A   Alpha

Diver below (when stationary)

I am undergoing a speed trial


The wind is blowing on to the wrong side of the sails.

There may be several reasons.


adv   Nearer to the stern;  further aft.

"The mizzen mast is abaft the main mast."

A bearing which is abaft the beam is more than 90° from ahead.


adv   At right angles (square) to the boat's heading.


On,  or in,  a boat.


To 'go about' is to change the tack;  to put the wind on the other side of the boat by turning head to wind.

Above board

The board is the uppermost strake or wale of the ship's side.

Pirates might hide behind the board while approaching a potential victim to keep the element of surprise:  they were not 'above the board'.

Open and honest;  not hiding anything.

The Latin ad means 'to' or 'toward'.

It seems likely that the 'a' at the beginning of many nautical words (eg abeam,  alee,  athwart) is an elision of the Latin ad and should be followed by an apostrophe.   So;  a'beam,  toward the beam:  a'lee,  toward the leeward side.   


Level with;  alongside;  abeam.

A breast rope is a mooring warp which is square to the centreline from the bow cleat or the stern cleat.

Absolute bearing

The bearing of a feature relative to North (either True or Magnetic).

cf Relative bearing.

Accommodation ladder

A temporary ladder or steps at the vessel's side,  to enable boarding.


A senior naval officer of flag rank.


The naval authority for a state's navy.

Admiralty curve

A graph,  often with a form close to a normal or Gaussian curve,  which illustrates the rise and fall of the tide at a Standard Port.

The horizontal axis is time,  usually extending from the time of Low Water through the time of High Water to the time of the next Low Water:  approximately 12.5 hours.

The vertical axis is an arbitrary scale from 0 (the height of tide at both Low Waters) to 1 (the height of tide at High Water).

The curve is relevant for any tide,  on any date,  for that Standard Port.

There is a graph to the left of the curve:   the horizontal axis is height of tide relevant to the Standard Port of the curve;  the vertical axis represents time relative to the horizontal scale of the curve.   The purpose of this graph is to convert the arbitrary scale of tidal height on the curve to a graph of tidal height against time for any particular tide,  and to convert the curves of rise and fall to straight lines.


The curve and the scale can be used to find

  • the height of tide at any specific time

  • the time of any specific height of tide.

If the charted depth is known then the depth of water can be calculated.

Not all tidal curves are symmetrical about High Water.

Where there are tidal anomalies (such as Poole,  Bournemouth,  Portsmouth) the time of High Water may not be clearly defined,  and the Low Water would be used as the central origin of the curve.


Secondary ports have no tide table and no Admiralty curve:  they have a Table of Differences from the Standard Port.

When the Secondary Port tide table has been calculated (from the Standard Port tide table and the table of differences) it can be applied to the Admiralty curve of the Standard Port.

The Easytide section of the UKHO website offers tidal curves for both Standard and Secondary Ports.

They are different in that the vertical axis is not an arbitrary scale but is a scale of metres Height of Tide for that port on a given day.


adv   Floating on the surface,  at the mercy of wind and tide,  with no propulsion.
A vessel adrift is still under way,  under the meaning of IRPCS,  Rule 3 "General definitions",  which says

   "(i)   The word "underway" means that a vessel is not at anchor,  or made fast to the shore,  or aground."

A vessel might be intentionally adrift (Is a vessel hove-to also adrift?) or it might be 'Not under command' (IRPCS,  Rule 27).   Wikipedia states that a vessel adrift is not under way:  this conflicts with the IRPCS definition,  which must take priority.

Items or people that cannot be found are said to be adrift.

See Weigh,  under


Advection fog

Can occur any time that warm, moist air blows over a surface cool enough to drop it's temperature below the dew point.

cf Radiation fog.



A wire which transmits and/or receives electromagnetic waves.   An antenna.

An aerial conveys the radio waves,  as electrical pulses,  to an electronic instrument such as a radio,  a GNSS unit or RADAR set where they are converted into signals intelligible to people.

Some sailing vessels utilise the backstay or a triatic stay as a radio aerial



On the surface of the water;  not aground or sunk.   A vessel afloat might be under way or anchored or moored.
Displacing a weight of water equal to its own weight.


Toward the stern;  abaft the midsection.



Between noon (1200) and 1800 (local time).   It includes the afternoon watch (1200 to 1600 (ship’s time)) and the first dogwatch (1600 to 1800 (ship’s time)).

