra-large carrier would be the rare case that hits the cap.

The vertical extent $v_c$ runs from the baseline upward without limit. There is no cap and no formula. The assumed side breach opens the hull from the keel all the way to the top of the ship’s side. The consequence is that side damage is assumed to flood a tank over its full depth, which is why side damage tends to dominate the wing-tank outflow in the Regulation 25 result. A breach that opens a wing tank top to bottom releases more than a partial-depth bottom breach of the same tank.

Side damage extent	Symbol	Value	Cap binds when
Longitudinal	$l_c$	$\tfrac{1}{3}L^{2/3}$ or 14.5 m, lesser	$L \gtrsim 287$ m
Transverse (from shell, normal to CL, at summer load line)	$t_c$	$B/5$ or 11.5 m, lesser	$B \geq 57.5$ m
Vertical (from baseline)	$v_c$	upward without limit	never (no cap)

Bottom damage assumptions

Bottom damage in Regulation 24.1.2 is more involved, because the regulation sets two conditions to be applied individually to two stated portions of the tanker. The hull is split at 0.3L measured aft from the forward perpendicular. The forward 0.3L, the part most exposed in a head-on grounding, takes the larger box; the rest of the ship takes a smaller one. The two regions are computed separately and the worse result for each tank governs.

The longitudinal extent $l_s$ is

l_s = \begin{cases} \dfrac{L}{10} & \text{for the forward } 0.3L \text{ from the FP} \\[4pt] \min\!\left(\dfrac{L}{10},\ 5\ \text{m}\right) & \text{for any other part of the ship.} \end{cases}

Note the form. Forward, the longitudinal bottom extent is a plain $L/10$ with no metric cap. Anywhere else it is $L/10$ capped at 5 m, and since $L/10$ exceeds 5 m for any ship longer than 50 m, the elsewhere value is effectively a fixed 5 m for the entire tanker fleet. The bottom longitudinal extent forward is therefore far longer than aft: a 220 m tanker assumes a 22 m bottom tear forward but only a 5 m tear elsewhere. This reflects the grounding mechanics the regulation models, a vessel running up onto an obstruction tears a long gash forward, then a shorter contact aft. A common error is to apply $\tfrac{1}{3}L^{2/3}$ here, the side longitudinal formula; the bottom longitudinal extent uses $L/10$ , a different and longer measure.

The transverse extent $t_s$ is

t_s = \begin{cases} \min\!\left(\dfrac{B}{6},\ 10\ \text{m}\right),\ \text{but not less than }5\ \text{m} & \text{forward } 0.3L \\[4pt] 5\ \text{m} & \text{elsewhere.} \end{cases}

Forward, the transverse bottom extent is $B/6$ capped at 10 m and floored at 5 m. The $B/6$ term reaches the 10 m cap at $B = 60$ m, beyond the fleet, so the cap rarely binds; the 5 m floor binds only for narrow ships where $B/6$ falls under 5 m, which means a beam under 30 m. Elsewhere the transverse bottom extent is a flat 5 m with no formula. So a wide tanker takes a wider bottom bite forward and a fixed narrow bite aft.

The vertical extent $v_s$ , the penetration up from the baseline, is the same for both regions:

v_s = \min\left(\tfrac{B}{15},\ 6\ \text{m}\right).

This is the dimension that decides whether a bottom breach reaches the cargo tanks above an inner bottom. The $B/15$ term reaches the 6 m cap at $B = 90$ m, so for the whole oil tanker fleet the bottom penetration is $B/15$ . A 32 m beam tanker assumes a 2.13 m vertical penetration from the keel; if the double bottom height exceeds that, the bottom breach stops in the double bottom and never opens a cargo tank. This is the structural reason the double bottom height in Regulation 19 interacts with the Regulation 24 bottom box: a double bottom taller than $B/15$ keeps the assumed bottom penetration out of the cargo block entirely.

Bottom damage extent	Symbol	Forward 0.3L from FP	Any other part
Longitudinal	$l_s$	$L/10$	$L/10$ or 5 m, lesser
Transverse	$t_s$	$B/6$ or 10 m (lesser), not less than 5 m	5 m
Vertical (penetration from baseline)	$v_s$	$B/15$ or 6 m, lesser	$B/15$ or 6 m, lesser

Sliding the box to the worst location

Regulation 24 hands a fixed box to Regulation 25, and Regulation 25 then asks a question the box alone cannot answer: where along the hull does the box do the most damage. The regulation’s instruction is to compute the outflow “with respect to compartments breached by damage to all conceivable locations along the length of the ship.” The box does not sit at one assumed station. It is slid, conceptually, to every position where it could fall, and at each position the analyst records which tanks it opens and how much oil leaves.

