Nautical glossary: shipping terms explained

Aft

Aft means towards or at the rear of a vessel; its opposite is forward.

Shipboard directions avoid the ambiguity of "front" and "back": forward and aft describe positions along the hull, abeam means at right angles to it, and astern means behind the vessel. The rear part of a ship is the after body with the afterdeck; on most passenger vessels this is where the engine room, steering gear and propellers sit. In listings the term appears in phrases such as "covered afterdeck" or "stern boarding platform aft".

See also: Stern, Bow

Air draught

Air draught is the distance from the waterline to the highest fixed point of a vessel – the height that actually has to pass under a bridge.

Masts, radar antennas and the wheelhouses of many inland vessels can be lowered or folded and do not count. What remains is the fixed structure, on passenger vessels usually the roof or railing of the top deck. The permissible air draught is dictated by the bridge clearances of the operating area, and those depend on the water level: at high water the space under the bridges shrinks until navigation is suspended. On canals with low bridges this single figure decides whether a second deck is possible at all. In listings it appears as "height above waterline" or "fixed point height" and matters as much as the draught.

See also: Draught, Water level gauge, Deck

AIS

AIS (Automatic Identification System) is a radio system through which vessels automatically exchange identity, position, course and speed with each other and with traffic centres.

Every AIS-equipped vessel transmits a data packet over VHF every few seconds to minutes, received by other vessels, traffic centres and locks. Inland navigation uses the Inland AIS variant, which additionally transmits vessel type, dimensions and loading condition. On the Rhine and many other European waterways Inland AIS is mandatory together with an Inland ECDIS device in information mode, passenger vessels included. A type-approved Inland AIS transponder is therefore part of the compulsory equipment and should appear on every inventory list. Public AIS portals also show where a vessel offered for sale has recently been operating – a useful first look before a viewing.

See also: Captain / boatmaster, CCNR

Anchorage

An anchorage is a place where a vessel can lie safely on its own anchor – designated on charts or by the waterway authority, or freely chosen where anchoring is permitted.

Whether a spot is suitable depends on water depth, holding ground (sand and clay hold well, rock and weed poorly), shelter from wind and current, and enough swinging room around the anchor. On rivers anchoring is only allowed in marked areas; it is prohibited in the fairway, off harbour entrances and under bridges. Passenger vessels rarely anchor in service – they run from landing stage to landing stage, and what matters to them is a permanent berth. On inland waterways one speaks of anchoring or waiting areas; the roadstead is the counterpart off seaports.

See also: Roadstead, Berth

Azimuth thruster

An azimuth thruster is a propeller drive that rotates through 360 degrees, combining rudder and propeller so that thrust can be directed anywhere.

Many know it by the manufacturer name Schottel; in German it is a Ruderpropeller. Because the entire thrust can be pointed sideways or even astern, a vessel with two azimuth thrusters turns on the spot and berths without a bow thruster – a major advantage for passenger vessels in tight harbours and in current. The drive is common on ferries, harbour tour boats and newer saloon vessels; ferries also use the Voith Schneider propeller with vertical blades. The drawbacks are higher purchase price and maintenance: seals and gears in the underwater unit have to be checked at every dry-docking. Conventional installations with fixed shaft and rudder blade are more robust and remain the rule on river vessels.

See also: Bow thruster, Stern

Ballast water

Ballast water is water a ship takes into tanks to adjust draught, trim and stability to its loading condition.

An empty cargo ship rides too high, its propeller does not bite and the wind pushes it off course, so it floods ballast tanks. Before loading, the water is pumped out again, often thousands of kilometres away, together with larvae, algae and micro-organisms. To curb the spread of invasive species, the IMO Ballast Water Management Convention has required on-board treatment on seagoing ships since 2017.

Inland passenger vessels do not change their cargo; their stability is set mainly with fixed ballast – concrete, steel or cast iron in the double bottom. After conversions such as an additional deck or a new galley, the ballast has to be re-balanced and verified by an inclining test. Buyers should ask when that was last done.

See also: Stability, Heel, Draught

Berth

A berth is the permanent mooring where a vessel lies between trips; for a passenger vessel it is commercially as important as the vessel itself.

