Sail Skills - Stability
Free guide to ship and small vessel stability.  Aimed at small vessel users but suitable for professional mariners.  Diagrams, text and animations explaining stability: from righting lever to movement of liquid in tanks – the free surface effect.  Demonstrates adding weights to a vessel, heel and list.  Especially recommended for RYA examination candidates.
A small sailing vessel heeled by the wind Diagram:  relating topweight in a sailing vessel to levers and moments
Diagram:  a vessel heeled showing righting lever (GZ) A vessel recovering from inversion
A part-filled inclined tank
Diagram:  a vessel kept afloat by her buoyancy
Diagram:  centre ofbuoyancy (CB) of a vessel heeling
Diagram:  a vessel at the angle of vanishing stability (AVS)
GZ Curve for a stiff and tender vessel - showing initial steepness of curves
Diagram: centre of gravity (CG) of a vessel heeling
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Centre of Buoyancy

Centre of buoyancy (CB) is a straightforward concept.  It is important in understanding a vessel’s stability

The CB of an object or vessel is an imaginary point through which the force of buoyancy is considered to act

The CB of a vessel or object is:

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within its underwater shape
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located at the centroid of the underwater shape

The centroid of a shape is where its centre of gravity (CG) would be if it were completely filled with a fluid such as water - with absolutely nothing else in it, such as machinery or equipment:
Diagram:  Centre of Buoyancy (CB) of a barge Diagram:  Centre of Buoyancy (CB) of a motor vessel
Centre of buoyancy (CB) of a motor vessel
Centre of buoyancy (CB) of a crude barge
The location of a CB is easy to visualise for most vessels such as a crude barge or motor vessel sitting quietly in still water:
The location of the CB depends on the shape of the underwater part of a vessel.  So, if the shape of the underwater part changes, the CB moves

For example, the shape of the underwater part changes when a vessel:

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heels - this is the most important when considering stability
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begins or stops planing
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changes her trim
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pitches

Remember, the CB remains at the centroid of the underwater part of the vessel:
Centre of buoyancy (CB) of an object or vessel is at the centroid of its underwater shape
Movement of centre of buoyancy (CB) when a vessel heels
The movement of the CB is an important factor in considering a vessel’s stability.  Designers give very careful thought to how the shape of the underwater part of a vessel will change - for example as a sailing vessel heels to the wind

Although movement of the CB athwartships (side-to-side) is the most important consideration in a vessel’s resistance to capsize, it’s worth showing the movement as a vessel meets and rises to a head sea:
Movement of centre of buoyancy (CB) when a vessel meets a head sea
This helps demonstrate that the CB moves in three dimensions - for example, a vessel could be simultaneously:

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pitching into a head sea while
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heeling to the wind and
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rolling in a cross-sea or another vessel’s wake


To summarise, the centre of buoyancy (CB) of a vessel or object:

