Iain has, I believe, the correct and simple answer. The displacement will change with any additional weight. ie if you add 100m of heavy chain the boat will displace more water. But the displacement will always equal the overall weight.
Yes – and it depends on the salinity and temperature of the water.
See the Plimsoll loading Line on ships.
TF – Tropical Fresh water
F – Fresh
T – Tropical
S – Summer
W – Winter
WNA – Winter North Atlantic
Right, Nigel, but the weight of water displaced remains the same - its the volume of water displaced that varies according to density - so the need for different markings on the Plimsoll line. The ships weight does not change when it moves from TF to WNA. Also, I beleive the WNA mark also reflects the kind of weather encountered in WNA!
Oh Dear !!!
The Plimsoll line is NOT the lettered markings on the hull side. That is a fixed line showing the max loaded draft, passing through a circle giving the Summer loadline, named after Samuel Plimsoll.
The marks you now refer to actually govern how much a vessel can be loaded to draft, depending on the sea area and season it intends to sail in.
Finally it makes no difference as to what salinity the water is to the amount of displacement .... YES the draft will change as salinity changes, but the weight of water displaced by the vessel will remain the same. You cannot 'magic away' weight without actually physically removing it !
Deadweight is basically the 'carried weight' of cargo / stores and people.
Displacement is the weight of displaced water when the vessel is free-floating
Light Displacement is the vessel empty and free-floating, basically Displacement - deadweight.
Gross and Nett Registered Tonnage are based on volume and have no relation to actual weight.
The different lines are actually mathematically derived, based on hydrostatic calculations of the hull form. The last one, WNA, this stands for Winter North Atlantic is the odd one out ... it is 1/48th less draft from Winter mark.
They are designed to give safe drafts for different sea and climate areas and take account of the seasons.
Example : If a vessel sails through a WNA area, it has to make sure that on crossing the boundary between W and WNA it must not have the WNA line submerged. So many vessels calculate fuel use and 'rise of vessel' during passage to allow for this .....
If a vessel arrives in port and a mark is submerged that shouldn't be, then it is liable to serious charges.
Finally open sea areas are classed as 1.025 kgs per litre density, except for particular areas such as Baltic etc.
Errr - yes, Nigel. I fort that was wot i'd sed... but then I aint too much of a teknikal type.
And of course you are absolutley right about Sam Plimsoll and his line, rather than the variable draft markings. In my little way I was trying to make the pont you confirm that, although the draft may vary with salinity and temperature etc, the overall weight of the vessel, and therefore its displacement does not.
(Oldharry now makes mental note not to enter into technical discussions late at night when Mrs O'h is pestering him to turn in for the night....!)
Oh silly me, confusing Weight with Volume!
And I missed out an & between Plimsoll line – a disk 12 inches in diameter, with a horizontal line 18 inches long passing through the centre, the upper edge of the horizontal line denoting the maximum depth to which a ship may be loaded in Summer when sailing in salt water, and load line!