Salt water is more buoyant because it's heavier ( it takes more weight to displace it, we recognize that as buoyancy)
Have you ever swam under a boat, did you feel extra pressure?
Engineering question
- Thread starter pussyluver
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Have you ever swam under a boat, did you feel extra pressure?
Just from memory but I believe the hydraulics pressure applies to an enclosed system. The water is would rise by the amount of the displaced water which is probably insignificant in this body of water. As long as the boat is not touching the bottom - there would be little indication that one passed by with the possible exception of pressure waves (the wake behind the boat) caused by it's movement. I (rightly or wrongly) equate the boat to an airplane flying overhead. The air is a fluid, if the weight of the plane was transferred directly to the earth as weight - it would steam roll everyone flat as a pancake.
Yes, but airplanes rely on uplift, so their weight transfer is different.
Put it this way....if you weighed a tub of water on it's own, then added a toy boat to the water, the weight of the tub should increase by the weight of the boat. The difference is, and to your point that the water rises due to the boat in it, the additional weight of the boat would be dispersed over the original river bed, plus whatever additional area it is 'sitting' on. In other words, yes it is insignificant, but there.
So even though the weight changes, the pressure stays the same because the water must rise along the shore, etc. to compensate for the boat's displacement?
This was my next question which you answered.
So the added weight is displaced across or along a large area, such that the added weight to the tunnel is negligible?
I'm way out on a limb but it seems to me that your 'uplift' is in reality simply a pressure difference from air flow over the wings. Buoyancy (if you push down on a toy boat you can feel it) is simply a similar stability in pressure from the weight of the boat down against the water pressure pushing it up. I don't think a boat travelling through the water crushes fish as similarly an airplane doesn't crush people under it. They are both fluid - water is just obviously denser. (again - just thinking out loud here).
On the bottom is correct.
The boat floats in the water by displacing an equivalent weight of water. The end result is a wash.
Visualize an oil tanker sitting in Port and the keel is 6 feet off the bottom. Can you scuba dive under the keel of the tanker? Of course you can.
Now the physics of it.....
Pressure = gamma x h, where gamma is the unit weight of water and h is the height of water.
Unit weight of water = 1000 kilograms / m^3 x 9.81 m/s^2 (acceleration due to gravity)
=9.81 kilonewtons/m^3
h = whatever the depth of the water is, say 10 metres deep
Pressure (at a depth of 10 metres)= 9.81 KN/m^3 x 10 m = 98.1 KN/m^2 = 98.1 kilopascals. That's about 14 psi for all you Luddites out there (or 1 atmosphere in scuba terms.)
That's it, that's all.
If the boat can float in 10 metres of water it does so by displacing an equivalent weight of water. The weight of the boat is equivalent to the weight of the water displaced. So the pressure you feel is just gamma h
The boat floats in the water by displacing an equivalent weight of water. The end result is a wash.
Visualize an oil tanker sitting in Port and the keel is 6 feet off the bottom. Can you scuba dive under the keel of the tanker? Of course you can.
Now the physics of it.....
Pressure = gamma x h, where gamma is the unit weight of water and h is the height of water.
Unit weight of water = 1000 kilograms / m^3 x 9.81 m/s^2 (acceleration due to gravity)
=9.81 kilonewtons/m^3
h = whatever the depth of the water is, say 10 metres deep
Pressure (at a depth of 10 metres)= 9.81 KN/m^3 x 10 m = 98.1 KN/m^2 = 98.1 kilopascals. That's about 14 psi for all you Luddites out there (or 1 atmosphere in scuba terms.)
That's it, that's all.
If the boat can float in 10 metres of water it does so by displacing an equivalent weight of water. The weight of the boat is equivalent to the weight of the water displaced. So the pressure you feel is just gamma h
ocean going cargo ships did get into london....the docks were there they are now gone and the area redeveloped.....
The Thames River is 37 ft. at its deepest depth. who knows how shallow it is at its shallowest place. As you can see in the map there are many bridges spanning the Thames. Most likely only small boats can navigate the Thames. No ocean going cargo ships will ever get into London. Most boats on the Thames probably do not displace much more than 3 or 4 feet of water in depth.
Salt water is 2-3% more dense than fresh water so there would be a small change in draught. Of course I don't know how 'fresh' the Thames is these days. Realistically though I think the weight of fuel used while traveling the river would have similar effect.
This reminds me of a discussion in school with a butterfly, sealed aquarium, and a scale. Unlike that example, a river is an open system and so any additional pressure would be spread along the entire river (if not the North Sea, etc) so would have no practical impact on a tunnel.
but is this displacement instantaneous? It seems to me the instant the boat goes over there must be a time when the pressure underneath is greater until displacement occurs
If you had a large ship going fast enough you would get a noticeable pressure wave but at the speed a large tanker would travel, the changes in force would be so gradual that their impact would be dissipating before any noticeable pressure would build up.
And yes, you can experience this yourself by swimming under a boat. You won`t notice any difference in force when the boat is above you.
OK what then about hydrodynamic pressure when the vessel is moving? I'm not sure about damaging an under river tunnel, but it will certainly detonate a naval mine.
and after many years and boats will it affect the tunnel ?
the answer must be yes and they must of taken this affect into account
Nothing happens before dislacement occurs because the boat isn't in the water. Displacement begins when it enters the water and is complete when the boat stops sinking and floats in a state of equilibrium because it has displaced and amount of water equal to its weight. The load of that weight is now supported by the entire body of water.
When you first put the boat into the water it will create a pressure wave. The strength of that wave will depend on the speed at which the boat enters the water. If it slides in the wave will be insignificant. If you dropped the boat from an airplane the wave from the impact would be more significant.
That pressure wave would travel out from the point of impact and be measurable everywhere.
Once the boat is already in the water the weight of the boat is equal to the weight of the water it's displaced so there is an equilibrium and no more change in pressure. After that it's just floating around. No more pressure waves.
If you start the engine the wake behind the boat represents the waves created by the movement of the boat. Those waves travel to the bottom as well. Same force transmitted downwards as across--pretty small.
Right, since the weight of the boat for it's surface area is lighter than the total weight of the body of water within its surface area...