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is an imaginary point through which buoyancy is considered to act.  The location of this point depends on the shape of the underwater part of the vessel or object
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acts at the centroid of the underwater part
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moves as the shape of the underwater part changes.  The athwartships movement is the most important when considering stability
Boating Books
Website Overview October 2012
The product is suitable for RYA certificates , RYA , Yachtmaster , Yachtmaster Coastal Certificate , Coastal skipper ,        Day Skipper , Advanced Powerboat , international Certificate of Competence (ICC) for operators of pleasure craft , RYA certificates training , RYA training , Yachtmaster training , Yachtmaster Coastal Certificate training , Coastal skipper training ,        Day Skipper training , Advanced Powerboat training , international Certificate of Competence (ICC) for operators of pleasure craft training , STWC candidates, Standards of Training Watchkeeping and Certfication ----------------------------The tides portion covers topics such as, Tides ,  flood tide ,  ebb tide , incoming tide , outgoing tide , spring tide , neap tide , moon , sun , Causes of tide , Tidal Lag , tide lag , Orbits , Distance , Declination , Landmasses , Rivers , Weather , Wind , Pressure , Wind & Pressure , Rain , Predicting tides , predicting tidal heights , tidal heights ,  admiralty , Tide Tables , Tidal Curves , Using Tidal Curves , Secondary Ports ,  High Water Curves , hw , lw , hw curves , HW Standard Ports , HW Standard Port , Standard port , Secondary Ports , secondary port , Rule of Twelfths , rule of 12ths , rule of , LW Water Curves , low water curves , LW Standard Ports , LW Secondary Ports , Swanage to Selsey Bill , swanage , Selsey bill , poole , solent , Swanage to Yarmouth ,  Yarmouth , Factors , tidal factors , Interpolation and Extrapolation , Interpolation Between Curves , Secondary Port Differences , Extrapolation , Using Predictions , Chart datum , charted clearance , vertical clearance , depth below keel , clearance below keel , Depth to Anchor , Under Keel , Overhead clearance , clearance overhead , Ground and Beach , Grounding , Beaching , Contours depth , Using Contours , Distance Off -----------------------The colregs portion covers the topics of International Regulations for preventing collisions at sea , also known as International Regulations for prevention of collisions at sea , IRPCS ,  I.R.P.C.S , colregs , col regs ,  collision regs , collision regulations , Regulations for collisions at sea , rule of the road , colregs rule of the road , International Regulations for preventing collisions at sea lights and shapes , colregs lights and shapes , International Regulations for prevention of collisions at sea lights and shapes , IRPCS lights and shapes ,  I.R.P.C.S , colregs lights and shapes , col regs lights and shapes ,  collision regs lights and shapes , collision regulations lights and shapes , Regulations for collisions at sea lights and shapes , rule of the road lights and shapes , International Regulations for preventing collisions at sea steering and sailing rules , International Regulations for prevention of collisions at sea steering and sailing rules , IRPCS steering and sailing rules ,  I.R.P.C.S steering and sailing rules , colregs steering and sailing rules , col regs steering and sailing rules ,  collision regs steering and sailing rules , collision regulations , Regulations for collisions at sea steering and sailing rules , rule of the road steering and sailing rules , International Regulations for preventing collisions at sea steer and sail , International Regulations for prevention of collisions at sea steer and sail , IRPCS steer and sail ,  I.R.P.C.S steer and sail , colregs steer and sail , col regs steer and sail ,  collision regs steer and sail , collision regulations steer and sail , Regulations for collisions at sea steer and sail , rule of the road steer and sail , Backscatter , Blast – Short , Blast – Prolonged , Breadth , Close-Quarters , Constrained by Draught , Dracone , Fishing , vessel engaged in fishing , Fog Signals , Give-Way Vessel , give way vessel , Length , Making Way , making-way , vessel making way , Non-Displacement  ,  non-displacement craft , hovercraft , Not Under Command , Power-Driven , power-driven vessel , power driven , power driven vessel , Restricted , Restricted In their Ability to Manoeuvre , Restricted Visibility , Seaplane , seaplane on the water , Stand-On Vessel , stand on vessel , Traffic Separation , traffic separation  scheme ,  TSS ,  TSSs ,  small vessels and traffic separation schemes , Underway , vessel underway , under way , vessel under way ,
Vessel , WIG Craft , Wing-in-ground , Steer & Sail Rules , Look-out , Lookout , Safe Speed , Risk of Collision , compass bearing , change compass bearing , Avoiding Collision , Narrow Channels , small vessels and narrow channels , Narrow fairways , Narrow channels or fairways , Vessels in Sight , Overtaking , Responsibilities Between Vessels , Sailing Vessels , sailing vessel rule of road , leeward vessel , windward vessel , Power-Driven , Restricted Visibility , Radar , Forward of Beam , Lights & Shapes , Arcs of Lights , masthead light , sidelight , side-light , side light ,  sternlight , stern-light , stern light , towing light , towinglight , tow light ,  towing-light , tow-light , Ranges of Lights , Shapes , anchor ball , cone , diamond , cylinder , Power-Driven vessel Under 50m ,  Power-Driven vessel Under 50m , Power-Driven Under 12m ,  Power-Driven Under 12 m , Power-Driven Under 7m & 7kts ,  Power-Driven Under 7 m & 7 kts ,  Power-Driven vessel Under 7 m & 7 kts , Power-Driven Lengths of Vessels , Lights Air-Cushion Vessel , lights air cushion vessel , Lights WIG Craft , Towing and Pushing , Power-Driven Vessel Towing , power driven vessel towing , Tow Alongside or Push , towing or