Question From Readers # 3: Reduction In GM Due To Free Surface

What is the reduction in GM due to free surface

A 7000 ton displacement tank ship carries two slack tanks of alcohol with a SG of 0.8 each tank i 50 ft. long 30 ft. wide. What is the reduction in GM due to free surface with the vessel floating in sea water, SG is 1.026?

Solution:

I will try to answer your question.

This is the formula we are going to use FSC =           r L B³          
                                                                       420 x Displacement  

1. Divide SG of liquid inside the tank by the density of water where the vessel is floating to get the value of   "r".
                            r =   0.8   = 0.78
                                 1.026

2. Proceeding now to the formula to get the reduction in GM.

                     FSC =           r L B³          
                                420 x Displacement
                           
                             = 0.78 x 50 x 30³
                                 420 x 7000
                             = 0.78 x 50 x 27000
                                 420 x 7000
                             = 1053000 
                                 2940000
                             = 0.36 ft      * as is if there is only one tank. But since there are two slack tanks,
                                  x 2             multiply it by 2.
                                0.72 ft    is our final answer.

I hope this answer to your question. If ever I am wrong in some way, feel free to correct me. And if you may, share this blog to your friends.

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