Intact Stability GM Quick Check: Metacentric Height, Free Surface Correction and a Worked Example

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Every loading condition on a ship or workboat starts with one question: is the vessel stable enough to sail? The fastest first answer is the transverse metacentric height, GM. This post sets out the hydrostatic fundamentals behind it, shows how free surface in slack tanks erodes it, explains how the inclining experiment fixes the lightship KG, and works through a loading condition for a small cargo vessel. It ends with the first checks against the IMO 2008 Intact Stability Code. It is a screening method, not a substitute for a full stability booklet. It also sits alongside our wider overview of marine engineering and naval architecture . Fundamentals: KB, BM, KM, KG and GM A floating vessel is in equilibrium when weight equals buoyancy and both act along the same vertical line. Buoyancy is the resultant of hydrostatic pressure on the hull, a topic we cover in pressure and fluid statics . When the vessel heels through a small angle, the underwater shape changes, and the centre of buo...
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Leveling

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LEVELING

Definition :

The procedure for determining differences in elevation between points that are some distance away from each other is called Levelling.

Types Of Leveling :

There are many types but we discussed some of them here. 

1. Differential Leveling :

It is used to determine the differences in elevation between points that are some distance away from each other by level together with a graduated measuring rod.


2. Reciprocal Leveling :

Reciprocal leveling is employed to determine the difference in elevation between two points when it is difficult to keep back sight(B.S.) and fore sight(F.S.) short and equal.

3. Profile Leveling :

It is a method of surveying that has carried out along the central line of a track of land.

4. Cross-section Leveling :

A profile leveling in which we determine the elevation of series of points or lines at right angle to lengthwise along a highway. 
  • Bench Mark Leveling ➤ using Bench mark for leveling 
  • Three-wire Leveling  ➤ using stadia cross hair
  • Trigonometric Leveling  ➤ using trigonometry 

Terms Related to Leveling :

  • Elevation : A vertical distance above the reference datum to the center of instrument. 
  • Bench Mark : A point of known elevation.
  • Temporary Bench Mark (TBM) : A semi-permanent point of known elevation.
  • Turning or Change Point : A point temporarily used to transfer an elevation.
  • Height of Instrument (HI) : Vertical distance between line of sight & mean sea level (MSL).

Methods Of Leveling :

1. HI or Line of Collimation Method.
2. Rise & Fall Method.

1. Leveling By HI Method :

The elevation of unknown points can be calculated by subtracting the staff reading from height of instrument to get reduction of level.
          HI = BS + RL 
     RL = HI -  BS (Staff Reading)      
 

2. Leveling By Rise & Fall Method :

Rise and fall method of leveling is used to determine the difference in elevations between points and also we can calculate the reduce level of points. 

Tigonometric Leveling

Trigonometric leveling is an efficient method for the determination of reduce level of points.

Note : Anyone who is interested to get more problems related to leveling, he or she will contact me by comment below and I will send you problems of leveling. 







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