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MoominPapa

CRT boaters update - way dodgy science

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So, I just got around to reading last week's CRT boaters update, which includes the following gem.

Quote

If your boat weighs 15 tonnes it will displace 15 tonnes of water. As you progress along the cut 15 tonnes of water needs to move from in front of your boat to behind it. The same happens with air – the volume that your superstructure displaces needs to move past your boat as well.

Here’s where it gets a a little more complex. There’s a concept in science known as Bernoulli’s principle which states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.

Both water and air have to move from in front of the boat to behind, but because the channel width is smaller for water, it has to move faster, so the water pressure is reduced more. This can readily be seen by watching the water-level on a vertical bank; several inches of wet bank can be seen at the mid-point of the boat, showing that the water-level has dropped because the air pressure is now greater than the water pressure.

So, the faster you cruise along, the bigger the pressure difference will be between water and air, making the water more turbulent and, potentially, causing more disturbance to moored boats.

I think all the stuff about air pressure is just wrong - the air pressure changes from  a boat cabin and above-water hull moving at 3mph are tiny, and anyway air pressure changes are not needed to explain changes in the water level: exactly the same water level changes would be experienced if there was a vacuum above the water. What does the team think?

 

MP.

 

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4 minutes ago, MoominPapa said:

So, I just got around to reading last week's CRT boaters update, which includes the following gem.

I think all the stuff about air pressure is just wrong - the air pressure changes from  a boat cabin and above-water hull moving at 3mph are tiny, and anyway air pressure changes are not needed to explain changes in the water level: exactly the same water level changes would be experienced if there was a vacuum above the water. What does the team think?

 

MP.

 

Yes, I agree with you.  One can assume that the air pressure is uniform (as it density is so much lower than water). The other factor that  most of these descriptions ignore is the action of the propellor in sucking water in and pushing it backwards. That's what causes the effect in the piece that you quote. One would get a different behaviour of the free water surface if the boat is being towed on a very long line; compared to a boat being self propelled. 

 

I was also disappointed that the article did not distinguish between these hydraulic (displacement) effects, and the effect of surface waves (just moving up and down). So there are occasions with boats that generate surface wash but no displacement effect  (eg kayaks), and vice versa (slow moving deep draughted narrowboat).  

 

Also it did not explain why you get sucked sideways if the channel is shallower on one side than the other. I think the Bernouilli effect is relevant here (it's similar to lift created by a wing).

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20 minutes ago, MoominPapa said:

So, I just got around to reading last week's CRT boaters update, which includes the following gem.

I think all the stuff about air pressure is just wrong - the air pressure changes from  a boat cabin and above-water hull moving at 3mph are tiny, and anyway air pressure changes are not needed to explain changes in the water level: exactly the same water level changes would be experienced if there was a vacuum above the water. What does the team think?

 

MP.

 

Yes, all the stuff about air pressure is a load of bollocks.  However, the same water level changes would not be experienced if there was a vacuum above the water because the engine would not work and the steerer would be dead. B)

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1 hour ago, Scholar Gypsy said:

Yes, I agree with you.  One can assume that the air pressure is uniform (as it density is so much lower than water). The other factor that  most of these descriptions ignore is the action of the propellor in sucking water in and pushing it backwards. That's what causes the effect in the piece that you quote. One would get a different behaviour of the free water surface if the boat is being towed on a very long line; compared to a boat being self propelled. 

 

I was also disappointed that the article did not distinguish between these hydraulic (displacement) effects, and the effect of surface waves (just moving up and down). So there are occasions with boats that generate surface wash but no displacement effect  (eg kayaks), and vice versa (slow moving deep draughted narrowboat).  

 

Also it did not explain why you get sucked sideways if the channel is shallower on one side than the other. I think the Bernouilli effect is relevant here (it's similar to lift created by a wing).

A simple way to experience if us to take a sheet of paper, A4 will do and hold one short end so that the sheet curves downwards. Then blow across the top of the sheet from where you are holding it. You should see the sheet rise up. If us easier to do than explain!

 

Btw  I'm not sure that it needs to be shallower one side but a result of just not being quite in the middle. Doesn't take much off centre to produce enough net difference to move the boat to one side bur once you start it is a self magnifying process.

