Master Yachts | Stability

Question 1

Which ships shall be inclined and intact stability booklet supplied according SOLAS II-1?

Answer 1

All passenger ships regardless of size and every cargo ship over 24m

Question 2

When shall a ship be inclined?

Answer 2

When deviation from lightship displacement exceeds 1% for ships >160m, and 2% for ships <50m in length - linear interpolation for intermediate lengths.

Question 3

Light weight survey shall be conducted to verify changes in lightship displacement and longitudinal centre of gravity, how often is this completed?

Answer 3

Periodical intervals, not exceeding 5 years

Question 4

Datum draught marks shall be displayed where?

Answer 4

On the bow and stern, and maybe a single line.

Question 5

In the REG Yacht Code, chapter 11, states minimum requirements for intact and damage stability for load lines less than…

Answer 5

85m

Question 6

Damage stability criteria is not required to be met for which yachts, according to Large Yacht Code, Part A, 11.3?

Answer 6

Short range yachts, and yacht in full compliance with the International Load Line Convention.

Question 7

Intact and damage control booklets and plans are to be approved by whom?

Answer 7

The administration, who may delegate to class.

Question 8

Who shall approve freeboard markings?

Answer 8

Assigning authority for the assignment of freeboard and issuance of the International load line certificate

Question 9

Where is the freeboard mark applied and what colour shall it be?

Answer 9

Port and starboard amidships on load line length and maybe an all season mark. It is to be a permanent disc and of contrasting colour.

Question 10

In what condition regarding the freeboard mark, shall the ship not operate?

Answer 10

When the mark is submerged when it is upright in calm water.

Question 11

Within what distance shall the datum draught marks be placed above the deepest load waterline?

Answer 11

1000mm

Question 12

According to the REG Yacht Code, unless it can be clearly demonstrated that no major change has occurred, what shall be conducted and when?

Answer 12

Light weight check at the renewal survey

Question 13

For short range yachts where damage stability has not been assessed, what note shall be added to the approved stability booklet?

‘This vessel has not been assessed…’

Answer 13

This vessel has not been assessed for damage stability, and therefore might not remain afloat in the event of damage or flooding.

Question 14

When a Master is considering stability for a passage, what four angles should be computed or estimated?

Answer 14

1. Angle of first deck edge immersion.
2. Angle of first immersion of coaming/bulwark.
3. Angle of first down flooding.
4. Angle of vanishing stability.

Question 15

List two major factors that can adversely effect stability.

Answer 15

• Adding weight high up.
• Moving weight higher in the vessel (including lifting from any point).

Excessive list or trim affects stability and handling of the vessel.

Question 16

State 6 stability considerations before putting to sea.

Answer 16

1. Remove bilge water.
2. Check freeing ports and deck drains are clear
3. Close all unnecessary openings.
4. Close sea cocks not required to be open
5. Secure all loose gear
6. Check freeboard mark

Question 17

Density in relation to stability is ‘measure of mass …’

Answer 17

‘…per unit volume’.

Question 18

What is the definition of Light Displacement?

Answer 18

The mass of the vessel when complete and ready for sea but with no crew, passengers, stores, fuel or water aboard.

Question 19

Definition of Load Displacement

Answer 19

The mass of the vessel when floating at the summer load line or datum load line in Sea Water.

Question 20

What is Load Deadweight?

Answer 20

The difference between the load and light displacements. The ‘lift’ capacity of the ship.

Question 21

What is TPC?

Answer 21

• Tonnes per centimetre immersion.

The change in displacement, in tonnes, that will cause the yacht’s draft to increase or decrease by 1 centimetre.

Question 22

What is Block Coefficient?

Answer 22

• Relationship of the underwater hull form to a circumscribing block of the same dimensions
• The circumscribing block’s dimensions are given by multiplying the yacht’s length x beam x draft.

Question 23

Explain Centre of Gravity.

Answer 23

Centre of Gravity G is the point through which the gravity force is said to act, its position depends on the distribution of weights within the vessel.

Question 24

Explain Centre of Buoyancy.

Answer 24

Centre of Buoyancy B is located at the centroid of the underwater volume, it is the point through which the buoyancy force is said to act.

Question 25

What is the righting lever?