The governing case is the location that maximizes outflow, not the average or the midship case. A box centered on a transverse bulkhead breaches the two tanks either side of that bulkhead at once; a box centered in the middle of a long tank may breach only that one tank. The worst location is usually the one that straddles a bulkhead between two full cargo tanks, because it opens both. The analyst has to test bulkhead-straddling positions, single-tank positions, and the corners of the cargo block, then take the worst. Regulation 24 even instructs that smaller-than-maximum damage be considered where it gives a worse result, so the box is not only slid along the hull but also, where relevant, shrunk to find a worse intersection of breached tanks.

For side damage the box is run down both sides of the ship at the summer load line, breaching wing tanks and, where the transverse extent $B/5$ reaches that far, the outboard edge of center tanks. For bottom damage the forward box is run across the forward 0.3L and the smaller box across the rest, breaching whatever sits above the assumed $B/15$ penetration. The combination that yields the largest hypothetical outflow becomes the design case the cargo-tank arrangement must satisfy. A tank plan that passes everywhere except one bulkhead straddle fails Regulation 26, so the designer reworks the subdivision until even the worst box location stays under the cap.

This sliding-box procedure is what converts a static geometric assumption into a subdivision requirement. The box is the same everywhere; the consequence depends entirely on where the bulkheads sit relative to it. Subdivision design for an oil tanker is, in large part, the art of arranging bulkheads so that no position of the Regulation 24 box opens enough tankage to breach the Regulation 26 outflow limit. The same logic that governs floodable length in passenger ship subdivision, treated in subdivision and floodable length, reappears here as an outflow-limiting subdivision rather than a buoyancy-preserving one.

Side outflow, bottom outflow, and the Regulation 25 formulas

The box feeds two outflow formulas in Regulation 25, one for side damage and one for bottom damage, and the split matters because the two assumed breaches behave differently. The hypothetical outflow for side damage, $O_c$ , sums the volumes of the wing tanks and the affected center tanks that the side box opens. The hypothetical outflow for bottom damage, $O_s$ , is computed as one third of the summed tank and center-tank volumes the bottom box opens, reflecting that a bottom breach with the tide does not empty a tank the way a full-depth side breach does. The factor of one third in the bottom formula is the regulation’s allowance for partial retention; oil sits above the breach and only a fraction escapes as the tide falls.

That difference traces directly back to the vertical extents in Regulation 24. The side box opens a tank from baseline to the top of the side, so the side outflow assumes the tank empties. The bottom box penetrates only $B/15$ up from the keel, so much of the tank sits above the breach, and the one-third factor in $O_s$ captures the retained fraction. The geometry in Regulation 24 and the outflow weighting in Regulation 25 are designed together. A designer who reads only the box dimensions, without the matching outflow treatment, misjudges which breach controls a given tank. Wing tanks are usually side-damage controlled; center tanks over a shallow double bottom can be bottom-damage controlled.

Permeability enters here too. The outflow calculations take cargo tank permeability as 0.99, slightly higher than the 0.95 commonly used in damage stability work, because the regulation treats a cargo tank as almost entirely fillable with cargo for outflow purposes. The damage box defines the breached volume; permeability decides how much of that volume was oil to begin with. The two together give the oil that leaves.

Worked example: the box for a 220 by 32 m product tanker

Take a product tanker with a freeboard length $L = 220$ m and a molded breadth $B = 32$ m. Work the Regulation 24 box by hand. Every step below is reproducible with a calculator.

Start with the side box. The longitudinal extent uses $L^{2/3}$ . Here $220^{2/3} = 36.44$ , so $\tfrac{1}{3}L^{2/3} = 12.15$ m. The cap is 14.5 m, and 12.15 m is less, so $l_c = 12.15$ m. The transverse extent is $B/5 = 32/5 = 6.40$ m against the 11.5 m cap, so $t_c = 6.40$ m. The vertical extent is from the baseline upward without limit; there is no number to compute, the box runs the full depth of the side. The side box for this ship is 12.15 m long, 6.40 m inboard, and full-depth.

