Thursday, June 12, 2014

A sound too far

We anchored in Sapelo Sound, Georgia, just north of where Blackbeard Creek divides Sapelo Island from Blackbeard Island (map).  We've passed this way before -- we are less than half a mile from the ICW and just a few miles from where we anchored in the Wahoo River when we came through here on that route back in December.  You may note that this is not Wassaw sound, where I projected we would be when I posted on Tuesday, and, in fact, is only about half way.

While I wrote that we would leave the St. Marys on the ebb and arrive at Wassaw at the end of the flood, when I reviewed my math Tuesday evening I realized that did not leave us enough time to make the full passage at our preferred cruising speed of 6.5 knots and still arrive in the daylight.  We decided to move the departure up a full hour, which meant leaving on the last of the flood.

Running uphill out the fairly narrow inlet, the best I could manage was four knots at normal engine speeds.  A full four miles of that set us back another half hour, which, once we cleared the jetty, could still be made up on the roughly 11-hour northward leg if conditions cooperated.

On the way out the inlet we were passed by the HOS Westwind, a contract vessel for the Military Sealift Command.  We had watched her come in the previous evening, just after dark, with no AIS and minimal lights, with no fewer than five Coast Guard patrol boats closing off the harbor for her passage.  The larger patrol boats had their removable deck guns mounted, and we sat on deck in great anticipation, thinking perhaps a submarine was coming in to base.  When we saw it was a freighter instead, we knew it was probably carrying weapons material, perhaps ballistic missiles or cruise missile "specials" (two of the Ohio-class fleet based at Kings Bay have been converted from ballistic-missile boats to cruise-missile boats).  She left the harbor without escort, having already offloaded her deadly cargo.

HOS Westwind sails into the dawn.  Still no AIS, though.

Figuring speeds, and thus transit times, on the boat is an inexact science.  Thanks to GPS, I can know to a high degree of precision what our Speed Over Ground (SOG) is, but that's only part of the story.  We have no instrument to tell us what our Speed Through Water (STW) is, and, without that, it is very difficult to gauge the effects of wind and current.  Instead, I keep a chart posted on the helm which lists the "expected" STW at various engine RPM, numbers which we derived through averaging several observations in calm conditions.  The numbers are imprecise, and I would estimate an error of +/- 5%.

Those numbers were also derived with a clean hull, having been figured not long after our relaunch with fresh bottom paint at the boatyard.  We did have a diver clean us back up again in Stuart before we headed south, but a full month in the warm, flowing, nutrient-rich waters of the Middle River in Fort Lauderdale had a devastating effect, and we have a very bad barnacle encrustation along the full length of the hull, including both stabilizer fins, and a bit of growth on the propeller as well.  I'm not sure what the exact effect is on fuel consumption and vessel speed at various engine RPM, but I am guessing it is as much as 10%, based on recent observations.

After we turned northward from the St. Marys jetty, I gave the boat a half hour to settle in at 1650 RPM, which my chart shows at 7.2 knots.  I normally run the boat at 1500 RPM, 6.8 knots, but I knew we had to make up some time and we would have to overcome the effects of the growth.  And early in the day, things looked good, with an indicated 6.5 knots SOG.  That showed an arrival time of between 8:00 and 8:30, which would be acceptable with sunset around 8:15.

As the morning wore on, however, the SOG progressively dropped, and when it got down to oscillating between 5.9 and 6.2, with an ETA pushing past 9pm, I cranked it up to just over 1700 RPM to see if we could still make it.  Above 1700 RPM, however, fuel burn becomes excessive for very little incremental gain in speed, and we seldom use these settings for anything other than dealing with challenging conditions such as coming in a narrow channel against high current, or occasionally making a short run at higher speed to make a tight bridge schedule.

