Mitre gates versus single leaf - which is safer?

[magpie patrick]

I'm thinking of narrow locks, or at least not very wide ones!

Most of you will know that bottom gates on locks occasionally "blow" - giving way when the lock is full, usually when hit by a boat. They may not have been hit very hard but a weakness in the gate is revealed and woooosh the boat goes down the lock very quickly along with a lot of water. 

I can only think of incidents of this happening with mitre gates - has it ever happened with  single leaf gate? Could it happen with a single leaf gate?
 

 

[Jen-in-Wellies]

A single leaf gate in a decrepit state would be more at risk of blowing out I'd have thought. An arch is stronger than a beam and a pair of mitre gates is a sort of arch in construction.

Other safety aspects would favour a single leaf. No risk of a bow, or rudder getting trapped in the mitre and the boat either hung up in falling water, or trapped down as the water rises. Also no risk of someone doing the bold step from a single closed mitre gate to the other and falling in.

A lock would need to be designed with single leaf gates in mind as they need more space to swing for the same maximum length boat.

 

Edited to add: I suspect it is rare to hear of a mitre narrow bottom lock gate blowing out because narrow mitre bottom lock gates are themselves rather rare! The few examples I can think of off the top of my head are on stop locks, so you only drop 6" or a foot in the resulting wooosh!

Jen

Edited April 21, 2021 by Jen-in-Wellies

[Detling]

1 hour ago, Jen-in-Wellies said:

 

 

Edited to add: I suspect it is rare to hear of a mitre narrow bottom lock gate blowing out because narrow mitre bottom lock gates are themselves rather rare! The few examples I can think of off the top of my head are on stop locks, so you only drop 6" or a foot in the resulting wooosh!

Jen

Maybe you need to travel more, virtually every narrow lock has mitre gates at the bottom and single top gates. South Stratford is an exception. The mitre resists the greater water pressure better as the pressure pushes the gate shut. This is very necessary on staircase locks such as Foxton.  Mitre gates also permit the jump from one gate to the other as it is only about 3 1/2 foot from the walkway to the other gate walkway, assuming you don't mind falling 10 feet into 3 foot of cold water

[Jen-in-Wellies]

4 minutes ago, Detling said:

Maybe you need to travel more, virtually every narrow lock has mitre gates at the bottom and single top gates. South Stratford is an exception. The mitre resists the greater water pressure better as the pressure pushes the gate shut. This is very necessary on staircase locks such as Foxton.  Mitre gates also permit the jump from one gate to the other as it is only about 3 1/2 foot from the walkway to the other gate walkway, assuming you don't mind falling 10 feet into 3 foot of cold water

The thread is about bottom gates. I'm talking about bottom gates. Read the first post. I've travelled by boat quite a lot thanks.

[Mike Todd]

2 minutes ago, Jen-in-Wellies said:

The thread is about bottom gates. I'm talking about bottom gates. Read the first post. I've travelled by boat quite a lot thanks.

I am not sure you are making yourself clear as you seem to be talking abut exactly what Detling refers to. Perhaps it would help if you explained the difference.

[GUMPY]

12 minutes ago, Jen-in-Wellies said:

The thread is about bottom gates. I'm talking about bottom gates. Read the first post. I've travelled by boat quite a lot thanks.

Northampton arm mitre bottom gates

Aylesbury arm mitre bottom gates 

That's 34 locks to start with?

Oxford canal mitre bottom gates.

I could go on ?

Could it be a terminology problem?

 

Mitred bottom gates

Edited April 21, 2021 by Loddon

[David Mack]

4 minutes ago, Jen-in-Wellies said:

The thread is about bottom gates. I'm talking about bottom gates. Read the first post. I've travelled by boat quite a lot thanks.

I would have said mitred bottom gates are more common than single bottom gates on narrow locks. From memory single bottom gates are mostly found on the Southern Oxford, South Stratford and some BCN flights. Most of the rest have twin bottom gates. 

Twin top gates are rare on narrow locks. The ones that come to mind are Bosley on the Macclesfield and Fairfield on the Ashton.

[Jen-in-Wellies]

5 minutes ago, Mike Todd said:

I am not sure you are making yourself clear as you seem to be talking abut exactly what Detling refers to. Perhaps it would help if you explained the difference.

Well, the first post says:

2 hours ago, magpie patrick said:

Most of you will know that bottom gates on locks occasionally "blow" - giving way when the lock is full, usually when hit by a boat. They may not have been hit very hard but a weakness in the gate is revealed and woooosh the boat goes down the lock very quickly along with a lot of water. 

I can only think of incidents of this happening with mitre gates - has it ever happened with  single leaf gate? Could it happen with a single leaf gate?

So @magpie patrick  was asking about bottom gates and that is what I replied to. I then went on to give my thoughts. I am fully aware that they single leaf gates are not commonly used on the bottom and are on the top, with some exceptions, like the Bosley flight that has mitres both ends. The reasons, I believe are more space required to swing the bottom gate past a boat in the lock and possibly less strength than a pair of mitres. Not sure how much more clear I needed to be?

[David Mack]

2 hours ago, Jen-in-Wellies said:

because narrow mitre bottom lock gates are themselves rather rare! The few examples I can think of off the top of my head are on stop locks, so you only drop 6" or a foot in the resulting wooosh!