Now the forward bottom box, applied over the forward $0.3L = 66$ m from the forward perpendicular. The longitudinal extent is $L/10 = 22.0$ m with no cap forward, so $l_s = 22.0$ m. The transverse extent is $B/6 = 32/6 = 5.33$ m, checked against the 10 m cap and the 5 m floor; 5.33 m sits between them, so $t_s = 5.33$ m. The vertical penetration is $B/15 = 32/15 = 2.13$ m against the 6 m cap, so $v_s = 2.13$ m. The forward bottom box is 22.0 m long, 5.33 m wide, and 2.13 m deep from the keel.

Now the bottom box for the rest of the ship, the 154 m aft of the forward 0.3L region. The longitudinal extent is $L/10 = 22.0$ m capped at 5 m, and 5 m is less, so $l_s = 5.0$ m. The transverse extent is a fixed 5.0 m. The vertical penetration is again $B/15 = 2.13$ m. The elsewhere bottom box is 5.0 m long, 5.0 m wide, and 2.13 m deep.

Read the three boxes side by side. The forward bottom tear is the longest assumed breach on the ship at 22.0 m, more than four times the 5.0 m aft tear, which is the regulation’s way of loading the grounding case toward the bow. The bottom penetration of 2.13 m is the dimension a naval architect checks against the double bottom height: if the inner bottom on this tanker sits higher than 2.13 m above the keel, the assumed bottom breach stops in the double bottom and opens no cargo tank, exactly the protective intent that Regulation 19 builds in. The 6.40 m side transverse penetration is the figure that decides how many wing tanks the side box reaches; against typical wing tank widths it usually opens the wing tank and stops short of the center tank.

These boxes are the inputs. The outflow itself follows from running each box to its worst location along this hull and summing the breached tank volumes through the Regulation 25 formulas, then checking the result against the Regulation 26 cap. The point of the worked example is the geometry, which is fully determined by $L = 220$ m and $B = 32$ m and nothing else.

How Regulation 24 relates to Regulations 25 and 26

The three regulations form a chain, and Regulation 24 is upstream of both. Regulation 25, hypothetical outflow of oil, is the calculation that consumes the box. It defines $O_c$ for side damage and $O_s$ for bottom damage and instructs that both be evaluated across all conceivable damage locations using the extents from Regulation 24. Regulation 26, limitation of size and arrangement of cargo tanks, then sets the permitted maximum hypothetical outflow and, through it, the maximum allowable volume of any single cargo tank. A designer cannot draw a cargo tank so large that the Regulation 24 box, slid to its worst position, produces an outflow above the Regulation 26 limit. The size of the box therefore constrains the size of the tanks.

Regulations 25 and 26 are parallel deterministic provisions in the same chain, and the deterministic outflow scheme reads as one connected body of rule that Regulation 24 opens. What matters for an author or a surveyor is the order of operations: extents first (Reg.24), outflow second (Reg.25), tank size cap third (Reg.26). Skip the first and the other two have no input.

The chain is deterministic in the strict sense. There is no probability weighting on where the damage falls or how severe it is. The box is fixed, the worst location is taken, and the resulting outflow is a single deterministic number compared against a single deterministic limit. This is the scheme’s strength, reproducibility, and its weakness, treated in the limitations below: it cannot tell a designer that one arrangement is statistically safer than another that passes by the same margin.

The probabilistic scheme in Regulation 23 and the 2010 cutover

The deterministic Regulation 24, 25 and 26 chain is no longer the rule for new tankers. Regulation 23, accidental oil outflow performance, adopted as part of the revised Annex I under MEPC.117(52), replaced the deterministic logic with a probabilistic mean-outflow scheme. Regulation 23 applies to oil tankers delivered on or after 1 January 2010, the delivery date defined in Regulation 1.28.8, which corresponds to a building contract on or after 1 January 2007 or, absent a contract, a keel laid on or after 1 July 2007. For those tankers the designer computes a mean oil outflow parameter that integrates outflow over a statistical distribution of damage locations and extents, rather than testing a single fixed box at its worst point.

