After six days of flying downwind from St. Thomas on the easterly trade winds, making landfall on one of the outlying Bahamian islands can be somewhat stressful. Most of the islands are low, windswept and covered by scrubby inconspicuous trees – hard to spot from more than a few miles out, even with radar.
Consider, for instance, the approach to San Salvador Island, in the eastern Bahamas. This is an approach that requires a steady hand on sextant and tiller, since, a few miles of error in either direction could lead one to miss the island altogether, which would quickly lead to much more serious problems. It’s the same approach as made by Columbus in 1492. Of course, aboard the schooner Ocean Star we have it much easier, since we are equipped with six sextants and a Raytheon R-70 radar set. But since we deliberately operate without GPS, loran or satnav receivers, there is still a bit of a challenge to the navigator, especially when the approach is made at night in winter (night lasts forever in winter) or in overcast conditions.
San Salvador itself is barely 10 miles in length, and its highest point is hardly taller than a good-sized schooner’s main mast. Approaching from the east, with the full tradewind breeze in one’s sails, a vessel will not begin to feel the bottom until a couple of miles out, and depths of 500 feet or more are consistent up to about one mile from the windward shore.
Landfalls of this nature are fairly common for Ocean Star and its crew. Since the vessel operates as a navigation training vessel, it is only natural that celestial navigation be its primary means of offshore navigation. After three seasons of operations we hope we have demonstrated to ocean-going yachtsmen that celestial navigation can, indeed, be aggressively managed to provide a basis of navigation as accurate as anyone needs for safe and reliable landfalls.
The approach to San Salvador, however, does not begin when one is 10 or 20 miles from the landfall. Rather, the approach must begin at the beginning of a voyageandmdash;when the first marks are made on a chart, even if 1,000 miles away.
The key to achieving consistently accurate landfalls on a year-round basis is to push celestial navigation and its all important companion, dead reckoning, to the point where they can provide reliable and accurate information in almost any situation. Most important, perhaps, is the need for star fixes. A navigator can cross the ocean perfectly well with just sun lines, but when accuracy and confidence are needed, as in the approach to a tricky landfall, there are no substitutes for star fixes and a good DR plot.
It’s hard to tell which is more important. A star fix will provide a very reliable and accurate position, taking all the error out of the day’s harvest of sun lines and estimated positions (EPs). But that only happens twice each day, and sometimes not even that often. Dead reckoning is the key to consistent, reliable, ever-faithful knowledge of position. Dead reckoning alone, when conducted religiously, will carry off a landfall well enough. When DR is combined with star fixes, the navigator can then get down to the fun business of taking wagers on the precise time of landfall.
Managing offshore navigation for confidence must begin with DR plotting. Without it there can be no confidence; without it there can be no celestial navigation at all since the sight reduction process generally requires a DR position. A ship’s DR should be plotted about once an hour, on the hour, when offshore in open water. In addition, a fresh DR position must be plotted each time a vessel changes course or speed, and each time the navigator intends to construct an EP on a celestial LOP.
Dead reckoning is best conducted with the navigator believing that he has already gotten his last star fix before landfall. The DR plot somehow becomes just a bit more accurate if the navigator believes, or at least acts as though he believes, that every twilight period from that time on will be clouded over. Information for the DR must, of course, come from the ship’s logbook. Helmsmen, watch captains and/or their equivalents must make logbook entries as if they were the navigators themselves, dependent upon every stroke of accuracy and timeliness. Here, as well, hourly entries are essential. Courses and speeds averaged over a number of hours are nowhere near as accurate as hourly entries, each one sensitive to changes in wind speed, boat speed, steering error, and other factors.
A DR plot is most accurate when the hourly segments between the semi-circular DR symbols are reflective of miles traveled rather than speed maintained. A speed entry, by its nature, requires an average speed maintained over the hourandmdash;in other words, a guess. A log reading, by contrast, provides the navigator with a precise number of miles traveled, usually measured down to tenths of miles. The best technique is simply to read the ship’s log each hour at the top of the hour, and make note of the reading in the logbook along with all other entries.
Dead reckoning provides the basic vehicle for offshore navigation. But it takes plenty of sextant work to build up an accurate and truly reliable system. To manage celestial navigation for confidence, the navigator must shoot and plot numerous sun lines during daylight and shoot stars whenever they are available. Sleeping through stars is definitely not allowed. And waltzing through daylight with just a noon sight would be equally unacceptable.
