14 replies to this topic

[Atom Fireworks]

Hey people,

Ime after some help for something totally non pyro related. I thought here would be the best place to ask the question as theres some smart people on this forum.

Right basically ime trying to produce a graph of water densities and different depths. By densities i mean if i had an object that sinks to the bottom of a lake that is 10 ft deep will it still sink to the bottom in say 100ft of water? As i understand it the deeper the water means theres more water ontop of it, this inturn means it must be denser the deeper you go. I have seen a graph for this with ocean water but ime talking about still water lakes in england. Ive read that the density or pure water is 1000kg/m³ obviously the density will depend on whats disolved in the water but as a base line and a proof of concept could anyone help me produce a graph based on pure water?

Ime useless with maths and graphs and i could be totally wrong about the concept however you people seem to be the best i know of that could possibly help.

The idea is that when fishing for carp in deep water they may not be on the bottom of the lake if its too deep, so we use a rig called a ZIG rig which in effect uses a bait that floats and its tethered to a weight, so you can set the depth you want the bait to sit at because it floats up from the bottom, this setup is effective in water between 5ft and 20ft but after that its impractical and doesnt produce fish asmuch, my theory is the fish are preocupied on the lake bed and because the bait is so far off the bottom the dont see or smell it so they have no reason to come looking. My idea is that small baits like hemp seed may suspend at certaind depths instead of sinking straight to the bottom like they do in 10ft of water, and if so i could employ this tactic on some really hard venues where the fash are so clever they stay in open water of around 100ft deep knowing the anglers cant catch them.

What do you think????

[Arthur Brown]

Water is for all practical purposes incompressible so a litre will weigh a kilo at whatever depth. The pressure is roughly half a psi for every foot of depth and most other (than liquids ) things will have some air content so will compress as they sink actually making them unstable -the more they sink the faster they will sink.

[Atom Fireworks]

Thanks for the input arthur sounds right, does anybody else have an opinion on it? from what ive read the density of water also has allot to do with whats dissolved into it like salt water for instance, I read an article on ocean water and densities and there was a graph for the water which showed the density increasing up until about 1500 meters then it flat lined and stayed the same for many thousands of meters therafter.

[Arthur Brown]

And then you get the temperature effects, as the temperature falls the density rises, but temperature falls linearly with depth near the surface, then it stabilises and may rise or fall with depth. Whole submarines hide under thermoclines as the thermocline hides the vessel's echo.

[Atom Fireworks]

Maybe my answer can by found with the way submarines operate, i now they take on water to sink but could this information relate to the small scale? the submarine has to weigh a certain weight and displace a certain amount of water to sustain the said depth, if it tried to go deeper by nose diving would it pop back up the depth it tried to dive from??

[helix]

As Arthur said water as a fluid is incompressible and its density will not vary with depth. The hydrostatic pressure increases with depth, this pressure acts all around anything immersed in the water. The pressure is just calculated from the water density (1000kg/metre cubed normally for pure water) gravitational acceleration (9.81 m/s squared) times the depth (in metres) gives you the pressure in newtons/ sq metre or pascals. The density does however vary with dissolved salts etc and also temperature.


From what I can remember, the buoyancy force is calculated from the mass of water that the object (ie your float or float of sorts) multiplied by gravitational acceleration F= MA


so if it had a volume of a litre it would displace a kg of water so its buoyancy force would be 1kg x 9.81 or 9.81 newtons you would then have to take of the effect of the weight of the float which would be acting in the opposite direction again using F=MA. I the difference would be the actual buoyancy force available.


The way to get it to float at a height would be to have an air pocket in the float which could be compressed as the thing sinks. As it sinks the hydrostatic pressure acting on the float would impart a force on the walls of the bottle and, providing the bottle was made from soft plastic, the air bubble would compress. As the air compresses and the bottle sides collapse in, the volume of the bottle will effectively reduce and therefore its buoyancy force will reduce. At a certain depth the buoyancy force will reach equilibrium with the force imparted by virtue of the floats weight.


I think that's correct ..... but its been a while (20 years) since I did hydrostatics

[alany]

The bulk modulus of water is ~2.2 GPa, the density variation would be small at reasonable depths, so the object would need densities near 1. You'd need to include the bulk modulus of the object in question too as it will become denser from the hydrostatic pressure. For solid metals (hundreds of GPa) you may be able to ignore the variation. Temperature is a major variable too, in short this calculation is nontrivial for a precise solution.

[Peret]

The way to get it to float at a height would be to have an air pocket in the float which could be compressed as the thing sinks. As it sinks the hydrostatic pressure acting on the float would impart a force on the walls of the bottle and, providing the bottle was made from soft plastic, the air bubble would compress. As the air compresses and the bottle sides collapse in, the volume of the bottle will effectively reduce and therefore its buoyancy force will reduce. At a certain depth the buoyancy force will reach equilibrium with the force imparted by virtue of the floats weight.