For tankers delivered before 1 January 2010, Regulations 24, 25 and 26 still govern. The cutover is by delivery date, not by survey date, so an aging single-purpose products carrier delivered in 2005 carries the deterministic Regulation 24 box for its whole service life, while a sister design delivered in 2011 is assessed under the probabilistic Regulation 23. Both regimes coexist in the worldwide fleet, which is why a class surveyor or a port state control officer reviewing a tanker’s stability and subdivision documentation has to first establish the delivery date and then apply the right regime. Mixing the two, checking a pre-2010 tanker against Regulation 23 or a post-2010 tanker against Regulation 24, produces a wrong answer and an invalid compliance record.

The two schemes share a purpose, limiting the oil that leaves a damaged tanker, but they differ in method. Regulation 24 fixes one box and tests the worst case; Regulation 23 weights many damage cases by their likelihood and tests a probability-weighted mean. The probabilistic approach better handles unusual subdivisions, where a single fixed box might miss a vulnerability that a distribution of damages would catch, and it removes the cliff-edge behavior of a pass-or-fail box at one location. The deterministic approach is simpler to apply and audit, which is why it remained the rule for two decades and still governs the large existing fleet. The same deterministic-versus-probabilistic split appears in subdivision and stability work generally, where damage stability under the older deterministic SOLAS rules gave way to the probabilistic damage stability framework now in SOLAS Chapter II-1.

Damage assumptions and the stability check in Regulation 28

Regulation 24 supplies its damage extents to a second consumer beyond the outflow chain: the subdivision and damage stability requirement. Regulation 28, subdivision and damage stability, requires every oil tanker of 150 gross tonnage and above delivered after 31 December 1979 to survive assumed side or bottom damage and remain within stated stability criteria for any operating draft reflecting actual partial or full load. The assumed damage in the Regulation 28 stability check uses extents of the same character as Regulation 24, a side box and a bottom box defined in L and B, applied at the worst location.

There is a subtle difference in the permeability assumption between the two consumers. The outflow calculation under Regulations 24 and 25 takes cargo tank permeability as 0.99, while the damage stability assessment commonly uses 0.95, because the appropriate value differs between an outflow question, how much oil leaves, and a flotation question, how much water enters and where the ship settles. The damage box geometry is shared in spirit; the permeability and the pass criteria differ because the two checks ask different questions of the same breach. An author or surveyor should keep the two consumers distinct: Regulation 24 feeds the outflow chain directly, and Regulation 28 imposes its own stability survival check using comparable but separately specified damage extents.

The result a designer aims for is a tanker that, after the assumed damage, both keeps its outflow under the Regulation 26 cap and keeps its residual stability within the Regulation 28 criteria. A subdivision can satisfy one and fail the other. A long center tank may pass the outflow check yet flood asymmetrically in a side-damage stability case and fail the heel criterion. Both checks read off the same family of assumed damage, which is why Regulation 24 is the connective tissue between the pollution-prevention and the survivability sides of tanker design. The residual righting energy that survives the breach is read from the GZ curve and righting arm for the damaged condition, and the whole exercise depends on accurate intact and damaged hydrostatics.

Interaction with double hulls and ballast tank arrangement

The Regulation 24 box does not act on a bare cargo block. It acts on a hull whose protective spaces are themselves regulated. Regulation 19, double hull construction, requires segregated wing tanks or spaces and a double bottom over the cargo length on oil tankers of 5,000 deadweight tons and above, with the protective space dimensions set by deadweight. The width of the wing ballast tank and the height of the double bottom decide whether the Regulation 24 box reaches a cargo tank at all. A side box with $t_c = 6.4$ m that lands on a wing ballast tank 2 m wide will reach the cargo tank behind it; a wing tank wider than 6.4 m would absorb the whole side penetration. A bottom box with $v_s = 2.13$ m that lands on a double bottom 2.5 m high never reaches the cargo above it.

So the protective spaces in Regulation 19 are sized, in part, against the damage extents in Regulation 24. The double hull is the structural defense, and the Regulation 24 box is the assumed attack it has to defeat. A designer reads the box first, then sizes the wing tanks and double bottom so that, across the worst box positions, the protective spaces flood and the cargo tanks stay intact wherever the geometry allows. The same protective spaces serve as segregated ballast tanks under Regulation 18, which requires segregated ballast capacity sufficient for a safe operating condition without carrying ballast in cargo tanks. Regulations 18, 19 and 24 thus describe one integrated protective shell from three angles: ballast function, structural geometry, and assumed damage.