pushing alongside , Vessel or Object Towed , Inconspicuous Object Towed , Sail and Oars , Sailing Vessels General , Sailing Vessel Optional Lights , sailing vessel combined lantern , Sailing Vessels Under 20m , Sailing Vessels Under 7m , sail vessel under 7 m , Sailing Vessels Under Sail & Power , Vessels Under Oars , Trawling ,  vessel engaged in trawling , Fishing Other Than Trawling , Fishing in Proximity – Trawling , Fishing in Proximity - Purse Seining , Not Under Command , Restricted In her Ability to Manoeuvre – General , Restricted In her Ability to Manoeuvre - Dredging or Underwater Operations – Obstruction , Restricted In her Ability to Manoeuvre - Diving Operations - Smaller Vessels , Restricted In her Ability to Manoeuvre - Mine Clearance Operations , Constrained by Draught , vessel constrained by draught , Constrained by draft , constrained by draft , Pilot Vessels ,  vessel engaged in pilotage duties , Anchored , vessel anchored , vessel at anchor , lights vessel at anchor , fog signal vessel at anchor , Aground , vessel aground , lights vessel aground , fog signal vessel aground , Power-Driven Vessels Underway , power driven vessel under way , Lights Air-Cushion Vessel , Lights WIG Craft , lights wig , Towing and Pushing , Towing Vessel - Tow Not Over 200m , Towing Vessel Under 50m - Tow Not Over 200m , Towing Vessel - Tow Over 200m , Towing Vessel Under 50m - Tow Over 200m , Tow Alongside - Single Vessel , Tow Alongside - Multiple Vessels , Pushing - Single Vessel , Pushing - Multiple Vessels , Vessel Being Towed - Tow Not Over 200m , Vessel Being Towed - Tow Over 200m , Inconspicuous Object Towed - Tow Not Over 200m , Inconspicuous Wide Object Towed - Tow Not Over 200m , Inconspicuous Long Object Towed - Tow Over 200m , Sailing Vessel , Sailing Vessel (Large) - Optional Lights , Sailing Vessel (Small) - Optional Lights , Sailing Vessel Under 20m - Combined Lantern , Lights Sailing Vessels Under 7m , Lights Sailing Vessel Under Power , Lights Sailing Vessel Under Power Under 20m , Lights Sailing Vessel Under Power Under 12m , Lights Sailing Vessel Under Power Under 7m & 7kts , Lights Vessels Under Oars , Lights Fishing , Lights Engaged in Trawling  Not Making Way , Lights Engaged in Trawling  Under 50m Not Making Way , Lights Engaged in Trawling  Making Way , Lights Engaged in Trawling  Under 50m Making Way , Lights Fishing Other Than Engaged in Trawling  Not Making Way , Lights Fishing Other Than Engaged in Trawling  Making Way , Lights Fishing Other Than Engaged in Trawling  Not Making Way - Gear Ahead , Lights Fishing Other Than Engaged in Trawling  Making Way - Gear Astern , Lights Fishing Other Than Trawling Making Way - Gear to Port , Lights Fishing in Proximity - Trawling - Shoot Nets , Lights Fishing in Proximity - Trawling - Haul Nets , Lights Fishing in Proximity - Trawling - Nets Come Fast , Lights Fishing in Proximity - Trawling - Pair Trawlers , Lights Fishing in Proximity - Purse Seining  , Lights Fishing in Proximity - Purse Seining - Gear to Port , Lights Not Under Command , Lights Not Under Command - Not Making Way , Lights Not Under Command - Making Way , Lights Restricted In their Ability to Manoeuvre , Lights Restricted in their Ability to Manoeuvre - Dredger Not Making Way , Lights Restricted in their Ability to Manoeuvre - Buoy Service Vessel Not Making way , Lights Restricted in their Ability to Manoeuvre - At Anchor , Lights Restricted in their Ability to Manoeuvre - Making Way , Lights Restricted in their Ability to Manoeuvre – Towing , Lights Restricted in their Ability to Manoeuvre - Obstructed Not Making Way , Lights Restricted in their Ability to Manoeuvre - Obstructed Making Way , Lights Restricted in their Ability to Manoeuvre - Mine Clearance - At Anchor , Lights Restricted in their Ability to Manoeuvre - Mine Clearance – Underway , Lights Constrained by Draught , Lights Constrained by Draft , Lights Pilot Vessels , Lights Pilot Vessel - At Anchor , Lights Pilot Vessel – Underway , Lights Anchored , Lights Anchored – Warship , Lights Anchored - Sailing Vessel , Lights Anchored - 100m or More , Lights Anchored - Under 50m - Sailing Vessel , Lights Anchored - Under 50m - Motor Vessel , Lights Aground , Lights Aground - Under 50m , Signals , Manoeuvring & Warn , maneuver and warn , Alter and Astern , signal alter course ,  signal alter course to starboard , signal alter course to port , signal going astern , signal operating astern propulsion , Concern , signal short and rapid blasts , get out of my way , signal get out of my way , Overtaking , overtaking vessel , vessel overtaking , View Obscured , signal in bend , Fog Signals , Fog Signal Power driven Underway ,  Fog Signal Power-driven Underway ,  Fog Signal Power driven Under way , Fog Signal Anchored & Aground , Fog Signal Pilots , Fog Signal Vessels Under 12 m , Signal to Attract Attention ,  International Code of Signals , Distress signals  , Flares & Shells , SOS , Flag & Ball , Orange Smoke , Arms Outstretched , sound & Light Signals , Fishing in Proximity , Sound Appliances , Distress Signals , Restricted In their Ability to Maneuver , Restricted in their Ability Maneuver - Dredger Not Making Way , Restricted in their Ability Maneuver - Buoy Service Vessel Not Making way , Restricted in their Ability Maneuver - At Anchor , Restricted in their Ability Maneuver - Making Way , Restricted in their Ability Maneuver – Towing , Restricted in their Ability Maneuver - Obstructed Not Making Way , Restricted in their Ability Maneuver - Obstructed Making Way , Restricted in her Ability Maneuver - Mine Clearance - At Anchor , Restricted in their Ability Maneuver - Mine Clearance – Underway , ----- the stability portion covers small and large vessel stability, including buoyancy, inverted stability, centre of gravity (center of gravity), GZ curves, righting moment and lever RM, free surface affect (Courtesy of the University of Tasmania, Australian maritime College ), defines heeling and list, angle of vanishing stability AVS and other aspects of stability