Age of Sail

A period between about 1571 (the battle of Lepanto was the last naval engagement in which oar-propelled galleys played a significant part) and about 1862 (when steam-powered ships began to defeat sailing ships in battle).   International trade and naval warfare were dominated by sailing (especially square-rigged) ships.   The Golden Age of Sail was between 1850 and the early 1900s when sailing vessels reached their peak of size and complexity.




A vessel immovably in contact with the bottom of the sea,  but not sunk,  is said to be aground.

IRPCS Rule 30 "Anchored vessels and vessels aground"  says:"

(d)   A vessel aground shall exhibit the lights prescribed in paragraph (a) or (b) of this Rule (one ball or two all-round white lights) and . . .

   (i)   two all-round red lights in a vertical line;

  (ii)   three balls in a vertical line."

Ships do not voluntarily take the ground;  they are in danger of breaking up.

Small vessels often take the ground,  as on a half-tide mooring,  but those with deep keels may fall over and become swamped by the flooding tide.

Dinghies are often pulled onto the beach.

Mooring chain

The photograph above was taken from Welkin,  aground in the Strood Channel waiting for the tide.   Being less than 12 metres she displayed no signals.   Having twin keels she stood upright in the mud,  in no danger of falling over.


In front of the vessel.


A call to draw attention,  or to hail another vessel.


Intentionally adrift,  without sails or engine,  as in a storm.


Air cushion vessel

A vessel having large downward-facing fans which generate a cushion of air lifting the vessel just above the surface of the water (or land). Such vessels have been used by the military as river patrol boats;  commercial carriers have also used larger models as passenger ships. By reducing or eliminating water resistance, the vessels are capable of great speeds.   See Hovercraft.

cf WIG.

Air mass

The temperaturepressure,  water content of an air mass tends to change slowly,  and is different from the air masses around it.   As a mass of air moves across the surface of the earth it forms a wind.

The boundary between two air masses is known as a front.   The warmer of the two masses rises;  the colder of the two falls.   Where a warmer air mass is moving toward a mass of colder air,   a warm front forms and the warmer air rises over the colder air.   In the Northern Hemisphere the rising mass of warmer air tends to spiral anticlockwise,  forming a cyclone around an area of lower pressure.   Where a colder air mass is moving toward warmer air it forms a cold front,  and sinks beneath the warmer air.


Automatic Identification System.   Vessels with AIS transmitters broadcast information about their headingspeed,  destination,  size,  cargo,  and many other things.   Vessels with AIS receivers can receive these signals;  the information is often displayed on a chart-plotter screen.

AIS has revolutionised the safety of large vessels.

Small vessels can be fitted with AIS transponders or AIS receivers so that,  in bad weather,  other vessels can be 'seen'.   There is a danger that the small vessel navigator will assume that the vessels on-screen (those with AIS transmitters) are the only vessels in the area.


adv   On,  or toward,  the leeward side of the vessel.    "Helm's a'lee" means that the tiller has been moved toward the leeward side of the boat to to initiate a change of tack or to heave to.


An alarm,  or  alarum,  was a violent or noisy disturbance which caused fear and anxiety.

The term was applied to a clock (an alarm clock) which could be set to ring a bell at a certain time,  usually to rouse someone from sleep.

Alarms can now be set on a wide variety of equipment,  including depth sounders,  RADAR,  AIS,  to alert mariners to certain pre-arranged conditions.



All round light

The IRPCS,  Rule 21,  “Definitions”,  says;
All round light” means a light showing an unbroken light over an arc of the horizon of 360 degrees

Rule 23:
"(b)  An air-cushion vessel (a hovercraft) . . .shall . . . exhibit an all-round flashing yellow light.
Rule 28:
A vessel constrained by her draught may . . . exhibit . . . three all-round red lights in a verticle (sic!) line . . .
Rule 29:
"(a)  A vessel engaged on pilotage duty shall exhibit:
  (i) . . . two all-round lights in a vertical line,  the upper being white and the lower red . . .

Rule 30:
A vessel at anchor shall exhibit . . . an all-round white light . . .

All standing

All the sails are a'back.   The vessel is brought to a sudden and unexpected stop.


A book or pamphlet which contains data and predictions.

A nautical almanac contains tidal data (streams,  heights & times),  and information about ports & harbours


In the rigging;  climbing the mast.

Above the deck.

High above.


By the side of another vessel or a wharf.


A device which converts rotary motion into alternating electrical current.

The auxiliary diesel (or petrolengines of sailing vessels and the propulsion engines of power-driven yachts have one or more alternators attached to them and driven by a pulley linked to the crankshaft pulley by a drive belt (often known (for historical reasons) as a 'fan belt').   The alternator provides electrical power to charge the vessel's batteries and to power the vessel's lights,  refrigerators,  etc.