This integration is why a tanker general arrangement cannot be drawn from any single regulation. The wing tank width answers to Regulation 19 for its minimum, to Regulation 18 for its ballast volume, and to Regulation 24 for whether it defeats the side box. The double bottom height answers to Regulation 19 for its minimum and to Regulation 24 for whether the bottom penetration clears the cargo. The Regulation 24 damage assumptions are the common reference geometry that the protective-space rules are written to defeat.

Limitations

Regulation 24 is a deterministic idealization, and its limits are worth stating plainly to anyone applying it. The box is rectangular; real damage is not. A clean parallelepiped never matches the irregular, raking gash of an actual grounding or the localized crush of a collision. The regulation accepts that mismatch in exchange for a reproducible, auditable shape, but the box should never be read as a prediction of what a casualty will look like. It is a compliance yardstick, not a damage forecast.

The scheme tests a single worst location and a single set of extents. It cannot weigh one subdivision as statistically safer than another that passes by the same margin, because it carries no probability information about where damage falls or how severe it is. Two tanks plans that both pass the Regulation 26 cap at the worst box position are treated as equivalent under this scheme, even if one would, across the real distribution of casualties, spill far less. That blind spot is the reason the probabilistic Regulation 23 was written for new tonnage; the deterministic box could not distinguish arrangements the way a mean-outflow integral can.

The extents are fixed in L and B and respond to nothing else. They do not vary with cargo density, with the structural strength of the side shell, with the presence or absence of a longitudinal bulkhead in the path of the box, or with the actual energy of a collision. A heavily framed side and a lightly framed side take the same assumed transverse penetration. The box is indifferent to how hard the hull would really be to breach, which means a structurally tougher design earns no credit in the geometry, only in whether its subdivision defeats the same fixed box.

The vertical side extent without limit is conservative by construction, and the one-third retention factor in the bottom outflow is a single blanket allowance rather than a tank-by-tank hydrostatic balance. Both are deliberate simplifications that trade physical fidelity for tractability. A practitioner using Regulation 24 should treat its outputs as the regulatory minimum the design must clear, not as a physical estimate of spill volume. For a physically realistic outflow estimate after a specific casualty, a hydrostatic balance and a flooding simulation are the right tools, and the Regulation 24 box is not.

Finally, Regulation 24 applies only to oil tankers delivered before 1 January 2010 within the meaning of Annex I. It is silent on chemical tankers under Annex II, on the probabilistic regime that governs newer oil tankers under Regulation 23, and on any vessel outside the Annex I cargo-area construction rules. Applying the Regulation 24 box to a ship the regulation does not cover, or to a post-2010 tanker that the probabilistic scheme governs, gives a compliance answer that is not valid. The first question in any Regulation 24 application is always whether the regulation applies to the ship at all.

Frequently asked questions

What is the assumed side damage extent under MARPOL Annex I Regulation 24?

Longitudinal extent is one third of L raised to the two-thirds power or 14.5 m, whichever is less. Transverse extent is B/5 or 11.5 m, whichever is less, measured inboard from the ship's side at right angles to the centerline at the summer freeboard level. Vertical extent runs from the baseline upward without limit.

What is the assumed bottom damage extent under Regulation 24?

It depends on location. For the forward 0.3L from the forward perpendicular the longitudinal extent is L/10, the transverse extent is B/6 or 10 m (but not less than 5 m), and the vertical penetration is B/15 or 6 m. Elsewhere the longitudinal extent is L/10 or 5 m, the transverse extent is a fixed 5 m, and the vertical penetration is again B/15 or 6 m.

Does Regulation 24 set a discharge limit?

No. Regulation 24 fixes only the geometry of the assumed hull breach. The discharge or outflow numbers come from Regulation 25 (hypothetical oil outflow) and the tank size caps from Regulation 26. Regulation 24 is the input box those two calculations use.

Does Regulation 24 still apply to modern tankers?

It applies to oil tankers delivered before 1 January 2010. For tankers delivered on or after that date, the probabilistic accidental oil outflow scheme in Regulation 23 replaces the Regulation 24, 25 and 26 deterministic chain.

Which length L is used in Regulation 24?

L is the freeboard length defined in Regulation 1.19: 96 percent of the total length on a waterline at 85 percent of the least molded depth, or the length from the fore side of the stem to the rudder stock axis on that waterline if greater. B is the maximum molded breadth amidships.