Definitions & Q&A's

Question 1

Law of flotation:

Answer 1

Law of flotation: “A body wholly or partially immersed in a fluid will experience an upward force equal to the weight of the fluid displaced”.

Question 2

Archimedes’ principle:

Answer 2

Archimedes’ principle:
1.“The upward buoyant force that is exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid”.

2.When a body is fully submersed in fluid it will displace the same mass as the mass of the object.

Question 3

Density:

Answer 3

Density is the measure of a substances mass per unit volume. Typically, density is
measured in either |t/m3| or |Kg/m3|. The common symbol used for density is the Greek
letter ‘p’, pronounced rho.

Question 4

Relative Density:

Answer 4

Relative Density (RD) or Specific Gravity (SG), is the ratio of the density of a substance
to the density of a given baseline substance. Typically, the baseline substance is surface
freshwater at 4°C. You may consider RD as a scaling factor.

Question 5

Marine Hydrometer

Answer 5

A Marine Hydrometer is an instrument used to measure
the density of a liquid. It consists of a weighted bulb and
a long stem. The stem is marked with a scale for
measuring liquid density.
The density of a liquid is measured by placing the
instrument in the liquid and noting how much of the
instrument’s stem is immersed. The denser the liquid
the higher the hydrometer will float in the liquid.
Different hydrometers are used for different ranges of
liquid density. Each hydrometer should have an
accompanying calibration certificate.

Question 6

Lightship Condition

Answer 6

Lightship condition is a ship complete in all respects, but without consumables, stores,
cargo, crew, and effects, and without any liquids on board except that machinery and
piping fluids, such as lubricants and hydraulics, are at operating levels.

Question 7

Light displacement

Answer 7

Light displacement is ship mass when in the lightship condition.

Question 8

Load displacement

Answer 8

Load displacement is ship mass when in a loaded condition. -

Question 9

Deadweight

Answer 9

Deadweight is the difference between light displacement and load displacement. It is the
total mass of consumables, stores, cargo, crew, and effects loaded on the ship.

🔺️ Load = 🔺️ Light + Deadweight.

Or…

Deadweight = 🔺️ Load - 🔺️ Light

Question 10

Buoyancy

Answer 10

Buoyancy is the upward force a floating ship experiences from the water it displaces. The
Buoyancy force is equal to the weight of the displaced water. The weight of displaced
water is directly proportional to the mass of displaced water, which is directly
proportional to the volume of displaced water; hence ‘buoyancy’ may be measured in
‘Newtons’ or expressed in |t| or |m3|. The correct units are ‘Newtons’, however the
commonly used units are |t| and |m3|.

The cente of buoyance:
the centre oid of the underwater volume of the vessel

Question 11

Reserve Buoyance

Answer 11

Reserve Buoyancy is how much further water a ship could possibly displace, and it is
measured commonly in |m3|. Reserve buoyancy is the ship’s volume between its
waterline and its freeboard deck. The freeboard deck is normally the uppermost complete
deck exposed to the weather and sea. Openings on the freeboard deck (usually the main
deck on a yacht) must have permanent means of closing (e.g., hatches, ventilation
openings) and below this deck all openings in the side of the ship are fitted with
permanent means of watertight closing (e.g., deadlights fitted to side scuttles).

Question 12

Water Plane Area (Aw)

Answer 12

TPC is a function of waterplane area (Aw) and density of displaced water (p). Waterplane
area is the cross-sectional area of the ship in way of the waterline, i.e., the two dimensional
area enclosed by the waterline of the vessel. The formula for TPC is provided
below.

TPC =
Aw/100 × ρ

Question 13

TPC

Answer 13

TPC is an acronym for ‘Tonnes Per Centimetre’. TPC is a measure of a ship’s sensitivity to
loading and discharging of deadweight. To change the mean draft by 1cm

Question 14

Displacement

Answer 14

Displacement is the term used for ship mass. The mass of a floating ship is equal to the
mass of water displaced by the ship, hence the term ‘displacement’.

Question 15

Freeboard

Answer 15

Freeboard is the distance from the waterline to the top of the freeboard deck at the
ship’s side (inc. the projected hight of deck coverings such as teak).

Question 16

Camber

Answer 16

Camber is the difference in height (crown) between the freeboard deck on the centreline
and the freeboard deck at the ship’s side.

