C&RT License Survey

[Allan(nb Albert)]

1 minute ago, magnetman said:

 

When the Boat is initially added to the water it creates a hole. Once it is in the water it acts exactly the same as a block of water the same shape as it. 

 

 

So a boat is a hole filled with water?

[IanD]

4 minutes ago, magnetman said:

 

When the Boat is initially added to the water it creates a hole. Once it is in the water it acts exactly the same as a block of water the same shape as it. 

 

 

Not *exactly* the same, I'd like to see you standing on the block of water (not ice!) like you can on the boat... 😉

[magnetman]

 

 

Water with no Boat = water. Add Boat. The Boat takes up the space previously taken by the water. All it is doing is behaving as if it were a block of water.

 

If the Boat was not there there would be a block of water there instead. Its impossible, while stationary, for the Boat to do anything other than what the equivalent block of water would have done. 

 

 

 

This simplifies the argument about the lock. Boat = block of water. Therefore a lock full of water will use the same amount of water as a lock full of Boat. 

 

 

[Guest]

So...........

 

What does a Sea Otter with water ballast do to all this equation?

 

Asking for a friend....

[phillarrow]

1 hour ago, MartynG said:

 

58 minutes ago, phillarrow said:

To add to my initial reply:

 

Add 1m for the depth of water in the lower canal, i.e. what's in the lock below your shaded bit.

 

Total lock volume in the first pic is now: 4 x 20 x 4 = 320. 

After the lock has emptied, it is 4 x 20 x 1 = 80.

Therefore, change in water = 240

 

In the second pic, total volume of water = 320 - 15 = 305

After emptying, total is 80 - 15 = 65

Therefore, change in water = 305 - 65 = 240!

 

The maths is here above. It really is this simple. The change in water volume/mass when using a lock is unaffected by anything in the lock.

[magnetman]

3 minutes ago, Allan(nb Albert) said:

So a boat is a hole filled with water?

Not yet.

It is a hole in the water which once established will behave exactly the same as the block of water it replaced, in circumstances where the Boat is floating and basically stationary. Like going through a lock. 

 

 

[Col_T]

 

10 minutes ago, magnetman said:

It is a hole in the water which once established will behave exactly the same as the block of water it replaced, in circumstances where the Boat is floating and basically stationary.

Genuine question: What is the relevance of the boat being “basically stationary” in all this? If it acts as a ‘block of water’ when stationary, what is it acting as when not stationary?

[Mike Todd]

16 hours ago, Arthur Marshall said:

Irrelevant. When the boat goes in, the gates and paddles are open. Nothing suddenly changes when you shut the gates, the situation is identical to when they were open.

If i have my toy boat in the bath, and move it from one end to another, nothing changes if I suddenly put a block down the middle of the bath. It can only act as a closed cube if it's shut before the boat goes in, which would involve either a crane, a helicopter, or a bizarre gravitational anomaly.

But is the hole deeper if it's a BIG rabbit? Or if a badger goes down instead?

When a boat enters the top of a lock its displaced water is pushed back into the upper pound. Hence the upper pound has more water in it when the gates are closed than before the boat arrived.

[IanD]

Just now, Mike Todd said:

When a boat enters the top of a lock its displaced water is pushed back into the upper pound. Hence the upper pound has more water in it when the gates are closed than before the boat arrived.

Yes, but this hasn't "used" any water -- no extra water needs to enter the upper pound to keep the water level the same.

[Mike Todd]

1 minute ago, IanD said:

Yes, but this hasn't "used" any water -- no extra water needs to enter the upper pound to keep the water level the same.

I did not suggest any such thing!

[IanD]

4 minutes ago, Mike Todd said:

I did not suggest any such thing!

True -- but how much water is *in* the upper or lower pound or the lock at any point doesn't really matter, does it? All closing the gates does is divide the upper pound back into two parts, one inside the lock and one outside, as opposed to one body of water when the gates are open -- nothing has changed.

 

What matters is how much water a boat going through a lock uses (transferred from upper pound to lower pound), because this has to be supplied from somewhere.

Edited December 18, 2023 by IanD

[Momac]

22 minutes ago, phillarrow said:

 

 

The maths is here above. It really is this simple. The change in water volume/mass when using a lock is unaffected by anything in the lock.

