Naval Bombardment

Potomac River
N 38 12 W 76 45

Well, in the past this blog has discussed aerial bombardment and hostile Indian attack. Today, we add naval bombardment, to Tarwathie's list of adventures. Naval bombardment? Really? Well, sort of.

Here's the story. As we cruised down the river today, we were hailed on the radio by a US Navy boat. They wanted to inform us that we were about to enter a firing range and that a live fire exercise was about to begin. The navy man gave us directions for how to skirt around the periphery of the fire zone. Needless to say, we were happy to comply.

As we did skirt the periphery thought, we were treated to a grandstand seat to sights that most people never get to see. BOOM would sound an explosion from near by. We would whip our heads around to look at the sound and we saw an enormous fountain of water thrown up in to the air by the shell. Five to tens seconds later we heard another, duller, BOOM as the sound of the cannon reached us. I should point out that "near by" meant about three miles. That's where the shells fell. The cannons
were about 10 miles behind us. Clearly, the shells traveled much faster than sound coming up range.

So that was our rather unique form of entertainment for this morning. Think of the people who live along the river bank near the target area. I imagine that those people and their pet parakeets and pet cats must really appreciate the BOOMs.

Going downstream on the Potomac, we're trying a navigation policy that we never tried before. We move day or night when the current is with us. We stop day or night when it is against us. Mentally calculating the times between tides though is making my head hurt. I'll try to explain simply.

Think of a tidal surge like a wave approaching a beach. Tidal cycle here come 11 hours apart. The tidal surges(waves)also travel up the river. It takes 6 hours for a tidal surge to move 100 miles up the Potomac to Washington. When we travel down the river, we meet one tidal cycle every 5.5 hours. When stopped, the tides pass every 11 hours as normal. We leave at high tide and stop at low tide, 2.25 hours later. Then we sit at anchor until the next high tide 5.5 hours after that. So on the
average, we are stopped 2/3 of the time and moving only 1/3 of the time. It all sounds counter intuitive. But it saves us a lot of fuel. With the current with us, we travel at 6 knots, with the current against us we only do 3.5 knots. Given a 10 knot wind with or against us, the two speeds become 7 knots and 3 knots.

It is easier to think about traveling up stream. If we could move 15 mph, then we could ride the same tidal surge up the river, just like a surfer on a wave. Then we would meet zero tidal cycles per hour.