The engines of some vessels may have more than one alternator.   One might recharge the engine starter battery;  another might recharge the battery bank which provides 'domestic' power;  yet another might recharge a dedicated battery for,  perhaps,  a bow thruster or an anchor windlass.




Height above the ground or above sea level.

Sailing:  Some GNSS instruments give a value for ‘altitude’:   since the boat is on the sea surface this is pretty meaningless.

Aviation:   height above the ground or above sea level.   The altimeter of a small aircraft is usually an aneroid barometer:  it measures air pressure.   In ‘standard’ air (air at normal temperatures and pressures) pressure is a useful measure of altitude.   In colder air (at higher pressure) the actual altitude is lower than the indicated pressure altitude.   In warmer air (at lower pressure) the actual altitude is higher than the indicated altitude.

Larger (commercial) aircraft usually have radio or radar altimeters which measure the time it takes a radio signal to travel from the aircraft to the ground and back again.

Meteorology:   Synoptic charts have isobars which are lines along which the surface pressure is the same.   These are ‘constant height’ charts because the pressures are all measured at the same height (altitude) (usually sea level).   Other forms of synoptic charts are ‘constant pressure’ because they measure the altitude at which the air pressure is the same.   These are especially useful in general aviation.

Astronavigation:   The altitude of a celestial body (such as the moon,  the sun,  a star) is the angle between the horizon and the body subtended at the observer’s eye.   This is the angle measured by a sextant.


The small float,  or ballast,  at the end of the outriggers (aka) of a proa or trimaran.


Toward the middle of the vessel,  as opposed to one side or the other.

When the rudder is amidships both it and the tiller are along the centre line of the vessel.


The bullets,  cartridges,  rounds and shells used in firearms and cannon.

Amphidromic point

Where a tidal basin,  such as the North Sea,  has circular tides there is a nodal point where the tide neither rises nor falls.

Estuaries and channels tend to have linear tides.   For example,  the tides of the English Channel flow in either an Easterly direction or in a Westerly direction.   The stream accelerates in one direction and then decelerates before the stream reverses and accelerates in the other direction;  it then decelerates before reversing again.   In the North Sea the height of the tide changes in a circular manner.   At roughly the centre of the North Sea basin there is an amphidromic,  or nodal,  point where the tide neither rises nor falls.



n   An anchor is a heavy object, often made out of metal, that is used to attach a floating vessel to the bottom of a body of water at a specific point to keep the vessel from moving with the wind and tide.  


There are two primary classes of anchors:  temporary and permanent.

  1. A permanent anchor is often called a mooring and is rarely moved;  it is quite possible that the vessel cannot hoist it aboard but must hire a service to move or maintain it.   Mooring anchors are usually in pairs,  some distance apart and linked by a heavy chain from which bridles lead upward to mooring buoys at the surface.

  2. Vessels carry one or more temporary anchors which may be of different designs and weights.   The bower anchor (as its name suggests) is deployed from one of the bows;  the kedge is a smaller anchor,  often deployed astern when the vessel moors to its own two anchors.   The kedge may be carried away in the dinghy and used to move the boat to a more secure anchorage.

v   To anchor is to stop the boat and keep it relatively still by lowering an anchor to the seabed with a suitable scope of rode.



A modern anchor is shaped so that,  when pulled horizontally,  it embeds itself partially or wholly in the seabed and resists movement through the seabed.   When pulled vertically it cuts its way out of the seabed.

When first lowered the anchor rests on the seabed;  it is then pulled horizontally so that it embeds itself and resists further movement.   To achieve this the anchor rode must be horizontal:  the scope is adjusted so that the rode is vertical from the stemhead and horizontal at the seabed.   This catenary is best achieved when the rode is all-chain and the scope is at least 4 times the depth of water.   If the rode is all line it must not float and the scope should be at least 6 times the depth.   If the rode must be line (perhaps because chain would be too heavy for a small boat or weak crew) then 5 or 10 metres of chain near the anchor will lie flat on the seabed.

Under normal conditions of wind and tide the vessel would be held by its chain,  not the anchor.   In a strong wind or a very strong tide the rode might become straight with the anchor holding the vessel.   The greater the angle of the rode with the seabed the more likely it will be that the anchor will pull out of the seabed,  and drag.

The anchor is weighed by shortening the scope until it is vertical,  when the anchor cuts its way out of the seabed.

