The Big Drop!

[Kirbstone]

Regular browsers here know that my principal weakness is boat-shoving.
Last Week-end we did our early Summer Tour, which involved passage of four boats through our highest lock.... on the Shannon at Ardnacrusha, near Limerick. (We had rowed 18 miles into the teeth of a persistent S.W gale from Killaloe on Lough Derg to get there).

Tour May 2011 Killaloe-Limerick 012.jpg

This is at a hydro-electric power station which was built in 1927, and predates Grand Coolie, Hoover &c &c., so the architecture while functional is somewhat stark.

Tour May 2011 Killaloe-Limerick 016.jpg

It consists of two chambers, the upper one having a drop of 60 feet, while the lower drops a further 40+ feet down onto the tidal tail race.

Tour May 2011 The big drop.jpg

The upper chamber is normally empty, hence no growth on the vertical sides. The lower chamber is normally kept full, hence there is considerable weed growth on the walls there.
Tom K.

[Kirbstone]

A few more pics:

Tour May 2011 Top of lower chamber.jpg

The lower chamber is very smelly because of the weed growth.

Tour May 2011 Killaloe-Limerick 023.jpg

We timed our lock exit at one hour past the top of the tide, so we had a good push the last three miles into Limerick

Tour May 2011 Tailrace.jpg

We sank a few pints on arrival !

Tom K.

[crfriend]

Awesome shots, Tom. Thanks!

I suspect I might get a wee bit claustrophobic at the bottom of the sixty-footer! What was the transit time?

[Kirbstone]

We got all four boats in in about ten minutes, and the big gate at the back took a few more minutes to drop closed. The lock keeper said he'd let us down slowly, so no one would panic, so the first drop took approx. 20 mins. Same again for the lower lock, with about ten minutes to traverse into the top of the lower lock in between. I suppose we were underway again in about an hour.
They told us that they can operate it much faster, giving a transit time of about half an hour....presumably for motor boats. I expect it would be slower going up.

Tom K.

[crfriend]

[... T]he first drop took approx. 20 mins. Same again for the lower lock [...]

That works out to about one vertical foot in 20 seconds -- not a bad pace at all. The keeper may have been right -- anything much faster could well be disorienting, especially given your little (and unstable) craft.

They told us that they can operate it much faster, giving a transit time of about half an hour....presumably for motor boats. I expect it would be slower going up.

I would expect it to be symmetric when it comes to speed -- everything's gravity driven and in these here parts 32 feet per second per second is not only a good idea, but it's the law. What you'd get with rapid fill and drain cycles is turbulence in the water, especially when one is near the floor of each lock segment.

Aside form the stink of rotting vegetation in the lower chamber it must've been an interesting trip. And you're right -- that is a rather severe looking structure. What were the keepers' reactions to seeing a group of shells? It probably wasn't a first, but it can't happen all that often.

[Big and Bashful]

I have always fancied going through those locks, spectacular! You lucky people! I would rather be in something bigger than a canoe though.

[Kirbstone]

OK guys, let's get a couple of things right. The vegetation in the lower chamber is all alive and growing. Not dead & rotting. The smell is just strong pungent veggie & very damp.
It's amazing how stable those craft are when kept on an even keel by four sets of lightweight oars, or sculls. They're not in the least scary. Not canoes either, 'B&B. At least canoeists can see where they are going. We rowers either totally rely on what the cox can see, or rubberneck it, in the case of a bow steersman in a coxless craft.
You're right about the rate of descent, Carl. About a yard per minute, which was comfortable without being boring. The lock keeper assured us that the ducting is so clever that there is minimal turbulence in the chambers at any time either going up or down. Motorboats are faster in and out than a gaggle of rowing boats. You book your passage through that lock, so they knew we were coming in advance, and no, I expect rowing craft only pass through once in a few years.
When Ireland gained independance from Britain in 1921 their first task was to harness our plentiful water to produce power and they prevailed on the Austrians to design & build the 'Shannon Scheme' as it was known in 1927. The turbines were made by Siemens. Four of them, and even now in 2011 it still generates some 15% of the country's power supply. They built three others: on the Lee, the Liffey and on the Erne, but none of these has an adjoining navigation lock.
Tom K.

[crfriend]

OK guys, let's get a couple of things right. The vegetation in the lower chamber is all alive and growing. Not dead & rotting. The smell is just strong pungent veggie & very damp.

OK, that was me. I'm used to stuff in tidal and brackish areas, including manmade works where the water level rises and falls, and for the most part it's a slimy stinking mass. It may be different in freshwater environments.

It's amazing how stable those craft are when kept on an even keel by four sets of lightweight oars, or sculls.

Does the stability continue if the craft is not in motion and the oars are stilled? This is, actually, an honest question; given the length-to-beam ratio I'd rather suspect the craft would be quite prone to tipping. If stability is achieved by active means, what happens when passive takes over?

