Magnetic compass

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Image
The magnetic compass is fitted on the upper bridge exactly center line of the ship   It is referred to as standard compass     1. D ry C ard    2. W et C ard   A compass point north because all magnets have two poles north pole, south pole.   The north pole of the one magnet attracted towards the south pole of another magnet     DRY CARD COMPASS   A light aluminum ring about 254mm in diameter is attached to a brass hub by silk cord which is threaded in and out through holes in the ring.   This tight silk cord is act like bicycle wheels.   There is no other connection or support between the ring and the hub.   Silk cord is used because it doesn’t shrink or stretch due to moisture or change in atmospheric temperature.     Card support: -     The bottom of the hub has small cap fi tt ed with a sapphire bearing surface.   This rests on a pivot tipped with iridium which is hard nonmagnetic metal....
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DAMAGE STABILITY

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Archimedes Principle:-

When a body floats freely in the water the mass of the body is equal to the mass of the water that the body presses away at the corresponding draught.



Change in draught

When a ship has one or more damage compartments the draught will increase to compensate the loss of buoyancy from the damaged compartments. A new equilibrium of the floating position occurs.



Heeling angle

If the damage is unsymmetrical around the center line, the ship will heel.

Change of trim

If the damaged compartments are in the fore ship or in the aft ship, a significant trim will develop.

Change in stability parameters

The sea “feels” the hull of the damaged ship as if the damaged compartment were non existent

Centre of gravity and Centre of buoyancy

The stability parameter of the ship is affected by the damage .the underwater hull is completely different and thereby the stability parameter .if the cargo and the fluids in tanks still is onboard the ship after the damage ,G will remain on its original location. B will change position so that B and G lie on the vertical line.

Metacentric radius BM

The metacenter, M, will change location since the shape of the underwater hull has changed. The metacentric height ,GM, will often decrease ,but can in fact sometimes increase ,depending on the width of the damage. The location of M depends on the displacement of the ship and the moment of inertia of the water plane area around the longitudinal axis through the cof by the formula



BM=moment of inertia of the water plane area /volume of the displacement.

 

If the damaged compartment extends above waterline, they will cut a hole in the water plane and reduce the moment of inertia.

Damage extending above original waterline

Since the displacement is constant the distance BM will be reduced .B will rise a little due to increased draught, but not so much as decrease of BM .This means that G will be closer to M after the damage and the ship will have a smaller GM.

 

DAMAGE NOT REACHING THE ORIGINAL WATERLINE

If the damage only affects, for example, double bottom the w/p area will be almost unaffected. A little increase of draught gives larger w/p area and a slightly increased moment of inertia.

This means that the distance BM will increase and the increased draught will push B upwards consequently GM will increase.



DAMAGE STABILITY CALCULATION

This can be done by two methods

1. Added weight method

2. Loss of buoyancy method

And thereby we can understand the GZ affected by damage.



LOSS OF BOUYANCY

In the loss of buoyancy method, which can be regarded as the true method, the ship will have a new under water hull form and a new equilibrium is calculated based on this.

Procedure:-

1. Calculate the volume of the flooded seawater, i.e., the loss of buoyancy, by multiplying the displacement to the original waterline of the damaged compartment with the volume permeability of the compartment.

2. Calculate the change in draught and the new location of longitudinal Centre of floatation, TCF for the remaining water plane area, when damaged compartments are taken away.

3. Calculate moment s of inertia for the remaining water plane area based on the new Centre of floatation, both transverse and longitudinal. Thereafter calculate the new BM.

4. Calculate the new location of B above the keel.

5. Calculate the new GM.

6. Calculate the heeling angle and trim.

Correction of GZ curve: - True GZ for the actual damage can be calculated only if the cross curves ,KN values ,are calculated for the damaged hull.



The loss of buoyancy method is not recommended when doing damage stability calculation manually. The method is more suitable for programming on computer calculations.

2. ADDED WEIGHT METHOD

For manual calculation, this method is more easier.it is similar to an ordinary load calculation where the flooded seawater is added weight. Based on this new stability parameter found out from hydrostatic tables and cross curves for the ship.



PROCEDURE:-

1. Estimate the mass of the flooded seawater and use this as an added weight.

2. The new draught and displacement for the ship is calculated based on this added weight

3. Estimate the location of cog of the flooded sw and calculate the effect on the cog of the ship.

4. Calculate the new gm for the new draught and location of G.

5. Calculate the free surface effect; since the added weight is liquid, the reduce in GM is found by the formula

GGv= (I/ ) + (Ay2 / )

I= moment of inertia of the water plane area

= volume of the displacement

A=AREA OF THE FREES URFACE

Y = the distance between the Centre line of the ship and the area Centre of the free surface.

6. Calculate the new trim and list.

7.correction of GZ curve

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