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Anyone been through Froghall Tunnel... looks tight!?


larrysanders

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29 minutes ago, Captain Pegg said:

A similar statement to this regarding gauges is made whenever there’s a thread about clearances in tunnels. In reality it’s an issue of the simplicity of using a fixed 2D gauge to represent an almost infinite set of combined 3D shapes rather than the gauge being deliberately set pessimistically. Bear in mind the worst case will involve the longest length boat permitted on the navigation.

 

JP

And wishful thinking about water levels ...

 

It has been a long time since I cruised a canal over 0 level on the weir boards.

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8 hours ago, TheBiscuits said:

And wishful thinking about water levels ...

 

It has been a long time since I cruised a canal over 0 level on the weir boards.

 

But any change in the water level is surely the same for the gauge and the actual tunnel?

 

If the level is down there will be more available height in the tunnel, but that extra available height will also be reflected by the gauge.

 

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14 hours ago, Goliath said:

Worth a look. 

Good set of lime kilns to see. 

 

Nothing to stop you walking it, though!

9 hours ago, Captain Pegg said:

Bear in mind the worst case will involve the longest length boat permitted on the navigation.

 

Yes, it has to be like that -- imagine if the gauge had said you were OK but you were just 1mm too high for the tunnel?

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9 hours ago, Captain Pegg said:

A similar statement to this regarding gauges is made whenever there’s a thread about clearances in tunnels. In reality it’s an issue of the simplicity of using a fixed 2D gauge to represent an almost infinite set of combined 3D shapes rather than the gauge being deliberately set pessimistically. Bear in mind the worst case will involve the longest length boat permitted on the navigation.

 

JP


Yes, but the thing they use does make a reasonable account to take the profile of the cabin top in to consideration.

I can't actually see why the length of the boat enters in to it, (provided you make a straight approach t the tunnel mouth - we failed to!)

I would say that "pessimistic" is actually a very good description of the gauge at the lock.  We failed that by several inches, particular on the extremes of the cabin top at the grab rails, but still traversed the tunnel

 

DSCF4770.JPG

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12 minutes ago, Jen-in-Wellies said:

Oversize boats are doing us all a favour trying to get through the tunnel and grinding away at the roof. Each one helps improve the situation for the next!

 

Jen ?

It's taking a while, though: when was the tunnel built?

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41 minutes ago, Athy said:

It's taking a while, though: when was the tunnel built?

I take it that the tunnel didn't start off that low. Particularly as the profile changes so much through it. Was it mining subsidence, or collapse of the arch from the ground on top? It is effectively a bridge for the road going over the top and the weight on top of the arch isn't that much compared to many other tunnels. If the canal to Uttoxeter ever gets restored beyond the basin it is going to seriously restrict the number of boats that can get there.

 

Jen

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1 hour ago, Jen-in-Wellies said:

Oversize boats are doing us all a favour trying to get through the tunnel and grinding away at the roof. Each one helps improve the situation for the next!

 

Jen ?

Thats a strange one, I thought these days if something looked bit of a challenge people just moored up and visited on foot or in some cases by train. ie. Oxford, moor at Hayward and take the train in.

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7 minutes ago, ditchcrawler said:

Thats a strange one, I thought these days if something looked bit of a challenge people just moored up and visited on foot or in some cases by train. ie. Oxford, moor at Hayward and take the train in.

I must admit I wimped out and walked the last bit from the tunnel west end to the basin. That was a few years ago and now I'd probably have a go. Taken the boat to some places since where it turned out there wasn't enough water depth, or air draft. Managed to get out so far with perhaps some damaged paint to repair.

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1 hour ago, larrysanders said:

any idea on why this tunnel was built so low?   have water levels gone up slightly since it was built or was it always like this?

Probably because it didn't need to be any higher, No leisure boats then with 6' 6" inside headroom 

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8 hours ago, Athy said:

Wasn't the tunnel dredged out to increase its usable height a few years ago?

I seem to remember that it was the removal of an obstruction just before the tunnel that allowed the water level to be lowered!

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9 hours ago, Machpoint005 said:

Yes, it has to be like that -- imagine if the gauge had said you were OK but you were just 1mm too high for the tunnel?

 

9 hours ago, alan_fincher said:


Yes, but the thing they use does make a reasonable account to take the profile of the cabin top in to consideration.

I can't actually see why the length of the boat enters in to it, (provided you make a straight approach t the tunnel mouth - we failed to!)

I would say that "pessimistic" is actually a very good description of the gauge at the lock.  We failed that by several inches, particular on the extremes of the cabin top at the grab rails, but still traversed the tunnel

 

It’s a four dimensional issue; you’re only thinking in two. The four dimensions are the three that the boat and tunnel have plus the fact that since the boat has a (mostly) fixed profile but the tunnel has a constantly changing one the boat has to be able to negotiate its three dimensions through the changing profiles of the tunnel. Therefore time is the fourth dimension.

