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Question From Readers #6: Change of Draft


What will be her draft in sea water considering there is no change in displacement

A ship has LBP 215 m, TPC 59.05 T, MTC 767.41 T-m, LCB 7.08 m with displacement of 31979 T and floating in a dock water density of 1.022 with draft 4.18 m FWD, 7.69 m AFT. What will be her draft in sea water considering there is no change in displacemet.

 Solution:

 In this kind of problem, we are going to use to two formulae and then a little analysis on what to do with the figure derived from the solution. First formula we are going to use is this:

 DWA = displacement / (4 x TPC)

 The second formula is:

 DWA = FWA x (1025 - new density) / 25

 Ok now let us to answer...

 Step 1.

 DWA = displcement / (4 x TPC) = 31979 / (4 x 59.05) = 31979 / 236.2 = 135.4

 Step 2.

 DWA = 135.4 x (1.025 - 1.022) / 25 =(135.4 x 0.003) / 25 = 0.4062 / 25 = 0.016 m

 Step 3.

A little analysis here. Our ship is originally from a water with density of 1.022 going to the seawater. So there must be a change in draft because there is a change in water density. Your question then is, what will happen to the draft? Will it increase or will it decrease? Since the ship is coming from brackish to sea water, the ship's draft will decrease. So we are going to use our answer in step 2 to subtract the original forward and aft drafts.

 FWD 4.18 m - 0.016 = 4.16 m

AFT 7.69 m - 0.016 =7.67 m

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