hdrifting about in locks

[Top cat]

I noticed when locking down in narrow locks that in many locks the boat starts to drift back once the lock has emptied, any ideas why that happends? I would have thought that the water flow would have pretty much ceased and anyway it should be pulling the boat forward.

 

TC

[John V]

could it be that there is a circulating current as it empties.....sort of like water down a plug hole. but when the water stops flowing out of the lock the circulating water then has some force in the opposite direction...sort of bouncing off the now stationary water in the paddle openings?

 

I'm not sure if that's clear....I know what I'm thinking but putting into a coherent statement is not easy!

[trackman]

This certainly seems a common effect in narrow locks at least. I've often wondered without success why it occurs, but at least it's a useful indication that the gates should open.

[Mac of Cygnet]

It also happens when you're waiting above the top gate (I rarely use lock landings when descending narrow locks), both indicating when the gate can be opened, and allowing you to do it!

[John V]

I suppose I must have noticed it without being conscious of it, because I recognised it from your description and then remembered having seen it it.

[MoominPapa]

The water flowing out towards and through the gates paddles has momentum, which keeps it flowing out briefly after the levels have equalised, so that the level in the lock goes slightly lower than the level in the canal. That causes the flow to turn around, and there's a short period of flow into the lock. That is what nudges the boat backwards when the lock is ready. It's quite a handy sign.

 

MP.

[Tony Dunkley]

It happens in bigger locks too, ship canal locks included, affecting any vessel in the lock in just the same way regardless of size and displacement.

Someone with a good grasp of Fluid Dynamics needed here I think.

 

 

Edit to add : -- Looks as though that someone turned up while I was typing this

Edited September 17, 2014 by tony dunkley

[John V]

yep that sounds more like it than my mangled attempt!!!

[junior]

It's very handy when single handing.

[Paul C]

Simple answer: Its an automatic feature of locks, to drift the boat back enough to allow the gates to be opened easily.

 

Complicated answer (I'm not 100% sure of this though). The water in front of the boat is still, because now the water in the lock is at the same level with the water downstream outside it, there's no flow through paddles or gate leak. The water behind the boat is turbulent, because the top gate and/or paddles invariably leak a little bit, and cause a small waterfall or something behind. Because of this, the boat is pushed towards the turbulent water. I'm not sure if its because of flows set up by the turbulence, or because the water is less dense when its turbulent (due to trapped air) so the still water effectively 'pushes' the boat to the turbulent water. Or it might be because of a dynamic effect of the still water creating a flow and pushing the boat shortly after the bottom paddles etc are closed.

[Detling]

Thanks for the ideas, I was wondering about the strange behaviour this after noon going down a flight. It is a very useful indicator that you can open the gates.

[ditchcrawler]

Have you never dropped a lock, taken the boat in and closed the bottom gates only to see a flow of water open them again before you reach the top paddles.

 

On the River Douro in Portugal with the big deep locks the water level comes back by a good foot after emptying.

 

In the UK Dropping down through Maston Doles top lock the back flow is very noticeable several seconds after the bottom gates have been opened and the boat rises quite noticeably as you try to leave

[PhilR]

It's very handy when single handing.

 

Agreed.

[Tony Dunkley]

Have you never dropped a lock, taken the boat in and closed the bottom gates only to see a flow of water open them again before you reach the top paddles.

 

On the River Douro in Portugal with the big deep locks the water level comes back by a good foot after emptying.

 

In the UK Dropping down through Maston Doles top lock the back flow is very noticeable several seconds after the bottom gates have been opened and the boat rises quite noticeably as you try to leave

 

That's the "flush" from emptying the lock coming back up the relatively short pound. Boats moving back from the bottom gates still happens when there's a long pound after the lock.

[Top cat]

Simple answer: Its an automatic feature of locks, to drift the boat back enough to allow the gates to be opened easily.

 

Complicated answer (I'm not 100% sure of this though). The water in front of the boat is still, because now the water in the lock is at the same level with the water downstream outside it, there's no flow through paddles or gate leak. The water behind the boat is turbulent, because the top gate and/or paddles invariably leak a little bit, and cause a small waterfall or something behind. Because of this, the boat is pushed towards the turbulent water. I'm not sure if its because of flows set up by the turbulence, or because the water is less dense when its turbulent (due to trapped air) so the still water effectively 'pushes' the boat to the turbulent water. Or it might be because of a dynamic effect of the still water creating a flow and pushing the boat shortly after the bottom paddles etc are closed.

