Why draft calculation is not done on a tanker ? or why cargo is calculated by ullage method and not by draft survey ?

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for very obvious reasons...on tanker we are not only interested in weight of oil cargo but finding out if there is any free water in the cargo and if there is calculate it separately.
draft survey cannot give amount of oil and free water (if any) separately.well people may raise points about accuracy but i believe this one would be a better answer.
posted by: ajeet kumar