Lock gate configuration on narrow canals

[MtB]

Something that has always puzzled me, is why, on narrow canals, are the bottom gates a pair of mitre gates and the top gate a single panel gate?

 

The only reason I can think of is cost. Single panel gates I imagine are cheaper, but the extra cost of a pair of mitre gates means the lock chamber can be built two feet shorter.

 

Is that it? Or is there a more obvious reason, concealed from me by the alcohol?

 

 

MtB

[twbm]

Bottom gates carry the weight of the canal depth plus the drop of the lock, and a the double gate creates a triangle i.e. strongest shape.

Edited January 27, 2013 by twbm

[Paul C]

I thought they were a pair on the bottom gates because a single gate would be heavy to use (I know, exceptionally, some locks have single bottom gates). (And that the equivalent on a broad lock bottom gates are a pair, slightly bigger still).

[nicknorman]

I thought they were a pair on the bottom gates because a single gate would be heavy to use (I know, exceptionally, some locks have single bottom gates). (And that the equivalent on a broad lock bottom gates are a pair, slightly bigger still).

Yes, both are right really. If you had one gate, it would have to be built stronger, so heavier than the two, and so much harder to operate. However, there are of course some canals with single bottom gate (S Stratford for one I think) and they are pigs to operate, so the single top, double bottom is definitely the best solution overall.

 

Its only a pest if you feel you don't want to hop over an open double gate to get to the other side.

Edited January 27, 2013 by nicknorman

[IanM]

Something that has always puzzled me, is why, on narrow canals, are the bottom gates a pair of mitre gates and the top gate a single panel gate?

 

The only reason I can think of is cost. Single panel gates I imagine are cheaper, but the extra cost of a pair of mitre gates means the lock chamber can be built two feet shorter.

 

Is that it? Or is there a more obvious reason, concealed from me by the alcohol?

 

 

MtB

 

On narrow locks there can be single top and single bottom gates, single top and double bottom and if I remember correctly at Ellesmere Port (there may be other locations) double top and double bottoms.

 

I think it's down to location and fashion rather than uniform convention

[AndrewIC]

A pair of bottom gates also requires less clear space to open into than a single gate, so a lock with a single gate will be slightly longer and use more water than one with a pair of bottom gates.

[Capt Ahab]

Bottom gates carry the weight of the canal depth plus the drop of the lock, and a the double gate creates a triangle i.e. strongest shape.

... and single bottom gatestend to be used on the shallower drops like the S Stratford and chunks of the BCN.

[alan_fincher]

I thought they were a pair on the bottom gates because a single gate would be heavy to use (I know, exceptionally, some locks have single bottom gates). (And that the equivalent on a broad lock bottom gates are a pair, slightly bigger still).

A single bottom gate on a narrow canal typically will weigh about the same as one gate of a bottom gate pair on a broad canal, and they are not considered too heavy to use!

[nicknorman]

A single bottom gate on a narrow canal typically will weigh about the same as one gate of a bottom gate pair on a broad canal, and they are not considered too heavy to use!

No, but a lot heavier than the top gate, or one of a pair of bottom ones.

 

The point about the increased lock length is a good one, I hadn't thought of that.

[furnessvale]

On narrow locks there can be single top and single bottom gates, single top and double bottom and if I remember correctly at Ellesmere Port (there may be other locations) double top and double bottoms.

 

I think it's down to location and fashion rather than uniform convention

There is at least one location with a single bottom gate and double top gates (Hall Green Macc Canal) but there is a specific reason for that.

 

George ex nb Alton retired

[alan_fincher]

A pair of bottom gates also requires less clear space to open into than a single gate, so a lock with a single gate will be slightly longer and use more water than one with a pair of bottom gates.

This is the explanation I tend to favour.

 

It costs more to build with double bottom gates, but the locks do use a bit less water as a result.

 

If you think about the Southern Oxford, towards the Southern end, where the funds to build it were more stretched, they economised and one of those economies was single gates at the bottom, (another of course being the typical Oxford lift bridge, instead of brick overbridges).

 

I'm guessing because of the river fed elements at the Southern end, slightly more water used for each lock operation was considered worth trading for being able to build the locks more cheaply.

 

There is at least one location with a single bottom gate and double top gates (Hall Green Macc Canal) but there is a specific reason for that.

 

George ex nb Alton retired

This one ?

 

[system 4-50]

I will try again, the wine has worn off. (Unfortunately.)

Single gates are simpler & therefore cheaper and more reliable and would be used at both ends but they take the load in line with the lock and so need a large mass of stonework behind them in line with the lock.

For the top lock this is not a problem as the stonework of the lock chamber itself provides that mass.

For the bottom lock where the ground level typically falls away quickly this is a problem, and a means of reducing the mass needed is desirable.

Double gates, ie mitred gates, transfer roughly half the imposed load perpendicularly to the line of the lock and therefore reduce the required supporting mass by half. Therefore mitred gates are used for the bottom gates.

If this theory is correct you would expect any lock that has problems with the solidity and mass of its lock chamber to have mitred gates at the top of the lock, and any lock that happens to have substantial supporting mass behind/below the lock to have a single gate at the bottom of the lock.

Any takers?

Edited January 28, 2013 by system 4-50

[BruceinSanity]

There is at least one location with a single bottom gate and double top gates (Hall Green Macc Canal) but there is a specific reason for that.

 

George ex nb Alton retired

 

And Bosley has double gates top and bottom, with no bridge at the top, a right PITA. The last time I fell in was trying to shove the offside top gate shut from the towpath side with a shaft that was just too short…

 

 

I've read somewhere (no idea if it's true) that the idea was that the lockie worked the towpath side and the boater the offside, so no need for a bridge. There were landing stages on the offside at the bottom to let the boater get off.

