Naval architects when designing a ship often face many difficulties in determining the displacement due to the lack of data for the weights of hull components, propulsion machinery, wood and outfit, equipment and etc.
Displacement of a ship is the volume of water that it displaces when floating at sea or river. The volume is calculated in cubic meter. If it is multiplied with the sea water density (1.025 metric ton/cubic meter) then we can get the displacement which equals to the ship weight.
On page 4 of Ship Design and Performance for Masters and Mates, Dr. C.B. Barrass gave a simple formula for calculating the displacement of ships. The formula is called deadweight coefficient.
CD = dwt/W
The following table is the typical deadweight coeffiecient for various types of merchant ships
The deadweight coefficient varies from one ship to another depending on its type and is calculated up to SLWL or Summer Load Water Line. It is the condition where a ship is fully loaded.
Worked Example:
For a new design, the deadweight of the ship is determined to be 12,000 metric tons. Information from previously built similar ships suggests CD to be 0.6. Estimate or calculate the fully loaded displacement (W) and the lightweight of the new ship:
W = dwt/CD
W = 12,000/ 0.6 = 20,000 metric tons.
Lwt = displacement - deadweight
= W - dwt
= 20,000 - 12,000 = 8,000 metric tons This is for only for preliminary design calculation.
If we see on the table above, the ship is container ship.
Also read: Ship rudder
