Basic Stability Guide 1…Terms and...

Basic Stability – Guide 1…Terms and abbreviations

Welcome Stability Terms Density Flotation Centre of Gravity

Welcome to Ship Stability at Master 4 level

This study guide is set up as follows:

Guide 1 (this guide)

• Terms and Abbreviations

• Density and Specific Gravity

• Laws of Flotation

• Centre of Gravity and Centre of Buoyancy

Guide 2 ( the second guide in this series)

This topic will cover Stability Calculations

Guide 3 (the third and final guide in this series)

This topic will cover stability calculations using MV Twosuch , an excerpt from a ship’s stability booklet used for examination purposes only

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by Lance Cunningham

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TDMMA1207B This unit involves the skills and knowledge required to manage the stress and dynamic factors affecting the stability of a small commercial vessel using basic stability criteria. By the end of the course you will be able to

• interpret basic stability criteria

• carry out required stability calculations

• correlate and interpret calculated stability data

• manage stability and stress conditions within safety parameters

• communicate the stability information to others as required

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Stability Terms

Your chosen career requires you to converse with many professionals in the area of Ship Stability and as such you will be expected to “speak the language”.

Terms and abbreviations are commonplace within many industries including the Maritime industry.

What follows is a glossary of common terms and abbreviations that you will become familiar with during this course and will provide you with a “common language “ used within the Maritime industry, specifically in the field of Ship Stability.

Click the “forward button” to see more glossary entries

AP After Perpendicular:The vertical line of reference that coincides with the after end of the stern post or if no stern post the turning axis of the rudder.

Stability Terms … A to B

BG Longitudinal separation of the centres of buoyancy and gravity

Aft Draft Marks The set of draft marks, closest to the stern

Baseline A horizontal line drawn the length of the ship at the top of the keel

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Stability Terms … B to D Stability Terms … D

BMo Height of the initial transverse metacentre above the centre of buoyancy B

Breadth Moulded A transverse measurement taken amidships at the point of maximum internal width

Centre of Gravity The point of a body through which the total weight of the body may be considered to act

Deadweight The weight of cargo plus fuel,stores,water ballast, fresh water, crew, passengers and baggage. The difference between the light and loaded displacements.

Depth Moulded A measurement taken from the baseline amidships to the heel of the upper deck beam at the ship's side.

Displacement The total weight of the vessel often abbreviated to 'W'

Draft Sometimes written as "Draught" The measurement of "how deep the vessel sits in the water" This is measured at specific points of the vessel...eg. the forward draft or after draft. continued…

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DWL Designed Waterline

Forward Draft Marks The set of draft marks closest to the stem of the vessel

Continued…Draft can be the mean draft, the forward draft and after drafts added together and divided by 2. Draft can be the draft measured at the longitudinal centre of floatation, known as the LCF Draft By measuring the LCF draft, we can obtain the Displacement of the vessel

Stability Terms … D to F

FP Forward Perpendicular: This is vertical line of reference that intersects the Summer Load waterline at the forward edge of stem when the vessel is on an even keel

Freeboard The vertical distance from the waterline to the freeboard deck taken at the ship's side

Freeboard Deck The uppermost complete deck open to the weather and sea which has a permanent means of closing all openings

Stability Terms … F

DWA Dock Water Allowance

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Free Surface Effect The movement of liquid within a ship's tanks when the tanks are partially filled. Has the effect of reducing the vessel's righting lever (GZ) causing a reduction in the stability of the vessel

Fresh Water Allowance The number of millimetres by which the mean draft will change when a vessel moves from fresh water to salt water.

Stability Terms … F

FSC Free Surface Correction. Calculated as the total free surface moment divided by the displacement. This value is added to the calculated height of the transverse centre of gravity KG solid (KG),to obtain the centre of gravity of the vessel corrected for free surface effect, KG fluid (KGf)

Stability Terms … F to G

GfMo Initial transverse metacentric height corrected for free surface known as the metacentric height

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GfZ The transverse righting lever being the shortest distance from G (corrected for free surface effect) to the line of action of B the centre of buoyancy

KG Height of the centre of gravity of the ship above the baseline , sometimes referred to as KG solid. No account taken for Free Surface Effect.

KGf Height of center of gravity, corrected for free surface above the base line. Sometimes referred to as KG fluid

KN The righting lever with the ship heeled to any angle when the centre of gravity of the vessel is assumed to be at the baseline

LBP Length between perpendiculars. The length between the forward and aft perpendiculars measured horizontally

LCB Longitudinal centre of buoyancy, the point where the force of Buoyancy will act upwards in the longitudinal plane of the vessel.

Stability Terms … G to K Stability Terms … K to L

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LCF Longitudinal centre of flotation, the centre of the waterplane area in the longitudinal plane, around which the vessel will trim .