[nicknorman]

I will try again, the wine has worn off. (Unfortunately.)

Single gates are simpler & therefore cheaper and more reliable and would be used at both ends but they take the load in line with the lock and so need a large mass of stonework behind them in line with the lock.

For the top lock this is not a problem as the stonework of the lock chamber itself provides that mass.

For the bottom lock where the ground level typically falls away quickly this is a problem, and a means of reducing the mass needed is desirable.

Double gates, ie mitred gates, transfer roughly half the imposed load perpendicularly to the line of the lock and therefore reduce the required supporting mass by half. Therefore mitred gates are used for the bottom gates.

If this theory is correct you would expect any lock that has problems with the solidity and mass of its lock chamber to have mitred gates at the top of the lock, and any lock that happens to have substantial supporting mass behind/below the lock to have a single gate at the bottom of the lock.

Any takers?

Can't fault that logic.

[Pluto]

A list of which canals have which type of gates would be useful, as one answer would be that mitre gates can be quicker to operate, on average, than single gates. It could be that early canals had single gates, and later canals had mitre gates, as by then time passing a lock had become more important. Mitre gates required more substantial stonework to resist the sideways thrust generated by the mitre, and this may have been cheaper than the longer lower entrance required by single gated locks, where the thrust was in line with the stonework. In this case, the stonework needed more dressing as it was exposed, compared to mitre gates, where the counterfort was covered. It could also be that individual engineers had their own preference. As I suggested at first, it is something which can only really be considered properly when a full list has been compiled. Mind you, lack of facts never stopped anyone commenting on this, and most other, forums.

[Jim Riley]

Hall green is just a stop lock, about 2" rise? so no difference in the water pressure on either gate.

[Keeping Up]

Hall green is just a stop lock, about 2" rise? so no difference in the water pressure on either gate.

 

Originally it was two stop locks, end-to-end facing in opposite directions, one owned by each canal company. The level difference could go either way and was a good deal greater than 2" normally until the level of the T&M was lowered to increase the headroom in the Harecastle Tunnel

[DaveC]

Originally it was two stop locks, end-to-end facing in opposite directions, one owned by each canal company. The level difference could go either way and was a good deal greater than 2" normally until the level of the T&M was lowered to increase the headroom in the Harecastle Tunnel

 

 

Harecastle tunnel is at the lower level of the stop lock. Therefore if the level in the tunnel was lowered then the difference in levels at the lock would have increased.

 

 

 

Dave

[alan_fincher]

Any takers?

Not convinced, I'm afraid.

I believe it is generally accepted that the lower end of the Southern Oxford was built single gated to save money.

 

So even if more masonry is required for single gates, (which I'm not convinced about, as only half the forces of a single gate get applied to either pier), it still seems you can spend less if you omit double gates.

 

The "uses less water" is still the most compelling argument to me, as water shortages were a constant cause of problems on many canals for much of their commercial lives.

 

Can't prove it though, unless someone has the original thoughts of the likes of Brindley documented somewhere!

 

Next question.....

 

Why do some (most?) narrow canals have the balance beams of single top gates on the tow-path side, but some choose instead to largely have them on the non tow-path side?.......

[Keeping Up]

Harecastle tunnel is at the lower level of the stop lock. Therefore if the level in the tunnel was lowered then the difference in levels at the lock would have increased.

 

 

 

Dave

 

 

Yes but until then it was well above the level of the other half of the stop lock, except when the level happened to be down in which case it could be more than 2" difference (in the current direction)

[trackman]

The plain fact is that bottom gates open into the lock. Mitred bottom gates need less room inside the lock and so the lock can be shorter, saving money in lock construction and saving water in use, a double benefit.

 

The top gates open out away from the lock (and any boat in it), and so it makes no difference to the lock length which sort of gate is used. As a single gate may be cheaper to build then a mitred pair, and given that a top gate will be relatively light, there is no benefit to using mitred top gates where there is no special need for this.

 

 

[Pluto]

The plain fact is that bottom gates open into the lock. Mitred bottom gates need less room inside the lock and so the lock can be shorter, saving money in lock construction and saving water in use, a double benefit.

 

The top gates open out away from the lock (and any boat in it), and so it makes no difference to the lock length which sort of gate is used. As a single gate may be cheaper to build then a mitred pair, and given that a top gate will be relatively light, there is no benefit to using mitred top gates where there is no special need for this.

When early English canals were built in the 1770s, measurement was not that standardised - you couldn't just go out a buy a tape or rule. To allow for slight local variations it is possible that these canals were built with lock chambers around 74 feet in length. As more canals were built, it became easier to standardise measurements, and locks built in the 1790s-1800s were slightly shorter, just long enough to take a boat. However, this must has caused problems in operation, as anyone with a full length boat will confirm, and canals built 1810-20 had locks around 75 feet in length, with plenty of room for opening and shutting gates.

 

I haven't done a full survey of lock lengths to confirm the above, but from what I have done there seems to be something like that affecting lock length. The difference in water usage by adding a couple of feet to a lock would be marginal, particularly when compared to other losses.

 

Another way of looking at the problem concerns the boats themselves. The very first narrow boats were 68 feet long, and this length continued to be used on canal boats around Manchester into the twentieth century. The length comes from the length of the majority of flats which could work up the rivers around Liverpool. The length of lock chambers were set at around 72 feet to allow clearance, and as measurement became more standardised, as suggested above, then narrow boats were built to the maximum possible. Again, this would have been a problem for those working the boats, accounting for the slightly longer locks on 1810-20 built canals.

[Mac of Cygnet]

I suddenly have a strong sense of deja vu:

 

Identical discussion 2009