B) Wind Surface moments

U sing the formulas we get:

For 0˚ to 30˚: = (3/8)*(10/57.3)*(3*GZ10° + 3*GZ20° + 1*GZ30°)

= (3/8)*(10/57.3)*(3*0.509 + 3*0.913 + 1*1.039)

= 0.347mrad

For 0˚ to 40˚: = (3/8)*(10/57.3)*(4*GZ10° + 2*GZ20° + 4*GZ30° + 1*GZ40°)

= (3/8)*(10/57.3)*(4*0.509 + 2*0.913 + 4*1.039 + 1*1.052)

= 0.594mrad

For 40˚ to 30˚: = (0˚ to 40˚) – (0˚ to 30˚)

= 0.594 – 0.347

= 0.247mrad

Step 5: Rolling period

W e calculate the rolling period with the formulas:

T = (2*C*B) / √ GM final with

C = 0.373 + 0.023*(B/d) – 0.043*(L/100)

C = 0.373 + 0.023*(19.4/9.45) – 0.043*(125.5/100)

C = 0.3662519

T = (2*C*B) / √ GM final

T = (2*0.3662519*19.4) / √(2.96)

T = 8.26sec

B Sirrah = 19.4mtr

d Sirrah = 9.45mtr

L Sirrah = 125.5mtr

If we make the calculation with the second formula for the Rolling period we would get:

T = √ ((bc*B) ^2 / GM final) where bc (block coefficient = 0.665)

T = √ ((0.665*19.4) ^2 / 2.96)

T = 7.50sec

If we look at the table onboard you see it is more likely to have a rolling period of 7.50sec than 8.26sec.

Step 6: Checking the results against the IMO-rules

1. GM > 0.15mtr? Yes, GM = 2.96mtr → OK

2. Md < 7.36mtr (winter) or < 7.51mtr (summer)? Yes, Md = 3.06mtr → OK

3a. 0˚ to 30˚ > 0.055mrad? Yes, is 0.347mrad → OK

0˚ to 40˚ > 0.090mrad? Yes, is 0.594mrad → OK

40˚ to 30˚ > 0.030mrad? Yes, is 0.247mrad → OK

3b. GZ at 30˚ > 0.20mtr? Yes, is 1.039mtr → OK

3c. GZ max > 25˚? Yes, GZ max is at 40˚ → OK

4. Is list 0˚? No, list is 2˚ to portside → OK (anti heeling can upright)

5. Do we have a positive deadweight reserve? Yes, 12025 – 4054.2 = 7970.8mt

Conclusion:

I f you create this situation in the BELCO / MACS3 program, you’ll see that there is a warning that the GM is not OK.

This is because the program is calculating with a GM limit.

With the manual calculation we only check whether the program is still OK and is having the same results, or close too, the results of our manual calculation.

Looking at the previous page checking’s, we see that our stability wouldn’t be OK because of the 2˚ list to portside. Fortunately we have an anti-heeling system onboard which has the capacity to adjust the list for us. The system can do this up to 4˚ list either side. Therefore we would have a good stability.

Taking in account good seamanship though, we would not sail with stability like this as the vessel will not sail smoothly.

A few remarks:

If looking at the stability printout from the MACS3 program, you may have noticed that the values for the Wind Surface moments are not the same as we calculated.

This is also applicable for the rolling period and for the FSM of the tanks.

The program has been programmed with formulas and values and we don’t know these formulas or values that they used.

If you follow the steps mentioned in this document, you’re sure that you can make a stability calculation and know what you’re calculating. You also understand then what the stability computer is calculating, and not just do something on that computer to make the 2 lights green.

I wish you good luck.

After all:

“The safety of the vessel and lives of you and others are affected by stability! So make sure the stability is OK!”

Appendix A: Water ballast 1

Appendix B: HFO 4

Appendix C: Container Stow Plan

Appendix D: Hydrostatic Particulars

Appendix E: Cross Curve tables

[1] Taken from NP100, ninth edition 2009

[2] Explanation provided by Oleksiy Repin, 2^nd officer.

[3] NtM is Notices to Mariners

[4] OOW is Officer on Watch

Дата добавления: 2015-11-14