Occasionally a fluke will jam under a rock,  and the anchor cannot be weighed.   If this is likely to happen an anchor buoy (or float) should be attached by a line to the crown of the anchor so that it can be tripped crown first rather than shank first.   Be aware that someone might mistake your anchor buoy for a mooring buoy!

Anchoring in a roadstead

Approaching the anchorage find a spot that gives enough swinging circle,  with the necessary scope,  without fouling other vessels.
When the rode is all chain the scope should be at least four times the depth.   When the rode is mostly line the scope should be at least six times the depth.
If the rode is marked it may be paid out from the windlass,  watching the marks as it goes.   If the rode is not marked it will need to be flaked out along the deck and measured.
If the windlass is electric,  and the rode is to be paid out under power,  ensure that the engine is running and that the breaker switch is accessible.   If the windlass is manual,  ensure that the manual brake works.   Remove the retaining pins and lashings from the anchor.

The boat should be moving (slowly;  ahead or astern) as the anchor is lowered and the rode paid out:  this avoids lowering the rode on top of the anchor and ensures that the momentum of the boat digs the anchor into the seabed.   At the chosen scope the anchor should be embedded in the bottom and the few metres of rode near the anchor should be horizontal along the bottom.
Take bearings and transits every half hour or so to check whether or not the anchor is dragging and the boat drifting.
A deep-keeled sailing boat should lie with the tide:  a shallow power boat will probably lie with the wind.
At the turn of the tide check the swinging circle to see if your boat fouls another:  if it does,  consider moving.

Anchoring in a narrow channel or creek

There will not be enough room to swing with the tide without grounding on the banks or fouling other craft.

Drop the anchor,  with enough scope,  from the uptide end so that the vessel is held.

Pay out double the calculated scope for the highest tide,  and drop another anchor from the downtide end of the vessel;  pay out the correct scope.

Recover half the rode from the uptide end of the vessel.

The vessel is not actually moored by two anchors;  whichever anchor is uptide holds the vessel so that she does not swing.

CQR anchor

A mud anchor used by a small river boat

A sea anchor is an unrelated device:  a drogue used to control a drifting vessel.

CQR does not stand for Cuthberts Quick Release.

The intention was that spoken CQR might sound like SECURE.

Bruce anchor

Sea anchor

Winslow conical sea anchor



  • Bower anchor is the principal anchor for the vessel,  being catted at the bows.

  • Sheet anchor is an emergency anchor,  of the same weight and size as the bower(s),  which is kept attached to a rode in case the bower(s) are lost.

  • Kedge anchor is a smaller anchor,  often deployed from the stern.   It may be used for moving the vessel around an anchorage.

  • Stream anchor is used as a second anchor to moor the vessel


A place where ships anchor.   A road,  or roadstead,  usually marked as such on charts.

In 1815 anchorage was the duty (or tax) levied from vessels at anchor in a roadstead.   See Falconer.



Anchor ball

A black sphere suspended in the forward rigging to show that a vessel is at anchor.

IRPCS Rule 30 "Anchored vessels and vessels aground" says

"(a)   A vessel at anchor shall exhibit where it can best be seen:
             (i)   in the fore part, . . .  one ball;

Anchor buoy

A buoy attached by a light line to an anchor to show the position of the anchor,  and perhaps to retrieve it.   The light line should be attached to the crown,  rather than the stock,  of the anchor;   if the flukes become jammed (in rocks,  or under a heavy chain) the line can be used as a tripping line and the anchor can be recovered crown first.   The line should be no longer than the maximum depth of water expected.

It is not unknown for other sailors to mistake an anchor buoy for a mooring buoy;  by attaching their mooring warps they might weigh your anchor.

Anchor rode

chain or a stretchy (usually nylon) line linking the anchor to the vessel.

The RYA tells us that if the rode is all chain it should be of length 4 times the maximum depth to be expected;   if mostly line,  it should be of length 6 times the depth.   But,

"Chain in the locker is wasted chain."

The anchor rode hangs in a catenary (almost a parabolic curve) which should be vertical from the vessel and horizontal at the anchor,  so that the anchor is pulled horizontally.   The flukes of the anchor are angled such that they dig into the sea bed.

When the rode is too short it might not be horizontal at the anchor:  it will then tend to pull the anchor upward,  out of the ground.

Because chain is heavy it creates a pronounced catenary;  in light winds and weak tides a vessel might lie to her chain rode rather than the anchor.   In strong winds and tides the rode might be pulled straight;  more rode will then be needed to increase the catenary.

Nylon line is lighter than chain.   Wind and tide acting on the vessel tend to pull an all-line rode straight,  with almost no catenary,  so that it is not horizontal at the anchor.   There are three possible solutions.

