Fact: The V made by the wake of a displacement hull is always 19.47 degrees to its centerline, whether it’s from a duck or a destroyer.
Remember that old TV game show that asked if you were smarter than a fifth grader? Well, it was rigged. The kids usually won because they have an unfair advantage. They’re only storing 10 years worth of information, most of which is new and right up front in their little Justin Bieber-obsessed heads.
In your defense, by the time you’re 40, you’ve forgotten more than they’ve learned. But since our brains can only hold so much, some things — such as Albania’s main export — have to be thrown overboard and replaced with more important stuff, such as what capacity of bilge pumps to install.
But there are some things that both you and a kid should know, and here’s your chance to put the brats in their places. Let’s see how you do.
Ice, Ice Baby
To have some fun with global warming, let’s say you decide to cool down Nevada’s Lake Mead and airlift in one of the many ice flows that have broken free from Antarctica. After the ice is lowered in, you tie your boat to a pier and wonder if you’ll have to adjust the lines as the huge berg melts. Will the lake’s level go up, down or stay the same?
Most kids will say that it goes up — a fact that makes a killer bar bet. Put an ice cube in a glass and fill the glass to the brim. Most folks, even when sober, will bet on the glass overflowing as the cube melts. It won’t. If anything, the level will fall slightly as air trapped in the ice is released when it melts.
A tugboat pushes up a huge bow wave in the harbor at 14 mph while a duck paddles serenely across the pond at half a mile per hour and the Navy’s new stealthy 600-foot LOA DDG-1000 class destroyer is in hot pursuit of bad guys at 30 knots. Now let’s say you’re in a helicopter looking directly down at each of the above with a gadget that lets you measure the precise angles of their wakes. Which angle will be the greatest?
Well, it’s no contest at all — they’re all the same. Back in 1887, the physicist William Thomson (soon to become Lord Kelvin) went through some tortuous computations to discover that the angles made by each arm of a wake’s V to the displacement hull’s centerline are always 19.47 degrees. Speed doesn’t matter, nor whether it’s a duck or a destroyer. Of course, with a planing hull, all the rules change.
You turn on the bilge pump and hear it go to work. But when most of the water is gone, you notice something odd: The faster the pump runs, the less current it draws. Could bilge pumps be the answer to the world’s energy problems.
Not likely. The more work a pump’s motor has to do, the slower it turns and the greater the current draw. Conversely, as the pump’s load decreases, it turns faster and draws less. But there’s something else going on called “back EMF” (electromotive force). The pump’s motor acts like a generator. As it spins, it creates voltage that opposes the incoming voltage. The faster it spins, the more voltage goes out, lowering overall voltage at the motor as well as the current flowing to it.
Didn’t get them right? Maybe you’re not as smart as a fifth grader after all. Which means I won’t be seeing you outside the inlet next season because you’ll be too busy in summer school. How embarrassing.