Low-capital, bridge-based tidal power

This past weekend took me on a bike ride across Portland, Maine’s Tukey’s Bridge. Thanks to the vagaries of the bicycle network in Portland (another long post), we looped under the bridge before crossing over it.

As we did, I noticed the tide going out. Portland’s back cove is not a particularly large body of water, it’s about one square mile. Nor is it deep—at low tide it mostly empties in to the Atlantic and becomes a large mud flat. But all the water has to go somewhere, and it certainly does: it flows under Tukey’s bridge. Portland’s tides—around 10 feet—are relatively large but nowhere near those up in the Bay of Fundy. But where the water rushes in and out of the Back Cove, there is a decent current which occurs several hours each day. Every day.

The flow is increased by the current incarnation of the bridge, which is built on fill to narrow the mouth of the cove considerably. So now the square mile of water has to flow through an opening only about 250 feet wide. There’s power in that water, and while it’s certainly not megawatts, the infrastructure seems to be such that you could pretty easily install a small-scale tidal power system to take advantage.

Tukey’s bridge has two main passages. What if, in one of them, you installed a paddle wheel contraption between the supports with a generator on one end? There is no need to worry about building supports; the bridge abutments are in place. There is no worry about navigation; small boats could easily pass through the other half of the bridge. There would have to be some sort of floating barrier to keep boaters from running through such machinery, but that would be doable. The paddles would not reach the floor of the cove, so they wouldn’t interrupt whatever ground fisheries there are there. The generator could be housed on the bridge abutment out of the water, which would keep it out of corrosive salt water.

So if we assume that all the agencies (and I’m figuring NOAA, Army Core, the DOT and others would be involved) could be satisfied, would this work as a demonstration project? Could it conceivably have a payback time that would make it economically feasible? It seems like a low-cost project. All you’d need is a generator, some power lines and some concrete to anchor it to the bridge.

Or do I have the physics of it all wrong anyway?

The Portland-Boston options

If you’re going from Portland, Maine to Boston, you have several choices. You could drive … if you like to be aggravated, spend a lot, have it take no less time and lose two (or more) hours of productivity. Or, you could take the bus or the train. This page has, previously, talked about buses and trains further south along the Northeast Corridor, but the conditions are different further north.

From Boston to DC, Amtrak is a luxury product competing with air travel, boasting faster travel times and more amenities. Buses are cheap (cheaper than Boston-Portland for twice the distance) but offer cramped seats, sometimes shady equipment and the opportunity to sit in traffic for six hours if you happen to hit rush hour (likely). From Boston to Portland, travel times are faster for the bus, costs are similar, and there is a sort-of symbiotic competition between the modes; the train actually provides mostly for trips which don’t traverse the whole of the route, the bus serves air travelers, and both serve as alternatives to driving (there are no Portland-to-Boston flights).

Here’s a quick comparison between the bus* and the train in several metrics.

(* We’ll discount the couple of Greyhound buses along this route, which take longer, have no Wifi, and an overall inferior product.)

First, why is driving a poor choice? The costs, mainly (even if you have free parking at your destination).

Car Bus Train
Fare $16-27 $20-25
Tolls 6.50
Gas $14
Other maintenance $11
Total $31.50 $16-27 $20-25


