Thanks.

You are welcome!

When a ship moves from water of higher density to lower density, or vice versa, a phenomenon known as "sinkage" or "rise" affects the ship's draft. This is often called "density effect" or "density correction." The change in water density influences the ship's buoyancy, causing the entire vessel to "sink or rise" by a few centimeters in the water. This will cause a change in the ship's draft both forward and aft including the mean draft. Thank you for the comment.

Hi! Thank you for your comment. In my last example, in the final step, I subtracted the dock water allowance from the departure draft to find the arrival draft, since the ship sails from a lower R. density (1.007) to a higher R. density (1.018). The effect on the ship upon reaching higher density, she will rise, and as a result, there will be a decrease in the ship’s draft. That is why DWA should be subtracted from the dep. draft to find the arrival draft as shown in my final step which is also stated in your comment, a decrease in the arrival draft. Maybe you are confused when I take the difference of the departure and arrival relative density because, in my solutions, I have shown (1.018 - 1.007) which is the arrival R.D 1.018 minus the dep. R.D 1.007. What I did is subtract the lesser from the greater R.D. so that I would have a positive DWA, but this procedure does not affect the ship’s draft yet. As you mentioned in your comment the ship will rise in this problem, and there should be a decrease in the ship’s draft “You are right”. That is why in my final step, I have subtracted the DWA from the dep. draft to determine the arrival draft because the effect is a decrease in draft. Now you can also follow this format (1.007 - 1.018) which is departure R.D 1.007 minus arrival R.D 1.018, you will have a negative DWA. A negative DWA should be subtracted from the departure draft, and the result will be a decrease in the arrival draft as shown in my final step. Thank you very much👨

thank you very much for your detailed answer sir, your channel is very helpful and insightful. plus the animation is superb. make it more easy to understand. have a great day.

Welcome.

Thank you. In the comment section, it's hard to explain how this formula was derived and why 4 became a constant. I will try to make a separate video about the derivation of the formula of freshwater allowance. Usually, onboard we can extract the value of the freshwater allowance in the stability manual provided by the shipbuilder. But try his one, Derivation of the FWA formula: Consider a ship floating in SW at load Summer draft at waterline WL. Let the volume of SW displaced at this draft be ‘V’. Now let W1L1 be the waterline for the ship when displacing the same mass of fresh water. Let ‘v’ be the extra volume of water displaced in FW. The total volume of freshwater displaced will be V + v. Mass = Volume x density Mass of SW displaced = 1025V Mass of freshwater displaced = 1000 (V + v) But the mass of FW displaced = Mass of SW displaced. 1000(V + v) = 1025V v = V/40 Assume that ‘w’ is the mass of SW in volume v and ‘W’ in volume V, Then, replacing the factor as obtained above we get: w = W/40 But w is a factor that is a product of the FWA and the TPC Now since the FWA is in mm and the TPC is in cm, they both have to be converted to metres Thus: W = (((FWA mm x 100) cm X TPC cm) / 100) metres Simplifying we have: w = (FWA x 100 x TPC) / 100 = W / 40 Or (FWA x TPC) = W / 40 But w = TPC x (FWA/10) Hence W/40 = TPC (FWA/10) or FWA = W/(4 x TPC). Where ‘W’ = Loaded SW displacement in tonnes.

@@nauticalacademy001 thank you for answer

FWA is the number of millimeters by which the mean draft changes when a ship passes from saltwater to freshwater, or vice versa, while floating at the loaded draft. This is typically measured when a ship is moving from seawater to freshwater, such as when entering a river or a lake. Dock Water Allowance (DWA): DWA is the number of millimeters by which the mean draft changes when a ship passes from saltwater to dock water, or vice versa, when the ship is loaded to the Summer displacement. Dock water is a type of water with a density between that of seawater and freshwater. DWA is typically measured when a ship is moving from seawater to dock water, or vice versa, while in a dock or a shipyard. FWA is the change in draft when a ship sails from seawater to freshwater or vice versa, while DWA is the change in draft when a ship sails from seawater to dock water or vice versa or from dock water to another dock water with different water densities. The key difference is the type of water involved, with FWA involving a transition between seawater and freshwater or vice versa, and DWA involving a transition between seawater and dock water or vice versa.

@@nauticalacademy001 so we can calculate the change of draft by FWA value and TPC value with the given formula DWA= change in draft= displacement * change in density/ 0,025