I had set a decision point at the St. Simons sea buoy, and after passing the buoy with no real prospect of making the Wassaw anchorage in the daylight without cranking the throttle up above 1800, we conceded defeat and set a new course for our fall-back anchorage.  Fortunately, the entrance buoy to this inlet lies just about a mile west of our plotted course line to Wassaw, and as far south as St. Simons we had only to make a correction of a few degrees west to our existing course.  I also immediately reduced throttle to 1400, both to conserve fuel and to delay our arrival here to at least mid-tide and rising.

It was a stretch to think we could make it all the way to Wassaw in one day at normal cruise speed, and it is really only even possible right at this time of the year, when the days are very long.  It makes no sense to do it at all if we have to exceed cruise speed.  The detour here to Sapelo Sound is about 20 miles round trip, about 12 gallons of fuel at conservative throttle settings.  Increasing speed by 10% on the more direct route burns an additional 15 gallons over normal cruise speed.  One of the many reasons why we often say the worst thing to have on a boat is a schedule.

We updated our float plan with our emergency contacts, and turned on our SPOT tracking device, knowing there was a good chance we could not even send a safe arrival message from here.  It turns out we both have limited cell coverage, and we are eking by on Louise's tiny Verizon data allotment, as my unlimited Sprint phone is out of data coverage here.

Dropping the throttle back to 1400 for a delayed arrival proved to be prescient, because we got hit by a thunderstorm just a few miles from our turn.  It had been a calm and pleasant day, so hot that I was driving from the flybridge when the safety broadcast came in over the radio.  I could see the storm forming to the south and west, and for a brief while we held out hope that it would miss us entirely.  That was not to be, although we only caught the outer edges.  We did put our cell phones and the handheld VHF in the microwave oven (a handy Faraday cage) as a precaution, and battened everything down before it hit, but I was worried that the wind, waves, and visibility issues associated with the storm would make what was already a somewhat challenging inlet into an impassable one.

The gathering storm.

Fortunately, by the time we made our turn at the sea buoy, the worst had passed, and we could see the inlet and markers clearly.  The sea state was calm enough that we were not concerned about being slammed into the bottom or pushed out of the channel, leaving just the normal concerns of navigating the bar before an incoming current.  There was a bit of added drama owing to two of the normal channel markers being off-station, which we knew ahead of time from the LNMs. Another of the major markers was missing entirely, and an unknown off-station marker added some confusion, but the charted depths were mostly correct and we made it over the bar and into deeper water without trouble.

We find it quite beautiful here in its isolation.  We are surrounded by beaches and wooded hammocks, with the only signs of man's presence being a couple of radio towers, the navigational markers, and the occasional shrimp boat.  Even though we are a stone's throw from the ICW, I saw nary a pleasure boat yesterday after we arrived, and this morning I counted a total of three.  As with our southbound transit, we are so far behind the "migration" that we are nearly alone.

Today we have a relaxing morning because tide and current dictate we arrive at the Wassaw entrance no earlier than 5pm.  That will let us run at a low, fuel-efficient RPM all day, and still get a nice push out the inlet here from the outgoing tide.  I will once again be challenged by crossing the bar with the current behind me, but we now have bread crumbs to follow over known-good depths.

Coming in to Wassaw Sound after five means we will again be anchored tonight.  We may end up at either a marina or a yard in the next couple of days to get some errands done and perhaps to have the bottom and running gear cleaned.


  1. I'm not a mariner but it would seem if a map based GPS in our car can give us our speed on the ground, something must be available for a GPS based speed on the water for mariners. Happy trails to you two.

    1. As I wrote, we do know Speed Over Ground (SOG), which is what you get from a GPS. But in boats and airplanes, the SOG is not the same as the speed through water (STW) for a boat, or the speed through the air (airspeed) in a plane, because the water and air are themselves moving relative to the ground. GPS can't help with that. Planes get airspeed from pitot tubes, and many boats get STW from paddlewheel devices. On a boat like ours, the paddlewheel would quickly become fouled with marine growth and stop spinning, so we don't have one.


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