  • Bus fares: $22 one way. $27 to Logan Airport. $32 same-day round trip and discounts for college students. Train fares depend on time of day, either $20 or $25 (a few very-off-peak trips are $15, college students can buy six trips for $76).
  • Tolls $3 in Maine, $2 in NH, $3 one way in MA (using Tobin Bridge).
  • Gas: 110 miles at 27.5mpg and $3.50 per gallon. Other maintenance: 10¢ per mile.
  • Add another $35 to get the IRS-computed cost of driving (55¢ per mile)
  1. Travel time — The bus is scheduled at 1:55, the train at 2:25. This seems like an easy win, right? Not entirely. Outside of rush hours or weekend getaway and drive-back times, the bus will probably arrive at its terminal faster than the train. During rush hour? The bus could spend an hour getting in to our out of Boston. Also, it partially depends on where you are going. If you are going to the Financial district or somewhere along the Red Line, the bus will get you nearer to the Red Line, although it’s a bit of a walk. For the Green Line or Orange Line, it’s more of a wash, and near North Station (say, for a basketball or hockey game) you’d be better off taking the train. If there’s bad traffic, the bus will spend quite a bit of time getting from one side of downtown to the other. So, verdict: Bus, but not always.
  2. Frequency — Here, the bus winds, rather handily. It runs every hour for most of the day. The train runs five trips daily, although there are more frequent trips during rush hours, quite useful for outbound commuting during rush hour when getting from South Station to Route 1 is particularly bad. Verdict: Bus, except perhaps at the peak of outbound rush hour.
  3. Guarantee of a seat — If you go to buy a ticket for the train and it says it’s sold out, it’s sold out, no ticket. (You could board and play dumb and buy a ticket on-board by phone, but you might wind up standing. I’m not sure if Amtrak overbooks, but if you have a ticket they will let you on. When trains are sold out they sometimes check tickets on the platform.) On the bus, you buy a ticket, and it’s good, well, forever, but there are no reservations: everyone lines up for the bus, and if there are more riders than there are seats, well, you wait for the next bus. At heavy travel times, this means that you have to show up half an hour before departure, negating any real travel time savings. Amtrak suggests you show up half an hour early, but I haven’t been the only one sprinting down the platform to make a train. Concord Coach would have to amend its ticketing policy to allow for seating reservations (i.e. sell tickets for specific times) to guarantee seats. Which would be nice. Verdict: Train.
  4. Comfort — Here, the train takes the cake, as it can exploit economies of scale in a way that the bus can not. A bus is, basically, an airplane with a top speed of 75 mph, legroom-wise (the windows are bigger). The train has seating pitch equivalent to airlines’ domestic first class and wider seats. And you can get up and walk around on the train. Verdict: Train.
  5. Luggage — The train and bus both have advantages here. On the train, you can carry on however much luggage you’d like and store it above you on the (large) overhead luggage racks. On the bus you can put luggage in the under-bus bins. You can take skis, for example, on either. Verdict: Both
  6. Bicycles — Both modes allow bicycles, with caveats. For the bus, the caveat is that the bicycle is only taken if there is sufficient room, which may not be the case at busy times (especially weekends when many passengers have luggage). On the train, bicycles are taken at all times, but there is a $5 charge, although you could probably get on without a bike ticket and no one would be the wiser. The bike on the train doesn’t go underneath with the potential to get rattled around, an issue if you have an expensive ride. Verdict: Both
  7. Arrival times — While Amtrak suggests you arrive half an hour before your train, your ticket reserves a seat. On the bus, that is not the case. (see above) If the bus is full when you show, you may be waiting for the next one. (They don’t specify this on the website, but suggest arriving especially early during the holidays.) Both experience delays, although not very frequently. Verdict: Both
  8. Airport service — Concord Trailways serves the airport directly, although they charge an extra $5 for the service, it is generally direct to and from Portland. Amtrak requires two transfers to the terminal, but for a $20 ticket it’s less than the bus, even with T fare. Verdict: Bus
  9. Food — On the bus, you get pretzels and a bottle of water. On the train, you can go to the cafe and buy a beer. It’s not free, but if you have your own water bottle Poland Springs is not that exciting. Verdict: Train
  10. Wifi/power/entertainment — This depends on how much you like PG-rated movies. If you do, the bus provides them for free. If you don’t, the train provides slightly better wifi. Both bus and train have power ports. The peak wifi connection speed on the bus is 45 KB/sec. On the train the use several providers and it’s over 100 KB/sec. Neither is fast, but one will load Gmail a bit faster. (Also, you can use your iPhone for ticketing on the train.) However, the train traverses some cell-signal-free areas in New Hampshire and Maine, the signal along I-95 is better. Oh, and if you make a phone call on the bus, you get yelled at; on the train you can go to the cafe or a vestibule and talk away. Verdict: Both
  11. Restrooms — If you have the choice, use the facilities at either terminal. If you require the restroom during the trip, the one on the train is slightly better. Verdict: Train
  12. From a transportation planning perspective — This is a bit of a harder question. Ostensibly, the bus breaks even but, of course, it is subsidized, significantly, by government-built and funded roads (or roads funded by the tolls of other travelers). The train has significant government investment in infrastructure, and a direct subsidy to cover operating costs (they’d have to double costs to break even). Both are energy efficient. Both are quite advantageous over automobiles. Taking the train may take a bit longer, but it’s a more comfortable ride, and, perhaps, non-drivers shouldn’t be forced in to uncomfortable conditions. The bus is at its top speed, but with more patronage and investment, perhaps, a train could make a non-stop trip in 1:30, which would easily negate the bus’s advantages. But, for now, we’ll go with a verdict of Both
So, which wins? Well, the train wins, 4-3-4. But really, the traveling public wins, with 25 daily departures along the route. Take whichever is more convenient, especially since travel time is probably the main factor for most trips and the bus (usually) wins out there. If you’re interested in comfort more than travel time, the train is where it’s at. For speed (and a movie) the bus is hard to beat, especially outside of rush hour. If you’re in Portland and a bus leaves in 10 minutes with a train in two hours, by all means get on the bus. Plan your day around the schedules, but it’s perfectly easy to treat the corridor like a transit system (albeit one with poor headways, although they are better than most MBTA commuter rail lines). Take whichever you’d like.

I will also point out that the Concord Coach provides one of the best intercity bus experiences I’ve had. Unlike many Boston-to-New York routes (and I’m including Bolt and Megabus, which will make random, 15 minute stops at gas stations in Connecticut for “snack” where is sure seems like there’s some payola from the gas station operator for bringing in 50 captive customers) the service is very professionally run. The buses leave from terminals, not streeetcorners, the staff answers questions, and it does not seem like a two-bit operation. Considering that 20 years ago the only bus service to Portland was provided by Greyhound, Concord Coach has proven that the market can be captured and expanded (in 1997 they only ran 9 round trips daily) with service and quality.