Unlike cargo ships, passenger vessels are tied to a location: scheduled and round trips start where the guests are. A good berth offers a landing stage with passenger access, shore power and water, waste disposal and a permit from the waterway or port authority or the municipality. Such berths are scarce, are often only leased and do not automatically pass to the buyer with the vessel – whether the berth can be taken over should be settled before the purchase contract is signed. A sheltered winter berth is usually needed in addition. Anchorages and roadsteads, by contrast, are places where vessels wait only temporarily.

See also: Anchorage, Roadstead

Bilge

The bilge is the lowest space inside the hull, where leakage, condensation, oil and dirt collect.

Every vessel takes on a little water in service – through shaft seals, condensation and rain entering ventilators. It runs into the bilge and is pumped overboard by bilge pumps as soon as a float switch triggers; a bilge alarm in the wheelhouse reports unusual inflow. Water from the engine room is oily and must not be pumped out: on the Rhine and inland waterways bilge water is collected and handed over to bilge-oil collection boats or reception facilities under the CDNI convention.

During a viewing, a look into the bilge is worth more than any look at the saloon: rust, oil film and damp frames reveal how the vessel has really been cared for.

See also: Plate thickness, Dry-docking

Bow

The bow is the forward part of the hull that parts the water; its counterpart is the stern.

The shape of the bow determines how much resistance and wash a vessel produces. Passenger vessels on rivers and lakes usually have a slender, slightly raked stem or a rounded pontoon bow that brings plenty of deck area forward at shallow draught. The bow houses the anchor windlass, the bow thruster and often a landing ramp over which passengers board at flat banks. Forebody, foredeck and stem all refer to this area; bow wave and bulbous bow belong to the same family of terms.

See also: Stern, Bulbous bow, Bow thruster

Bow thruster

A bow thruster is a transverse propeller in the forebody that moves the vessel sideways, allowing it to berth and unberth without tug assistance.

The propeller sits in a tunnel running through the hull from port to starboard; thrust can be switched in either direction. It is driven electrically, hydraulically or by its own diesel engine, with outputs on passenger vessels ranging from about 50 to several hundred kilowatts. Shallow inland vessels often carry a pump-jet instead of a tunnel, drawing water from below and discharging it through a rotatable nozzle.

For a passenger vessel that comes alongside a dozen times a day, a powerful bow thruster is not a comfort feature but a prerequisite for scheduled operation with a small crew. Year of build, type of drive and the unit's last service therefore belong in every listing; a repair almost always requires dry-docking.

See also: Azimuth thruster, Dry-docking

Bulbous bow

A bulbous bow is a bulb-shaped protrusion of the stem below the waterline that reduces the wave-making resistance of seagoing ships at cruising speed.

The bulb generates its own bow wave that runs against the hull's wave and partly cancels it; properly designed, this saves up to ten percent of propulsion power on cargo ships, ferries and cruise ships. The effect only works within the narrow range of speed and draught the bulb was designed for – at slow speed or in shallow water it does nothing or even slows the ship. That is why inland and passenger vessels practically never have one: their speed is too low, the water too shallow, and on rivers debris and grounding are a constant threat. If you see a bulb on an excursion vessel, you are probably looking at a former coastal or seaside-resort ship.

See also: Bow

Bunkering

Bunkering is taking fuel on board a vessel – and, in the wider sense, lubricating oil and fresh water too.

The word dates from the steamship era, when coal was shovelled into bunkers. Inland vessels bunker at bunker stations and from bunker boats that come alongside; passenger vessels with smaller tanks often from a tanker lorry at the berth. Low-sulphur diesel is standard, increasingly joined by HVO and GTL as almost soot-free alternatives that many engines accept without modification. Tank capacity – listed as "bunker capacity" or simply "diesel 2 × 3,000 l" – together with consumption determines range and thus whether the vessel is suitable for multi-day trips. More on fuels and drives in the guide to propulsion and emissions.

See also: Berth

Cabin vessel

A cabin vessel is a passenger vessel with overnight cabins for its passengers – the ship of river cruising.

ES-TRIN distinguishes the cabin vessel from the day-trip vessel; because guests sleep on board, stricter rules on fire protection, escape routes and alarm systems apply. Typical river cruise ships on the Rhine and Danube are up to 135 metres long and 11.45 metres wide – the dimensions of the locks – and carry 100 to 200 guests. Smaller cabin vessels with 20 to 60 beds operate on the Elbe and Havel, in the Netherlands and in France. When buying, the condition of the hotel outfit and the energy supply for air conditioning and galley count as much as certificate and survey status. Day-trip vessels can rarely be converted into cabin vessels economically because of their deck and hull heights.

