Engineering question

[pussyluver]

Was watching Nova tonight and it was about building a subway line in London. Part of the line is under the Thames and they had to rebuild a section of tunnel because there wasn't much space between the roof of the tunnel and the bottom of the river bed.

My question is, there are some large boats that sail along the river, when designing the tunnel would they have to take into consideration the size/weight of the boats that would be sailing above it? Or would that be dissipated by the boat displacing the water?

Not sure if that makes sense, but it piqued my curiosity!

 

[Ceiling Cat]

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.

 

[anon1]

I watched it also and was thinking the same thing. The military ships would have a deeper draught than the civilian pleasure craft.

 

[pussyluver]

Sorry for boring you CC... Figured with all your trivia posts you might have an inquisitive mind.

 

[pussyluver]

I watched it also and was thinking the same thing. The military ships would have a deeper draught than the civilian pleasure craft.

Definitely... My guess is if it was in a sealed system the boats would exert pressure downward... But I'm guessing in an open system like a river the weight is disapated to the point of being negligible and not taken into consideration

 

[anon1]

I would think that the Thames River has draught restrictions on waterway usage and the section in question meets the minimum requirements.

 

[huckfinn]

Definitely... My guess is if it was in a sealed system the boats would exert pressure downward... But I'm guessing in an open system like a river the weight is disapated to the point of being negligible and not taken into consideration

The weight is carried by the water and transferred to any surface it touches.....including the bottom, sides, shore, etc etc.

So, if you think about the number of square feet the water is sitting on, the weight of boats needs to be taken into account, but it would be an insignificant amount resting on the tunnel.

 

[anon1]

The draught (how low it sits in the water) of a ship is dependent on it's weight. That can't change from ocean to river unless gravity changes.

 

[KBear]

My question is, there are some large boats that sail along the river, when designing the tunnel would they have to take into consideration the size/weight of the boats that would be sailing above it? Or would that be dissipated by the boat displacing the water?

Think your asking if a pressure gauge was put at the bottom of the river, would the pressure increase if a boat was above it. The answer would be no. Pressure measured at the bottom would be based on the depth below the waters surface and that would not change if a boat was above it. They might have to consider the pressure waves caused by large boats passing over.

If this was a boat in a bucket, the pressure at the bottom would go up when you put the boat in the bucket, as the water depth would increase as the boat displaces the water.

 

[wigglee]

the downward pressure would follow the path of least resistance, to the sides and length of the river. As long as the boat doesn't run aground, there is no problem and boat size is limited there.

 

[huckfinn]

The draught (how low it sits in the water) of a ship is dependent on it's weight. That can't change from ocean to river unless gravity changes.

Would salt content in the water change that too?

 

[huckfinn]

Think your asking if a pressure gauge was put at the bottom of the river, would the pressure increase if a boat was above it. The answer would be no. Pressure measured at the bottom would be based on the depth below the waters surface and that would not change if a boat was above it. They might have to consider the pressure waves caused by large boats passing over.

If this was a boat in a bucket, the pressure at the bottom would go up when you put the boat in the bucket, as the water depth would increase as the boat displaces the water.

The pressure may not change, but the 'weight' of the water would - or the water plus boat(s).

 

[Aardvark154]

The controlling structure on the Thames is London Bridge at the upstream end of the Pool of London, which is a fixed span with a clearance of 29.2 feet (8.9 m) only small vessels (as in the photograph in CC's post) can proceed up river from that point.

 

[Aardvark154]

Was watching Nova tonight and it was about building a subway line in London. Part of the line is under the Thames and they had to rebuild a section of tunnel because there wasn't much space between the roof of the tunnel and the bottom of the river bed.

My question is, there are some large boats that sail along the river, when designing the tunnel would they have to take into consideration the size/weight of the boats that would be sailing above it? Or would that be dissipated by the boat displacing the water?

Not sure if that makes sense, but it piqued my curiosity!

Certainly there would be hydrodynamic pressure from the passage of the vessel (the principle behind pressure trigger naval mines), however, this not being an area in which I have expertise, how great that pressure would be in the situation you raise I don't know.

 

[thumper18474]

Wouldnt a boat with a narrow beam sit lower in the water than a boat with a wider beam?( with same weight and length?)
and how low are the props and keeels sitting under the water
Any cargo vessels allowed would have to take into account how deep it sits loaded ..no?

 

[oldjones]

More critical to keeping the boats from running aground (or opening the tunnel roof at this narrow point) is the accuracy of the depth soundings on the charts and the pilot's knowledge of the draught of her vessel (which does change if the water's salt; Google 'Plimsoll line'. And yes, it is named after the running shoe inventor). Sonar helps too, but what keeps boats safe are the humans who operate them and the waterways they t**********se.
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Oops! TERB censors r a v e r wherever it finds it

 

[Kirby2006]

I would think that dredging operations would be the greatest danger to the tunnel and that is why they had to lower it.
The weight/ pressure/ displacement theory just doesn't hold water (sorry!).

 

[onthebottom]

One boat weight measure is displacement (the amount of water displaced by the wieight of the boat). As a boat floats over an object the pressure on that object is consistent, the boat simply displaces the weight of the water.

 

[Yoga Face]

One boat weight measure is displacement (the amount of water displaced by the wieight of the boat). As a boat floats over an object the pressure on that object is consistent, the boat simply displaces the weight of the water.

I would think the weight of the boat causes pressure to whatever is underneath

yes the boat does replace the weight of the water but now we have both the weight of the water plus the weight of the boat as the weight of the boat does not instantaneously transfer into a rise in water level and until that happens the boat has increased the pressure on the subway tunnel


as salt water makes boat more buoyant I would think it makes a difference as boat does not displace as much water but unsure as to the dynamics of it all


good question


someone look it up

 

[oldjones]

I would think the weight of the boat causes pressure to whatever is underneath

yes the boat does replace the weight of the water but now we have both the weight of the water plus the weight of the boat as the weight of the boat does not instantaneously transfer into a rise in water level and until that happens the boat has increased the pressure on the subway tunnel


as salt water makes boat more buoyant I would think it makes a difference as boat does not displace as much water but unsure as to the dynamics of it all


good question


someone look it up

If I recall my Physics teacher introducing us to hydraulics, the boat's added pressure on the liquid will be distributed equally throughout the liquid.

Like anything floating, the boat sinks until the weight of water it displaces is equal to the weight of the boat (which includes the cargo). The displacement causes a rise in the surface level of the entire body.

Salt water is denser more than fresh, and warm water less dense than cold so Plimsoll lines show maximum permissible safe lading for fresh and salt water in summer and winter.

That's classic physics that hadn't much hope of measuring fleeting effects like a local increase in water pressure caused by a boat launch, and could only track a local surface rise by the wet marks on the quay wall afterwards. But they do accurately describe the steady state version we deal with day after day.

I watched that part of the piece. The engineer was brought up short by the imprecision of their on-land measurements, nothing to do with ships that had safely navigated the Thames at that point under those conditions for over a century. As she said herself, the diggers' best information and measurements indicated the Victorian tunnel roof was well below the bottom of the River. Until The Day came when they could control the River, shut down traffic and actually look, they couldn't know what really was the case.

Fill your clawfoot tub with soapy water, now think how tricky it would be to drill a hole in the floor from the room below that just penetrated the cast iron without goring your sweetie and ruining her bubble bath, when all you know is what's in the tub and what's in the room next door to the one below below. James Bond and Q could do it, in real life it took an engineer who built NASA launch pads did to do it.

Such aplomb, such calm confidence in the face of potential disaster. She was great.