 

If the gauge was exactly the same size as the smallest part of the tunnel how would a boat that exactly fitted that profile be able to cope with any adjacent undulations in the profile of the walls or shifts in the centre line of the tunnel (both of which occur continuously along any masonry lined tunnel I know)? It would get stuck.

 

Many boats have a larger profile at the front and given more generous clearance at the back can negotiate the irregularities of the tunnel walls. However the gauge needs to cope with a boat of 70’ length (with perhaps a 60’ cabin) and a constant cabin shape so it has to make allowance for such a boat to cope with the irregular nature of the walls.

 

The gauge also has to cope with the fact that tunnel walls move with time as they are flexible structures and any measurement survey will have a degree of accuracy of measurement of which the worst case has to be used.

 

When I set up Vulpes to transit Dudley Tunnel I could make either the front or rear of the cabin the highest point according to how I trimmed the boat (within the rules that prohibit ballast other than in tanks and lockers designed for the purpose). I trimmed the boat level for the passage which meant both front and back of the cabin were broadly equally close to the tunnel walls which limited the manoeuvrability within the tunnel in a way that wouldn’t restrict a boat that trimmed downward all the way the along its cabin as many boats do. Being 35’ long with a 24’ cabin this wasn’t an issue and neither handrail ever touched the wall. However if there had been an additional 35’ of hull and cabin in the middle between the highest and widest points then the envelope required for the boat to negotiate the irregular profile of the tunnel would clearer be bigger. However as far as the tunnel gauge would be concerned it would show exactly the same clearance to the worst case locations at the front and rear of the cabin.

 

For me it’s entirely logical that many boats can strike the gauge yet still negotiate the tunnel. It may make the gauge appear pessimistic but I still contend it’s really because it’s a simplistic model of the actual problem. In any case I doubt CRT have a particularly sophisticated and accurate model of the tunnel but even if they did and wished to continue using a gauge - which let’s face it is a very practical way of doing things from the boater’s perspective - it would still appear by nature to be pessimistic even if it were statistically very accurate.

 

For starters there are many different ways you could go about representing an irregular 3D shape in 2D form; all of which would be simplifications of reality to a greater or lesser degree.

 

JP

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1 minute ago, nicknorman said:

We failed the gauge by a couple of inches, but made it through, just. Neutral, me pushing through by hand on the roof at the back, Jeff ditto at the front. Very pleasant on the other side, so worth it.

That was our method of reversing out of the tunnel when the solar panel hit the low bit on attempt #1!

 

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10 hours ago, Jen-in-Wellies said:

Taken the boat to some places since where it turned out there wasn't enough water depth, or air draft. Managed to get out so far with perhaps some damaged paint to repair.

That's one of our favourite games, with a photo to prove it.

 

We are not trying for the Silver Propeller list, because it misses out too many great bits!

 

To stay a bit on-topic, we have photos of us grounded in the shallows in the basin at Froghall, touching the stop planks on the next (disused) lock on the Uttoxeter canal,

 

20180628_163248.jpg.3f6d1a7f2d4e8d6ebc9a44c7f5f052f5.jpg

 

and also going up past the cafe until we grounded at the end of the arm.  Someone came out of the cafe to tell us we were going the wrong way ...

 

20180628_165045.jpg.fd09dbaf5fda99d069357a4c7d7d4ccd.jpg

 

 

Going the other way up the Caldon we went right up to the feeder at Leek, well past the sign saying "Not navigable to boats over 45 feet."

 

20180629_161448.jpg.54ca494a57d7f4ee8a1c5e0702cba542.jpg

 

We were quite amused by this.  It was blindingly obvious that nothing longer than a canoe has winded in the feeder pool for years!  It took us about two hours to wind, because I didn't fancy reversing half a mile down a 5' wide channel with reeds both sides ...

 

20180629_164257.jpg.8578fb307f7803c39d83c97542c3c229.jpg

 

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5 hours ago, Mike Todd said:

How does that work?

As I recall, the water sits lower in the tunnel so there is more air draught. If you just reduced the water level without dredging, boats would be more prone to grounding.

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We failed to pass through on the first occasion - having to reverse out.  Next time, the water tank was filled and the well deck stacked with coal - and we got through.  The water level may have changed, of course.

 

Has anyone managed to pass under the gauge with brushing it away?

 

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47 minutes ago, Tacet said:

We failed to pass through on the first occasion - having to reverse out.  Next time, the water tank was filled and the well deck stacked with coal - and we got through.  The water level may have changed, of course.

 

Has anyone managed to pass under the gauge with brushing it away?

 

Post 29

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48 minutes ago, Tacet said:

We failed to pass through on the first occasion - having to reverse out.  Next time, the water tank was filled and the well deck stacked with coal - and we got through.  The water level may have changed, of course.

 

Has anyone managed to pass under the gauge with brushing it away?

 

 

1 minute ago, matty40s said:

Post 29

Sorry - I meant to say without brushing the gauge away.  Ie. Has pretty much every boat passing through the tunnel previously failed the gauge test?

 

 

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