The lock keeper assured us that the ducting is so clever that there is minimal turbulence in the chambers at any time either going up or down. Motorboats are faster in and out than a gaggle of rowing boats.

That is clearly not your typical 19th Century narrowboat canal lock. By 1927, proper baffling and ducting techniques were very well understood and, no doubt, applied at that installation. However, even in modern designs, if one starts flow too quickly on a lift, or doesn't slow down somewhat on the drop, turbulence will happen. Engineering drawings for modern locks should be readily available on the Intarwebs for perusal; I am basing my commentary on original blueprints I saw at a place I used to work at a few years back that specialised in large-scale civil engineering projects.

You book your passage through that lock, so they knew we were coming in advance, and no, I expect rowing craft only pass through once in a few years.

This was going to be another question of mine, as it takes a while to "prep the system" to accept a craft at either end for transit. Having the upper chamber empty and the lower one full most of the time would minimise water-wastage, but still requires filling or emptying chambers in advance of a boat's arrival. So one does not just ring them up on the mobile as one approaches. (How many feet by how many feet by how many feet divided by maximum flow-rate plus gate time yields "prep time". Yes, "make reservations".)

The turbines were made by Siemens. Four of them [...]

The penstocks are visible in Google's aerial view, as are a couple of approach and departure shots of the navigation locks. That the station does as well, percentage-wise, of the country's needs is a testament to the folks who first hatched the "scheme" and saw it to fruition! I've not looked hard for the locks' dimensions, but they look rather like narrowboat locks -- much longer and narrower than most things we have here in the New World..

[Kirbstone]

The Inland Waterways Assn. of Ireland (IWAI) publish lock/boat dimensions for all the Irish navigations, and they are a disparate lot. It appears that each canal company had their own idea how big the locks should be.
Above Killaloe the Shannon Navigation locks are 102 ft.X 30 ft.. built in 1844. The Grand and Royal canals from there to Dublin were 61ft X13ft and 75ft X 13.3 ft respectively.(lock size).
These canals date from some 200-odd years ago. The original canal into Limerick was built some 250-odd years ago (Pic)

Lock Bridge plaque. Limerick.jpg

At Ardnacrusha they arrived at 100ft X 18ft as dimensions for the lock chambers there, and the works were so big that all preceding canals and locks were made derelict or sunken beneath the new water levels.

Needless to say, navigation systems and locks on the New World rivers were constructed on a vastly different scale altogether, but drew knowledge from these old world canals.

Incidentally, when the lock keeper arrived, having been summoned by mobile phone, he appeared to take only a few minutes to fill the upper chamber for us. Perhaps when there are no boats inside they can open up to full throttle, so to speak.

Tom K.

[crfriend]

At Ardnacrusha they arrived at 100ft X 18ft as dimensions for the lock chambers there [...]

OK, a little bit of arithmetic yields a navigable capacity of 807,840 US gallons (the little ones), so at a speed of about a yard a minute yields a flow rate of 40,932 gallons per minute. That's not a whole lot when it comes to large civil engineering works.

Incidentally, when the lock keeper arrived, having been summoned by mobile phone, he appeared to take only a few minutes to fill the upper chamber for us. Perhaps when there are no boats inside they can open up to full throttle, so to speak.

I saw mention elsewhere of a chap who claims that he's locked completely through in 25 minutes, but that it's not for the faint of heart. So, rather clearly, they can modulate the fill/drain speed quite nicely.

[Kirbstone]

Vast water volumes are nebulous mathematics to me. A good pot of tea normally is about a pint or a pint-and-a-half.

Re: Passive/active stability. All narrow rowing boats with riggers require active stability when a crew is aboard. Getting into one at the slip, question no 1 is: 'is anyone holding a rigger down?'....otherwise don't get in!!
Once the buttons of the oars are locked into the swivel gates all is well, and novice oarspeople may venture out. Oarblades flat on the water and oar handles held nearly together in the middle will keep the boat on an even keel. Swinging the blades off the water and venturing up the slide is learned with practice, usually with an experienced oar aboard to keep things on an even keel, so to speak.
The people who take part in lengthy tours are all very experienced rowers, so stability isn't an issue.

Tom K.

[AnneHopkins]

Oh I wanna be there also!

[Kirbstone]

Don't know where you are,Anne . Our last tour had 19 participants of whom some twelve were women rowers. Our next organised tour will be from Belmullet to Enniskillen on the upper Erne, overnighting at a place called Carrybridge in Co. Fermanagh. That will be the Sat/Sun.Sept.17th/18th next. It won't be as drmatic as the big drop, as amazingly the Erne is actually flat all the way from B'mullet to Beleek, some 70 miles N.W.

Want to come?

Tom K.