Guidance

Stability Examination Syllabus

Published 3 January 2018

1. Course details

1.1 Duration

The course must take place over five days or 30 hours of formal instruction.

1.2 Assessment

Assessment will be by written 2.5 hour examination candidates must achieve an overall pass mark of 60%. The Exam will consist of two parts. A minimum mark of 40% must be achieved in each part.

Part A will consist of four questions involving calculations and explanation, Part B a question on curves of statical stability and one question on loll, drydocking and longitudinal stability

2. Syllabus

2.1 Basic principles

2.1.1 Basic principles of hydrostatics and related terms

1. Can calculate the draught and freeboard for a box shaped vessel given length, breath, depth, displacement and relative density.
2. Can calculate the displacement of a vessel given the length, breadth, draught, relative density and block coefficient.
3. Can determine the draught from the displacement (or vice versa) using hydrostatic tables found in a vessels stability book.

2.1.2 Fineness of hull form and resistance to forward motion

1. Can define block coefficient.
2. Can outline the influence block coefficient has on resistance to forward motion.
3. Can outline how fluid flow causes resistance to forward motion with regards to skin friction, and wave making.

2.1.3 Statical stability

1. Can explain the term “righting lever” and “righting moment”.
2. Can draw a transverse sketch of a vessel in stable, unstable and neutral equilibrium when heeled to a small angle, showing the positions of:
   1. Centre of gravity “G”;
   2. Metacentre “M”;
   3. Point “Z”;
   4. Centre of buoyancy “B”;
   5. The buoyant force and gravity force vector.
3. Can define, with reference to the sketch in 2.1.3.2, how the forces through the centre of gravity and the centre of buoyancy react.
4. Can describe how the vessels hull shape can affect stability.

2.1.4 Initial stability

1. Can define the transverse metacentre (M) and initial metacentric height (GM).
2. Can show how the distance from keel to metacentre (KM) is influenced by the beam of a vessel.
3. Can describe the relationship between the size of a vessels GM and stiff and tender motion.
4. Can state the dangers of a vessel being too tender or too stiff.

2.1.5 Loading, discharging and shifting weights

1. Can calculate the final position of the distance from keel to centre of gravity (KG) and the GM when loading, transferring and discharging weights by taking moments about the keel using stability information data sheets.
2. Can apply a correction for free surface in stability problems contained in 2.1.5.1 above.
3. Can solve simple loading and discharging problems to show a vessel is in a stable condition using Simplified Stability Information making allowance for Free Surface Effect (FSE).

2.2 List and related problems

2.2.1 List

1. Can explain the effects of moving a weight off the centreline.
2. Can draw a diagram to show that the force lines through the centre of buoyancy and centre of gravity lie in the same vertical line when at an angle of list and that the ship oscillates about this equilibrium angle.
3. Can show that an angle of list is influenced by the size of GM.
4. Can calculate an angle of list when a single weight is moved, loaded or discharged from the centre line.
5. Can calculate the weight required to remove an angle of list by:
   1. Moving a single weight already on board;
   2. Loading a single weight;
   3. Discharging a single weight.
6. Can explain the effect of list, heel or rolling on the draft of a box-shaped vessel.

2.2.2 The inclining experiment

1. Can state the reasons for conducting an inclining experiment.
2. Can describe the procedure for conducting an inclining experiment.
3. Can prepare a check list of precautions to be observed before and during an inclining experiment in order to ensure an accurate result.

2.2.3 The effect of slack tanks on the centre of gravity

1. Can explain how a slack tank can cause a virtual reduction in GM.
2. Can draw a diagram to show how this virtual reduction in GM results in a reduction of GZ.
3. Can explain the factors affecting free surface effect with reference to free surface moments (FSM), relative density (RD) of liquid, KG of the tank in vessel, and the effect of longitudinal sub- division.
4. Can state that the depth of liquid in the tank doesn’t influence the FSM from the tables.
5. Can calculate the virtual GM given the solid GM and free surface correction.
6. Can obtain the FSM from a stability data sheet and calculate the free surface correction given the relative density of the liquid and displacement of the vessel.

2.3 Curves of statical stability

2.3.1 Curves of statical stability

1. Can identify on a GZ curve of a vessel in stable equilibrium the following information:
   1. Range of positive stability;
   2. Maximum GZ and angle at which it occurs;
   3. Angle of vanishing stability;
   4. Approximate angle of deck edge immersion;
   5. Dynamical stability;
   6. Approximate initial GM;
   7. Range of stability.
2. Can sketch a curve for a vessel in stable equilibrium given;
   1. Initial GM;
   2. Maximum GZ and angle at which it occurs;
   3. Range and the angle of vanishing stability.
3. Can distinguish between GZ curves for stiff and tender vessels.
4. Can explain how a change in KG of the ship affects the shape and main features of the curve (with reference to comparison between departure and arrival conditions).
5. Can explain how a change in freeboard or beam can affect the shape and main features of the GZ curve.
6. Can state the criteria for minimum stability identified in the current large yacht code with regards to GM, maximum GZ and angle at which it occurs.
7. Can explain the difference between dynamic and static stability.
8. Can state that a simplified stability curve or table of maximum KG’s may be provided to show that the minimum stability criteria are met.
9. Can explain the effect of a steady and gusting beam wind on a motor and sailing vessel and how the respective angles of heel can be assessed from the GZ curve using a constant wind-heeling lever.
10. Can state the criteria for minimum stability identified in the current large yacht code with minimum freeboard, maximum angle of equilibrium, and minimum range of positive stability after free-flooding of any one compartment .

2.3.2 Stability data supplied to yachts

1. Can describe the content of the stability data booklet supplied to yachts.
2. Can use the information given in a yacht stability booklet to determine load condition.

2.4 Loll, dry-docking and longitudinal stability

2.4.1 Angle of loll

1. Can explain that an initial upright vessel with a negative GM when subjected to an external force will create a capsizing moment.
2. Can explain that provided a vessel with a small negative GM has sufficient stability it will lie in equilibrium at an angle of loll.
3. Can explain the dangers to a vessel lying at an angle of loll in still water when it is subjected to wave action at sea or movements of a mass on board.
4. Can explain the methods that can be used to correct an angle of loll and achieve a positive GM.
5. Can explain how an angle of loll may be corrected or reduced on a vessel fitted with an empty tank subdivided at the centre line.
6. Can distinguish between list and loll.

2.4.2 Dry-docking

1. Can explain the process of slipping and lifting.
2. Can explain the use of a docking plan.
3. Can explain the preparation of the yacht and the dry-dock prior to dry- docking.
4. Can explain the need for an acceptable trim and adequate GM when dry- docking.
5. Can define critical period and critical moment.
6. Can explain the meaning of P force and the effect it has on the stability of the vessel.
7. Can explain the importance of duplicating arrival tank soundings and other weight distributions when departing the dry-dock.
8. Can explain the importance of aligning the support structure and lifting equipment with the vessel’s main strength members.

2.4.3 Longitudinal stability

1. Can define forward perpendicular, after perpendicular, length between perpendiculars, length overall.
2. Can define trim, change of trim, longitudinal centre of floatation, longitudinal centre of gravity, longitudinal centre of buoyancy and the moment to change trim one centimetre (MCTC).
3. Can calculate the chance of trim of a vessel when a weight already on board is shifted longitudinally when given the longitudinal centre of flotation (LCF) and MCTC.