narrow locks

[john b.]

Hi all,

On the Shroppie this weekend, after my wife had trudged round another lock to shut the double bottom gates she asked me why do we have these double gates and not just a single one. Embarrassingly I had to say I dunno. Who's going to be the first clever one to tell us?

[Iain_S]

Hi all,

On the Shroppie this weekend, after my wife had trudged round another lock to shut the double bottom gates she asked me why do we have these double gates and not just a single one. Embarrassingly I had to say I dunno. Who's going to be the first clever one to tell us?

 

If the bottom gates are double, the lock can be 3'6" shorter

Iain

[Night Hawk]

Has it got something to do with the fact that the gates are much longer and heavier so with two of them, they're easier to shift!

[NB Alnwick]

Has it got something to do with the fact that the gates are much longer and heavier so with two of them, they're easier to shift!

 

On the Oxford, south of Banbury, they have single gates at each end - apparently this was to cut costs!

[Chris J W]

On the Oxford, south of Banbury, they have single gates at each end - apparently this was to cut costs!

 

And the last half-dozen on the Diggle Flight of the HNC (West) have them, too.

[alan_fincher]

If the bottom gates are double, the lock can be 3'6" shorter

Iain

 

I have always though this argument one of the most plausible.

 

Water supply was one of the biggest threats to many canal schemes, with boats often unable to move for extended periods, or at least reduced lock opening hours. Anything that saved around 5% of the water on each lockage would surely have been very valuable, even if it cost more to build the lock.

 

I have also heard the "cost cutting south of Banbury" argument for the single bottom gates on the Oxford too. Again quite plausible, particularly coupled with the many lift bridges that are obviously cheaper than brick built over bridges on other cuts.

 

I don't know the Oxford well, but isn't it probably case that the bit below Banbury has less problems with water supply, because the river flows in and out, so slightly more water used each time a lock was used might have been considered less important.

 

It's often possible with double gates to step from one to the other if one is closed and one open. Depending upon your agility you don't always have to walk around.

 

Unscrupulous use of a short shaft is also often a good alternative to something more energetic!

[NB Alnwick]

It's often possible with double gates to step from one to the other if one is closed and one open. Depending upon your agility you don't always have to walk around.

 

The way I open the narrow lock double gates (when they are ready) is to open the nearest one just enough so that it doesn't slam closed again, then walk to the middle and push the partially open gate with my trailing foot until it is fully open - I then proceed across the closed gate and open it in the conventional manner. When the gates need closing, I close the nearest one, then walk across it and step across from the closed gate onto the open gate, then close that one in the normal way. It usually works but I won't try it on locks with dodgy handrails or when it is very slippery.

Edited July 12, 2009 by NB Alnwick

[Heffalump]

Has it got something to do with the fact that the gates are much longer and heavier so with two of them, they're easier to shift!

 

longer gates in a narrow lock?

[NB Alnwick]

longer gates in a narrow lock?

 

I think he meant that the bottom gates are much deeper, therefore heavier - when compared to the top gate(s). The depth of the top gate is usually about five feet (depending on the depth of the canal) whereas the depth of the bottom gate(s) will exceed the full depth of the lock.

[Heffalump]

ohh, I thought I was missing something! Bloomin Jamaican beer

[magpie patrick]

If the bottom gates are double, the lock can be 3'6" shorter

Iain

 

 

No on really knows but that's the best guess, saves water. Where single bottom gates were used the motives seem to have varied, on the Oxford and Southern Stratford they cut costs, on the BCN single handed working was made easier, and at Diggle all the furnitue was put on the non towpath side, including both paddles, so as to leave the route clear for the tow rope. The footbridges there are a modern addition.

 

One thing most people forget is that single leaf gates were almost unheard of before narrow locks came along

[MartinClark]

I think the question that really needs answering is why the hell some narrow locks have seemingly unnecessary double gates at the top end of the lock (such as the Bosley flight).

 

I can't see any possible advantage to this and can certainly think of one disadvantage!

 

Unlike some of you, I don't do stepping over the gap between narrow gates - I don't have the confidence to step over gaps above water like that, even when the walkways are not slippery!

[magpie patrick]

I think the question that really needs answering is why the hell some narrow locks have seemingly unnecessary double gates at the top end of the lock (such as the Bosley flight).

 

I can't see any possible advantage to this and can certainly think of one disadvantage!

