The schedule method of collision avoidanceJan 1, 2003
|From Ocean Navigator #87 |
A fast-moving ferry on a regular schedule, like the Scotia Prince, shown above, can be tracked using its printed timetable.
The Gulf of Maine and Bay of Fundy may have less traffic than Long Island Sound, for example, but they still contain hazards. Two of these are the ferries operating out of Yarmouth, Nova Scotia, for Portland and Bar Harbor, Maine. These vessels represent the principle traffic on the run between Yarmouth and Down East. The question for the radar-free, passage-making sailor is how to best avoid them.
One might think that carrying a radar reflector and a VHF is protection enough and, indeed, we have always been impressed by the unfailing professionalism and politeness of these ships' bridge crews. In nearly every passage we have made, we have spoken with the vessels to arrange a passing agreement long before any urgency presented itself.
However, as prudent mariners we recognize our responsibility to plan these encounters so urgency is never an issue. We keep two simple tools on board for this: the schedule of each ferry. From the schedules, we determine each ferry's departure and arrival times. From the chart we get the distance between the ports. Then we determine the approximate speed of advance of the ferries (SOA = distance/transit time). We then plot the projected positions of the ferry from its departure time to its arrival, using the likely course the ferry will take. (In practice, we plot positions at half- or whole-hour increments). Then we do the same for Restless, assuming we'll make good some speed along the likely track line, again plotting positions at hour increments. Often the track lines cross a given place near thesametime.It is then and there that we will find ourselves near the ferry. Around this point we plot expected positions of the ferry and Restless at 10- or 15-minute intervals to produce anaccuratetime of expected meeting.
"Huh," one of our crew responded when he heard of our approach, "doesn't sound very precise." That's true, but then it doesn't need to be. On the day-long passage from Yarmouth to Stonington, Maine, we want to know which hour we are most likely to encounter the ferry Bluenose, making its way toward us at 24 knots of relative speed. Even if we are off in our calculations by 12 miles (a pretty hefty sum) the high speed of closure between our vessels makes this apparently large distance a short time: only 30 minutes! That is the beauty of this methodeven approximate times of approach are likely to be correct, within a half hour, given the high relative speed between our vessels.
Armed with this estimated approach time, the watch is ready, with fog horn in hand, radio in pocket. Approximately 15 minutes before the encounter window, the watch records our speed and course through the water, our DR position, and our SOA on a slip of paper. When the ferry hails us, we are prepared for a brief, polite, professional, and very satisfying conversation with the bridge watch of the Bluenose or the Scotia Prince.
We consider ourselves pretty good navigators (others use less flattering terms). On our most recent trip, we predicted the time of first foghorn from Bluenose at 1215 hours, bearing 030 relative to the bow. We heard the fog horn at 1209, two points off the starboard bow. Radar? No, just damn fine plotting.
Larry McKenna is a navigation instructor, freelance writer, and sailor who lives in Overland Park, Kan.Edit Module