See also: Passenger vessel, Saloon vessel

Captain / boatmaster

The captain – legally the boatmaster in inland navigation – bears responsibility for vessel, crew and passengers and needs a certificate of qualification for the relevant operating area.

In EU inland navigation the Union certificate of qualification as a boatmaster has been the basis since 2022; it is issued on the strength of navigation time and an examination and is valid across Europe. Stretches with specific risks, such as the Rhine from Iffezheim to the Dutch border, require additional proven route knowledge; sea waterways in zones 1 and 2 require further endorsements. Passenger vessels must also carry trained personnel for passenger safety, known as passenger navigation experts.

Anyone buying a vessel without intending to skipper it should therefore plan for a boatmaster with the right certificate from the outset – qualified boatmasters are in short supply. "Captain" is the customary form of address on board and popular with guests; it does not appear on the certificate.

See also: Pilot, Navigation zones

CCNR

The CCNR (Central Commission for the Navigation of the Rhine) is the international organisation that regulates navigation on the Rhine – from construction rules for vessels to crew qualifications.

Founded in 1815 at the Congress of Vienna and based in Strasbourg, it is the oldest international organisation still in existence. Its members are Germany, France, the Netherlands, Belgium and Switzerland; its legal basis is the Mannheim Convention of 1868, which guarantees freedom of navigation on the Rhine. For vessel owners the most relevant instruments are the Rhine Vessel Inspection Regulations (the basis of the Rhine ship certificate), the Rhine Police Regulations with the traffic rules, and the crew regulations. Since the CCNR and the EU apply the common ES-TRIN standard, technical requirements on the Rhine and on other EU waterways are largely identical, and the Rhine certificate is recognised on EU inland waterways.

See also: ES-TRIN, Ship certificate

Deck

A deck is a continuous horizontal level in a ship; on passenger vessels the number of decks determines capacity, comfort and bridge clearance.

The main deck sits at boarding level and usually carries the saloon with galley and bar. Above it come the upper or promenade deck and the open sun deck; below lies the lower deck with engine room, tanks and – on cabin vessels – the cabins. The trade speaks of one-, two- or three-deck vessels; every additional deck adds seats but raises the centre of gravity, the windage and the air draught. On many vessels part of the sun deck is roofed or can be closed with glass panels to extend the season. Which type suits which use is covered in the guide to passenger vessel types.

See also: Saloon vessel, Air draught

Displacement

Displacement is the weight of the water a floating vessel displaces – and, by Archimedes' principle, therefore the weight of the vessel itself.

It is stated in tonnes and changes with draught: the deeper the vessel sits, the more water it displaces and the heavier it is. Light displacement describes the vessel without cargo, stores and people, full displacement the condition at maximum permitted draught; the difference is the deadweight. The tonnage certificate of an inland vessel tabulates exactly this relationship. On passenger vessels displacement says something about construction and steel weight – a heavily built hull from the 1960s displaces considerably more than a modern lightweight hull of the same length – and is needed for dry-docking, craning and overland transport.

See also: Tonnage, Draught, Gross tonnage (GT)

Draught

Draught is the vertical distance from the waterline to the lowest point of the vessel; it decides which waters a vessel can navigate.

It is usually given as a range: the light draught of the unloaded vessel and the maximum draught when fully loaded, shown by draught or gauge marks on the hull. Passenger vessels typically draw between 0.8 and 1.6 metres and can therefore use lakes, tributaries and canals where loaded cargo vessels drawing up to three metres no longer go. At low water what counts is not the draught alone but the margin to the fairway depth plus a safety clearance to the bottom; propellers and rudders often sit lowest. Add to that the squat effect: in shallow water a moving vessel sinks several decimetres deeper. In a listing, draught belongs with length, beam and air draught to the four dimensions that define the possible operating area.

See also: Air draught, Water level gauge, Displacement

Dry-docking

Dry-docking means taking a vessel out of the water in a dock or on a slipway so that the underwater hull, propellers, rudders and sea valves can be inspected and repaired.

