Spearguns and buoyancy

Quote from hau

Do you think an air filled barrel acts like a diver who is positive on the surface yet sinks past a certain depth?
Just asking

it would have to wouldnt it?,, air in lungs is same as air in a barrel...
i think i notice my gun sinks faster at more depth (with shaft in)

Quote from hau

Do you think an air filled barrel acts like a diver who is positive on the surface yet sinks past a certain depth?

I don't know. But it doesn't matter because based on my experience I wouldn't want my gun floating in 15ft nor in 60ft.

Quote from hau

Do you think an air filled barrel acts like a diver who is positive on the surface yet sinks past a certain depth?

I don't think so. The only way that the buoyancy of an object changes if if the volume of that object changes.

A diver's air filled lungs compress as he descends, occupying less volume, and providing less buoyancy.

An air filled speargun is a rigid structure by comparison. If the barrel is 1.000" in diameter at the surface, then it should be 1.000" in diameter at normal diving depths. If the weight does not change, and the dimensions of the gun (volume) don't change, then the buoyancy of the gun should not change.

That's what I was thinking, for buoyancy to be affected the barrel would have to compress which it doesn't do. But I think there's another factor that affects buoyancy. That is to say if the object is just barely buoyant at the surface, if you take it down to certain depth, even if it doesn't compress it may no longer be buoyant at that depth. I'm not sure about this..

Hau - That's because his wetsuit compresses with depth. The tiny air bubbles that form the insulation compress and lose buoyancy at depth, so the diver is heavier. The bubbles expand at the surface, so the diver is more buoyant.

Quote from Dan

But I think there's another factor that affects buoyancy. That is to say if the object is just barely buoyant at the surface, if you take it down to certain depth, even if it doesn't compress it may no longer be buoyant at that depth. I'm not sure about this..

If we want to be thorough, then we can add that anything which changes the density of the surrounding water changes the buoyancy of an object suspended in that water, giving us the obvious differences between net buoyancy in a freshwater pool vs. the ocean. The density of seawater changes with salinity and temperature, so there could be changes as you pass a thermocline, for example, or if you are diving near a river where there might be a layer of fresher water on top. But I think these changes in density would be pretty small in typical open water diving.

When a body is completely submerged in a liquid it displaces a volume of liquid equal to the volume of the object.
This volume of liquid has a mass and this mass is equal in magnitude to the buoyant force on the object.

Buoyant force though has nothing to do with whether the object sinks or floats though, well not really.

You could have a tennis ball submerged in water and it will always float back up but a ball of lead the same volume will sink, but both bodies have the same buoyant force.

If you had the right equipment it should be easy to do some measurements to find the amount of weight you need to add to make the gun neutral if it is floating.

I think.....

Quote from Kyle c

Buoyant force though has nothing to do with whether the object sinks or floats though, well not really.

Buoyant force has everything to do with it. If the buoyant force (which you correctly defined) is greater than the object's weight, then the object floats. If the buoyant force is less than the weight, then the object sinks.

Yes but the factor is more deciding on whether it sinks or floats is based on the density of the material used.

In the case of spear guns when is the buoyant force ever greater than the weight of the gun?

I know this is on a case by case basis pipe guns sink, wood guns can sink or swim depending on the material, pneumatic probably float if there is air in it.

Let me know what you think I am only making assumptions:confused1:, I'm an engineering student so going on pure fluid mechanics, no real world experience here. Never even shoot a spear gun.

Quote from Kyle c

Yes but the factor is more deciding on whether it sinks or floats is based on the density of the material used.

Negative. Ships are built from steel and even concrete, and they float. As a ship sits lower and lower in the water, the hull displaces more and more water. When the volume of water displaced by the hull is equal to the weight of the ship, then things are at equilibrium.

Aluminum is heavier than water. An aluminum ingot will obviously sink because its weight is greater than the weight of the volume of water it displaces. But if I take a hollow aluminum tube and seal it to make a speargun, then the COMBINED composite weight (aluminum + air) can be less than the weight of the displaced water, and the tube will float..

But a tube sealed off and a tube filled with water has the same buoyant force you have not changed the volume.

They do have different volumes. An unsealed tube floods when submerged. So it only displaces a volume of water equal to the volume of the metal in the tube.

But a sealed tube displaces more water. Air is trapped inside the tube (between the plugs). So the displaced volume is equal to the volume of the metal (as above) PLUS the volume of the trapped air inside the tube.

I don't want to argue and I hope we are having a healthy discussion here.

But I'm sorry that is wrong, if you fill a tube with water cap off the ends and put it into a tank where you can measure the displaced volume this will be the same as a sealed tube with air.

Volume = Pi * Length * ( Outer Radius (squared) - Inner Radius (squared))

Quote from Tin Man

They do have different volumes. An unsealed tube floods when submerged. So it only displaces a volume of water equal to the volume of the metal in the tube.

But a sealed tube displaces more water. Air is trapped inside the tube (between the plugs). So the displaced volume is equal to the volume of the metal (as above) PLUS the volume of the trapped air inside the tube.

Which part is wrong?

I can sense seaweed..

Sorry I am not sure how to quote but the part with, "They do have different volumes."

Its the same piece of pipe filled with water and one not filled the water the shape has not changed the dimensions have not changed the Volume is the same.

Damn looks like I am trouble maker, first I pissed off Dan with the fishing spots now Tin man who makes and uses spear guns, I have no future in the spear fishing community

There is a plume of freshwater coming out of an outflow pipe here in Hollywood. If you ever wander into it on your dive you sink like a rock.

Buoyancy force is the weight of the volume of water displaced minus the weight of the object like Jeff said.

So in the case of a floating speargun the weight of the water it displaces is greater than it's own weight.

Gun sinking past a certain depth can be due to the airspace between the handle and the plug as well as the muzzle and plug. If they flood as you descend it can change the buoyancy enough. Guns are close to neutral so any little airspace can change their characteristics.

Ok, let's see which parts we can agree on.

We have two identical spearguns, consisting of aluminum tube barrels (ignoring all the other parts like handles, shafts, etc.). Barrel "A" is plugged, but has a hole in it, so it floods with water when submerged. Barrel "B" is identical to Barrel "A", but has no hole.

I submerge barrel "A" in a bucket of water, so that I can measure the volume of water it displaces. Water fills the barrel through the hole, so the barrel only displaces a volume of water equal to the volume of the metal which forms the tube. Essentially Pi/4 * (OD^2 - ID^2) * length.

If I submerge barrel "B" in a similar bucket, it will displace more water. It will displace a volume of water equal to your equation, or as I stated it, the volume of the metal, plus the volume of the trapped air. The displaced volume in this case is Pi/4 * OD^2 * length

Barrel "B" is therefore more buoyant because it displaces more water when forced under water in the bucket. If I don't force it down, it may even float if it's weight is not sufficient to overcome its buoyancy.