Question 17

Sheer

Answer 17

Sheer is the difference in freeboard deck height fore and aft relative to the freeboard
deck hight at midships. .

Question 18

Flare

Answer 18

Flare is the outward (convex) curvature of the ship’s forward shell plate above the
waterline.

Question 19

Draft

Answer 19

Draught (or draft) is the distance between the top of the keel plate and the waterline.

Question 20

Depth

Answer 20

Depth
The vertical distance between the baseline and the
freeboard deck at amidships.

Question 21

Beam

Answer 21

Beam is ship width at its widest point. Moulded beam is ship width measured from the
inside of the shell plate.

Question 22

Panting

Answer 22

Panting refers to the cyclic shell plate deflection typically in fore region of a ship, caused
by cyclic variation in pressure on the shell plate. Pressure on the shell plate in the fore
region of a ship varies as the ship passes through waves. The cyclic nature of loading can
cause the shell plate to fail in fatigue. Panting beams may be used to brace frames and
therefore increase the fatigue life of bow structure. Other mean of increase the fatigue
life of bow structure included extra frames (Panting frames), reduced frame spacing, and
thicker shell plate.

Question 23

Pounding

Answer 23

Pounding refers to the load experienced in the fore region (and occasionally aft regions)
of a ship when the hull ‘slams’ into the water. The fore region (and occasionally aft
region) of the hull on small/medium ships may briefly be emersed from the water when
traversing a large wave/ swell, and then slam back into the water moments later. The
forward approximate 25% of a vessel hull is often strengthened to withstand pounding
loads.

Question 24

Vibration

Answer 24

Vibration typically stems from engines, gear boxes, propellers, turbines, generators,
pumps, electric motors, washing machines etc. Vibration causes discomfort to passengers
and crew, and occasionally structural/component damage due to cyclic loading and
subsequent fatigue. Mounting engines etc on shock mounts typically dampens loads, both
in frequency and size. The is also often scope to change the frequency (cycles per second)
of vibration, in the case of an engine, by altering the throttle (engine speed). There are
three main forms of vibration.
 Synchronous, where items of structure resonate with noise from machinery
 Local, where items of structure vibrate with noise from machinery
 Wave induced, normally because of pounding

Question 25

Tonnage

Answer 25

Tonnage is a measure of a vessel’s carrying capacity, tonnage is linked to internal volume.
‘Tun’ is an old English word for a cask of alcohol. Traditionally, a vessel’s carrying
capacity was measured in Tuns, where 1 Tun equals 2.78 m3
. Sailing cargo vessels used
almost all their internal volume for carrying cargo, hence the term ‘Gross’ (total)
Tonnage.

Question 26

Describe the loads and stresses in the mast and rigging of a sailing vessel

Answer 26

The stress in a sailing yachts hull will vary with time when at sea under the power of
sail(s) (under sail). When under sail, a sailing yacht will experience varied aerodynamic
forces on the sail(s) and in turn varied reactionary forces in the rigging and mast.
Whereas, when a sailing vessel is alongside and sails are furled, the loading on the rigging
and mast will be static. Load on sails, rigging and mast cause a sailing yacht to sag.

Question 27

Describe the effect of structural discontinuities (stress raisers)

Answer 27

The stress can be increased by stress concentration points (also known as stress raisers
or structural discontinuities). If a piece of structure has any form of hole in it (or a
change in shape) then the stress around the hole will be greater than the stress in the
structure away from the hole. The actual increase in stress depends on how round the
opening in the structure is. Note that on all ships, you very rarely see sharp angles,
corners, or square holes in the structure.

Question 28

Impressed Current System.

Answer 28

Impressed Current System. - This system requires that an adequate current be set up in
the opposite direction to the natural current. This can be thought of as “cancelling” the
current between the anode and cathode. Non-consumable anodes are fitted along the
underwater surface of the ship and an electric current passed from these to the hull
(which now becomes the cathode). An optimum current is essential and so the efficiency
of the system depends on careful regulation of the current and maintenance of the hull
potential.

Question 29

Freeboard Deck

Answer 29

The Freeboard Deck is normally the uppermost complete deck exposed to the weather
and sea, which has permanent means of closing all openings in the weather part thereof,
and below which all openings in the sides of the ship are fitted with permanent means of
watertight closing.

