Wednesday, June 25, 2014

The ten-minute mile

We are anchored in Adams Creek, very near the junction of Cedar Creek (map), where we anchored southbound the last time we came through.  We are just two miles from the Neuse River, where we will once again be in more or less open water, rather than the rather narrow confines of the system of creeks and dredged cuts which constitute the majority of the Intracoastal Waterway (ICW).

I say "just two miles," as if that is an inconsequential distance, but a good day for us is 25-40 miles, so that's 5-10% of a day's run.  We can do 80-90 nautical miles of ocean travel in a day at this time of year, when days are 14+ hours long, but that's the exception and not the rule for our preferred pace of travel.  Passages of greater than about 90 nautical miles must, of necessity, have a nighttime component.

For the less nautically-inclined, a nautical mile is just over 6,076 feet, or about 1.15 times as large as a statute mile.  While distances along the ICW happen to be measured in statute miles, as are distances along many inland rivers, distances in the ocean are measured in nautical miles, and all our instruments are calibrated this way.  Speed is measured in knots, which is just a fancy word for nautical miles per hour.

Vector's "cruise speed" is about eight knots through the water as designed, but at that speed we burn nearly eight gallons an hour, or just about one nautical mile per gallon.  If we throttle back about 25%, our speed drops to six knots, but our burn goes down to three gallons per hour, or about two nautical miles per gallon.  That lets us go twice as far on the same amount of fuel, at the expense of 33% more time to get there.

Scenery going by at eight knots looks a lot like scenery going by at six knots, and part of the zen of cruising is to enjoy the scenery going by and not be in a hurry to get somewhere.  Taking a full eight hours to get someplace rather than six hours seems to us a small price to pay to save half the fuel, and so unless we are trying to make a tight bridge schedule, or make an anchorage before sundown, we normally run the boat at the lower speed.

This speed, of course, is speed through the water.  The water is seldom stationary relative to the ground, so our actual ground speed will be more or less than this depending on which way the current is going.  Sometimes, we can plan a day's trip to have favorable current for more of the day than unfavorable current, and we take advantage of this as much as we can.  Mostly, though, our average speed ends up being right around six knots.

At six knots, a nautical mile takes ten minutes to transit.  This makes it very easy to look at objects on the chart and know how long it will take to reach them -- if an object is a half mile ahead of us, it will take five minutes to reach it; one tenth of a mile, one minute, etc..  Conveniently, six knots is also almost exactly ten feet per second, so something 100' away is just ten seconds, and it takes just about five seconds for our 52' boat to pass any fixed point.

Today's cruise started out calmly in Lookout Bight, but once we left it's protection we had some swell until we reached the protection of Beaufort Inlet.  That distance was "just" six nautical miles, which looks like a hop, skip, and a jump, but it takes us an hour.  It was just rolly enough today to prompt me to add another couple hundred rpm, which not only got us off the ocean a bit sooner, but also gave the stabilizers more to work with.

Once into the inlet I dropped back to our normal running RPM, but we were on a falling tide, and with almost two knots of current against us we clocked just over four knots until well inland.  Today's total mileage was just 25 nautical miles, but it took us five hours, for an average speed of five knots.

It is remote and mostly quiet here, with the exception of a fleet of a half dozen shrimp boats trawling up and down the creek.  It was just 1:30 when we dropped the hook, and we could have continued a bit farther into the Neuse, but a storm system was moving in and we did not want to be caught trying to anchor in weather.  We enjoyed an afternoon of downtime, and this anchorage was really more convenient than anything further along anyway.

Tomorrow we will turn east down the Neuse River, but we will continue straight when the ICW makes a northward turn onto Bay River, and instead continue down the Neuse into Pamlico Sound.  We should be anchored tomorrow night somewhere east of Bluff Shoal.

1 comment:

  1. We got schooled in running against a current yesterday morning. Leaving Campbell River against a 5.5 knot current took a LONG time. Roughly an hour after we untied the lines we had made it around the breakwater and back to a point roughly opposite where we were tied up. The alternative would have been waiting until the wind got up in Georgia Strait and getting bumped for our 4 hour trip so we paid the fuel penalty. There was a tug beside us as we exited the marina. He called traffic and told them he was more or less standing still so he would let them know when he started actually moving. That happened about an hour later.


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