Daily License Questions about Ship Stability and Trim: I just think people need to have some knowledge in their heads, professional or otherwise. After 2007 our world has changed due to the advent of Smart Phones. Retaining knowledge is like being able to play a musical instrument, if you do not use it you will lose it! The questions I post with will hopefully help you keep what you already know or remind you of what you should know. William E. George
If the ship navigates from saltwater to fresh water density, what changes occur? Why?
ReplyDeleteGood question! We all should know for example when transiting the Panama Canal the draft of the vessel increase with literally no change in displacement due to fuel consumption.
ReplyDeleteThe increase in draft at the LCF can be calculated to be equal to the Brackish Water Allowance, BWA:
BWA = (1.025 - Water density) x Displacement / (TPC or TPI depending on the units)
When a vessel is in equilibrium LCG and LCG are aligned and no change of trim occurs. If you inspect the hydrostatic tables you will notice for different drafts the LCB will move toward the end of the area of the waterplane that is larger. For a Panamax Vessel at deeper drafts this will cause LCB to move Aft. If LCB moves aft there is a Trimming Arm or Trimming Lever created and the vessel will trim towards the end with LCG until LCB moves under it once again.
Change of Trim = ((LCG-LCB) x Displacement) / MTC or MT1 using appropirate units.
Keep in mind the Trimming Lever = LCG - LCB will be very small, but unless the vessel is shaped like a shoe box LCB will move due the increase in draft and the vessel's trim will change.
I was going to bring this calculation up in future posts. If a Panamax vessel has zero trim before entering the Panama Canal its change of trim will be about 11 centimeters or a 4.33 inches by the bow. So for zero trim in transiting the Canal you need to plan to have LCG at the LCB for the Tropical Fresh Water Draft.