Answer 25

• The Righting Lever is the horizontal distance between the line of force of gravity and the line of force of buoyancy
• Measured at G, it is given the designation GZ.

Question 26

What is the righting moment?

Answer 26

This is, in effect, two forces; one force acting vertically downward through the vessel’s centre of gravity (G) and the other acting vertically upward through the centroid of the new underwater volume B1. Its effect is to bring G and B into line, effectively righting the vessel.

Question 27

Explain effect on GZ when G is lowered.

Answer 27

• The righting moment grows larger, increasing the rate of return to the upright (stiff ship).
• A lower VCG also produces a more stable ship.

Question 28

Explain effect on GZ and the ship when G is raised.

Answer 28

Ship becomes less stable, the righting lever and hence the righting moment becomes smaller, this leads to slower return to the upright.

Question 29

Define the Metacentre.

Answer 29

If the vessel is heeled to a small angle and a vertical line is projected up from the centre of buoyancy (B1), where this projected line cuts the centre line of the vessel is the called the Metacentre, sometimes called the Initial Metacentre.

Question 30

Define Metacentric height.

Answer 30

This is the height of the Metacentre above the ship’s centre of gravity (G), measured along the ship’s centre line. Often referred to as GM.

Question 31

When lifting a weight by crane or derrick, where does the centre of gravity of the mass shift to, and what shift occurs in the vessels centre of gravity?

Answer 31

The centre of gravity of the weight shift moves to the head of the crane or derrick, and the vessels centre of gravity moves vertically upwards, parallel to the effective movement of the weight.

Question 32

What is angle of heel?

Answer 32

This is usually a temporary condition caused usually by wind or wave action forcing the boat to incline for a period. Caused by forces external to the vessel.

Question 33

What is angle of list?

Answer 33

• This is caused by movement of masses within the vessel which cause an inclining moment producing a list.
• An asymmetric distribution of mass about the centreline.

Question 34

How can a list be recognised?

Answer 34

A list can be recognised by the fact that the vessel is permanently inclined over to port or starboard, and rolls about the listed angle.

Question 35

How can a list be corrected?

Answer 35

A list may be corrected by moving masses from the low (listed) side to the high side, adding weights to the high side, or removing weights from the low side

Question 36

What is the ultimate objective of an incline experiment?

Answer 36

To ascertain the lightship KG, the lightship displacement is also determined.

Question 37

Formula for finding KG (stability).

Answer 37

KG = Total Vertical Moments(tm)/Total Mass(t)

Question 38

Formula for finding GM (stability).

Answer 38

GM = KM - KG

Question 39

Formula for finding GGh (stability)

Answer 39

GGh = Total Horizontal Moments(tm)/Total Mass(t)

Question 40

What is the formula for finding Under Water Volume (stability).

Answer 40

U/W Volume = Length x Breadth x Draft x Coefficient(Cb)

Question 41

Free surface effect is proportional to the …

Answer 41

… breadth of the tank cubed.

Question 42

If it is not possible to limit the breadth of tanks, what measures can be installed to limit free surface effect?

Answer 42

Longitudinal sub divisions called swash plate baffles are sometimes fitted in tanks.

Question 43

What are two features incorporated into yacht design that addresses the accumulation of water on upper decks to reduce free surface effect?

Answer 43

Freeing ports together with deck camber, and, deck sheer where the deck slopes from fwd and aft to the centre of the vessel.

Question 44

What is LCF and the axis it relates to?

Answer 44

Longitudinal centre of flotation. Located at the centroid of the waterplane area, it is the trimming axis of the vessel.

Question 45

Define trim

Answer 45

The difference between the draft at the bow and the draft at the stern.

Question 46

Define change of trim.

Answer 46

Change of trim is any resultant change in the difference between the draft forward and the draft aft.

Question 47

What is MCTC?

Answer 47

Moment to change trim by one centimetre. The mass to move forward or aft by 1m which will cause a change in trim of 1 centimetre.

Question 48

Define LCB.

Answer 48

Longitudinal centre of buoyancy. The longitudinal position of the point through which the buoyancy force is said to act vertically upwards, located at the centroid of the underwater volume.

Question 49

Define Loll.