I think this is your statement that puzzles the rest of us. Most of us would regard them as the most common type.

[Jen-in-Wellies]

2 hours ago, Jen-in-Wellies said:

Edited to add: I suspect it is rare to hear of a mitre narrow bottom lock gate blowing out because narrow mitre bottom lock gates are themselves rather rare! The few examples I can think of off the top of my head are on stop locks, so you only drop 6" or a foot in the resulting wooosh!

I see the problem. For mitre read single leaf, which is what I intended to say! Couldn't see it for looking.

Edited April 21, 2021 by Jen-in-Wellies

[Tacet]

Neither type of gate commonly fails in the spectacular sense.  A single bottom gate would have to break across its narrow width where one of a pair could wind and pass the other, if struck with sufficient force.

 

Possibly the greater risk with a single gate is the fixed stop on the hanging- or clapping-side giving way.  But with a 7' boat in a similar width lock, it is challenging to largely concentrate the thump to one side only.

 

 

 

[PeterF]

50 minutes ago, David Mack said:

I would have said mitred bottom gates are more common than single bottom gates on narrow locks. From memory single bottom gates are mostly found on the Southern Oxford, South Stratford and some BCN flights. Most of the rest have twin bottom gates. 

Twin top gates are rare on narrow locks. The ones that come to mind are Bosley on the Macclesfield and Fairfield on the Ashton.

Diggle flight at the western end of Standedge tunnel single top and bottom gates, ground paddles to fill and empty the locks, paddles on the same side as the lock beams. Much less walking around to do these locks. They are also relatively deep, sure they are around 10ft each.

[David Mack]

1 hour ago, Tacet said:

 

Possibly the greater risk with a single gate is the fixed stop on the hanging- or clapping-side giving way.  But with a 7' boat in a similar width lock, it is challenging to largely concentrate the thump to one side only.

 

The 'fixed stop' as you term it is usually part of the masonry of the lock chamber, although the mating face with the gate may be fitted with a cast iron or timber piece to improve the seal. So a complete failure of the 'fixed stop' is very unlikely. Failure of these does occur but that results in leakage and a relatively slow emptying (or filling) of the lock, and not the catastrophic failure envisaged in the OP.

[Tacet]

1 hour ago, David Mack said:

The 'fixed stop' as you term it is usually part of the masonry of the lock chamber, although the mating face with the gate may be fitted with a cast iron or timber piece to improve the seal. So a complete failure of the 'fixed stop' is very unlikely. Failure of these does occur but that results in leakage and a relatively slow emptying (or filling) of the lock, and not the catastrophic failure envisaged in the OP.

Accepted point.  Less likely to occur at the top gate as the length and depth of the lock wall masonry provides support.  But if there is skimping in support beyond the end of the lower gates, the masonry could part company.

[Pluto]

The attached may be of interest as it gives a few details of some French experiments on lock gate strength. In theory, mitre gate locks can be shorter as some of the forces on the gate are directed at right angles into the sides of the chamber, while for single gates they are fully in line and may require an entrance to the lower end of the lock which is longer than that for those with mitre gates.

1836 Barlow on lock gates.pdf

[Machpoint005]

Top gates are usually only four or five feet deep. Bottom gates can be suge great things.

[ditchcrawler]

4 hours ago, Detling said:

Maybe you need to travel more, virtually every narrow lock has mitre gates at the bottom and single top gates. South Stratford is an exception.  

I can think of lots more, but then I don't go far but I did go through one today.

[system 4-50]

2 hours ago, Pluto said:

The attached may be of interest as it gives a few details of some French experiments on lock gate strength. In theory, mitre gate locks can be shorter as some of the forces on the gate are directed at right angles into the sides of the chamber, while for single gates they are fully in line and may require an entrance to the lower end of the lock which is longer than that for those with mitre gates.

1836 Barlow on lock gates.pdf 744.65 kB · 1 download

Hooray! I got shouted down when I suggested this some years ago when the forces involved were discussed on this forum previously.

[magpie patrick]

Thanks guys and gals! My original concern was over gate failure, I'm not ptoposing to change existing structures but from time to time I see proposals for deep locks that drop straight into a tunnel where surge discharge isn't possible, and gate failure would lead to substantially raised water levels under a headroom restriction. I was pondering whether a single leaf gate might be safer than mitre gates in these circumstances - not so much the risk of failure as the consequence

 

At Tuel Lane I think boats are barred from entering the tunnel until the lock is ready for them, at Dalmuir Drop Lock the operation is supervised with barriers under the bridge for precisely this reason. Both are, of course, staffed

[Mike Todd]

20 hours ago, Tacet said:

Neither type of gate commonly fails in the spectacular sense.  A single bottom gate would have to break across its narrow width where one of a pair could wind and pass the other, if struck with sufficient force.

 

Possibly the greater risk with a single gate is the fixed stop on the hanging- or clapping-side giving way.  But with a 7' boat in a similar width lock, it is challenging to largely concentrate the thump to one side only.

 

 

 

We came through Huddersfield Broad several years ago and I was quite concerned about the bottom gates as they gave the appearance that one might slip past the other. I havce not heard either that it did so fail or that any replacement has yet ben needed so I guess my concern was unwarranted!