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This technic is especially useful with a loaded boat on narrow waterways as the amount of water displaced as the bow goes into the bridge is compensated by the lift from backing off. Don’t remember going out of gear just wound off and as soon as got the lift from the following wave wound on again, by then you knew if there was anything in the bridge.

However this trick didn’t stop a friend getting stuck under bedworth bridge hard and fast just behind the mast. Needed tirfors and eventually a JCB to pull him back off what turned out to be a safe taken from the local co-op and dumped off the bridge when they couldn’t open it.

i still slow down at every bridge.

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On 18/05/2018 at 11:10, Nightwatch said:

Only 18hp, mine 50 he. Shirley nearly a third the size!!! Another smilie not available.

When do you use 50HP then?

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8 minutes ago, Nightwatch said:

Some of it, all the time. The rest not really. 

Most of the time, you're using enough to use about 1 litre of fuel an hour. Pretty much the same as any other narrowboat with any other engine

 

I don't have an 18HP available, so you're safe

 

Richard

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58 minutes ago, Dav and Pen said:

This technic is especially useful with a loaded boat on narrow waterways as the amount of water displaced as the bow goes into the bridge is compensated by the lift from backing off. Don’t remember going out of gear just wound off and as soon as got the lift from the following wave wound on again, by then you knew if there was anything in the bridge.

I would only knock out of gear if I suspected / knew that the bridge was full of soft rubbish that would catch on the propeller, otherwise there is no point :captain:

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I used to find the nearer the engine "ole" was to the entrance of the narrows of the bridge  the better the "lift" was on the sharp shut off of the throttle

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1 hour ago, X Alan W said:

I used to find the nearer the engine "ole" was to the entrance of the narrows of the bridge  the better the "lift" was on the sharp shut off of the throttle

 

I found this too when I was using the technique last year on the southern Stratford. A Game of chicken almost, keep the power on until the last possible second before the stern grounds in the bridge 'ole. 

 

I do wonder why the water is always shallower in bridge 'oles in the first place though. I can't believe it is all caused by locals chucking their rubbish in there.

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3 minutes ago, Mike the Boilerman said:

 

I found this too when I was using the technique last year on the southern Stratford. A Game of chicken almost, keep the power on until the last possible second before the stern grounds in the bridge 'ole. 

 

I do wonder why the water is always shallower in bridge 'oles in the first place though. I can't believe it is all caused by locals chucking their rubbish in there.

Because the boat sucks all the water out as it passes through. It's Bernoulli again

 

Richard

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1 minute ago, RLWP said:

Because the boat sucks all the water out as it passes through. It's Bernoulli again

 

Richard

Not strictly true, as the water will flow faster through a narrows. However, and rubbish being carried will be deposited on either side of the narrows as the water slows as the cross section increases. Dredging used to be done fairly regularly either side of bridgeholes as that was both where the rubbish was deposited, and where they could easily use the dredger without hitting the bridge.

Re slowing in a bridgehole, this would also happen with horse boats, a good horse knowing that it was easier if it slowed down as the boat entered the bridge.

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9 minutes ago, Pluto said:

Not strictly true, as the water will flow faster through a narrows.

Water flowing faster is at a lower pressure, so the atmosphere pushes the surface down. Exactly the same as you see alongside a boat in motion - only more so as the water speed is greater

 

Bernoulli

 

Richard

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2 hours ago, RLWP said:

Water flowing faster is at a lower pressure, so the atmosphere pushes the surface down. Exactly the same as you see alongside a boat in motion - only more so as the water speed is greater

 

Bernoulli

 

Richard

You, or perhaps I should say Bernoulli, are right regarding the effect on the water surface. But a boat passing along a canal with parallel banks, because of the reduced cross section, creates a lowering of water levels as suggested by Bernoulli, with the water then rushing forward to fill the space previously occupied by the boat. This carries rubbish along the canal. At a bridgehole, this rubbish is drawn through the narrows and is deposited as the canal widens. Somewhere there is probably a theorem regarding the speed of flow to the weight of sediment that flow will support - but I'm not going to look for it. As an aside, the Yellow River in China is so called because of the amount of sediment it carries, which can reach over 50% by volume at some periods in the year.

