Ship Draft: Complete Guide to Vessel Draft, Draft Marks, and Measurement

One number decides whether a ship enters port or runs aground. That number is draft. This guide covers everything about ship draft: what it is, how to read draft marks, how to use the Plimsoll line, and how draft connects to cargo weight.

What Is Ship Draft?

This number is not fixed. It changes hour by hour, sometimes minute by minute. Cargo operations move it. Ballast pumping moves it. Even sailing from salt water into fresh water moves it without touching a single tonne of cargo. A ship entering the Mississippi River from the Gulf of Mexico will gain several centimetres of draft simply because fresh water is less dense than seawater. The ship hasn't gotten heavier. The water has gotten lighter.

Why does draft matter so much? Three reasons dominate everything else:

• Navigation safety. A ship needs more water under its keel than its draft. Get that wrong and you have a grounding. Every port approach, every channel transit, every berth assignment starts with the question: "What's your draft?"
• Cargo calculation. The entire draft survey profession exists because Archimedes got it right around 250 BCE: a floating body displaces water equal to its own weight. Measure the draft, calculate the displacement, and you know the cargo weight. Every iron ore shipment, every grain cargo, every coal vessel is settled this way.
• Port access. A ship that draws 17 metres cannot visit a port with 15 metres of channel depth, no matter how much cargo it could carry. Draft constraints shape global shipping routes more than any other single factor.

Draft vs. Depth vs. Freeboard: The Distinctions

These terms get mixed up constantly. Even experienced shipping people sometimes say "depth" when they mean "draft." Here is the clear distinction:

Term	Definition	Does It Change?
Draft (Draught)	Vertical distance from waterline to bottom of keel. How deep the ship sits in the water right now.	Yes. Changes with every tonne of cargo, ballast, fuel, stores, and water density.
Moulded Depth	Vertical distance from keel to top of main deck beam, measured at the ship's side. A construction dimension.	No. Fixed when the ship was built, listed on its plans and certificates.
Freeboard	Distance from waterline up to main deck. Depth minus draft.	Yes. Freeboard is the inverse of draft: when draft goes up, freeboard goes down.

Think of it this way. A ship has a moulded depth of 25 metres. That number is welded into the hull structure and never changes. On a given day, its draft is 16 metres. Right now, its freeboard is 9 metres (25 minus 16). After loading 150,000 tonnes of iron ore, its draft is 18 metres and freeboard is 7 metres. The depth hasn't changed. The draft and freeboard have moved in opposite directions.

A practical warning: when someone tells you "the ship has a depth of 12 metres," ask them whether they mean draft or moulded depth. If the ship's moulded depth is 12 metres and its draft is 12 metres, freeboard is zero. Water is coming over the deck. This is an emergency. The distinction is not pedantic.

Why Draft Changes

1. Cargo Load

The obvious one. Load cargo, the ship sinks deeper. Remove cargo, it rises. But the relationship is not as simple as "one tonne equals one centimetre." The number is called TPC: Tonnes Per Centimetre Immersion. It tells you how many tonnes of additional weight will sink the ship by exactly one centimetre.

TPC changes with draft because the ship's waterplane area changes with draft. A ship with vertical sides (like a bulk carrier with slab-sided hull sections) has a nearly constant TPC across its working draft range. A ship with flared sides (like a container ship with a pronounced bow flare) has a TPC that increases as draft increases: the wider the waterplane, the more weight it takes to sink one centimetre.

For a Capesize bulk carrier at laden draft, TPC is roughly 100-115 tonnes per centimetre. For a VLCC tanker, it is 120-140 tonnes per centimetre. For a cruise ship, it is closer to 40-60 tonnes per centimetre. These numbers mean that a 1 cm error in reading draft on a Capesize translates to approximately 100-115 tonnes of phantom cargo. At the current iron ore price of roughly $100 per tonne, that is $10,000-$11,500 per centimetre of reading error.

