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Originally Posted by lesrae
3. Displacement (is this the same as weight - there you go, first indication of my total ignorance!)
Displacement is a measure of the weight of water that any floating vessel displaces, the theory is that so long as the physical weight of your ship is less than it’s displacement, it will float.
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No.
Displacement (W) is the weight of the volume (V) of the water displaced by a ship. (Volume of water times density of water, or VD) If W exceeds VD, the ship sinks. If its VD exceeds W, it rises. If VD = W, the object has neutral buoyancy.
Archimedes' principle: "Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."
Time for some maths!
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Let's assume a barge-shaped vessel with a surface of 10 meters by 10 meters, floating in water of density = 1025 kg / cubic meter (IE, seawater). If said barge weights 200 metric tons, then:
W / D = V
200 MT / 1.025 = 195.12 cubic meters
The immersed volume of the barge is then 195.12 cubic meters; knowing that it measures 10 meters by 10 meters, we can calculate its draft, since V = L*B*d:
195.12 / (10*10) = 1.95 meters.
Now suppose 100 MT of cargo is loaded unto the barge.
W / D = V
300 MT / 1.025 = 292.68 cubic meter
V / (L*B) = d
292.68 / (10*10) = 2.92 meter draft.
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As you can see, an increase in weight led directly to an increase in volume displaced. Once the ship or submarine has no more volume left to displace, it sinks. If it's got more volume than weight (IE, a submarine blowing ballast), it rises until VD = W.
In short, the displacement of a ship is its weight.