12/14/2005
Archimedes Helps Lift Boats
Dr. Bortrum is on vacation. This column will remain online until January 4, 2006, when a new column will be posted.
Having recently transited the Panama Canal and marveled at the enormity of the project, I naturally was intrigued by a short 3- paragraph article by A. R. Williams in the December National Geographic. The article, titled “A Ferris Wheel for Boats”, had pictures of a strange looking alternative to locks as a feature of canal travel. I was eager to find more details on this contraption and found them on http://thefalkirkwheel.co.uk, the Falkirk Wheel Web site.
Back in 1825, the Erie Canal was completed here in the U.S. and was a rousing commercial success. In the early 1800s, there was also quite a bit of commercial boat traffic on canals in Scotland. Two of these canals were the adjoining Forth and Clyde and the Union Canals in Scotland. The level of the Forth and Clyde was 115 feet below that of the Union Canal and there was a series of 11 locks that would step a boat up or down those 115 feet. Over the years, rail and highway transport cut into the canal traffic and in 1933 the locks were dismantled and the canals shut down.
However, after World War II, there was increasing interest in recreational canal travel. (We know a couple here in New Jersey that hire canal boats and make their own way traveling the waterways of Britain and France.) With the coming of the new millennium in 2000, there was a desire in Britain to mark the occasion in special ways. One of the proposed ways was the “Millennium Link” project in which the link between the Forth and Clyde and the Union Canals would be restored. The obvious way would have been to restore the locks but the Brits wanted something that would truly be a spectacular and long lasting symbol of the arrival of a new millennium. The result was the Falkirk Wheel, the first rotating boat lift, completed in 2002 by British Waterways, in conjunction with various consulting design and engineering firms, at a cost of over $150 million.
Before going into details of the Wheel, let’s see what it can accomplish. In less than ten minutes, with a turn of the wheel, it can lift /lower as many as eight 65-foot-long vessels (four up, four down) the 115 feet difference in the levels of the two canals. Those 65-foot vessels are typically tour boats, but private boats work their way in among the tours. The Falkirk Wheel is a tourist attraction in itself and the tour boats carry over 150,000 passengers a year. A “ride” on the wheel in a tour boat will cost you 8 pounds if you’re an adult, 4.25 pounds for a child, unless under three, in which case there’s no charge.
It’s safe to say that, for a cruise ship the size of our Panama Canal cruise ship, with some 3,000 passengers and crew, the old fashioned lock approach will never be replaced by a wheel. How to describe the Falkirk Wheel? Picture a crescent wrench handle with a hole at the end, the hole allows you to hang the wrench on a hook or nail. Now join two such handles so that you have a symmetrical “arm” with holes at the ends. Take what looks to me from the pictures to be 4 or 5 of these arms, line up the holes and put a rod or spindle through the centers of the arms. As the spindle rotates, the arms also rotate like a Ferris wheel. In the Falkirk Wheel, there are only two “seats”, so to speak.
The “seats” are actually two long “gondolas” supported on the inner surfaces of the two sets of holes in the arms. Each gondola can hold four of those 65-foot long tour boats. Let’s sail our boat into a gondola. When the gates to our gondola are closed, we’re still floating in the water contained in the gondola. If we’re to be raised, our counterparts in the gondola opposite us will be lowered. As when we ride on a Ferris wheel at the county fair, we want to be sure we stay upright in our seats. To do this, our gondola rides on little wheels that fit a single curved rail on the inner edges of the holes in which our gondola is riding. As the wheel rotates the gondola stays level. As a backup, there’s a mechanical system that assures we move properly if the wheels stick for some reason.
You might think that it would take a lot of power to raise and lower 600 tonnes (a British tonne is more than our ton) of water in just a few minutes. But there’s a neat little trick that dates back to Archimedes and his Eureka moment. Archimedes deduced his famous principle that a body is buoyed up by the weight of the water that it displaces. Let’s start out with our two “empty” gondolas filled to the brim with water, but no boats. Now let’s sail four boats into one gondola but none into the other opposite gondola. What happens? The boats come in and the water they displace spills out of the gondola. The opposite gondola is still filled with water.
What are the weights in the two gondolas? They’re both the same! Why? The water displaced by the boats weighs the same as the boats – Archimedes says so. That means that the weight of the remaining water in the gondola plus the weight of the boats is the same as the weight of the gondola when it was filled with water. Since the two gondolas are the same size, no matter how many boats are in each one, they both weigh the same.
Now, this may seem like much ado about nothing. However, I picture the situation to be sort of like a see-saw. If you’ve ever been on a see-saw, you know that, if the guy or gal on the other end is a lot lighter than you are, you have to exert some energy to raise your end off the ground. But, if you weigh almost the same as that guy or gal, it takes just a light push on either end to move the see-saw up or down. With both gondolas weighing the same, the Falkirk Wheel is virtually in perfect balance and it takes only 1.5 kilowatts to turn the Wheel. That’s only the power you need to light 15 100-watt light bulbs, or as they put it on the Falkirk Wheel Web site, the energy of two boiling kettles.
Speaking of energy, I saw two encouraging items in the news recently. In the December 11 Sunday New York Times there was a report that New York City has replaced the lights in some 11,000 traffic and pedestrian walk signs with LEDs, light emitting diodes. This saves 90 percent of the energy formerly used to power those signs. Having worked on LEDs at Bell Labs back in the 1960s, I’m pleased to see them put to such good use.
The other encouraging energy item was in yesterday’s December 13 Star-Ledger. It was about the East Coast’s first wind farm on the outskirts of Atlantic City. There are four large wind turbines (the article, by Alexander Lane, called them windmills) and a fifth is planned. When the wind really blows, the five are expected to generate 7.5 megawatts of electricity, enough to power 2,500 homes. I just happened to be in Atlantic City a couple weeks ago and saw the four turbines. It was my first time in Atlantic City in probably 20 years and I can’t help wondering whether even those 7.5 megawatts would be enough to power the huge casino I spent several hours in feeding the slots. There must have been a zillion flashing lights! Or were they LEDs?
Our editor, Brian Trumbore has kindly suggested that old Bortrum take the next two weeks off and I may well take him up on the idea. So, if I don’t see you until January, have a Merry Christmas, Happy Chanukah and/or Happy Holidays and a Happy New Year.
Allen F. Bortrum
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