2. Ballast Water

When a ship has no cargo, it still needs to sit deep enough in the water to operate safely. The propeller must stay submerged. The bow must not pound in waves. The ship must have enough stability to avoid excessive rolling. Ballast water solves all three problems.

Seawater is pumped into dedicated tanks low in the hull, adding thousands of tonnes of weight without any cargo onboard. A VLCC tanker in ballast carries 80,000-100,000 tonnes of ballast water. Even with zero oil in its cargo tanks, it draws roughly 9 metres. A Capesize bulk carrier in ballast draws 7-9 metres with about 50,000-70,000 tonnes of ballast. The ship literally cannot move safely without this extra weight.

Ballast water itself is regulated by the IMO's Ballast Water Management Convention (see: IMO, BWM Convention, entered into force 2017). Ships must treat ballast before discharging it to prevent the spread of invasive species. The convention has been in force since 2017 and affects how and where ballast can be exchanged, which in turn affects draft during different legs of a voyage.

3. Water Density: Salt, Fresh, and Everything In Between

Here is the factor most people miss. Salt water is denser than fresh water. The standard density for seawater is 1.025 tonnes per cubic metre. Fresh water is 1.000 t/m3. That 2.5% difference means a ship floats higher in salt water than fresh water at the exact same weight.

Real example: a Panamax bulk carrier drawing 12.0 metres in seawater (density 1.025) will draw approximately 12.3 metres in fresh water (density 1.000) with the same cargo, ballast, and stores onboard. The 30-centimetre difference is called the Fresh Water Allowance (FWA). The ship hasn't changed weight at all. It just sits deeper because the water pushes back less.

Temperature complicates this further. Cold water is denser than warm water. In the North Atlantic in February, seawater density might be 1.027-1.028. In the Persian Gulf in August, it can drop to 1.020-1.022. A ship sailing from Rotterdam to Fujairah will physically sink deeper by the time it arrives, simply because the water is warmer and less dense. Its draft will increase by several centimetres with zero change in weight.

How to Read Draft Marks on a Ship

The Metric System (Used on Most Ships Worldwide)

Metric draft marks follow a simple rule: each digit is 10 centimetres tall, with a 10-centimetre gap between consecutive digits. The bottom edge of each digit marks the depth in even decimetres.

Here is how to read them step by step:

1. Identify where the waterline cuts through the marks. Look for the highest visible number above the water and the highest visible number below.
2. Read the last fully visible digit above the waterline. Suppose the waterline is somewhere through the number "12." The bottom of the digit "1" marks 1.0 metres. The bottom of "2" marks 2.0 metres. But each full digit represents 10 cm. So the bottom of "1" is actually 10 cm, "2" is 20 cm, and the sequence continues: "10" = 1.0 m, "12" bottom = 1.2 m, "14" bottom = 1.4 m, etc.
3. Estimate the fractional part. If the waterline is halfway up the digit "12" (a 10 cm tall digit), the fractional part is 5 cm. The draft reading is 1.25 metres.
4. Repeat at all six positions. Forward port, forward starboard, midship port, midship starboard, aft port, aft starboard.

The most common reading mistake is reading the top of the digit instead of the bottom. This adds exactly 10 centimetres to the reading. On a Capesize bulk carrier where TPC is about 110 tonnes/cm, that single error creates 1,100 tonnes of phantom cargo. At iron ore prices, that is about $110,000.

The Imperial System (American and Older Vessels)

Imperial draft marks use Roman or Arabic numerals with each digit 6 inches tall. The bottom of each digit marks that depth in feet. The gap between digits is 6 inches. So if the waterline cuts through the bottom of the numeral "30," the draft is 30 feet exactly. If it cuts halfway up the digit, add 3 inches: 30 feet 3 inches.

Some older vessels still show Roman numerals on draft marks. You might see "XXIV" meaning 24 feet. This is increasingly rare but worth recognizing when you encounter it, particularly on smaller vessels and traditional craft.

