capnsensible
Well-Known Member
Seawater pressure increases by 1 bar every 10 meters, important to divers and submariners!
Three feet underwater.
- Thread starter BurnitBlue
- Start date
Seawater pressure increases by 1 bar every 10 meters, important to divers and submariners!
AntarcticPilot
Well-Known Member
For practical purposes, water is incompressible, certainly over the range of pressures between sea-level and a few metres down. So, it's density does not vary with depth UNLESS there is serious temperature or salinity stratification. I understand that in some coastal waters with a lot of fresh-water input, submarines can "float" on the interface between fresh-water and the salty water beneath it. In the open sea, such strong stratification is unlikely, except where there is a saline outflow and a less saline inflow as at the Straits of Gibralter, the Bosphorus and similar places.
Almost certainly whatever it was was floating at the surface, but the part the boat struck was well below the surface. Given the shape of a shipping container, it is highly unlikely that it would float level - almost certainly, it would float with a corner up. The free water surface inside the container would ensure that.
DJE
Well-Known Member
I looked it up. It's about half a part per billion change in length per Pascal increase in pressure. That works out as 1.5% increase in density for 1000 metres increase in depth. Yep practically incompressible!
BurnitBlue
Well-Known Member
You are quite right. Sorry the 3 feet comment was made by Rivonia post nu 7 liveaboard from his own experience not by the skipper of the sinking yacht.
My query was about the many comments I have read about yachts colliding with containers etc just BELOW the surface. The submarine comment I made was to a remark by the skipper of the Benny 500 who wrote that his yacht was HIT twice more. Was this a subliminal thought that passed through his mind. After all the area is alive with warships re Syria.
My query was about the many comments I have read about yachts colliding with containers etc just BELOW the surface. The submarine comment I made was to a remark by the skipper of the Benny 500 who wrote that his yacht was HIT twice more. Was this a subliminal thought that passed through his mind. After all the area is alive with warships re Syria.
Sailfree
Well-Known Member
I have previosly commented that I dislike this internal hull eggcrate type contruction and why I will renovate my current boat instead of buying new. Benneateau pioneered it but to be cost effective all production boats have now copied it. Also the reason why a modern 43 is cheaper than my 9yr old 43!
Thick hull reinforced with good strong GRP ribs - christ I am sounding like a MAB fan now!!
NPMR
Well-Known Member
Being dumb ......
Where do they get this high pressure air from, under water, and if it's stored on board, why doesn't it make the boat float anyway??
AntarcticPilot
Well-Known Member
The high pressure air is stored in pressurized containers. It is highly compressed, so it occupies far less volume in the compressed state than it does when pushing water out of the bouyancy compartments. Archimedes principle will tell you that it is the volume that matters in this case.
BurnitBlue
Well-Known Member
The answer to my query is mixed. Unless I have read the replies wrong then pressure at shallow depth is not an issue to cause sufficient variation in density because water is incompressable. Stratification of salinity is the only issue.
From the above posts can I conclude that YES an object can under certain circumstances "float" under the surface at whatever depth the difference in salinity occurs.
From the above posts can I conclude that YES an object can under certain circumstances "float" under the surface at whatever depth the difference in salinity occurs.
AntarcticPilot
Well-Known Member
But these circumstances are highly unlikely in the open sea; only in coastal waters with a LOT of freshwater coming off, or in polar waters where the surface water is much colder than the bulk of the water column.
Buck Turgidson
Well-Known Member
Fish and scuba divers manage to float at all kinds of depths below the surface. Buoyancy control was day 1 of my scuba training.
BurnitBlue
Well-Known Member
Thanks. As I always thought, some part of the submerged object must be above the surface. However, in theory that small part could be submerged in calm water during a very heavy rainfall. Just tying up a few loose ends. Thanks everyone.
On a different but related note, why is it that when a yacht has been sunk after a collision and the crew take to a liferaft I have never heard of the crew identifying the container. After all, there is not much else the survivers can do except to look around them for the cause of their predicament.
Once when crossing the Atlantic I came alongside a large steel red bouy. I wrote down the markings and later was informed that is was a bouy that had broken loose from Bermuda. It was floating high as normal and undamaged. I was close to Barbadus (few hundred miles) so it had been in the circular current for a year or more.
Edit to wonder why all container stories mention the word submerged. For sure, some of them must be floating high.
Wonderful world full of mystery questions.
Last edited:
Uricanejack
Well-Known Member
correct about the presure for the first 10m its not a straight line though its about anothe 20m to reach 2 bar. my 20 m may be way out.
Presure at surface is 1 bar.
at about 30 ft 10m 2 bar
and continous to increase as you go deeper.
An increase in the presure does not compress the water. 1m3 of water at the surface will still be 1m3 of water at 10m or 1000m. unless the tempretureof the water changes. cool the water it will contract. so the same water will ocupy less space or the space will contain more water.
the tempreture does drop considerably with depth.
bignick
Well-Known Member
No, the original comment about 1bar increase per 10m depth is correct. As described water is virtually incompressible so for each extra 10m depth you go down you have an additional 10m column of water on top of you. It's virtually linear.
prv
Well-Known Member
JumbleDuck
Well-Known Member
I have seen it estimated that the Titanic's less fortunate passengers would have been doing about 30mph when they hit the sea bed.
Seajet
...
It's not exactly a frequent event, but since coming across that largely submerged heavy boat - post 14 - I have wondered about carrying a disposable buoy - probably an old plastic can as that seems OK for ' pro ' fishermen - to tie on and mark any such object to hopefully help others at least in daylight to avoid or find it.
I don't know how one would tie on to a container though, and whether in the yacht or liferaft I doubt I'd fancy getting close ( again ! ) in any swell.
I don't know how one would tie on to a container though, and whether in the yacht or liferaft I doubt I'd fancy getting close ( again ! ) in any swell.
prv
Well-Known Member
Easy enough through the twistlock holes on each corner. But as you say, likely to be a fairly low priority!
Pete
capnsensible
Well-Known Member
ffiill
Well-Known Member
We tend to forget that not all objects float on the surface as gone are the days when a piece of "log wood"
was put over the side;the sand timer set off/counting by the second done whilst the knots were counted on the line.
Log wood a hard wood primarily from around the Honduras area has totally neutral buoyancy which is why it was used as a "log" on the ship for measuring distance.
I certainly have seen pieces of pine once waterlogged behaving in a similar manner.
was put over the side;the sand timer set off/counting by the second done whilst the knots were counted on the line.
Log wood a hard wood primarily from around the Honduras area has totally neutral buoyancy which is why it was used as a "log" on the ship for measuring distance.
I certainly have seen pieces of pine once waterlogged behaving in a similar manner.
bbg
Well-Known Member
I think we also have to remember the sea is not as flat as a billiard table. Something that is very slightly positively buoyant will react with the waves. As the waves pass over it might be completely underwater. Then it will be buoyant and try to rise. It may be that, as it rises a trough passes and then quite a bit of it is out of the water. Its weight will push it down, possibly well underwater. If it is only very slightly buoyant it may get 1 or 2 metres below the surface before it starts to float back up again.
i guess you would call something like this "awash".
i guess you would call something like this "awash".