Stability Terms … L

LCG Longitudinal centre of gravity, The point in the longitudinal plane , where the centre of gravity may be considered to act.

Lightship Displacement The weight of the vessel in it's constructed form. Hull, fittings and components only.

LOA Length Overall. The extreme length of the vessel

Stability Terms … L to M

Load Displacement The weight of the ship's hull and its contents, machinery, cargo, liquids, stores (everything on board) when floating at its summer marks

Load Line A marking on the hull indicating the maximum depth a vessel can load to under seasonal conditions. Allocated by a classification society or authorised government body.

MCTC Moment to change trim 1cm over the length between perpendiculars.

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Stability Terms … L to U

Mean Draft The mean draft is the arithmetical mean of the fore and aft drafts. That is the fore and aft drafts added together and divided by 2.

Mo Initial transverse metacentre

TPC Tonnes per centimetre immersion, the amount of weight in tonnes required to change the draft of the vessel by 1 cm

USK

Underside of the keel.

Stability Terms … V

VCB The height of the vertical centre of buoyancy above the baseline

VCG Vertical Centre of Gravity. Obtained by dividing the sum of the vertical moments by the displacement. Often referred to as the KG solid

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Density

Density is defined as Mass per unit Volume and uses units of ‘tonnes per cubic metre’

1. Fresh Water can be considered to have a density of 1 tonne per cubic metre

2. Salt Water can be considered to have a density of 1.025 tonnes per cubic metre

Relative Density

Relative density is a pure number …no units attached. This is the ratio of one substance compared to Fresh Water

Specific Gravity

Specific Gravity is just another term for Relative Density. It is usually abbreviated to SG

Example

Calculate the density and relative density of a block of timber. The timber measures: 3.0m x 0.5m x 0.8m and weighs 0.8 tonnes

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =𝑀𝑎𝑠𝑠

𝑉𝑜𝑙𝑢𝑚𝑒

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =

0.8 𝑡𝑜𝑛𝑛𝑒𝑠

3𝑚 × 0.5 × 0.8𝑚

𝑫𝒆𝒏𝒔𝒊𝒕𝒚 = 0.67 𝑡𝑜𝑛𝑛𝑒𝑠 𝑝𝑒𝑟 𝑚3

𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑆𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝐹𝑟𝑒𝑠ℎ 𝑊𝑎𝑡𝑒𝑟

𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =0.67 𝑡/𝑚3

1.0 𝑡/𝑚3

𝑹𝒆𝒍𝒂𝒕𝒊𝒗𝒆 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 0.67

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Flotation Archimedes determined a principle that a vessel will displace it’s own mass when immersed in a fluid. A solid block of steel weighing 6 tonnes and having a cubic capacity of 2m³ is put into fresh water,. What would you expect to occur? Case 1: We know that the block will displace 2 cubic meters of fresh water that will weigh 2,000kg. Think of this acting upwards. The weight of the steel block is 6,000kg acting downwards. The resultant of 6 tonnes acting downwards and 2 tonnes acting upwards is 4 tonnes acting downwards…the result is… the block will sink. Case 2: If the steel is hammered out so that it makes a box of 12 cubic metres, it will still weigh 6 tonnes but now we have an upward force of 12,000kg and a downward force of 6,000kg … the box will float.

Solid block sinks

Hammered out box floats

6t

6t

2m³

12m³

Case 1

Case 2

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Centres of Forces We have come across two forces in our study of Flotation: 1. The force of Gravity 2. The force of Buoyancy

These forces act at defined points within the vessel The force of gravity acts vertically downwards at the centre of gravity of the vessel The distance of the centre of gravity measured from the keel is defined as KG The force of buoyancy acts vertically upwards at the centre of the underwater volume The distance of the centre of buoyancy measured from the keel is defined as KB

KB

KG

K = Keel

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You have covered the following topics in this guide

• Terms and Abbreviations

• Density and Specific Gravity

• Laws of Flotation

• Centre of Gravity and Centre of Buoyancy

Guide 2 ( the second guide in this series) will cover

Conclusion

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• Loadline, Fresh Water Allowance

• Dock Water Allowance

• Draft, Mean Draft, Trim

• Displacement and Block Coefficient

• Hydrostatic Tables, TPC

• Movement of G in the transverse plane

• Movement of G in the longitudinal plane

• Free Surface and Loll

by Lance Cunningham

by Daver6

tayloj

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tayloj

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http://www.flickr.com/photos/lancecunningham/7011585367/

tayloj

Placed Image

tayloj

Placed Image

http://creativecommons.org/licenses/by-nc/2.0/

http://www.flickr.com/photos/daver6/4416342560/

Basic Stability Guide 1…Terms and...

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