Common Reading Errors

• Parallax error. Reading draft marks from above (standing on deck looking down) makes the waterline appear to cut higher on the marks than it actually does. Always read from as close to water level as possible, ideally from a small boat or quay at eye level with the marks.
• Wave error. In any swell, the waterline oscillates. Experienced surveyors watch the marks through several wave cycles and estimate the midpoint. In rough conditions with 30-50 cm waves, this becomes guesswork. Even a good surveyor can be off by 2-3 cm per mark in these conditions.
• Damaged or fouled marks. Welded draft marks on older ships can be bent, partially obscured by marine growth, or repainted incorrectly after hull work. Always check that the marks look intact and legible before trusting the reading.
• Night reading. Marks illuminated by flashlight from above create shadows that distort the apparent waterline position. Side lighting from a boat is more reliable. Many modern ports use LED floodlights positioned at water level specifically for night draft readings.

For the complete procedure with detailed diagrams, wave compensation techniques, and calibration methods, see our detailed guide to reading draft marks.

The Plimsoll Line and Load Lines

History: Samuel Plimsoll and the "Coffin Ships"

The Plimsoll line exists because of one man's outrage. In the 1860s and 1870s, British shipowners were systematically overloading vessels and collecting insurance when they sank. These were called "coffin ships" because the crews rarely survived. Ships were insured for more than their value, creating a perverse incentive: send an overloaded, unseaworthy vessel into rough seas, collect the payout when it goes down.

Samuel Plimsoll (10 February 1824, 3 June 1898), a British Member of Parliament, campaigned for mandatory load lines from 1868 onward. The shipping lobby fought him ferociously. Plimsoll was temporarily expelled from Parliament for calling members "ship-knackers" on the floor of the House of Commons. But public sentiment was on his side, and in 1876 the UK Merchant Shipping Act made load lines mandatory on all British vessels.

In 1930, the first International Load Line Convention was adopted. The current rules come from the 1966 International Convention on Load Lines (LL 1966), adopted on 5 April 1966 and entering into force on 21 July 1968. The convention was amended by the 1988 Protocol and is periodically updated by IMO resolutions. Every commercial ship on international voyages must have a valid International Load Line Certificate, and the marks must be permanently visible on the hull. (See: IMO, International Convention on Load Lines, 1966)

The Six Marks and What They Mean

Mark	Stands For	When It Applies	Why It Exists
TF	Tropical Fresh Water	Rivers and lakes in tropical zones	Warm fresh water is the least dense condition. The ship sits deepest here. Highest mark on the ladder.
F	Fresh Water	Fresh water at temperate temperatures	Fresh water (1.000 t/m3) provides less buoyancy than salt. Higher than seawater marks.
T	Tropical Seawater	Tropical ocean zones (defined by latitude and season)	Warm seawater is less dense than cold. Higher than S by 1/48th of summer load draft.
S	Summer Seawater	Temperate saltwater, summer months. The primary reference mark.	This is the baseline. All other marks are offsets from S.
W	Winter Seawater	Winter months in temperate zones	Cold water is denser (ship floats higher). Lower than S by 1/48th of summer load draft.
WNA	Winter North Atlantic	Vessels under 100m length, North Atlantic, winter months	North Atlantic winter storms are exceptionally dangerous for small ships. Gives 50mm more freeboard than W (ICLL Regulation 40(6)).

The tropical and summer marks differ by 1/48th of the summer load draft. For a ship with a summer draft of 12 metres, that is 25 centimetres of extra allowable immersion in tropical waters. The reason: tropical waters are less dense because they are warmer, so the ship naturally sits lower; but tropical seas also tend to be calmer, so less freeboard is needed for safety.

The WNA (Winter North Atlantic) mark applies specifically to vessels under 100 metres in length. Above 100 metres, the ship is considered large enough to handle North Atlantic winter conditions with the standard W mark. Below 100 metres, the additional 50 mm of freeboard (approximately 2 inches on the draft) is required by regulation under ICLL.