Question 30

Watertight

Answer 30

Watertight means “capable of preventing the passage of water in any direction”. A
watertight bulkhead can withstand pressure from flood water from either side. A
watertight valve at an overboard discharge point can prevent fluid escaping a ship or
entering a ship involuntarily.

Question 31

Weathertight

Answer 31

Weathertight means “water will not penetrate and enter the hull in the worst sea and weather conditions”.

Question 32

Freeboard

Answer 32

Freeboard is the distance between the waterline and the Freeboard Deck. There is a
direct link between a vessel’s freeboard and Safety regarding personnel, Stability,
Strength of hull structure, and prevention of ingress of water.
The minimum freeboard for a vessel is determined through the process of “assignment of
freeboard”. The process involves considering ship Type, Tabular Freeboard, Block
Coefficient, Length to Depth ratio, sheer profile, and extents of Superstructure (fore & aft
and transversely.

Question 33

Assigned Freeboard

Answer 33

Question 34

State the purpose of the Load Line(s)

Answer 34

The loadlines rules, and the load line marks ensure a ship is adequate regarding,
- Reserve of buoyancy under normal service conditions
- Hull girder strength
- Protection of the crew
- Subdivision and stability (intact and damage)
- Watertight enclosure of all exposed parts of the ship
- Limited deck wetness

Question 35

Define freshwater allowance (FWA)

Answer 35

A rise in the density of displaced water will cause a reduction in ship draught, and a fall in
density will cause an increase in ship draught. The change in draught a ship experiences
when moving between fresh water and sea water is known as Fresh Water Allowance
(FWA). FWA can also be considered the maximum allowable immersion of the seasonal
loadline when alongside in fresh water.

Question 36

State the importance of reserve buoyancy and the necessity for maintaining its
integrity

Answer 36

A minimum freeboard is necessary to reduce the amount of shipped water on deck whilst
at sea, and therefore reduce the risks to crew, passengers, ship, and the environment
whilst on passage. Ship draught will increase, and freeboard will reduce, if a ship’s
watertight integrity is breached below the waterline and the ship is then flooded (bilged).
It is therefore critical to maintain ship watertight integrity and ship watertight
subdivision. It is critical to maintain watertight subdivision to limit the longitudinal,
transverse, and the horizontal extent of any flooding following either a hull breach or a
system failure below the waterline.

Question 37

State the items which affect the watertight integrity of the ship

Answer 37

Hatches, doors, side scuttles, skylights, windows, ventilators, exhausts, and air pipes must
all be properly maintained to prevent ingress of water, thereby preserving the vessel’s
reserve buoyancy.

Question 38

Describe the routine maintenance of items which affect the watertight integrity of
the ship

Answer 38

Doors, hatches, vent covers and coamings - Hinges and closing mechanisms should all be
greased and free moving. Chalk, hose, or ultrasonic tests should be undertaken
periodically to ensure that full contact is made between rubber gaskets and sealing
surfaces. Coamings are required to be of a minimum height to prevent water on deck from
entering when hatches and doors are opened.

Tank vents – Closing arrangements should be periodically checked to ensure they are freemoving
and able to prevent ingress of water in a seaway as well as release pressure during
loading and discharge operations.

Scuppers and freeing ports – Water freeing arrangements such as scuppers and freeing
ports must be kept clear to enable shipped water to run clear of the vessel quickly.

Question 39

State the importance of non-return valves on tank vent and sounding pipes fitted
below the freeboard deck

Answer 39

Tank sounding pipes, especially those fitted low down on a vessel, will have both a
screwed watertight cap and a valve to prevent flood water ingress into the tank. Tank
sounding pipes high up on the vessel may only have a screwed watertight cap. Regardless
of location, fuel and freshwater sounding pipes typically have a lock on the cap to reduce
the likelihood of intentional or accidental sabotage.

Tank vent pipes are very often only fitted with a goose neck to prevent ingress of shipped
water and rainwater into the tank. Vent pipes for fuel tanks are also fitted by a flame
arrestor gauze.

Question 40

Describe the importance of water freeing arrangements to the watertight integrity
of the vessel

Answer 40

Water trapped on deck causes a rise in G and reduction in GM. It is therefore very
important that water is efficiently and automatically shed from upper decks by adequate
scuppers, freeing ports, deck camber, and deck sheer. Scuppers and freeing ports need to
be maintained in good working order.