Depending on the yard, this is done in a graving dock (a basin that is pumped dry), a floating dock or on a slipway where the vessel is hauled out on cradles. Typical work includes cleaning and grit-blasting, measuring plate thickness, renewing sacrificial anodes, checking shaft, bearings and seals, and applying new antifouling. For passenger vessels, dry-docking is part of every renewal of the ship certificate, which is valid for at most five years. Costs range from a few tens of thousands of euros to six figures depending on size and findings – which is why the dates of the last and the next due docking are among the first questions to ask when buying.

See also: Ship certificate, Plate thickness

ES-TRIN

ES-TRIN is the European standard of technical requirements for inland vessels; it defines how a passenger vessel must be built and equipped to obtain a certificate.

The abbreviation stands for "European Standard laying down Technical Requirements for Inland Navigation vessels". The standard is drawn up by the European Committee for drawing up Standards in the field of Inland Navigation (CESNI) and made binding through EU Directive 2016/1629 and the CCNR's Rhine Vessel Inspection Regulations, so the same rules apply on the Rhine and across the EU. Chapter 19 contains specific requirements for passenger vessels on stability, escape routes, fire protection, life-saving appliances and accessibility. The standard is revised regularly; existing vessels benefit from transitional provisions that may expire for older vessels – a point buyers should clarify with the inspection body.

See also: Ship certificate, CCNR, Navigation zones

Freeboard

Freeboard is the vertical distance between the waterline and the upper edge of the deck at the ship's side; it shows how much reserve remains before water comes over the deck.

More freeboard means more reserve buoyancy and safety when heeling or in waves; less freeboard means more cargo or lower superstructures. ES-TRIN prescribes minimum values for passenger vessels and additionally requires a residual freeboard and a safety clearance – the height that must remain to the lowest openings in the hull when the vessel is fully occupied and heeling to one side. The permitted passenger capacity follows indirectly from this. Converting to more decks or heavier outfit reduces freeboard, and compliance with the limits then has to be demonstrated anew.

See also: Draught, Heel, Stability

Gross tonnage (GT)

Gross tonnage (GT, in German Bruttoraumzahl or BRZ) is a dimensionless measure of a ship's total enclosed volume – not a weight.

It is calculated under the International Convention on Tonnage Measurement of Ships of 1969 from the volume of all enclosed spaces and in 1994 finally replaced the gross register ton (GRT, 100 cubic feet). Because port, canal and pilotage dues and many safety regulations are tied to gross tonnage, it is the most important size figure for seagoing ships.

Inland vessels are measured differently: their tonnage certificate states displacement and carrying capacity in tonnes, and passenger vessels are described by length, beam, decks and permitted number of persons. If a listing for an inland passenger vessel quotes a GT figure, it usually stems from an earlier seagoing or coastal certificate.

See also: Tonnage, Displacement

Heel

Heel is the sideways inclination of a vessel about its longitudinal axis – caused by wind, turning, uneven loading or passengers all moving to one side.

For passenger vessels the last case is the decisive one: when a vessel comes alongside, the guests crowd to the landward side. ES-TRIN therefore assumes a dense crowd of persons on one side and requires that the vessel heels only to a limited degree while retaining a residual freeboard. This is demonstrated in the stability calculation and, after every major conversion, by an inclining test in which weights are shifted on deck and the resulting inclination is measured. Heel should be distinguished from list, a permanent inclination caused by flooding or incorrect trim, and from rolling, the periodic motion in waves.

See also: Stability, Freeboard, Ballast water

Keel

The keel is the backbone of the hull – the longitudinal beam or plate strake at the lowest point from which the frames rise.

Seagoing ships and sailing yachts have pronounced keels that hold course and provide stability. Inland vessels and most passenger vessels, by contrast, are flat-bottomed: their "keel" is a reinforced centre strake and the bottom is flat, giving plenty of displacement at shallow draught and an even bearing surface in dock. Two expressions live on from the term: keel-laying marks the official start of construction – not to be confused with the delivery year that certificates record as the year of build – and "keel up" describes a capsized vessel. The wake is the trail a vessel leaves behind, in German still called Kielwasser, "keel water".

See also: Draught, Plate thickness

Knot

A knot (kn) is the unit of speed used at sea and equals exactly one nautical mile per hour, or 1.852 km/h.