Timber Load Lines

Ships carrying timber deck cargo get a separate set of load line marks, prefixed with "L" (for lumber). These sit higher than the standard marks because timber deck cargo provides additional buoyancy and reserve freeboard: if the deck cargo is washed overboard, the ship actually rises rather than sinking further. The timber load line marks include LTF, LF, LT, LS, LW, and LWNA, with the same seasonal logic as the standard marks but all offset upward.

Seasonal Zones

The world's oceans are divided into seasonal zones defined by the Load Line Convention. A ship moving from summer to winter zones must not be loaded beyond the winter mark when entering the winter zone, regardless of the actual weather at that moment. The zones are mapped in detail in the convention's annexes and enforced by port state control inspections worldwide.

Loading beyond the applicable Plimsoll mark is a violation of international law under SOLAS and the Load Line Convention. It voids marine insurance. It can result in the ship being detained by port state control. And it is genuinely dangerous: Samuel Plimsoll's 19th-century campaign was not about bureaucracy. It was about stopping ships from sinking with their crews still onboard.

For a deeper treatment of load lines, seasonal zone maps, and how to verify a load line certificate, see our complete Plimsoll line and load line guide.

Ship Draft by Vessel Type

Container Ships

Container ships have grown enormously in the past 30 years, but their draft has grown far less than their length or beam. A 1995 Panamax container ship was about 294 metres long with a 12-metre draft. A 2023 ULCV like MSC Irina is 400 metres long with a 17-metre draft. Length increased 36%. Draft increased 42%. Beam increased 89%. The design strategy for mega container ships is to go wider and flatter rather than deeper.

Class	Example	TEU Capacity	Max Draft	Length	Beam
Feeder	--	1,000-3,000 TEU	8-11 m	~150-210 m	~25-30 m
Panamax	--	4,000-5,000 TEU	12.0 m	~294 m	32.3 m
Neopanamax	CMA CGM Jacques Saade	13,000-14,500 TEU	15.1 m	~366 m	51.2 m
ULCV / Megamax	MSC Irina (24,346 TEU)	20,000-24,500 TEU	16.5-17.0 m	~399-400 m	61.3 m

MSC Irina, one of the world's largest container ships at 24,346 TEU, has a design draft of 17.0 metres. In operational practice, it rarely exceeds 16.4 metres because most ports on its rotation cannot handle 17 metres of draft without a tidal window. Even at 16.4 metres, it requires at least 18.0-18.5 metres of charted channel depth to maintain safe under-keel clearance.

Bulk Carriers

Bulk carriers show the widest draft range of any vessel type. A Handymax in ballast might draw 5 metres. A loaded Valemax draws 23 metres. The 18-metre swing between ballast and laden on a Capesize is one of the most dramatic weight changes in commercial shipping.

Type	Example	DWT Range	Ballast Draft	Laden Draft
Handymax	--	40,000-65,000 DWT	5-7 m	12.0-13.5 m
Panamax (bulk)	--	65,000-99,000 DWT	6-8 m	14.0-15.0 m
Capesize	--	150,000-200,000 DWT	7-9 m	17.0-18.2 m
VLOC (Valemax)	Vale Brasil	250,000-400,000 DWT	9-11 m	20.0-23.0 m

The Vale Brasil-class VLOCs (402,347 DWT, 362 m LOA, 65 m beam, 23.0 m moulded draft) are the deepest-draft bulk carriers in regular service today. These ships can only call at a handful of deep-water terminals: Ponta da Madeira (Brazil), Sohar (Oman), Teluk Rubiah (Malaysia), Villanueva (Philippines), and Rotterdam (Netherlands). The ships were purpose-built for the Brazil-to-Asia iron ore trade, and their draft limits their route to those specific terminals. The Valemax fleet numbers 68 vessels as of 2020, built to the Chinamax dimensional limits. (See: DNV Vessel Register, MS Ore Brasil)

Oil Tankers

Tankers operate at the deepest end of the commercial draft spectrum. Their hulls are designed as slab-sided, full-section boxes optimized for volume, and the laden draft numbers show it.