Aboard Ocean Star, we have a minimum requirement of one sun line each watch and no excuses on star fixes. But it would be a rare watch that passed with only one sun line plotted on our chart during the four hours. In the event of total overcast, of course, we are blanked out and just running on DR, but that doesn’t usually seem to be a long-lasting problem on ocean voyages in temperate zones. A sun line once every two hours is probably the more typical average, along with a noon sight.
When it comes to sun lines, however, we often insist that only andquot;tripleandquot; lines can be plotted on the chart. That is, the watch officer or navigator must shoot three sights within a few minutes and then reduce and plot all three. No averaging, no picking the most likely – we want to see them all reduced and plotted or don’t bother. The result, hopefully, will be three parallel lines falling almost on top of each other. With those, there’s no questioning their accuracy. With a single sun line and a single plotted LOP, by contrast, there will always be that doubt about the ultimate quality of the line.
In the event that the three lines are not contiguous or at least close neighbors, then the plotter is much better equipped to analyze their differences by comparing them to the ship’s DR position for the time of the sights.
Similarly, when it comes to star sights, we often encourage the practice of securing triple LOPs on the brightest and easiest stars or planet availableandmdash;just to add an extra measure of confidence. For example, at evening star time, a bright planet like Jupiter or Venus might be shining brightly for 15 or 20 minutes before civil twilight. That’s an excellent time for the navigator to grab three contiguous LOPs as the anchor line for his eventual fix. Those three adjacent lines can be very helpful, as well, when it comes time to picking a point within a triangle.
Similarly, at morning sights, the navigator might want to shoot three LOPs of the brightest star, or perhaps even of his favorite or most convenient star. Or, he might save a bright planet for last and grab three shots of the planet when most other bodies have just faded away.
My own style at star time is to get three lines from every body I shoot, a practice which thereby limits me to a selection of three or four bodies (which would be nine or 10 sextant shots). Youngsters who can jump around and get accurate shots on seven or eight different bodies (sometimes more) in one twilight period may scoff at this technique, but I find that it generates sufficient accuracy with a lot less moving around and much less readjusting of sextant, light, notepad, and pencil. It also produces an imposing level of confidence in my fixes if all the lines from each body are contiguous.
Neatness counts, as always, and that means plotting skills are important. A navigator must be adept with both universal plotting sheets (UPS) as well as position plotting sheets (PPS)andmdash;the latter being best for accuracy with their more advantageous scale. Generally speaking, a pencil point is equal to about one mile on a UPS but only about a half mile on a PPS. Here again, it’s the DR plotting that is most important since it makes up the bulk of hour-after-hour pencil work. It’s important that a plotting sheet not be marked up with azimuth lines and other unnecessary marks for every sight taken, especially at star time. It is sufficient to plot the azimuth and intercept andquot;invisiblyandquot; and to limit the pencil marks to actual LOPs.
The practice aboard Ocean Star is for star shooters to work out their own fixes on a UPS and then to transfer an appropriately labeled fix, with their initials, to the ship’s working chart. This keeps clutter to a minimum and makes it easier for the captain or watch captain to evaluate the various fixes submitted.
Plotting on any chart should always be done in such a way that the captain or some other navigator can always walk over to the chart and immediately discern the meaning of all those pencil marks. Plotting symbols and labels constitute a universal language of navigation and every navigator should strive to stick to the conventions.
To keep a plot current and allow for constant and immediate analysis of position, progress and navigational strategy, we make frequent use of EPs on LOPs. Rather than leave a good sun line untouched and waiting in readiness for some future running fix, we like to get immediate use from an LOP. Our usual practice is to pick the most probable position or EP on the LOP by dropping a perpendicular between the LOP and a DR position for the time of the sight. Normally, assuming no great discrepancy, we would begin a new DR track from the EP.
This practice does not preclude the ability to plot running fixes. It simply allows us to make more immediate use of every good LOP. This is one more reason why it builds confidence to make sure that all sun lines are backed up by the triple guarantee.
Plotting in current waters such as those of the Gulf Stream adds an extra dimension to the navigator’s job. In current waters, a vessel’s DR position and DR track do not, by definition, include set and drift of current. An LOP from sun or star, however, does. The navigator must bridge the gap between these two by adding set and drift vectors to his DR. (See sidebar.)
Looking back, I would say that the real key to conducting celestial navigation with accuracy and confidence is frequency of sextant work and diligence in plotting. To really feel confident, a lone navigator has got to have the following: his sextant out six to eight times a day, and plenty of pencil lead down on his chart.