However  the boat replenishes 15T of water back into the lock  as it leaves the lock. Therefore saving 15Tonnes of water being transferred from the upper pound  for the process compared to the lock empty case.

 

 

[magnetman]

4 minutes ago, MartynG said:

However  the boat replenishes 15T of water back into the lock  as it leaves the lock. Therefore saving 15Tonnes of water being transferred from the upper pound  for the process compared to the lock empty case.

 

 

This doesn't happen. 

 

When the Boat goes into the lock it is entering what is about to become a closed container. When it exists the lock it is simply moving through a channel. 

[phillarrow]

3 minutes ago, MartynG said:

However  the boat replenishes 15T of water back into the lock  as it leaves the lock. Therefore saving 15Tonnes of water being transferred from the upper pound  for the process compared to the lock empty case.

 

 

So, this is quite an interesting way of viewing it.

 

We could say that the boat "removes" 15 tonnes of water from the lock into the upper pound when it enters the lock, and then it "replenishes" 15 tonnes of water back into lock from the lower pound. Therefore, the nett effect is that the upper pound has gained 15 tonnes of water, the lower pound has lost 15 tonnes of water, but the lock has neither gained nor lost anything.

 

I can see some logic in that argument, which would suggest that the total effect on the entire system IS affected by the mass of the boat!

 

I'm going for a little lie down and then I'll come back later!

[magnetman]

23 minutes ago, Col_T said:

 

Genuine question: What is the relevance of the boat being “basically stationary” in all this? If it acts as a ‘block of water’ when stationary, what is it acting as when not stationary?

A moving Boat will displace water because it is moving. This changes the situation completely. 

 

The key is that when the Boat is in the lock it is not moving and it is in a closed container. When it is exiting the lock it is moving and it is in a channel, which while it could be viewed as a closed container does not behave like a closed container due to other factors like leakage and weirs. 

 

When the Boat -moves- out of the lock it will displace water so the impression is that it is somehow replacing water used during the lock cycle. 

 

The point is that the Boat will -move- water in any channel when it itself is moving so the fact that the channel started with a lock is not relevant. 

 

 

[Allan(nb Albert)]

21 minutes ago, Mike Todd said:

When a boat enters the top of a lock its displaced water is pushed back into the upper pound. Hence the upper pound has more water in it when the gates are closed than before the boat arrived.

...  thus the lock has less water in it to be let into the lower pound such that the boat can continue its journey ...

 

[magnetman]

No because the Boat is a hole in the water so it must expel more water in order to achieve a level before exiting. 

 

The Boat can not do anything which a block of water the same shape can't do. Its impossible because it is occupying the space otherwise occupied by water. 

 

The idea that this water is then replaced as the Boat exits, while understandable, is erroneous.

 

[IanD]

34 minutes ago, phillarrow said:

So, this is quite an interesting way of viewing it.

 

We could say that the boat "removes" 15 tonnes of water from the lock into the upper pound when it enters the lock, and then it "replenishes" 15 tonnes of water back into lock from the lower pound. Therefore, the nett effect is that the upper pound has gained 15 tonnes of water, the lower pound has lost 15 tonnes of water, but the lock has neither gained nor lost anything.

 

I can see some logic in that argument, which would suggest that the total effect on the entire system IS affected by the mass of the boat!

 

I'm going for a little lie down and then I'll come back later!

 

All that you've done is swapped 15 tons of boat over with 15 tons of water without any change to the water levels above or below the lock as a result of this -- you could do this with a crane and a pump without working the lock at all. Then you could reverse the process to put the boat back above the lock and the water below it -- and repeat this whole process as often as you like without having the slightest effect on any water levels or "using" any water.

 

You could also empty the lock without a boat in while you were doing this and use up exactly one lockfull of water (volume=area x drop)

 

Or you could dispense with the crane and the pump, use the lock to lower the boat, and still use exactly the same lockfull of water.

[phillarrow]

Just now, IanD said:

 

All that you've done is swapped 15 tons of boat over with 15 tons of water without any change to the water levels above or below the lock as a result of this -- you could do this with a crane and a pump without working the lock at all. Then you could reverse the process to put the boat back above the lock and the water below it -- and repeat this whole process as often as you like without having the slightest effect on any water levels or "using" any water.