Class	Example	DWT Range	Laden Draft	Cargo Capacity
Aframax	--	80,000-120,000 DWT	12-15 m	~750,000 barrels
Suezmax	--	120,000-200,000 DWT	15.5-17.0 m	~1 million barrels
VLCC	--	200,000-320,000 DWT	18.5-21.0 m	~2 million barrels
ULCC	Seawise Giant (historical)	320,000+ DWT	22.0-24.6 m	3-4 million barrels

The Seawise Giant (later Jahre Viking, Knock Nevis) remains the deepest-draft ship ever built at 24.6 metres laden. At 458.45 metres long and 564,763 DWT, it was the largest ship ever constructed. When fully loaded with 4.1 million barrels of crude oil, only about 5 metres of its 29.8-metre hull depth was visible above water. It could not transit the English Channel, Suez Canal, or Panama Canal. Most ports on Earth were physically inaccessible. It spent its final operating years as a stationary floating storage and offloading unit (FSO) off Qatar, and was scrapped at Alang, India in 2010.

Today's VLCCs typically offload at offshore single-point moorings (SPMs) or lightering zones rather than entering constrained harbours. The oil is either piped ashore through submarine pipelines or transferred to smaller tankers. The ship's extreme draft becomes irrelevant because it never actually enters a port.

Cruise Ships

Cruise ships are surprisingly shallow for their size. A modern mega cruise ship that towers 60-70 metres above the waterline draws only 8-9.5 metres. The design logic is straightforward: cruise itineraries need port flexibility, and every metre of draft eliminates dozens of potential destinations.

Ship / Class	Gross Tonnage	Draft	Length	Beam
Oasis-class (Royal Caribbean)	225,000-237,000 GT	9.3 m	361.8 m	47.0 m (waterline)
Icon-class (Royal Caribbean)	250,800 GT	~9.0 m	364.7 m	~48.0 m
Queen Mary 2 (Cunard)	149,000 GT	10.0 m	345.0 m	41.0 m (waterline)
Mid-size cruise ship	~70,000-100,000 GT	7.5-8.5 m	~260-290 m	~32-36 m

The stability secret: cruise ships maintain stability through extreme beam. An Oasis-class ship has a beam of 47 metres at the waterline, more than five times its 9.3-metre draft. That width-to-draft ratio provides enormous righting moment without needing deep hull immersion. The ship sits on the water like a wide, flat platform rather than a deep, narrow hull.

The Queen Mary 2 is the exception at 10 metres. She is an ocean liner, not a cruise ship. Her deeper draft, reinforced hull (40% more steel than a standard cruise ship), and heavier structure are purpose-designed for regular North Atlantic crossings in winter conditions, not for island-hopping in calm tropical waters. Her draft of 10 metres was an intentional design choice to permit access to most cruise terminals while maintaining adequate seakeeping for transatlantic service. (See: Ship Technology, "Queen Mary 2 Cruise Liner")

Naval Vessels

Warships typically draw between 5 and 12 metres depending on size. An aircraft carrier like USS Gerald R. Ford draws about 12 metres. A destroyer like USS Arleigh Burke draws about 9.5 metres. A frigate draws 5-7 metres. Naval architects for warships face different trade-offs from commercial vessels: speed requires fine hull forms that tend toward deeper draft, while operational requirements demand the ability to enter smaller ports.

For a detailed breakdown by vessel type with many more examples, see ship draft by vessel type and ship measurements guide.

