Topic: Swimming and Armor

[markc]

  It is an area that there is very little skill "skill as in DP" is involved. So you cannot just spend DP to make the chance go away.



MDC

[JimiSue]

The salinity of the Dead Sea apparently changes the buoyant properties of people and such, but I bet it doesn't change the buoyant properties of a helmet full of water even a little bit.

Oh yes it does The Eureka incident was the realisation that anything will float so long as its volume can displace an amount of water equal to the mass of the object. With regular fresh water, one cubic metre (or meter, for you Americans) weighs one metric tonne. therefore if an object is one cubic metre in size and weighs less than a tonne, it will float.

I don't remember the exact amount that seawater is denser than fresh water (it changes anyway by season and location) but let's say it's 5% denser than fresh water. Therefore, one cubic metre of water now weighs 1.05 tonnes.

The Dead Sea has denser water still. This is due to intense evaporation, which leaves the minerals behind in solution. If the dead sea is 20% more dense than fresh water, that cubic metre weighs 1.2 tonnes. Because a person's volume doesn't change significantly, he needs to displace less water in the Dead sea than in fresh, and therefore floats higher out of the water than normal.

Exaclty the same principle happens for the helmet. In salt water its volume displaces a greater mass of water so it sinks less quickly. In a super-salty water environment it might even float (yes, unlikely, since you don't get much that is denser than steel or iron, but technically, it's possible if you're trying to float in molten uranium or something (which I'm sure, carries a health warning).

Sorry for the science class, I have a pathological need to educate about sciencey things. It's a curse, really.

[jdale]

Wolfram Alpha gives the densities as

997 kg/m^3 for fresh water
1020-1035 kg/m^3 for sea water
1240 kg/m^3 for Dead Sea water (had some help from Wikipedia here)

So your memory is pretty good. About 3% higher for sea water vs fresh water, and about 24% higher for Dead Sea water.

People are not a lot more dense than water - especially with a lungful of air. Floating is possible, especially in saline water.

Steel, however, is about 7859 kg/m^3, so the buoyancy difference for your armor is not going to make much difference.


PS: it's not just you.

[Marc R]

When you float in water, on the inhale, you stick out of the water essentially to the tops of your lungs. You always right to head up, because head down you only stick out of the water to a point between knees and hip. This puts too much weight over your pivot point and you topple back to head up.

There are warnings about bathing in the dead sea, as due to the high salinity, you can float head down with your legs up, since you bob out past your waist to the bottoms of your lungs and reach a stable state where you don't topple.

A few people drown every year not realizing you need to actively turn yourself head up, you won't just naturally revert to head up orientation.

[JimiSue]

The statement was that it wouldn't make ANY difference. I wasn't saying it would make a LOT of difference, but extra salinity will have a little effect even on steel