The unit comes from the hand log: a line knotted at fixed intervals was paid out astern and the crew counted how many knots ran through their hands while a sand glass emptied. Because the nautical mile corresponds to one minute of arc along a meridian, speeds in knots can be worked directly on the chart – which is why the unit has survived at sea. Inland navigation, by contrast, calculates in kilometres per hour; inland vessels usually make 12 to 20 km/h over the ground. Passenger vessels on large lakes and near the coast often state their speed in knots. Conversion is simple: knots times 1.852 gives km/h, km/h divided by 1.852 gives knots.

See also: Nautical mile

Lee / leeward

The lee is the side of a vessel or object facing away from the wind – the sheltered side; leeward means in that direction.

Its opposite is windward, the side the wind comes from. Anyone seeking shelter on deck goes to leeward; anyone berthing a vessel works with the wind. If it blows off the landing stage, the stage lies in the lee of the vessel and the vessel is pushed away – you come in with momentum or use the bow thruster. If the wind blows onto the stage, it pins the vessel against it, making berthing easy and leaving hard. A shore to leeward of the vessel, a lee shore, is dangerous because an engine failure will set the vessel onto it. Memory aid: the lee is where people like to stand on deck – out of the wind.

See also: Windward

Nautical mile

The nautical mile (NM) is the unit of distance used at sea and in aviation and measures exactly 1,852 metres.

It derives from the geometry of the globe: one nautical mile corresponds to one minute of arc, a sixtieth of a degree of latitude. That is why a distance can be taken straight from the latitude scale at the side of a chart – and why the nautical mile has outlived the metric revolution. It has nothing to do with the statute mile of 1,609 metres. Ten nautical miles are 18.5 kilometres; a vessel making ten knots covers them in one hour. Inland waterways are measured in kilometres instead: the river kilometres on the Rhine, Danube and Elbe – visible on the kilometre boards along the bank – are the coordinate system of inland navigation. A position there is given as "Rhine km 554", not in degrees and minutes.

See also: Knot

Navigation zones

Navigation zones classify Europe's inland waterways into zones 1 to 4 according to wave height and weather risk; the zone determines where a vessel may operate under its certificate.

Zone 4 covers sheltered waters such as canals, small rivers and many lakes, zone 3 the large rivers such as the Danube, Elbe and Moselle, zone 2 estuaries and sea areas with limited wave action – parts of the Baltic and its lagoons, for instance – and zone 1 the most exposed river mouths. The Rhine has its own regime, zone R, under the CCNR. The lower the zone number, the higher the requirements for stability, freeboard, closures and equipment. A vessel with a zone 3 certificate may not sail the Baltic; a later zone extension is costly and not feasible for every hull. Which zone suits the intended operation is explained in the guide to the ship certificate.

See also: Ship certificate, Freeboard

Passenger capacity

Passenger capacity is the maximum number of persons a passenger vessel may carry as laid down in its ship certificate; it is the vessel's most important commercial figure.

The inspection body derives it from usable deck area, escape routes, life-saving appliances and the stability calculation; certificates often contain several values, for example a higher number for zone 4 and a lower one for zone 2 with its rougher water. The figure caps revenue per trip and decides whether charter, scheduled service or catering can pay. It can only be increased through conversion and a new survey. In a listing it appears as "certified for 250 persons" – not to be confused with the seats in the saloon, which are usually far fewer.

See also: Ship certificate, Stability, Navigation zones

Passenger vessel

Under European inland navigation law a passenger vessel is a vessel built and equipped to carry more than twelve passengers.

The twelve-person threshold separates the passenger vessel from the pleasure craft and the small craft; above it the special construction and equipment rules of ES-TRIN apply, together with the obligation to hold a ship certificate stating a maximum number of persons. A distinction is made between day-trip vessels without overnight cabins and cabin vessels for river cruises. In everyday language the ships are called excursion boats, saloon vessels, ferries, restaurant or event ships depending on their use; legally they are all the same thing: passenger vessels.

Navium is an online marketplace where owners offer exactly these vessels with no broker fees – from the compact saloon vessel for 100 persons to the three-deck ship for more than 600 guests. View current passenger vessels.

See also: Cabin vessel, Saloon vessel, Passenger capacity

Pilot

A pilot is a navigator with local knowledge who comes aboard to advise the boatmaster in difficult waters; responsibility for the vessel remains with the boatmaster.