Port Draft Restrictions

Why Ports Have Draft Limits

The fundamental rule: a ship needs more water under its keel than its draft. The gap is called under-keel clearance (UKC), and the IMO recommends a minimum UKC of 10% of the vessel's draft in open water and 15% in confined channels, with an absolute minimum of about 0.5-1.0 metres depending on local regulations.

For a container ship drawing 16 metres, a 15% UKC requirement in a confined channel means the channel must be at least 18.4 metres deep. For a VLCC drawing 20 metres, the channel needs 23 metres. These are not small engineering requirements. Dredging a channel deeper by even one metre can cost tens of millions of dollars.

The squat effect makes it worse. When a ship moves through shallow water, water flowing under the hull accelerates and creates a low-pressure zone. The ship physically sinks deeper: a 16-metre-draft vessel at 12 knots in a restricted channel might squat an additional 0.5-1.0 metre. Ship masters and pilots account for squat in their UKC calculations, which is why ships slow down in approach channels even when it adds time to the transit.

Major Port and Canal Draft Limits

Waterway / Port	Maximum Draft	Notes
Panama Canal	15.24 m (50 ft) TFW	Reduced to as low as 13.41 m (44 ft) during the 2022-2024 drought. The 2019 drought saw limits as low as 13.16 m (43 ft). Normal operations restored August 2024. (See: Panama Canal Authority, Advisory to Shipping A-28-2023)
Suez Canal	20.1 m (66 ft)	Deepest canal in the world by allowable draft. Can handle fully laden Suezmax tankers and most ULCVs. No locks, open seawater passage. (See: Suez Canal Authority)
Port of Rotterdam	~24 m	Deepest port in Europe. Can handle laden VLOCs and VLCCs without tidal restrictions. No locks, direct North Sea access. (See: Port of Rotterdam Authority)
Shanghai (Yangshan)	~16 m	Deep-water offshore terminal. Some berths can handle deeper draft with tidal windows.
Port of Ningbo-Zhoushan	18+ m	One of the deepest ports in China. Handles Valemax VLOCs at dedicated terminals.
Port of Singapore	~23 m	Deep natural water plus minimal tidal range. Handles the deepest commercial vessels year-round.
Port of Felixstowe (UK)	16.0 m	Deepened channel specifically for ULCV calls. Berth 8/9 can handle 17+ m draft with tidal windows.

Panama Canal: When the Rain Stops

The Panama Canal illustrates how draft limits are not just about channel depth. The canal's operation depends on Gatun Lake, a freshwater reservoir that supplies the water needed to operate the locks. When drought reduces the lake level, the canal authority must reduce the allowable draft.

In 2022-2024, a prolonged drought caused Gatun Lake levels to drop significantly. The Panama Canal Authority reduced the maximum draft from the standard 15.24 metres (50 feet TFW) to as low as 13.41 metres (44 feet) on 30 May 2023. For a laden Neopanamax container ship drawing 15 metres, this meant the ship literally could not transit. Shippers either had to reduce cargo, unload containers and rail them across the isthmus, or reroute around Cape Horn or through Suez. The draft restriction triggered supply chain disruptions across global container trades. Normal 50-foot draft operations were restored in August 2024, alongside the restoration of 36 daily transits by September 2024. (See: Panama Canal Authority, OP Notice to Shipping N-1-2023 and N-1-2024)

Tidal Windows

When a ship's draft exceeds the charted depth of a channel, it does not always mean the port call is cancelled. It means the ship needs a tidal window: arriving and departing during the highest part of the tide when the water is temporarily deep enough.

Tidal range varies enormously by location. The Bay of Fundy in Canada has a tidal range exceeding 16 metres. The Mediterranean typically has less than 0.5 metres. A port with a 5-metre tidal range can handle much deeper-draft ships than its charted depth suggests, provided the ships arrive on schedule. But miss the tide window by 60 minutes and you are waiting 12 hours for the next one. At vessel operating costs of $70,000-$100,000 per day for a large container ship, the delay is expensive.