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1 hour ago, Mac of Cygnet said:

Yes, all the stuff about air pressure is a load of bollocks.  However, the same water level changes would not be experienced if there was a vacuum above the water because the engine would not work and the steerer would be dead. B)

Electric boat and steerer wearing space-suit :)

 

MP.

 

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Does this mean that CRT are working on a 15T licence standard and anything above this will be charged extra accordingly due to the extra displacement of CRT property required for passage.?😁

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2 hours ago, MoominPapa said:

So, I just got around to reading last week's CRT boaters update, which includes the following gem.

I think all the stuff about air pressure is just wrong - the air pressure changes from  a boat cabin and above-water hull moving at 3mph are tiny, and anyway air pressure changes are not needed to explain changes in the water level: exactly the same water level changes would be experienced if there was a vacuum above the water. What does the team think?

 

MP.

 

 

Agreed.

 

There should be a term for this techno-babble written by people with no understanding of physics.

 

We could call it.... errrr..... techno-babble?

 

 

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Perhaps introducing a vacuum above the Atherstone and Glascote flights may improve upwards passage speeds as the bloomin locks may fill a bit quicker.....regardless of how many passengers(with luggage) are on the boat.

In fact, Mr Dyson could be involved, as Mr Hoover is long gone and only remembered by his dam. (False news)

Edited by matty40s

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10 hours ago, Mac of Cygnet said:

Yes, all the stuff about air pressure is a load of bollocks.  However, the same water level changes would not be experienced if there was a vacuum above the water because the engine would not work and the steerer would be dead. B)

Surely in a vacuum the water would be boiling, unless the temperature was absolute zero, so water movement would be the least of your worries.

  • Greenie 1

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26 minutes ago, Loddon said:

Surely in a vacuum the water would be boiling, unless the temperature was absolute zero, so water movement would be the least of your worries.

You only need to get down to about 200K for it to be solid in a vacuum, not 0K, but the point is still valid.

 

MP.

 

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16 hours ago, Mike the Boilerman said:

 

Agreed.

 

There should be a term for this techno-babble written by people with no understanding of physics.

 

We could call it.... errrr..... techno-babble?

 

 

There is already such a term.

Bachelor of Arts.

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17 hours ago, matty40s said:

Perhaps introducing a vacuum above the Atherstone and Glascote flights may improve upwards passage speeds as the bloomin locks may fill a bit quicker.....regardless of how many passengers(with luggage) are on the boat.

In fact, Mr Dyson could be involved, as Mr Hoover is long gone and only remembered by his dam. (False news)

We have in the past used a far less technological method of speeding the atherstone locks. 

Given the passing of time and the statute of limitations i can confirm that both our previous motors inventories contained a large steel wedge with a length of chain to  fasten them to the rail.

of course this was under a previous waterways regime.

100% faster more reliable than a verlokie and silent.

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3 hours ago, Manxcat said:

There is already such a term.

Bachelor of Arts.

MBA = Master of the Bullsh!t Arts.

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On 12/09/2019 at 00:03, matty40s said:

Does this mean that CRT are working on a 15T licence standard and anything above this will be charged extra accordingly due to the extra displacement of CRT property required for passage.?😁

Well as it would affect widebeams more I’m all for it....might need to raise the tonnage so heavy narrowboats are exempt.....🤣🤣

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On 11/09/2019 at 23:29, Mike Todd said:

A simple way to experience if us to take a sheet of paper, A4 will do and hold one short end so that the sheet curves downwards. Then blow across the top of the sheet from where you are holding it. You should see the sheet rise up. If us easier to do than explain!

 

Btw  I'm not sure that it needs to be shallower one side but a result of just not being quite in the middle. Doesn't take much off centre to produce enough net difference to move the boat to one side bur once you start it is a self magnifying process.

what has that got to do with anything?

 

differences in air pressure have absolutely nothing to do with the hydraulics of a boat moving in a restricted channel, which are the result only of displacement of water by the boat.

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C R T

 

C = Speed of light in a vacuum

R = a programming language

T = Written or published text

 

Reverse it 

 

Words written in computer gibberish at the speed of light !

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