1. Units and Measures used in Stability

Question 1

Length on Load Waterline (LWL):

Answer 1

This is the distance along the hull at the level of the water from the FWD Perpendicular to the very aft when floating in the fully loaded condition.

Question 2

Length Overall (LOA):

Answer 2

Is the extreme length of the vessel. Critical for choosing a berth but not used in stability.

Question 3

Forward Perpendicular (FP):

Answer 3

The vertical line (perpendicular to the waterline) at the point where the forward edge of the stem touches the water when the vessel is floating at the Summer Load Water line.

Question 4

After Perpendicular (AP):

Answer 4

The vertical line or line perpendicular to the waterline on the after side of the rudder post or the centre line of the rudder stock(if no rudder post fitted) where it touches the water line when the vessel is floating at her Summer Load Draft.

Question 5

Length Between Perpendiculars (LBP):

Answer 5

The distance between the Forward (FP) and Aft Perpendiculars (AP)

Question 6

Amidships:

Answer 6

The point midway between the forward and aft perpendiculars. The centre of the load line circle should indicate this position.

Question 7

Moulded dimensions:

Answer 7

the inside measurement, taken from the internal side
of the plates

Question 8

Beam or Breadth (B):

Answer 8

Measurement across ship at midships from the inside of the plating on each side.

Question 9

Extreme Breadth (EB):

Answer 9

the maximum beam taken over all extremities.

Question 10

Moulded Base Line:

Answer 10

line which passes through the upper edge of the keel
plate.

Question 11

Define Depth (D):

Answer 11

Distance from the bottom of the keel to Freeboard Deck at the ship’s side.

Question 12

Define Draught or Draft (d):

Answer 12

Distance from the bottom of the keel to the waterline.
Do not confuse D (Depth) and d (draft)!

Question 13

Define Draught Datum Line:

Answer 13

The line which passes through the lowest point of the hull and is extended to the forward and after perpendiculars. This is the line from which the draught marks are measured.

Question 14

Define Freeboard (FBD):

Answer 14

Vertical distance from the top edge of the Deck Line (Freeboard Deck) measured amidships to the waterline.

Or measured vertically from Water line to the Freeboard Deck or Weather deck.

Question 15

Define Freeboard Deck:

Answer 15

Freeboard deck/ Weather deck and Main deck are all the same!

Upper most continuous deck to which all WT(watertight) bulkheads are carried., running from bow to stern.

Whole ship must be watertight below the freeboard deck.

Question 16

Define Assigned Freeboard:

Answer 16

The Distance measured vertically in mm, from the upper edge of the deck line to the upper edge of the related load line.

Question 17

Difference between Watertight and Weathertight

Answer 17

Watertight = A door, hatch or other means of closure is said to be watertight meaning that it will prevent water passing in either direction.

Weathertight = This is a similar means of closure and will prevent water entering the hull of the ship in the worst sea and weather conditions likely to be encountered by the ship in service. Water can pass in one direction only.

Question 18

Reserve Buoyancy:

Answer 18

The measured, enclosed watertight volume between the waterline and the freeboard deck.

It can also be expressed as a percentage of the total volume of the vessel.

Why is it important and considered in the Load
Line Certificate calculations?
In a seaway the vessel will be hogged and sagged
with the passing of the waves. When sagged the vessel is displacing the same amount but with less buoyancy acting upon it. This can lead to the ship being
overwhelmed. The Reserve Buoyancy keeps the vessel safe from sinking.

Question 19

Define Watertight and Weather tight:

Answer 19

Watertight is A door, hatch or other means of closure is said to be watertight meaning that it will prevent water passing in either direction.

Weathertight: This is a similar means of closure and will prevent water entering the hull of the ship in the worst sea and weather conditions likely to be encountered by the ship in service. Water can pass in one direction only.

Question 20

Define Centreline:

Answer 20

Do not mix up between CL and Amidships.

CL is a line indicating the longitudinal plane cross-section along the length of the vessel through a point at half the beam.

Question 21

Define the Sheer of a vessel:

Answer 21

The upward curvature of the vessel towards the bow and stern. This helps maintain freeboard when the vessel is pitching.

Question 22

Define Gross Tonnage GT and Net Tonnage NT:

Answer 22

Gt is the Total Enclosed volume of the vessel.

NT is the GT - non revenue size (non money making volume of the ship)

Question 23

Define Light Displacement, Load Displacement, Deadweight Tonnage and Displacement:

Answer 23

Light Displacement is the total weight of an empty ship at summer load line.

Load displacement is the weight of ship and everything on it at summer load line.

Deadweight Tonnage is the max weight of everything on ship when floating at summer load line, excluding the weight of the ship. DWT = Load - light.

Displacement is the weight of the ship and everything on it at any draught.

Question 24

Outline the main diference in requirements for Damage Stability required by a ship under the SOLAS Convention and the REG-YC Convention;

Answer 24

SOLAS / Load line convention

• Must Remain Afloat
• 2 or more watertight compartments flooded allowed.

Red Ensign Group - YC

• Only 1 watertight compartment allowed to flood.

Question 25

Draw an All Seasons Load Line set up.

Answer 25

Plimsol Mark with Fresh water mark and deck line.

Question 26

Draw a Standard Load line set up

Answer 26

Standard load line includes Plimsol Mark with the following marks;

Tropical Fresh

Fresh

Tropical

Summer

Winter

Winter North Atlantic