Question 41

Outline the process of chemical corrosion (e.g., acids and alkalis)

Answer 41

Chemical corrosion occurs when there is a chemical reaction between metal and an acid
or alkali, including certain paints, battery acid, cleaning materials (e.g., Caustic soda) and
paint & varnish remover (e.g., Nitromors).

Question 42

Describe the process of oxidisation of a steel plate

Answer 42

Oxidisation (also known as Dry corrosion) occurs when there is a chemical reaction
between a metal and a gas, such as steel and oxygen in the air with moisture.
Steel and oxygen combine to make iron oxide ‘Rust’. Rust is flaky, friable, and porous.
Aluminium and oxygen combine to make aluminium oxide. Aluminium oxide is hard,
abrasion-resistant, and chemically inert.
Dry corrosion is prevented by sealing the metal against oxygen – painting! Correct
protection depends on correctly applying the paint.

Question 43

Describe the process of galvanic corrosion between dissimilar metals by explaining
the electro-chemical cell

Answer 43

Galvanic Corrosion (also known as wet corrosion) occurs when two different materials are
electrically connected. If there is a potential difference between the two electrically
bonded metals, then ions (charged atoms or molecules) will flow from the anode metal
(akin to negative terminal on a battery) to the cathode metal (akin to the positive
terminal on a battery) – constituting a galvanic cell. One of the metals (the most anodic)
will waste i.e., corrode by discharging positively charged ions into the solution
(electrolyte) and will leave the metal with a negative charge or potential.

Question 44

Centre of gravity

Answer 44

The Centre of Gravity ‘G’ of a body may be defined as the point at which its total weight
(i.e., gravitational force) is considered to act. The total weight acts at the centre of mass,
the centroid of a body’s mass will depend upon the distribution of mass within the body.

Whats the centre of gravity:
The centreoid of mass on board moving vertically downwards

Question 45

What is metacentric hight

Answer 45

The distance measured from the centre of gravity and the metacentre as measured from the centre line (also know as GM, an indication of a vessels stability)

Question 46

What is the Metacentre

Answer 46

The point of intersection of the centre line when the buoyance force acts through vertically through the centre line

Question 47

What is the water plane area of the vessel and how would you calculate it?

Answer 47

2D plain, Length x Breath = Aw
On the water line.

Question 48

Reserve buoyance

Answer 48

The area of voume of intact area above the waterline to the freeboard deck inside expressed as a percentage of the total volume of the vessel

Question 49

Water tight deck

Answer 49

The water tight deck is the upper most water tight deck below which all fixes and fittings have to be watertight

Question 50

Summer deadweight

Answer 50

Summer ship displacement - light ship displacement

Question 51

Summer displacement

Answer 51

Displacement of the vessel when fully loaded up to her summer load line

Question 52

Gross Tonnage

Answer 52

Internal volume less certain exemptions calculated at a
rate of 2.83 m3 per tonne.

Question 53

Scantlings

Answer 53

The size/strength of structural elements, dimensions of
frames, girders, stiffeners, stringers, and plates.

Question 54

Assigned Freeboard

Answer 54

The vertical distance between the freeboard deck and
the SLL (Summer Load Line) at amidships

Question 55

Assigned Freeboard

Answer 55

The vertical distance between the freeboard deck and
the SLL (Summer Load Line) at amidships

Question 56

Measurement Tonnage and
Displacement

Answer 56

Gross Tonnage is a volume measurement not the mass
of the ship it does not indicate the vessel’s size. Harbour
authorities use Gross or Net tonnage to calculate the
port and light dues, registration costs, etc. The Mass of
the ship is its Displacement. This varies depending on
draught.

Question 57

Freeboard Deck☆

Answer 57

Freeboard Deck
The uppermost continuous watertight deck having permanent means for closing all
openings to maintain watertight integrity. Below which all openings in the vessel’s side
have permanent means for watertight closure.

Question 58

Superstructure ☆

Answer 58

Superstructure
The parts of a ship, other than masts and rigging, built above its hull and main deck.

Question 59

Superstructure deck ☆

Answer 59

Superstructure deck
A deck forming the upper boundary of a superstructure.

Question 60

Reserve Buoyancy ☆

Answer 60

Reserve Buoyancy
Volume of enclosed watertight spaces between the Load Line and the freeboard deck. The
Reserve of Buoyancy must prevent the deck line submerging when the largest watertight
compartment is bilged.