I don't believe that's true. I agree with Arthur Brown - as the gas compresses, it displaces less water, so it sinks further, ad infinitum. A submarine that is not under way can only be held at a certain depth by actively blowing and flooding the tanks, otherwise it will either sink or surface - I checked this with my stepson, who is a submariner. Most fish have a gas-filled swim bladder to control their buoyancy, but like submarines it's an active system - dead fish either sink or float (sharks are an exception - they don't have a swim bladder and if they ever stop moving, they sink). So I don't think a simple passive depth float is possible. An active one wouldn't be too difficult to construct, though. I can envisage a miniature fishing bathysphere equipped with pressure (depth) sensors, a plumb bob to sense the bottom, and of course a baited hook. When it sensed a tug on the line it could blow its tanks with a CO2 cartridge.

[helix]

I don't believe that's true. I agree with Arthur Brown - as the gas compresses, it displaces less water, so it sinks further, ad infinitum. A submarine that is not under way can only be held at a certain depth by actively blowing and flooding the tanks, otherwise it will either sink or surface - I checked this with my stepson, who is a submariner. Most fish have a gas-filled swim bladder to control their buoyancy, but like submarines it's an active system - dead fish either sink or float (sharks are an exception - they don't have a swim bladder and if they ever stop moving, they sink). So I don't think a simple passive depth float is possible. An active one wouldn't be too difficult to construct, though. I can envisage a miniature fishing bathysphere equipped with pressure (depth) sensors, a plumb bob to sense the bottom, and of course a baited hook. When it sensed a tug on the line it could blow its tanks with a CO2 cartridge.

Having thought about it your correct, the buoyancy force would reduce as the depth increased causing it to sink further/ faster. I think your correct it would have to be an active solution unless you just used a weight to sit on the bottom and a float with a specific length of nylon between to give you a set height to fish.

[Atom Fireworks]

Thanks for your input guys, so it would seem i am barking up the wrong tree about deeper water being more denser because of the force from the water above, how much of a difference would temperature make on the density of water?

We allready have systems that enable us to fish at any depth with a single hookbait, my question is can free offerings be " self suspended" in deeper water because of their mass and weight being equal to the denser water, so they no longer sink but are suspended at what ever depth they become equal to the waters density due to the cooler water.

In extreme cases ime talking maybe a water that is 100 ft deep but more practically ime talkin about venues with 50 ft of water where we may find fish at a depth of 25 feet, we can put a hookbait there but no free offerings, this is my aim with this bit of research,

Many Thanks

Jay

[Arthur Brown]

Remote control boat with remote reading echo sounder and fish finder AND remote controlled winch with distance log! This way you can motor about and look for the fish then send a free feed down to exactly where they are. Adds £2K to the tackle box and isn't really cricket!

[Atom Fireworks]

Remote control boat with remote reading echo sounder and fish finder AND remote controlled winch with distance log! This way you can motor about and look for the fish then send a free feed down to exactly where they are. Adds £2K to the tackle box and isn't really cricket!

I allready have one of those mate, ha ha i even have a 4 man white water rafting dinghy with an electric outboars and propper sonar but when you find the fish in " suspended animation" in 100ft of water its a little tricky getting them to feed with only 1 hook bait

[Peret]

We allready have systems that enable us to fish at any depth with a single hookbait, my question is can free offerings be " self suspended" in deeper water because of their mass and weight being equal to the denser water, so they no longer sink but are suspended at what ever depth they become equal to the waters density due to the cooler water.

There are some very deep and still mountain lakes, fed by snow melt, where the water at the bottom is extremely cold - below freezing - and correspondingly dense. Lake Tahoe in California is one such. Occasionally, sunken boats have been found floating part way down on the cold water (and unusually well preserved by the cold). This only happens at considerable depth, many hundreds of feet down, where the pressure is high enough to let the water become super cooled without freezing. So it is possible, but it's unlikely you could find such a lake in Britain, or run out enough line to reach the thermocline if you did.

[crystal palace fireworks]

There are some very deep and still mountain lakes, fed by snow melt, where the water at the bottom is extremely cold - below freezing - and correspondingly dense. Lake Tahoe in California is one such. Occasionally, sunken boats have been found floating part way down on the cold water (and unusually well preserved by the cold). This only happens at considerable depth, many hundreds of feet down, where the pressure is high enough to let the water become super cooled without freezing. So it is possible, but it's unlikely you could find such a lake in Britain, or run out enough line to reach the thermocline if you did.

Blimey Peret, its been a few decades since I visited Lake Tahoe, is it still the second largest lake in the world? and has a tide of its own?

Sorry guys for going slightly off topic for a moment!

Edited by crystal palace fireworks, 29 January 2011 - 08:12 AM.

[Peret]

Blimey Peret, its been a few decades since I visited Lake Tahoe, is it still the second largest lake in the world? and has a tide of its own?

I don't know about the second largest in the world - it's 496 square km, 192 sq m - but it is a number of "-est"s. Largest mountain lake in the US, highest (1897 meters), second deepest (501 meters, beaten only by Crater Lake Oregon). I don't know if it has tides, but it has waves. It has a very small outflow for its size so water remains in the lake for an average 650 years, then it runs 200 miles down the Truckee River before it ends up in Pyramid Lake, which has no outlet.

It's such a spectacular place, I advise everyone who visits me to get up there for a few days. I get up there about once a year myself. It's not exactly just up the road for me, it's 8 hours drive, but then everything is 8 hours drive from here