The Ship Flashcards
Describe how NT and GT became the adopted measurement of ship size
- When engines were added to ships, the old ‘tuns’ became adapted to NRT and GRT (net/gross registered tonnage) – this measures the enclosed area within a ship, with 1 ton = 100 cubic feet; gross includes the entire enclosed space whereas net excludes space taken up by the engine/other machinery
- A new system of measurement was introduced in 1994 as some countries used different methodology for the above; a ton is now equal to 95-105 cubic feet depending on size/type of vessel. To distinguish this from the old units, ‘registered’ was dropped from the names, leaving them as GT and NT
- The old GRT/NRT are still used in some places to describe minimum staffing requirements
- These measurements (although confusing) are considered to give a good indication of a boat’s revenue earning capacity
Name and describe different ways of measuring ships
- Displacement tonnage
- The weight of a body is equal to the weight of water it displaces
- Loaded displacement tonnage is not used outside of warships
- **Light displacement **
- The actual weight of an empty ship
- Typically used in ship sales for scrap as it measures the qty of steel/other metals
- Deadweight (dwt)
- Difference in tonnes between the light and loaded displacement
- Commercially important as it measures the total qty of cargo/supplies a boat can carry (described as DWAT, deadweight all told)
- DWCC is also important – deadweight cargo capacity – this measures the cargo a boat can carry when its stores are full (however operators can increase DWCC by carrying less fuel)
What is a loadline?
- Max depth to which a ship can be loaded – aka its max draught
- Max draught differs depending on location, time of year etc – but is generally quoted on summer marks
What convention standardised loadlines?
The International Convention on Load Lines (ICLL) was created in 1966 and came into force in 1968
What does a loadline look like?
It’s is shown amidship on both sides of a vessel via a painted disc, 12” in diameter with an 18” line drawn horizontally through it.
* Ships can have up to 6 additional marks alongside its loadline mark, identified by painting symbols alongside. From lowest to highest, these are; TF = tropical zone, fresh water; F = fresh water; T = tropical saltwater, S = summer (non-tropical), W = winter (not tropical), WNA = winter north Atlantic
* Ships that carry lumber may be allowed to load lower as their cargo is inherently buoyant; this is shown by adding L for lumber to the above descriptors
Describe freeboard and its importance in relation to load lines
- Loadlines are often associated with draught, however from a safety perspective it focuses on ensuring sufficient freeboard. Freeboard is the height from the waterline to the main deck. Sufficient freeboard = sufficient reserve buoyancy (the volume of ship above waterline). The level of the lowest deck still above waterline is also painted onto the ship, just above the loadline itself
- Different parts of the world/times of year vary in risk, meaning minimum safe freeboard also varies. Tropical is the least dangerous and winter north Atlantic is most dangerous
Saltwater is more buoyant than freshwater so ships can be loaded deeper in fresh water as once they reach saltwater their level will rise
Define and describe draught
- The distance between the bottom of a ship (keel) and the waterline
- The greater the weight of laden cargo, the deeper the draught; this is creates a dwt scale which is painted on a ship’s bridge or on a bulkhead in the accommodation – typically expressed as tonnes per centimetre or tons per inch (US vsls). E.g. adding 1dwt tonne = an increase in draught of 1.2cm
- For bulk carriers this can be used to measure the weight of cargo loaded, by taking the difference between its unladen draught and the draught once laden; this is known as a draught survey and can be carried out by the crew and terminal personnel or independent surveyors
How are loadlines determined and what are the commercial implications of this?
The level of the loadline is determined as a vessel is constructed and is overseen by the vessel’s classification society, who issue the loadline certificate of a ship and supervise its placement. The classification society’s initials are included on the line
The level of the line is calculated via a formula however owners can opt for a lesser draught – this can be beneficial if they expect to be carrying light loads as this reduces a ship’s GT/NT, reducing size related port charges
Define bale and grain cubic measurements
Each ship has 2 different cubic capacities; grain cubic (for free flowing cargo that can fill all corners of the hold, surround any supports etc) and bale cubic (this measures around any beams etc for non-free flowing cargo)
Why are a ship’s interior dimensions important and how has this developed?
Some cargoes are less dense than others making the hold’s interior dimensions important (eg a tonne of steel takes up less space than a tonne of timber) – to calculate how much a vessel can hold you’ll need to know both the cubic capacity of the ship and the cargo’s stowage factor (cubic meters per tonne).
In older boats, the bale capacity is about 10% lower than the grain, however in modern ships these two are much closer due to the addition of an interior skin over side beams
The cubic capacity of a ship will depend on its trade, eg an iron ore vessel won’t need as much hold space as a grain vessel to carry the same tonnage
Define deadweight and measurement cargoes
Cargoes with a stowage factor of less than 40cu ft/ton are referred to as deadweight cargoes (the weight is more of a limiting factor than volume) and cargoes with a stowage factor of over 40 cu ft/ton are known as measurement cargoes (because the interior measurements limit the qty of cargo carried rather than the weight of the cargo)
Describe the relationship between cubic capacity and dwt
Regardless of size, older ships could carry less weight per cubic foot – an old ship might have the same cubic capacity as a modern ship, but the modern ship can carry a heavier dwt. This is shown as a ratio, with older ships having 40cu ft/ton dwt, and modern ships being around 50 cu ft/ton.
Understanding stowage factor is essential to commercially maximise a vessel; e.g. British deepmined coal is generally much more dense than American shallow-mined coal. British vessels loading American coal assumed it would be a deadwight cargo, however found the hold full before a vessel’s deadweight capacity was reached
