Integrated - Procedure to calculate final GM and Draughts.doc

STABILITY – INTEGRATED PHASE 5

Calculating Final GM

When you have completed the Loading Sheet, you will have the following Totals at the bottom of the sheet:

ü      Total Displacement

ü      Total Vertical Moments (VMmts)

ü      Total Longitudinal Moments (LMmts)

ü      Total Free Surface Moments (FSMs) 

You will probably have to calculate Final / Fluid / Effective GM (i.e GMf)

GMf = KM – KGf   

You get the KM by entering the Hydrostatic Tables and obtaining the KMT corresponding to the Total Displacement from your Loading Sheet (interpolating if required).

KGf is obtained as follows:

KGf = KGs + FSE

KGs =                   Total VMmts                            and               FSE  =    Total FSMs

                            Total Displacement                                                            Total Displacement

So calculate KGs and FSE in order to determine KGf. Then subtract the KGf from the KM to determine the GMf.

Calculating Sailing Draught / Draughts

You may also have to determine either:

a)      The Mean Sailing Draught  or

b)      The Final Sailing Draughts

If you have to determine a) The mean Sailing draught:

·         Enter the hydrostatic tables and obtain the TMD (draught figure on the left hand column) corresponding to the final displacement.

If you have to determine b) The Final Sailing Draughts, you must do a Trim Calculation i.e. Calculate the COT, apportion it between for’d and aft (i.e. calculate ta and tf) and apply this to the NEW TMD. So,

·         Enter the hydrostatic tables and obtain the following figures, corresponding to the final displacement:

ü      TMD

ü      MCTC

ü      LCF

ü      LCB

You will also need the Final (new) LCG which you calculate using Totals from the Loading sheet as follows:

LCG                =                  Total LMmts

                                          Total Displacement

Calculate the Change of trim:

              COT (cm) = Displacement x (LCB ~ LCG)

                                                        MCTC

Calculate change of trim aft ta :

              ta = LCF  x  COT

                     LBP                                          (LBP will be given in Q or you get it from the

                                                                      Ship’s particulars in the stability booklet)

Calculate change of trim for’d tf :

              Tf = COT - ta

You must now determine if the ship is trimmed by the head or by the stern.  This is so you know whether to add or subtract the ta and tf.

You do this by considering the positions of LCB and LCG relative to each other.  It is best to draw a sketch showing LCB and LCG in m foap, with Bf and Wf etc.

·         If LCG < LCB, then the ship will be trimmed by the stern so you add ta and subtract tf.

·         If LCG > LCB, then the ship will be trimmed by the head, so you subtract ta and add tf.

NOTE:  The ta and tf will be applied to the new, final TMD which you obtained from your hydrostatic tables, corresponding to the final displacement.

e.g. if the TMD obtained from the hydrostatics was 5.4 m, ta was calculated as 22.1 cm and tf was calculated as 18.3 cm, with LCB = 71.3 m foap and LCG = 69.9 m foap :

LCG is < LCB so the ship is trimmed by the stern.  Therefore, add ta and subtract tf from the TMD

                                          F’ord                                          Aft

TMD                                          5.400 m                            5.400 m

COT                                          0.183 m -                            0.221 m               +

Final Draughts              5.227 m                            5.621 m

              - 1 –

Stability1/Integrated