Ports with minimal tidal range (Baltic, Mediterranean) invest more heavily in deep-dredged channels because they cannot rely on tides to provide extra depth. Singapore benefits from both deep natural water and a minimal tidal range: it simply does not need tidal windows because the baseline depth is sufficient year-round.

From Draft to Cargo Weight: Draft Survey Basics

What a Draft Survey Measures

A draft survey measures the ship's displacement before and after cargo operations. The difference between the two displacements, after correcting for all non-cargo weight changes (ballast, fuel, fresh water, stores), is the cargo weight.

The physics is Archimedes' principle: a floating body displaces water equal to its own weight. If you know exactly how much water the ship displaces, you know exactly how much it weighs. And if you know how much it weighed before loading and after loading, you know how much cargo came aboard.

The Procedure at a Glance

A full draft survey involves these steps:

1. Read six draft marks. Forward port, forward starboard, midship port, midship starboard, aft port, aft starboard. Read each to the nearest centimetre.
2. Average port and starboard at each position. This cancels out any list (sideways tilt).
3. Correct for draft mark position. Draft marks are physically offset from the ship's forward and aft perpendiculars. A trim correction adjusts the readings to the perpendiculars.
4. Calculate quarter mean draft. This is a weighted average formula that suppresses the effect of hull bending (hog or sag). The formula: QM = (F + A + 6M) / 8, where F = forward mean, A = aft mean, M = midship mean. The midship reading gets six times the weight because it is at the centre of buoyancy.
5. Enter hydrostatic tables. With the quarter mean draft, look up displacement, TPC (tonnes per centimetre immersion), LCF (longitudinal centre of flotation), and MCTC (moment to change trim one centimetre).
6. Apply trim corrections. First correction uses LCF position relative to midships. Second correction (Nemoto correction) accounts for the change in trimming moment as the ship trims.
7. Correct for water density. Hydrostatic tables are calculated for standard seawater (1.025 t/m3). If the actual water density is different, multiply displacement by (actual density / 1.025).
8. Subtract deductibles. Remove ballast water weight (from tank soundings), fuel oil, diesel oil, fresh water, lubricating oil, stores, crew and effects, and the ship's constant (unrecorded weights). What remains is the lightship weight plus cargo.
9. Compare initial and final surveys. Cargo weight = (final displacement minus final deductibles) minus (initial displacement minus initial deductibles).

Accuracy and Error Sources

The theoretical accuracy of a well-conducted manual draft survey is approximately plus or minus 0.5% of cargo weight. On a 180,000-tonne iron ore cargo, that is 900 tonnes of uncertainty. At $100 per tonne, that is $90,000.

In practice, with an experienced surveyor in calm conditions, accuracy can reach 0.2-0.3%. In rough water with an inexperienced surveyor, errors of 1-2% are common. The main error sources are:

• Draft reading error. 1-3 cm per mark in choppy water. At 110 tonnes/cm, that is 110-330 tonnes per centimetre of error.
• Density measurement error. An uncalibrated hydrometer reading 1.023 instead of 1.025 creates a 0.2% displacement error.
• Ballast soundings. Undetected ballast in "empty" tanks, especially in double-bottom tanks with poor trim, can hide 50-200 tonnes.
• Uncertain ship constant. The constant (paint, rust, sludge, unaccounted liquids) changes over time and is hard to verify. Errors of 50-200 tonnes are common.

Our complete draft survey calculation guide walks through the full procedure with worked examples, formulas, and common pitfalls.

Frequently Asked Questions

What is ship draft?

What is the difference between draft and depth?

Draft is the distance from waterline to keel. It changes with loading. Depth (moulded depth) is the distance from keel to the top of the main deck beam at the ship's side. It is a fixed construction dimension that never changes. Freeboard is depth minus draft: the safety margin between the water and the deck. If someone says "the ship has a depth of 12 metres," ask whether they mean draft (how deep it sits right now) or moulded depth (how tall the hull was built).