  • More line.

  • A chum.

  • More chain between the line and the anchor.

When the rode is too long the vessel might sheer or sail around its anchor,  first one way and then the other:  the vessel might run foul of other vessels in the anchorage​;  it might run ashore when the tide changes.


Anchor lights

The white all-round lights displayed by a vessel when at anchor in poor visibility.

IRPCS Rule 30 "Anchored vessels . . . " says

"(a)   A vessel at anchor shall exhibit where it can best be seen:

    (i)   in the fore part,  an all-round white light . . ;

   (ii)  at or near the stern and at a lower level than the light prescribed in sub-paragraph (i),  an all-round light.

(b)   A vessel of less than 50 metres in length may exhibit an all-round white light where it can best be seen instead of the lights prescribed in paragraph (a) . . "

Anchor watch

Crewmen assigned to watch over the vessel whilst at anchor.

They generally have 3 duties:

1   To notify the OOW if the vessel drags her anchor or drifts or appears to foul another vessel.

2   To notify the OOW if the vessel is about to be boarded.

3   To search for fires aboard,  and to raise the alarm if any are found.

Anchor sentinel

A proprietary hook for attaching a chum,   angel, or  kellet to an anchor rode.

Anchor sentinel

Anderson turn

Used by a power driven vessel to return to a point it had previously passed through,  when that point is still close.

If the turn is in response to a MOB, stop the engines.
Put the rudder over full.   If in response to a MOB, put the rudder toward the person.
When clear of the person, go all ahead full, still using full rudder.
After deviating from the original course by about 240 degrees (about 2/3 of a complete circle), slow the engines 2/3.
Stop the engines when the target point is 15 degrees off the bow. Ease the rudder and back the engines as required.
See also Williamson turn,  Scharnow turn,  crash stop.


An instrument for measuring the speed (usually in knots,  sometimes in miles per hour) or strength (usually in Beaufort) of the wind.

There are several forms.

  • The most usual form,  at the top of a vessel's mast,  consists of three small cups arranged horizontally around a vertical axis.   The cups are moved around the axis by the wind;  the speed of rotation is translated electronically into wind speed or force.

  • A vertical plate,  with a counterweight,  is blown over by the wind;  it moves across a scale which shows the speed or force of the wind.

  • A small fan is rotated by the wind;  its speed of rotation is translated electronically into speed or force.

For any given sailing rig and hull there is a speed (hull speed) above which the vessel cannot sail faster;  higher wind speeds exert heeling forces rather than driving forces,  and it becomes advisable to reef the sails to avoid damage to the vessel.

Aneroid barometer

A mechanical device for measuring air pressure.

Most of the air is withdrawn from a metal box,  creating a partial vacuum.   The box is prevented from collapsing by a spring.   The outside of the box is connected to a needle on a dial.   An increase in atmospheric pressure causes the box to collapse slightly against the spring,  moving the needle.   A decrease in atmospheric pressure causes the box to expand with the spring,  moving the needle the other way.   The card of the dial is calibrated to atmospheric pressure in millibars or mm of mercury.

Changes in atmospheric pressure predict changes in weather.



See anchor sentinel (above),   Chum,  Kellet,  Sentinel


An angle is the corner where two lines meet.   The lines meet at the corner and diverge away from the corner.   The lines may be straight or they may be curved.

The divergence,  or convergence,  of the two lines of an angle may be measured in several units,  the most common in nautical navigation being degrees,  minutes and decimal minutes.   A full circle is 360 degrees (denoted °):  a minute of arc (denoted ′) is 1/60th of a degree.

In recent history each minute of arc was divided into 60 seconds of arc (denoted ") so that an angle might be measured in degrees,  minutes and seconds.

Angle from North.jpeg

A point (now obsolescent) is 11.25°,  and is often measured as a relative bearing from the heading.   Thus,  'two points on the starboard bow' would mean 22.5° to the right from the ship's heading.   There are 32 points in a circle.

The clock notation is occasionally used at sea.   Dead ahead would be at 12 o'clock;  dead astern would be at 6 o'clock;  on the starboard beam would be at 3 o'clock.

A radian (not used in navigation) is the angle at which the subtended arc is equal in length to the radius of the arc.

A milliradian (or 'mil') is 1/6400 of a degree and is an angular measurement used in the adjustment of gun sights.   An arc of 1 mil is 3.6 inches at 100 yards or 3 feet at 1000 yards.

