Cycling UK Forum

buryman wrote: ↑2 Oct 2021, 8:40am In the simplest model of flotation a ship floats because the upward buoyant force is greater than the weight of the ship.

The buoyant force must equal the weight of the ship.

: - )

It depends how real worldy we want to be. There will always be transient vertical forces and the buoyant force needs to be greater than the weight to tolerate those.

Jonathan

Greater volume underwater than above it in most cases.
The top bits of a ship tend not to have a great deal of volume.
Have a look at an oil tanker as an extreme example.

That's because it's full of oil

Mick F wrote: ↑2 Oct 2021, 9:08am Greater volume underwater than above it in most cases.
The top bits of a ship tend not to have a great deal of volume.
Have a look at an oil tanker as an extreme example.

Does that oil tanker have more volume above or below the waterline?

But this issue is already fully covered above... all ships have greater volume below the waterline than above except for the ones that don't.

Jonathan

On the face of it, a ship would not be unstable because it had more volume above the waterline than below, but rather because it had more mass. Theoretically, you could add a huge plastic superstructure, and thus a lot of volume above the waterline, to any viable ship, and the only reason for it suddenly to turn turtle would be that a strong wind caught it.

That said, practical ships are probably made substantially from similar materials above and below the waterline. But the heavy engines and keel will tend to be low down, and the hull is likely to need more (i.e. heavier) reinforcement than the superstructure. And there may need to be careful calculations about the distribution of any load. But within those limits, I can't see why it wouldn't be feasible to have more than half the volume up high.

drossall wrote: ↑2 Oct 2021, 4:17pm On the face of it, a ship would not be unstable because it had more volume above the waterline than below, but rather because it had more mass. Theoretically, you could add a huge plastic superstructure, and thus a lot of volume above the waterline, to any viable ship, and the only reason for it suddenly to turn turtle would be that a strong wind caught it.

That said, practical ships are probably made substantially from similar materials above and below the waterline. But the heavy engines and keel will tend to be low down, and the hull is likely to need more reinforcement than the superstructure. And there may need to be careful calculations about the distribution of any load. But within those limits, I can't see why it wouldn't be feasible to have more than half the volume up high.

Exactly. Volume is a canard.

Jonathan

Another way to think it out is that two shipyards work together to build a ship. The first has all the materials. So the hull is loaded with those and towed to the second, where the materials are made into the most amazing, high-volume lightweight superstructure. The ship has the same mass as before, so will have the same waterline, irrespective of what has been done with the materials.

drossall wrote: ↑2 Oct 2021, 4:22pm Another way to think it out is that two shipyards work together to build a ship. The first has all the materials. So the hull is loaded with those and towed to the second, where the materials are made into the most amazing, high-volume lightweight superstructure. The ship has the same mass as before, so will have the same waterline, irrespective of what has been done with the materials.

And lots of other thought experiments will give the same result, including mine of building a shell roof over the containers and including that in the ship's volume. Or putting up a giant marquee on a supertanker.

Jonathan

jb wrote: ↑2 Oct 2021, 9:09am That's because it's full of oil

That is exactly my point.
Ships carry cargo and the volume below the sea level, is higher than the volume above it.
Ships wot don't carry cargo are heavier low down as well. They all have a greater volume below the water-line.
Go on a ship and see.

There are exceptions, I agree - cruise liners perhaps as they are like floating skyscrapers. Maybe other exceptions.

I've said my piece now.
Over and Out.

Mick F wrote: ↑2 Oct 2021, 4:29pmShips carry cargo and the volume below the sea level, is higher than the volume above it.
Ships wot don't carry cargo are heavier low down as well. They all have a greater volume below the water-line.
Go on a ship and see.

There are exceptions, I agree - cruise liners perhaps as they are like floating skyscrapers. Maybe other exceptions.

All ships either carry cargo or don't carry cargo. If the ships in each of those categories have more volume below the waterline how can there be any "exceptions"?

Thanks

Jonathan

I thought there was an exception to every rule?

drossall wrote: ↑2 Oct 2021, 5:34pm I thought there was an exception to every rule?

Is that self-referential?

: - )

Signed: A Cretan barber

I can't help thinking there's a misunderstanding or cross purpose sneaking around in this discussion somewhere.

jb wrote: ↑2 Oct 2021, 6:03pm I can't help thinking there's a misunderstanding or cross purpose sneaking around in this discussion somewhere.

Do you mean the misunderstanding that this is a thread about a Suranne Jones drama?

This is our yacht Polibre being lifted in at Glasson Dock earlier this year.
The dark blue bit goes under the water. The white bit stays out of the water. As you can see the white bit is bigger than the blue bit. Sometimes the white bit tries to lean over and get in the water but we mostly manage to keep it more or less upright.
The light blue stripe is supposed to be just above the waterline: loaded with supplies and kit for a longish cruise it can get about an inch lower.

There is nothing magic about this: big ships behave in exactly the same way. The volume of water displaced by the part of the hull that is immersed weighs exactly the same as the complete yacht. Add weight and she sinks a bit to displace a greater weight of water, remove stuff and she floats higher. You don't notice this much on Poly, but you certainly do with four people climbing into the dinghy.

Water density varies, so a vessel will displace slightly varying amounts of water depending on where it is. google 'plimsoll line' for more about this.