I don't think it's turbulence from leaking top gates as the same effect was noticed on the few locks where the top gates seal almost 100%.

 

TC

[Scholar Gypsy]

My first thought was the "flush" effect - ie the wave you create when opening the paddles bouncing off the next lock/bridgehole and coming back uphill.

 

My second thought - given people have observed this with a long pound/reach below - is that there is also a Bernoulli effect. Water that is flowing (ie below the lock gates) will be lower than the level in the wider pound further down (where there is to a first approximation no flow). You certainly see that effect when emptying a lock (and any boat waiting outside that is too near the gate will flow "downhill" and hit the gate). But I am not sure I buy that, as the flow out the lock reduces as it empties.

 

Of course working out the flow when the lock empties is quite a hard problem to solve anyway. It is tempting to imagine it goes vertically down until it reaches the bottom of the lock, and then horizontally along the bottom of the lock and out through the paddles, but that is not correct (unless you had baffles in the lock, a bit counterproductive). Viscosity and turbulence effects need to be taken into account.

 

I have experienced a similar effect with a guillotine gate - eg the remaining manual ones on the Nene.

 

I hope to take a fluid dynamics professor on the boat soon, and will ask him....

[by'eck]

 

That's the "flush" from emptying the lock coming back up the relatively short pound. Boats moving back from the bottom gates still happens when there's a long pound after the lock.

 

That sounds more plausible than the other suggestions.

 

Even if its a long pound the effect will still be there as the small flow dynamics eventually reverse some distance from the lock. Not so pronounced but similar to the bore effect as current reverses in a tidal river maybe.

[mayalld]

I don't think it's turbulence from leaking top gates as the same effect was noticed on the few locks where the top gates seal almost 100%.

 

TC

I have noted it in most locks, regardless of the length of the bottom pound, so I suspect not the wave returning.

 

My theory is that as the water levels equalise, the gates "relax" (the point where they drop slightly and no longer sit against the quoin).

 

Once this happens, the gate seal has gone, and instead of flowing through the paddles the remaining water flows round the gate.

 

This means a slower and more diffuse flow and the end of the force that was pulling you forward.

 

Even if we don't understand it, we can agree that it is handy.

[magpie patrick]

The water flowing out towards and through the gates paddles has momentum, which keeps it flowing out briefly after the levels have equalised, so that the level in the lock goes slightly lower than the level in the canal. That causes the flow to turn around, and there's a short period of flow into the lock. That is what nudges the boat backwards when the lock is ready. It's quite a handy sign.

 

MP.

 

This is more or less correct, there are very complex equations to describe it but I can't remember them off the top of my head and even if I could I scarcely understand them.

 

If you move water, or any other fluid around quickly it sloshes about. Like any other substance water has inertia and momentum. It takes more energy to start it moving than to keep it moving, and needs energy expended to stop it moving. In a deep lock, when the gate paddles are opened to empty it the water leaving the lock actually pushes water away to the extent of lowering the level by the bottom gates a couple of inches (this is very apparent at Marple if two strong crew members whip both bottom paddles up quickly). You can achieve the same effect in a garden pond with a high pressure hose (though your goldfish won't thank you for it...)

[Tacet]

My theory is that as the water levels equalise, the gates "relax" (the point where they drop slightly and no longer sit against the quoin).

 

Once this happens, the gate seal has gone, and instead of flowing through the paddles the remaining water flows round the gate.

 

This means a slower and more diffuse flow and the end of the force that was pulling you forward.

 

Even if we don't understand it, we can agree that it is handy.

I'm not sure about that; a cessation of a force pulling the boat forward is not the same as a backward force.

 

The reflected wave effect can be useful in lifting a boat into a lock, when the levels are low. The flush itself is partly counter-productive due to its direction but on the way back, its all good.

[KarlosMacronius]

Seems to happen more in wide locks than narrows (thought its been a while since I was in a narrow lock).

 

I don't buy the rebounding wave theory as I've had it happen when theres a good straight length of canal in front of the gate. water couldn't possibly bounce back that quickly over 2 miles or so.

I'm going with the lock emptying to slighty less than the outsid level, even if it does seem counter intuitive...

 

It sometimes opens the gates too!