Genuine curiosity?

Question

Oil tankers...Like the Exon valdis... bad example but you get the picture. when full of fuel (oil diesel petrol whatever) does it sit higher in the water acording to the pimsole line. I mean 50,000 tons of cargo that is lighter than water ?
No reason for the question just curious.

Im assuming the pimsole line is at water level when empty. its unladen weight.

yes but im assuming also that the tanks will be filled above the water line to the top in fact. where is the pimsole line

Hi Bruzer. thanks If I understand that correctly . I can not fill the ship with enough oil or feathers for that matter. as it will not sink to the line. but will overflow. giving me a max allowed. cool
good answer. I can see 10 points coming your way. we will wait and see if there is another out there who disagrees. but I see your point.

Andew D . i think you will find flour is heavier than water and will sink. It does not matter the weight or the volume of oil on water it will not sink unless the vessel it is held in is hevier. my point is if the ship starts at a certain level. and you fill it with something lighter than water will it rise in the water.
Its probably just me cant grasp the sitch but it is a curiosity to me

Answer 1

Simply put:
Air is lighter than oil, oil is lighter than water.
A vessel displaces as much water as it weighs.
Just because the oil weighs less than water does
not mean it has a negative displacement, it just
displaces less volume.
The 'empty` vessel is filled with air.
- It displaces the least water.
Fill the vessel with oil,
- it displaces more water, (Rides deeper).
What may be confusing you is that an empty supertanker
would float so high as to be unstable.
They take on water as 'ballast` so that they can travel safely.

Answer 2

If the vessel were fully loaded, it would sit lower in the water. But the water would sit "higher" on the plimsoll line.

Take a clear empty bottle, and fill it 1/2 way up with beach sand. The sand is the ballast; it will make the bottle float upright. To make a plimsoll line, mark the bottle lengthwise with bold dots/dashes every 1/2 inch. Place the bottle in a full sink. The plimsoll line will indicate the "unloaded" level for the vessel.

Now, whatever extra is loaded into the bottle [unless it's hydrogen, a vacuum, or something like that], the bottle will sink and the water level will "rise" on the plimsoll line. Even if it's light like flour, the extra cargo will make the bottle sink a little lower.

So what's the difference between oil and water? Just like flour is lighter than sand, oil is lighter than water. Less weight, less sinking. More weight, more sinking. Semantically, you can't really say "..50,000 tons of cargo that is lighter than water...," but I think I get what you're after. 50,000 tons of anything, whether it's oil, water, or flour, will all make the boat sink the same. 50,000 barrels, on the other other hand, is another matter. 50,000 barrels of styrofoam would make the ship sink less than 50,000 barrels of water, oil, or mercury for that matter.

Incidentally, tankers do take on water ballast for empty runs, but they take on only what they need for ballast. This weights less than a full load.

That said,... What would sit lower, an oil-laden tanker or a water-laden tanker - same design, same volumes of cargo? The water-laden tanker would sit lower. It weights more (more weight for the same cargo volume), and it displaces more water to hold it above the sea.

Answer 3

The primsoll line is the maximum safe draft of a boat. The draft is the distance between the bottom of the boat and the surface of the water. That means fill the boat up any more and it may sink. Fill the boat such that the primsoll line is under water than there is too much weight in the boat and it might sink. Another way to look at is the primsoll line defines the minimum freeboard. Freeboard is the distance between the surface and the main weather deck. THE MINIMUM freeboard. fill the boat more and the freeboard distance is less. That means too much weight and......prepare to abandon ship.

Oh, if you fill the boat with oil or feathers it dosn't matter What matters is the weight in the boat and the deapth of the boat relatve to the primsoll line.

Answer 4

It depends on whether the tanker takes on water as ballast when the oil is offloaded.

oil is light than water but heavier than air. So if the tanker was truely empty (i.e the oil hold was filled with air), the boat would be higher than when filled with oil.

However, i suspect that the tanker will take on water in a ballast tank as it offloads oil, so it could go either higher or lower depending on the size of the ballast.

Answer 5

It doesn't really matter what there is in the tanks.
VERY simply the ship is loaded until a particular mark on the 'Plimsoll' line is level with the water line.
Looking at modern tankers you will find there are different levels for different conditions ie.
TF - Tropical Fresh Water
F - Fresh Water
T - Tropical Seawater
S - Summer Seawater
W - Winter Seawater
WNA - Winter North Atlantic etc. etc.
See this link http://www.rhiw.com/y_mor/plimsoll/plimsoll.htm

Answer 6

Oil is lighter than water, but not lighter than the air in an empty hull.
The total weight and volume of the hull determines how high the ship floats, if it floats.