In navigation one of the lines of an angle might be a meridian from the observer to the North (or South) Pole.   the other line might be a line of sight (using a hand-held compass) to a feature such as a buoy or lighthouse.   The difference in direction of the two lines,  the angle,  would be measured in degrees (to the nearest 5).   A navigator might use a series of such angles to fix the position of the boat.

Both lines would be treated as though they were straight lines.

Another navigator might measure the vertical difference in direction (the angle) of the horizon and a star:  a series of calculation would fix the position of the boat.


Longitude is an angle between two semicircular planes,  one from pole to pole along the Prime Meridian,  the other from pole to pole along the meridian of the observer.

Latitude is an angle between two lines,  one from the Equator to the  centre of the Earth,  the other from the observer to the centre of the Earth,  on the same meridian


When a sailing vessel heels under the influence of the wind (or a power-driven vessel lists for any reason) its mast forms an angle with the horizon and with the vertical (square to the horizon).   This angle of heel tells the sailor when to reef his sails.

At the Angle of Vanishing Stability (AVS) a vessel will neither recover her upright position nor capsize further.   At an angle greater than the AVS a vessel will become stable upside down.

The structure of any vessel incorporates very many angles,  very few of which are right angles.

The yards of a square sail are square (at right angles) to the mast.

Angle of attack

The angle between the chord of a sail and the apparent wind.

If the angle of attack is too small the wind will flow along the sail with equal pressure on both sides,  and will not drive the sail.

If the angle of attack is too high the air on the leeward (downwind) side will become turbulent,  the sail will stall and drive will be lost.   The situation is different for a sail (such as the crab claw,  lateen and steeply sloping staysails) where the turbulence on the leeward side becomes a steady vortex,   This increases the difference in pressure on the two sides of the sail and increases its driving power.

The angle between the chord of a vertical hydrofoil (such as a centreboard,  daggerboard or leeboard) and the flow of water.

If the vertical hydrofoil is in line with the centreline of the boat the angle of attack arises from the leeway of the boat:  the difference in water pressure on the two sides reduces the leeway.

Centreboards and daggerboards cannot normally be adjusted for angle of attack,  but leeboards often can;   correct adjustment can make them into lifting foils,  ie,  the difference in pressure on the two sides pushes the leeboard (and therefore the boat) up to windward.

If the leeway is too great and the forward movement of the boat is small (the angle of attack is too high) the water on the windward side will become turbulent,  the board will stall and forward movement will be lost.

Angle of Vanishing Stability

The angle of heel of a boat (at rest in calm water with no wind) at which it shows no tendency either to right (become upright) or to capsize.

Also known as the Limit of Positive Stability.

The stability of a vessel is usually described in the form of a graph of Angle of Heel against Righting Moment.

  • The Angle of Heel is the angle which a vertical mast makes with a line at right angles to the sea surface.   The mast of a vessel with zero Angle of Heel is normal to the sea surface;  it is upright.   The mast of a vessel with 90° of heel is horizontal:  parallel to the sea surface.

  • A moment (in this context) is a force acting around a fulcrum.   It is the product of the force and the distance of the force from the fulcrum.   The Righting Moment of a vessel (its tendency to become upright) is a combination of the moment of its ballast acting downward under the force of gravity and of the moment of its buoyancy acting upward.

The Angle of Vanishing Stability is the angle of heel at which ​the Righting Moment is zero.   The vessel will neither heel further nor become upright.

At zero angle of heel the Righting Moment is also zero.

As the angle of heel increases the Righting Moment increases to a maximum and then decreases to zero at the Angle of Vanishing Stability:  in this diagram the AVS is about 130° of heel.

At angles of heel greater than the AVS the Righting Moment is negative:  the vessel is stable upside down.


The Centre of Gravity (CG) is a point at which the entire mass of the vessel is thought to act downward;  its moment is the product of the downward force and its horizontal distance from its fulcrum,  ie CG x Z.

The Centre of Buoyancy (CB) is the point at which the buoyancy of the entire vessel is thought to act upward;  its moment is the product of its upward force and its horizontal distance from its fulcrum,  ie CB x Z.

On the left hand side of the curve the Righting Moment is positive and the vessel tends to become upright.

On the right hand side of the curve the Righting Moment is negative and the vessel tends to capsize.

Righting moments

Fishing with a rod and line.   The rod is held relatively still (by contrast with fly fishing and spinning) with the hook (and bait) underwater.   The line may have a float which becomes agitated or submerged when a fish bites.

People engaged in angling are anglers.

Sea anglers fish from a vessel at sea.

Beach anglers cast their lines from a beach.

Coarse anglers fish for inedible fish in rivers and canals.



