Ship Board Operations - Draft Survey 1

Draft Survey - 1

Introduction

Purpose: The purpose is of a draft survey is to determine the present amount (weight) of the cargo on board, thereby calculating the cargo loaded or discharged.

The formula -  Cargo weight = Present displacement - (Light ship + Ballast + Stores  + Fresh water + Constant)

Objective: Objective is to find the cargo weight presently on board. For that  Light ship, ballast, stores, constant are known. Therefore, finding the present displacement will solve the equation.

The method in brief:

1. Read the drafts at 6 points and correcting them for their respective perpendiculars. 

2. Using the corrected drafts above, obtaining the "Mean of mean of mean draft" or the "Quarter mean draft" (QMD)
3. Refer the hydrostatic table or the ship's stability booklet with the quarter mean draft and obtain the displacement.

4. Calculate the 1st trim correction or "Layer correction" and applying it to the displacement on step 3.

5. Calculate the 2nd trim correction and applying it to the displacement on step 4.

6. Correct the displacement above for sea water. (if the vessel is not floating in sea water)

7. Subtract all known wights onboard including the light ship displacement to get the present cargo weight on board.