On Germany's sea waterways – the Elbe, Weser, Ems and the Kiel Canal, for instance – larger ships are obliged to take a pilot; the sea pilots are organised in pilot brotherhoods. In inland navigation the boatmaster's mandatory route knowledge largely replaces the pilot; on demanding stretches such as the Rhine Gorge between Bingen and St. Goar, Rhine pilots can be taken voluntarily. For a passenger vessel the subject mainly arises during the delivery voyage after purchase: if the route leads through unfamiliar waters or across the sea, a pilot or an experienced delivery skipper is cheaper than any risk.

See also: Captain / boatmaster

Plate thickness

Plate thickness is the thickness of the hull's steel plating; it is measured ultrasonically and is the most important yardstick for the condition of an older hull.

Steel corrodes from both sides – outside in the water, inside in the bilge and under insulation. ES-TRIN prescribes minimum thicknesses for bottom, bilge and side plating; if a plate falls below the value it has to be renewed or doubled. Every survey for renewal of the ship certificate therefore produces a thickness measurement report with grid values for the entire hull. For buyers this report says more than any photograph: it shows how much reserve the hull has and where steel work will be due in the coming years. Many doubling plates point to a weakened hull, where renewing whole strakes may be more economical than further patching.

See also: Dry-docking, Bilge, Ship certificate

Port

Port is the left-hand side of a vessel when facing the bow; at night it is marked by a red sidelight.

The term always refers to the vessel itself, looking forward – regardless of where you stand or which way you face. That is what makes it unambiguous where "left" and "right" cause confusion on board. The name goes back to the age of steering oars: the oar hung on the right-hand side, so ships came alongside the quay with their left side to protect it – the side that faced the port. Two memory aids have proved their worth: "port" and "left" both have four letters, and there is no red port wine left.

In practice, port matters when berthing: passenger vessels usually have boarding gates on both sides, but many landing stages can only be approached with one side. A listing will then say "boarding port and starboard"; the usual abbreviations are P or Pt for port and S or Stbd for starboard.

See also: Starboard

Roadstead

A roadstead is an anchoring area off a port where ships wait safely until a berth, a lock or the tide lets them in.

"Lying in the roads" means waiting at anchor outside the harbour – off Rotterdam, Hamburg or Antwerp dozens of ships lie in the roads whenever there are delays. Roadsteads are marked on charts and often subdivided by ship size and cargo. On inland waterways one speaks of waiting or lay-by berths above locks; larger passenger vessels use them when a landing stage is occupied, for instance. The German word Reede shares its root with "ready": a ship in the roads was ready to sail, and the German Reederei, a shipping company, originally meant the fitting-out of a ship.

See also: Anchorage, Berth

Saloon vessel

A saloon vessel is a day-trip vessel whose main deck is taken up by an enclosed, glazed saloon with seating and catering.

The type has shaped excursion traffic on rivers and lakes since the late nineteenth century and remains the most common design: saloon with galley and bar, open or roofed sun deck above, boarding gates on both sides. Most vessels fall between 20 and 60 metres in length and 50 to 300 certified persons. Because the saloon is independent of the weather, scheduled trips, charters and events can be combined on the same vessel – one reason for the steady demand for second-hand saloon vessels. Related terms are excursion vessel, panorama vessel with particularly large windows and party boat; the guide to passenger vessel types gives an overview of all designs.

See also: Passenger vessel, Deck

Ship certificate

The ship certificate – today the Union inland navigation certificate – is the official operating licence of an inland vessel; it confirms compliance with the technical rules and lays down navigation zones and permitted number of persons.

It is issued after a survey by the inspection body – for the Rhine as a Rhine vessel certificate under the Rhine Vessel Inspection Regulations, otherwise as a Union certificate under EU Directive 2016/1629. For passenger vessels it is valid for at most five years; before expiry the vessel is surveyed again, dry-docking included. Without a valid certificate no passenger may be carried – a vessel without one is therefore worth little more than its scrap and conversion value. When buying, the certificate, its remaining validity, the last survey report and any conditions attached are among the first documents to examine. Everything else is in the guide to the ship certificate.

