Suction in Mud on Sea Bed. How to Calculate?

[savageseadog]

How much suction will develop between a flat surface like a railway wheel lying on the bottom and mud on the sea bed?

In air if one sticks a rubber sucker to a window air pressure holds the sucker to the window proportional to pressure and area but how would it be possible to calculate it for mud

The variables would be:
1) Area of contact
2) Density and nature of mud
3) Depth of water(?)

 

[boomerangben]

Suction anchors are used in the offshore oil industry. I haven't myself had anything to do with them, but as I understand it, they are more like flat plates with a skirt that penetrates a short depth into the mud. I think they also place them on the seabed and suck the trapped water out, thereby generating the suction.

In principle, the force holding the railway wheel to the seabed would be pressure of the water above x area of wheel less the percentage of a vacuum generated by the mud. How to determine the vacuum would be the difficult bit.

I have often wondered about this as an efficient way of laying a mooring. But there is quite a lot of science to get around.

 

[Searush]

The power of suction in air is simply air pressure, in mud, as mentioned it will be increased by the depth of water & mud. Aren't railway wheels spoked? If so there is far less suction in the as the mud/water can easily circulate the spokes.

 

[skodster]

I would discover what everyone else uses for a similar type of boat as yours, make sure they don't have problems, and do the same.

Sod the science...take the short cut.

 

[Ubergeekian]

The power of suction in air is simply air pressure, in mud, as mentioned it will be increased by the depth of water & mud. Aren't railway wheels spoked? If so there is far less suction in the as the mud/water can easily circulate the spokes.

With a very few exceptons, railway wheels haven't had spokes since the end of steam. Jumblie's mooring is a one ton wheel, well sunk into the mud. A reasonable estimate of vertical hold is water pressure x area, so for a 1m diameter wheel 5m down that would be four tons, plus the ton (less buoyancy) of the wheel. Less sideways, I'd have thought, so unlike with a "normal" anchor you'd want to keep the load as vertical as possible.

 

[Searush]

Well, I never! You see I only ever travel on Steam Trains, so I never see these new-fangled modern things.

 

[NormanS]

With a very few exceptons, railway wheels haven't had spokes since the end of steam. Jumblie's mooring is a one ton wheel, well sunk into the mud. A reasonable estimate of vertical hold is water pressure x area, so for a 1m diameter wheel 5m down that would be four tons, plus the ton (less buoyancy) of the wheel. Less sideways, I'd have thought, so unlike with a "normal" anchor you'd want to keep the load as vertical as possible.

Please explain where water pressure comes into it. The water pressure above the wheel will surely be balanced by the water pressure under it.

 

[Ubergeekian]

Please explain where water pressure comes into it. The water pressure above the wheel will surely be balanced by the water pressure under it.

Not when you try to move it up. In principle it's the same as the farting noise you get when you spoon out a trifle

 

[savageseadog]

Please explain where water pressure comes into it. The water pressure above the wheel will surely be balanced by the water pressure under it.

I tried to think that one through. I would say the suction must depend on the water permeability of the mud or sand on which a plate lies. As the plate lifts, the reduced pressure that develops in the area beneath the plate will create a suction force into which the water will try to flow to create an equilibrium. The denser and more glutinous the mud the less water can flow. Up to the point of applying lifting force the water will have penetrated to both sides, given time, so in static terms there is no suction until the moment a lifting force is applied. What's beaten me with the problem is what will the suction force be given perfect mud? It can't have anything to do with the depth of water because the equilibrium is local to the upper and lower sides of the plate.
It's pretty obvious that some sinking and burying of the plate in the mud is going to secure greater suction by obstructing the flow of water and increasing the forces holding the plate down. That's as far as it goes for me.

 

[boomerangben]

If you have perfect mud, the moment you lift the plate, you create a vacuum. If that vacuum is perfect, then the force acting on the plate upwards is zero, downwards you have atmospheric pressure, water pressure and gravity. If the mud isn't perfect, then the pressure within the mud will act on the underside of the plate. The more gloopy the mud, the less or rather the slower the balancing of pressures. Sand would be useless since water flows readily through it.

The moment the edge of the wheel lifts above the gloop the pressures equalise and you are relying on the weight of the wheel alone. My understanding of the large anchors is that the skirt is essential (in effect the anchor is shaped like a screw bottle top). It prevents water leaking in, traps a plug of mud adding to the effective weight of the anchor and provides friction.

I think if I had access to railway wheels for an anchor (or indeed a boat to anchor) I would view suction as bonus but would put enough wheels down there so that the weight of the wheels alone would stop the boat wandering off.

 

[earlybird]

Not when you try to move it up. In principle it's the same as the farting noise you get when you spoon out a trifle

If your previously stated theory of depth of submersion holds, is it easier to spoon up trifle atop Mt. Everest?

 

[boomerangben]

If your previously stated theory of depth of submersion holds, is it easier to spoon up trifle atop Mt. Everest?

Yes, until the trifle freezes and there'd be nothing so frustrating as frozen trifle after climbing all that way.