Answer 49

This occurs when the centre of gravity G is slightly above the metacentre M causing a small negative metacentric height (GM). When disturbed from the upright this results in a small capsizing lever (and moment). The vessel will incline further to an angle called the ‘angle of loll’, where the righting lever is zero.

Question 50

What actions should be taken when experiencing a condition of Loll?

Answer 50

Priority is to increase the GM; fill tanks low down on the low side first. There will be a rise in VCG as there will be free surface effect. Once tank pressed, the FSE is removed, the GM should be positive - there will be a list to address.

Question 51

Why would there be a risk of capsize if the high side is ballasted first when a vessel experiences Loll?

Answer 51

The added weight to the high side may pull the vessel back to the upright but once through it may lurch to the other side, accentuated by FSE, and the listing moment, resulting in capsize.

Question 52

Statical stability: define conditions for a Stiff vessel, referring to GM and the righting moment.

Answer 52

A stiff vessel has a large GM, large righting moment at all angles of heel, resulting in fast and snappy return to upright position. Uncomfortable for crew, and can result in transverse stresses.

Question 53

Statical stability: define conditions for a Tender vessel.

Answer 53

GM is small and correspondingly the righting moments. Boat returns to upright position slowly. Vessel may hang at angle of heel and the danger is it may be pushed over further by a gust of wind or wave. Little strain on fabric of the hull, and motion is more comfortable for crew.

Question 54

The REG Yacht Code intact stability criteria are:

Answer 54

Initial GM, corrected for free surface effect at least 15cm.
GZ at least 20cm at angle of not less than 30 degrees.
The maximum GZ should occur at angle of heel not less than 25 degrees.

Question 55

What is dynamical stability?

Answer 55

This is defined as the area under the statical stability curve and represents the work that has to be done in order to heel the vessel from the upright position to an angle of heel. The units of dynamical stability are metre radians (m rad).

Question 56

How is the increasing of the freeboard represented in a GZ curve?

Answer 56

Increasing the freeboard of a vessel increases the angle of deck edge immersion. Consequently the range of positive stability is increased as the centre of buoyancy moves further to the low side for given angles of heel

Question 57

As fuel and stores are consumed on a voyage, what will occur concerning the GM and centre of gravity?

Answer 57

The vessels centre of gravity will rise and there will be a reduction in GM. Tanks pressed on departure will not be slack and cause a further rise in G due to free surface effect.

Question 58

What information is contained within a docking plan?

Answer 58

Side elevation, bottom plan, parts of the vessel designed to take the framework of a cradle, where side blocks and shores can be positioned, position of tank plugs, transducers, stabilisers, suctions, discharges.

Question 59

Explain how the vessel should be trimmed when dry docking, and how the vessel will settle on the blocks under the keel.

Answer 59

The vessel should be trimmed by the stern. As the water is pumped out of the dock, the stern should land first followed by the stem.

Question 60

As the water is pumped from the drydock, where is the buoyance lost and what force replaces it, and what movement in G occurs?

Answer 60

Buoyance is lost around the waterline, and replaced by a vertical force (P) at the keel blocks. The up thrust is equivalent to removing mass at the keel so G will move away from the keel upwards, reducing GM.

Question 61

What measures should be taken prior to dry docking in regard to weight distribution?

Answer 61

Ensure that the tanks and cargo are distributed as such that there is sufficient GM to give a very stable vessel and that the vessel is upright. Press up or empty all tanks where possible.

Question 62

What measures should be taken in preparing and during the filling of the dock, post dry docking?

Answer 62

Restore the vessel to the same condition that she was in on arrival at the dry dock. Ask the docking company to stop filling the dock immediately if anything untoward is suspected, so that it can be investigated

Question 63

What is the ‘critical period’ and ‘critical instant’ when dry docking a vessel?

Answer 63

The “Critical Period” is the time between the stern first touching and the stem landing, the “Critical Instant” is the moment immediately before the stem touches. The significance is that when the stern touches the keel blocks the stability of the vessel starts to reduce, and continues to reduce as the water level drops further. As it is not possible to fit the side-shores until the vessel has taken the blocks along her entire length (and therefore stopped moving) this period of reduced stability is potentially hazardous. The maximum virtual reduction in metacentric height without side support occurs at the “Critical Instant”, which makes this the most hazardous part of the operation.