How do you read draft marks on a ship?

Draft marks are numerals welded on the hull at six positions: forward, midship, and aft on both port and starboard sides. For metric marks, each digit is 10 cm tall with a 10 cm gap between digits. The bottom of each digit marks the even-decimetre depth. If the waterline cuts through the bottom of "12," the draft is 12.0 metres. If it is halfway up the digit, add 5 cm: 12.05 metres. The most common error is reading the top of the digit instead of the bottom, which adds 10 cm to the reading. Read from close to water level to avoid parallax error. Our detailed guide to reading draft marks covers the full procedure.

What is the deepest draft ship in the world?

The Seawise Giant (later Jahre Viking, Knock Nevis), the largest ship ever constructed at 564,763 DWT and 458.45 metres long, had a laden draft of 24.6 metres (80.7 feet). Only about 5 metres of its 29.8-metre hull was visible above water when fully loaded. The vessel was scrapped at Alang, India in 2010. Among active vessels, Vale Brasil-class VLOCs (Very Large Ore Carriers) draw up to 23 metres fully loaded. The largest operating container ships (ULCVs like MSC Irina) draw 16.5-17 metres. The deepest recorded port calls on record include MSC Elenoire and MSC Verona at 17.1 metres draft, achieved using tidal windows.

How does water density affect ship draft?

Salt water is denser than fresh water (1.025 vs 1.000 t/m3), so ships float higher in seawater. A ship drawing 12.0 metres in seawater will draw about 12.3 metres in fresh water at the same weight, a gain of 30 cm. This difference is called Fresh Water Allowance (FWA). Temperature also matters: cold water is denser than warm water. A ship sailing from the cold North Atlantic to the warm Persian Gulf will gain several centimetres of draft from the density change alone, with zero change in weight. Draft surveys must correct for actual water density using a hydrometer reading, or the cargo weight will be wrong by about 2.5% at minimum.

What is a Plimsoll line?

The Plimsoll line is the International Load Line: a circle bisected by a horizontal line, plus a vertical ladder of marks, painted amidships on both sides of every commercial vessel. It shows the maximum legal draft for different water conditions. The marks are TF (Tropical Fresh), F (Fresh), T (Tropical), S (Summer), W (Winter), and WNA (Winter North Atlantic). Named after Samuel Plimsoll (1824-1898), a British MP who campaigned for mandatory load lines in the 1870s to stop shipowners from deliberately overloading "coffin ships." The current international rules come from the 1966 Load Line Convention. Loading beyond the applicable mark violates international law and voids insurance. Full guide to the Plimsoll line.

How much does draft change when loaded?

The change varies dramatically by vessel type. A Capesize bulk carrier goes from about 7-9 metres in ballast to 18 metres fully laden, a swing of 9-11 metres. A VLCC tanker changes from 8-10 metres in ballast to 19-21 metres laden. A large container ship changes by 6-8 metres between light and loaded conditions. Cruise ships change the least: only 1-2 metres, because their payload (passengers, provisions, fuel) is a small fraction of total vessel weight compared to cargo ships. The number that governs this is TPC (Tonnes Per Centimetre Immersion), which ranges from about 40 tonnes/cm for small vessels to 140 tonnes/cm for the largest tankers.

What is the draft of a typical cruise ship?

Most large cruise ships draw between 8 and 9.5 metres, which is surprisingly shallow for vessels that tower 60-70 metres above the waterline. Royal Caribbean's Oasis-class ships draw 9.3 metres. The new Icon-class draws approximately 9.0 metres. The Queen Mary 2 is the notable exception at 10.0 metres because she is an ocean liner designed for North Atlantic crossings, not a cruise ship. Cruise ships maintain stability through extreme beam (width) rather than deep draft: an Oasis-class ship has a beam of 47 metres at the waterline, more than five times its draft. This wide, shallow design maximizes port access for itinerary flexibility.

References and Further Reading

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