 

You could also empty the lock without a boat in while you were doing this and use up exactly one lockfull of water (volume=area x drop)

 

Or you could dispense with the crane and the pump, use the lock to lower the boat, and still use exactly the same lockfull of water.

 

Yeah I get that, and I've been with you and magnetman on this ever since I woke up yesterday!

 

However, my tired brain is struggling with the last movement of the boat, which has then set me back to the very start!

 

Here's the bits we agree on (I think!):

1) The volume of water used to cycle a lock is fixed and independent of whatever is, or isn't, inside the lock.

2) The level within the lock is a red herring - not least because it's also independent of the boat.

3) The nett effect of boats going through locks is that they basically cancel each other out over time...kind of!

 

But...the overall effect of one boat going through one lock once is the bit I'm struggling with.

 

In order for a boat to move along any stretch of water, it has to displace a 'buoyancy-equivalent' amount of that water.

When the boat goes into a lock to go downwards, it displaces that amount from the lock into the top pound.

Then the lock cycles, which is a fixed amount.

Then the boat leaves the lock, displacing the same amount of water from the lower pound back into the lock to fill the boat-shaped hole it has left behind.

Then the gates close.

 

Are those five statements true as you see them?

 

If they are, then doesn't this mean that the overall effect of a boat going down a lock is that the top pound and lock together have gained a boat-sized amount of water from the bottom pound? 

[IanD]

6 minutes ago, phillarrow said:

 

Yeah I get that, and I've been with you and magnetman on this ever since I woke up yesterday!

 

However, my tired brain is struggling with the last movement of the boat, which has then set me back to the very start!

 

Here's the bits we agree on (I think!):

1) The volume of water used to cycle a lock is fixed and independent of whatever is, or isn't, inside the lock.

2) The level within the lock is a red herring - not least because it's also independent of the boat.

3) The nett effect of boats going through locks is that they basically cancel each other out over time...kind of!

 

But...the overall effect of one boat going through one lock once is the bit I'm struggling with.

 

In order for a boat to move along any stretch of water, it has to displace a 'buoyancy-equivalent' amount of that water.

When the boat goes into a lock to go downwards, it displaces that amount from the lock into the top pound.

Then the lock cycles, which is a fixed amount.

Then the boat leaves the lock, displacing the same amount of water from the lower pound back into the lock to fill the boat-shaped hole it has left behind.

Then the gates close.

 

Are those five statements true as you see them?

 

If they are, then doesn't this mean that the overall effect of a boat going down a lock is that the top pound and lock together have has gained a boat-sized amount of water from the bottom pound? 

 

Yes, all five statements are correct 🙂

 

But the top pound has also lost a boat-sized boat to the bottom pound, so the two cancel out. The mass of (water+boat) above and below the lock doesn't change if you swap the boat and equivalent volume of water over (e.g. using a crane and pump), so neither do the water levels.

 

Which means that the boat moving through the lock doesn't in itself "use" any water, filling and emptying the lock does -- and this is always the same amount.

Edited December 18, 2023 by IanD

[magnetman]

Good explanation. The Boat is a block of water. 

[phillarrow]

16 minutes ago, IanD said:

 

Yes, all five statements are correct 🙂

 

But the top pound has also lost a boat-sized boat to the bottom pound, 

 

Of course it bloody has! What kind of dumbass can't see that, it's bloody obvious!!! 😳😳

 

I told you I was tired! 😉

[Sailbadthesinner]

The only thing that matters is the volume of water passing through the paddles as the lock empties or fills. This is always 240 m3 in the lock being discussed, whether or not there is a boat in it. The boat going in or out of the lock does not affect this.

 

[David Mack]

3 hours ago, beerbeerbeerbeerbeer said:

aren’t we only concerned with the reduced volume of water brought down with the boat from the pound above?

A better definition of the water used is the volume of water which it is necessary to add to the upper pound, to restore that pound to its original level. And if the lock is returned to its original state, that will be equal to the volume of water which flows over the overflow weir on the lower pound (if it isn't used for lockage at the next lock down).

[Momac]

2 hours ago, Mike Todd said:

When a boat enters the top of a lock its displaced water is pushed back into the upper pound. Hence the upper pound has more water in it when the gates are closed than before the boat arrived.

Correct.