I think it should be:
New Draft = 160 - (W / 1.05)cm
You were correct above with subtracting the change in draft, due to the removal of weights onboard, the ship will rise causing the draft to go down.
Rise or fall (cm) = weight removed or added (tonnes) / Tonnes per Centimeter (TPC).
What do you think?
Your vessel at departure from Brisbane had a draft of 1.6m and had 5,340litres of fuel, 5000 litres of fresh water and 8 passengers embarked.
Your vessel has a crew of 3 persons and uses 120 litres of fuel per hour at a speed of 16 knots and you routinely use fresh water at a rate of 1 litre per person per hour. Your destination is Sydney which is a distance of 188 nautical miles.
a) How long will you be at sea considering a constant speed
b) What will be your draft on entry to Sydney Harbour
c) What draft will the vessel have on its return to Brisbane provided the same conditions remain
Vessel Tonnes per Centimetre (TPC) in fresh water is 1.05
Specific density of sea water is 1.025
Specific density of fuel 0.78
Assume Brisbane is salt water and Sydney harbour is fresh water