The positive terminal of an electrical accumulator,  cell or battery.

Where the electrical cell consists of different metal parts of a boat,  with sea water as the electrolyte,  an anode,  consisting of a zinc mass,  is inserted into the circuit.   The zinc anode is lower in the electrochemical series than most other metals and is preferentially degraded,  thus protecting other metallic parts from electrolytic degradation.

Anodes should be replaced when they become significantly degraded.

If an anode degrades more quickly than expected efforts should be made to isolate,  or insulate,  other metallic components of the boat from one another.

Most coatings are electrical insulators,  so anodes should never be painted or varnished.


v   A vessel answers her helm when she responds to changes of the wheel or tiller;  she changes direction as signalled by the helmsman moving the wheel or tiller.

In order to be effective a rudder needs to have water flowing across it,  either because the vessel is moving through the water or because the propeller is pushing water across the rudder.   If the vessel stops moving through the water the rudder becomes ineffective and the vessel will not answer the helm.

n   A reply to a question.

n   A reply (in the affirmative! (Aye aye)) to an order.


An aerial.   A wire from a radio receiver or transmitter which gathers or broadcasts radio waves.

Antennas are included in radios (both fixed and handheld),  GNSS equipment,  AIS transeivers,  RADAR sets.   A radar reflector is itself an antenna which both receives and redirects radio waves.

An antenna may be internal to the instrument,  as it is in hand-held equipment,  or external,  leading from a fixed instrument to the top of the mast.



A mass of outwardly and downwardly spiralling air,  rotating clockwise in the Northern hemisphere and anticlockwise in the Southern hemisphere.

Large scale anti-cyclones are centred around regions of high atmospheric pressure;  they are driven by relatively colder,  higher pressure air falling through warmer air and by the Coriolis force.

See Cyclone


When boats are kept in the water for a long time marine plants and animals grow on the boat's bottom. This marine growth, or fouling, increases the friction between the boat and the water, and slows it down.  In tropical waters wood-boring molluscs, such as Teredo navalis, tunnel into, and eat the wood.

During the Age of Sail ships' bottoms were coated with sheets of copper, which tended to poison marine growth.

During the C20 paints were developed which were toxic to marine creatures.   Many of these paints are now considered too toxic to be used, and are, in some cases, being replaced with copper-based paints.

Renewing the antifouling is a tedious and unpleasant annual job. Being toxic, the paint must be kept away from the worker's skin, eyes and mouth, and must not be allowed to drain into the water.   See Breaming.

See paint,  coat.


Apparent wind

The direction of the wind that seems to blow across the deck of a moving boat.   The apparent wind is the resultant of the true wind and the movement of the boat.   Sails are trimmed to the apparent wind.

The direction of the true wind is the point of the compass from which the wind blows: a NorthWesterly wind blows from the NorthWest.   Apparent wind is shown on an instrument display which includes a diagram of the vessel and shows the relative bearing of the wind direction (the direction from which the wind appears to blow across the deck).


n   An approach to a harbour is the route a vessel takes to the SWM of the fairway or channel leading to the harbour.   The channel is often marked and buoyed,  but the approach might not be.   Details of an approach are given on a chart and in a pilot book.

v   To approach is to get nearer,  to move toward.   To make an approach is to use pilotage techniques to approach a harbour.



A curved piece of timber exactly shaped and fixed abaft the stem and above the forward part of the keel.


A channel of water raised above the ground.   Typically,  an aqueduct carries a canal across a valley.

The Pontcysyllte and Chirk aqueducts carry the Llangollen canal over deep river valleys.



Part of a fair (smooth) curve.   An arc of a circle is part of the circle.

By contrast,  a segment is part of a disc.


A person who fires arrows from a bow,  usually a longbow.



A person who designs structures.   In this nautical context,  a naval architect designs ships and boats.


A large fleet of ships with intention to invade,  and conquer,  another state.

The Spanish Armada was sent by Spain,  in 1588,  to invade and conquer England.

The British and Americans,  with other nations,  sent an armada of ships to Northern France to recapture it from German occupation.



A place where weapons are made and stored.

Arsenal Football Club was named after the Woolwich arsenal,  where the munitions workers formed a football club in 1886;  only its name now links the Club to Woolwich arsenal.


The square-rigged steering sail on the steeply raked foremast or bowsprit of a Roman trading vessel.

See Corbita


On the beach or land,  as opposed to aboard ship.

On arrival at a port sailors are allowed to "run ashore".



Aspect ratio

The ratio of the length of the luff of a sail to its foot.
The ratio of the depth of a keel to its chord (from front to back)


AR = Depth/chord


Sails and keels of high aspect ratio are more efficient than those of low aspect ratio of equal area.