See also: ES-TRIN, Navigation zones, Passenger capacity, Dry-docking

Stability

Stability is a vessel's ability to right itself after heeling; on passenger vessels it is the central safety parameter and is demonstrated both by calculation and by test.

What counts is the interplay between the centre of gravity and the centre of buoyancy, summarised in the metacentric height. The lower the weight – engines, tanks, fixed ballast – the more stable the vessel; every additional deck, roof terrace or air-conditioning unit on the sun deck works against it. ES-TRIN requires a stability calculation for passenger vessels covering several load cases, including a crowd of persons on one side and wind pressure. The basis is an inclining test, which has to be repeated after every conversion. Buyers should therefore ask not only whether a stability document exists but whether it reflects the vessel's current condition – later conversions without a new calculation are among the most common defects found on second-hand passenger vessels.

See also: Heel, Freeboard, Ballast water

Starboard

Starboard is the right-hand side of a vessel when facing the bow; its sidelight is green.

The name is historical: before stern rudders existed, the steering oar – the "steer board" – hung on the right-hand side, since most helmsmen were right-handed. To protect it, ships berthed with their left side to the quay, which is how that side came to be called port. Memory aid: starboard is right, where the steering board once was, and green. The same colours recur on the water: looking downstream, green buoys mark the right-hand and red buoys the left-hand edge of the fairway. On rivers traffic generally keeps to the right, so vessels pass each other port to port. The usual abbreviation is Stbd.

See also: Port

Stern

The stern is the rear end of the hull, where rudder and propeller sit and where passenger vessels often carry a second boarding platform.

The shape of the stern often reveals a vessel's age and use: the vertically cut transom stern is today's standard and makes room for a terrace or stern boarding gate, the rounded cruiser stern is found on older saloon vessels, and the tunnel stern with its raised bottom brings more water to the propeller in shallow water. Because propeller wash and rudder act at the stern, a vessel turns about a point far forward – the stern swings out, which has to be allowed for when leaving a landing stage. The direction is "aft", the wave behind the vessel is the stern wave, and the rearmost deck area is the afterdeck.

See also: Bow, Aft, Azimuth thruster

Tonnage

Tonnage is an umbrella term for the size of a ship – depending on context a volume measure (gross tonnage), the carrying capacity (deadweight tonnes) or the displacement in tonnes.

Three quantities are routinely mixed up: gross tonnage measures enclosed volume, deadweight (dwt) the weight of cargo, fuel and stores a ship can carry, and displacement the total weight of the floating ship. A cargo ship is traded by deadweight, a cruise ship described by gross tonnage, an inland vessel by its tonnage certificate, which relates carrying capacity and displacement to draught. For a passenger vessel tonnage is secondary: what counts are passenger capacity, dimensions and zones. Displacement helps at most in costing dock and crane work. If a listing states "tonnage: 180 t", ask which of the three quantities is meant.

See also: Gross tonnage (GT), Displacement

Water level gauge

A water level gauge is an official measuring station for the water level; on rivers its readings decide every day how deep and how high a vessel can navigate.

A gauge reading is not a depth but a height above a fixed datum; the fairway depth follows only from the reading plus the depth the waterway authority guarantees at the equivalent low water level (GlW). The best-known gauge is Kaub on the Middle Rhine, whose low readings regularly make headlines because cargo vessels can then only sail part-loaded. Passenger vessels with their shallow draught are affected less often at low water, but may lose landing stages that dry out. Conversely, high-water marks restrict navigation: from mark I restrictions apply, from mark II navigation is suspended – which for excursion operators means cancelled trips. Current readings are published daily by the German waterway administration via ELWIS.

See also: Draught, Air draught

Windward

Windward is the side of a vessel facing the wind – the side the wind blows from.

Windward and leeward belong together like port and starboard, but they do not describe fixed sides of the vessel: they move with the wind. A vessel that tends to turn into the wind by itself is said to be weather-helmed, or "luvgierig" in German. Passenger vessels with high superstructures and flat hulls tend to behave the opposite way in a crosswind and are set to leeward, which demands attention on narrow canals and when berthing – tall decks present a large windage, and a powerful bow thruster compensates for it. The prevailing wind direction is also worth checking when choosing a berth on a lake: a landing stage sheltered to windward by the shore stays calm, one facing open water takes the full waves.

See also: Lee / leeward, Bow thruster