 

Unlike some of you, I don't do stepping over the gap between narrow gates - I don't have the confidence to step over gaps above water like that, even when the walkways are not slippery!

 

 

It seems to have been a whim of some engineers and there were once a lot more than there are now! Shropshire Union Main Line and parts of the Montgomery for example. On the monty at Welshpool there were originally cast iron gates, which in theory had an unlimited life (they lasted the working life of the canal before being filched for the museum at Stoke Bruerne). Perhaps they couldn't make a single leaf cast iron gate.

 

The first narrow locks had single top and double bottom gates, and this design became near universal

[dor]

I always thought it was because the gates would be very heavy if they were a single gate - some of the shroppie ones are havy enough to open or close as half gates on some of the deeper locks. However, that doesn't explain how it is possible to handle a gate on a wide lock, which presumably would be the same sort of weight as a full width bottom gate on a narro lock.

[furnessvale]

Unscrupulous use of a short shaft is also often a good alternative to something more energetic!

 

Nothing unscrupulous about a shaft on double gates. Especially Bosley top gates where I had a special long but thin shaft, hence I could easily handle it one handed. This was ideal for opening/closing the top gates when single handing.

 

George ex nb Alton retired

[Pluto]

One thing most people forget is that single leaf gates were almost unheard of before narrow locks came along

What about single leaf flash locks, such as those on the Stecknitz Canal, built 1398? Flash locks had single gates or staunches which could be removed. There are also the small Dutch canals, and those of the fen areas, where single leaf gates were raised vertically.

 

Double gates could speed the passage of a boat as they could be closed slightly earlier than single gates, especially useful if you had a large crew, that's in number, not size.

[Steilsteven]

Someone's bound to shoot me down in flames for this but I believe they were designed like this so that when a boat was entering an empty lock from downhill, the water pushed ahead of the bow would open the gates.

 

Keith

[magpie patrick]

Someone's bound to shoot me down in flames for this but I believe they were designed like this so that when a boat was entering an empty lock from downhill, the water pushed ahead of the bow would open the gates.

 

Keith

 

Errm, no. That may have been a benefit (I don't know whether it is) but the very first narrow locks in this country were considerably narrower than anything previously built for general navigation (rather than flooded mines or whatever) in this country. For whatever reason, when the first locks were built, on the T and M, the S and W, and the Birmingham main line, they all had single top gates and double bottom gates.

 

There were one or two locks with single gates, I believe Pallingham on the Arun had a single top gate, but the norm, before narrow canals were built, was for locks to have two gates at each end. So we have to assume the Brindley came up with the single top gate and the double bottom gates as a standard design concept.

[nickd]

It's because of the length of the lock, with two 1/2 width lock gates the length of the lock can be shorter than 1 full width gate (for a full length narrowboat).

 

Regards,

 

Nick

[magpie patrick]

What about single leaf flash locks, such as those on the Stecknitz Canal, built 1398? Flash locks had single gates or staunches which could be removed. There are also the small Dutch canals, and those of the fen areas, where single leaf gates were raised vertically.

 

Double gates could speed the passage of a boat as they could be closed slightly earlier than single gates, especially useful if you had a large crew, that's in number, not size.

 

Yes, there were single gates on flash locks here as well, and other countries make their locks quite a bit bigger before resorting to double gates (Dalsland Kanal, Sweden, and Canal Du Berry, France, both have single gates at both ends, although in the case of the Dalsland water supply is not an issue.

 

The point I was making was that the standard use of single gates was an innovation (the standard bit, not the gates themselves), whereas double gates were routine.

[Pluto]

The point I was making was that the standard use of single gates was an innovation (the standard bit, not the gates themselves), whereas double gates were routine.

Double gates probably became standard with the development of the mitre gate by Leonardo da Vinci in the 1490s. It is thought that earlier chamber locks, those on the Canal de Bereguardo c1458, may have had gates which were lifted vertically. There were also double gates which folded back onto a cross beam, the beam being swung out of the way for boats to pass after the gates were opened. Such gates would tend to leak, the major benefit of mitre gates being their self-sealing properties. It could have been this which encouraged some narrow canal builders to use double lower gates where water pressures could cause leakage problems for single gates. Upper gates could have been single, the cheaper alternative as just one gate anchor and its associated stonework were needed, as they were considered to be less likely to leak.