Where the chord is not uniform along luff or depth,

 the formula is

AR=2 x B x B / S

where B is the (vertical) length of a board (leeboard,  centreboard or daggerboard) and S is its area.


Abaft the vessel.


Or celestial navigation.

The science of fixing position by measuring the angles (the altitude) between the horizon and a heavenly body such as the sun,  the moon or a star at a specific time.

The fix results in curved position lines on the surface of a sphere,  with spherical angles between the lines.

This contrasts with 'plane sailing' using a flat (planar) chart (usually Mercator's or a polar projection) and rhumb lines.


The study of stars and planets and of their relative movements.

Until the late C20 astronomy was important in fixing a ship's position;  a sextant was used to measure the altitude of stars and planets and,  in conjunction with an accurate clock,  tables of the altitudes of stars and planets could be used to fix a position.   With the advent of GNSS astronavigation is virtually obsolete,  although many professional and recreational sailors practice to maintain proficiency.


Across the breadth of the ship.




A book of maps or charts.

A tidal stream atlas contains 10 to 12 charts of the same area,  each referring to a different tidal hour.   Dots (sometimes commas) on the chart show points at which the tidal stream has been measured.   Some of these dots correspond to tidal diamonds on the chart represented in the atlas.   On either side of the dot (which is not a decimal point) are two figures:  the first is ten times the drift at the Neap tide;  the second is ten times the drift at the Spring tide.   An arrow shows the set of the tide,  which can be measured with a plotter,  in degrees True.   Each page of the tidal stream atlas represents the tidal stream during a one-hour period.   Each 1-hour period is before or after the High Water hour (not the time of high water) at the reference port for that atlas.   The High Water hour is from half an hour before HW to half an hour after HW.

A harbour atlas contains chartlets of the harbours and ports of a given coast;  where the book also gives historical and tourist details of a harbour,  and details of the approaches,  it is called a pilot book.

The concept of a book of maps (more portable and easy to consult than a folio of charts),  all to the same standard,  colouring and type-set,  belongs to the 16th Century cartographer Gerardus Mercator.   He used Atlas as the title after the legendary figure who carried the world on his shoulders in analogy to Mercator's mapping of the entire world into 6 volumes of book.


The layer of gases surrounding a planet;  the layer of air around the Earth.

The weight of the air contributes to atmospheric pressure;  air at different temperatures has different pressures.   Since warmer air has a lower density (and pressure) than colder air it rises above the colder air and,  with the Coriolis effect,  causes the winds that blow around the Earth.   Colder air can contain less water vapour than warmer air;  a fall in air temperature below the Dew Point results in precipitation of all forms of water;  a rise in air temperature results in the absorption of water vapour from clouds,  seas and lakes.   Small changes in temperature and pressure form or dissipate cloud,  fog and mist.



An anchor is atrip when it has been tripped from the sea bed,  is hanging on its rode but has not yet reached the boat.

See Aweigh.


The Aurora Borealis is the Northern Lights.

The Aurora Australis is the Southern Lights.

The interaction of cosmic particles from the sun with the magnetic field of the Earth causes the ionisation of gases,  and the release of coloured light,  high in the atmosphere.



An adjective describing something kept in reserve;  available should it be needed.

Most sailing yachts have auxiliary engines:  the vessel may then be known as an auxiliary yacht (the noun 'engine' being assumed).




A call which means ‘stop whatever you are doing’.   See belay.


Angle of Vanishing Stability.

The angle of list (not heel) in calm water with no wind at which a vessel will neither recover upright nor capsize further.


The sea is washing across the surface.

Flotsam shipping containers float awash.

Rocks awash may have waves breaking over them.


Referring to an anchor which has been raised fom the seabed but not yet catted to the vessel.

See Under weigh.

An anchor aweigh is also atrip;  it has been tripped from the ground.

Aye aye

The reply to an order by a superior officer.

It indicates that the order has been heard and will be obeyed.

It is normal to repeat the words of the order to show that it has been heard correctly and understood.

Azores high

The Azores is a group of Portuguese-owned islands to the West of Portugal.

At latitude 37'7°N,  they are on the Northern edge of the Hadley cell and the Southern edge of the Ferrel cell where a stable air mass falls from the upper atmosphere toward the surface,  causing a more-or-less permanent region of high atmospheric pressure and gentle anti-cyclonic winds.

With fluctuations in the position of the jet-stream the Azores High can move North toward British latitudes.


John Starkie

December 2020

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