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8 minutes ago, Pluto said:

You, or perhaps I should say Bernoulli, are right regarding the effect on the water surface. But a boat passing along a canal with parallel banks, because of the reduced cross section, creates a lowering of water levels as suggested by Bernoulli, with the water then rushing forward to fill the space previously occupied by the boat. This carries rubbish along the canal. At a bridgehole, this rubbish is drawn through the narrows and is deposited as the canal widens. Somewhere there is probably a theorem regarding the speed of flow to the weight of sediment that flow will support - but I'm not going to look for it. As an aside, the Yellow River in China is so called because of the amount of sediment it carries, which can reach over 50% by volume at some periods in the year.

OK, I see what you are saying. The passage of the boats draws debris into the bridge hole. 

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3 hours ago, Mike the Boilerman said:

 

I found this too when I was using the technique last year on the southern Stratford. A Game of chicken almost, keep the power on until the last possible second before the stern grounds in the bridge 'ole. 

 

I do wonder why the water is always shallower in bridge 'oles in the first place though. I can't believe it is all caused by locals chucking their rubbish in there.

I was always "advised " that boats passing through the bridge hole narrows the water swill drawn in also drew in sllt/leaves + the Joe Public introduced junk/crap doesn't help, in the days of a regular loaded run after several boats/trips the bridge holes in the sticks with no local yoof or other land based folk improved a lot & in a lot of cases you could keep some power on  however if you do it will pull the boat down passing though the narrows similar to exiting an empty lock with an empty boat with the counter not sat flat on a bit of forward motion has to be produced to stop the boat cavitating /fussing about until the counters down

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1 hour ago, RLWP said:

OK, I see what you are saying. The passage of the boats draws debris into the bridge hole. 

More either side of the bridgehole, with debris being dropped as the canal widens. It is also where stuff chucked off a bridge can end up, so that was where dredging was often done. If the canal was cleaned down to its bed either side of a bridge, the debris carried along and in the bridgehole would tend to end up in the deeper dredged 'hole', and that could be easily dredged again to keep the length clear. It does require knowledge of whether the canal at that point has clay puddle, so that the depth of dredging does not get over-enthusiastic. Dredging away from a bridgehole also meant that any stop plank grooves and sill were not damaged.

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1 hour ago, Pluto said:

. But a boat passing along a canal with parallel banks, because of the reduced cross section, creates a lowering of water levels as suggested by Bernoulli, with the water then rushing forward to fill the space previously occupied by the boat.

And this is exactly what causes the "surfing" wave that throws you through the bridge'ole when you knock the throttle back abruptly - the engine'ole will be just about coming into the bridge when you do it as Alan etc say as it would be too early otherwise.

There can be interesting results when you try to overtake too. We were with a single motor, loaded on the N. Oxford, and a small cruiser was trying to overtake. I would generally be helpful if possible, but I was coming to a bridge and he really did need to wait till we were through it. But he kept a-coming - until his bow got about level with the engine'ole door. Whereupon he just got sucked over and glued to me. I decided to let him see the folly of his ways and just kept on until I was a boat length from the bridge, when I did ease off a little so he came unstuck. Once throught the bridge I waved him by, but told him to go by steady or the same thing would happen again. I suspect it was time for clean underpants the next time he stopped.

  • Greenie 1
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5 hours ago, Mike the Boilerman said:

 

I do wonder why the water is always shallower in bridge 'oles in the first place though. I can't believe it is all caused by locals chucking their rubbish in there.

When The Coventry Canal Society do canal clearance on the five and a half with Panther we concentrate on bridge holes and narrows.

Bridge holes because there is often a side path leading to the road.  The scenario follows as, nick a bike to get home from town, dispose of it by going down the side of the bridge and step off just before it goes in.

Likewise with supermarket trollies that have been liberated.  

I also reckon when you want to dump a stolen motorcycle you're less likely to be seen under a bridge.  However I reckon a lot of jetsam bumbles along underneath a boat until it gets scraped off by the other treasures already in the narrows or the bridge, thus adding to it. 

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also in the narrows of a bridge a lot more water (and other rubbish) has to go past your prop making it that much more likely that you will foul something

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