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I spent several years sailing open, engineless boats up and down the
coast of Maine for the Outward Bound School on Hurricane Island in
Penobscot Bay.  The habits formed there have carried over into my
cruising practice in Maine, Nova Scotia and Newfoundland on my 21'
Drascombe Longboat. A handheld GPS has found its way into my tool kit,
but I have stubbornly stuck to the basics whenever I am crossing bays or
operating in the frequent fog of northeast waters. I found a way to
integrate the GPS into the DR routine without creating blind reliance on
waypoints, but I still enjoy doing it the old fashion way.  Described
below is my routine in the fog when making the occasional 2 to 3 hour
bay crossing in fog.

A paper chart are carefully folded to the area to be traversed and
inserted into a Chartkit waterproof/transparent chart cover. A 1/4"
plywood sheet cut to the proper size and inserted with the chart makes
the whole unit rigid enough to plot on my lap. I use clips around the
edges to prevent the chart from shifting under the cover (because I'll
be marking courses and bearings on the cover). A transparent plotting
rule can pick up a true course/bearing from any latitude or longitude
line, so you don't need a compass rose folded into the visible area.  A
hand bearing "hockey puck" hangs around my neck, a grease pencil,
plotting rule and dividers are available in a secure spot. I have a box
compass that I line up with the floor boards which is close enough to
the centerline.  Precision is an illusion when the currents run up to 2
knots in the bays. On a 2 hour crossing, you need to be as accurate as
you can while being prepared to fetch up well to the left or right of
where you intended, planning in advance how you will sort things out in
either case.  In a small, shallow draft boat where you can LISTEN
because you have no engine, the margin for error is greater than in a
heavy vessel under power and deaf to the world because of the roar of
the diesel. Shut her down and listen every 10 minutes is my advice.

Time is by the watch. Speed is by a wood chip on a 30' string. Number of
seconds for 30' of string to run out divided into 18 yields speed in
knots (18/6 seconds = 3 knots).  A grease pencil makes a very thick
line, but I would argue that is a good reflection of how broad an
estimate you get from pure DR work. Plot the starting point, draw out a
line for the course steered, measure speed every 15 minutes and plot a
DR position (the bird's eye on the course line).  Whenever I have reason
to place the estimated position differently from the DR position
(estimate of set and drift, a bearing on a known mark, or GPS as
described below), I plot a square EP and restart my DR.

When I got a handheld GPS, I didn't want to loose touch with the craft
of keeping a DR. I also found it impractical to input the latitude and
longitude of all the "waypoints" I might want to use in a day of
cruising.  The solution was to input into my GPS one or two "reference
waypoints" for the visible fold of the chart.  These reference waypoints
are simply crossing points of the chart's latitude and longitude lines
which are easy to find and easy to input into the GPS unit.  Enter a
reference waypoint as the "go to" waypoint in the GPS unit and most
units will give your bearing and distance off the waypoint.  You don't
actually have to "go to" the waypoint. Plot the bearing to the reference
waypoint and measure out the distance off and you have a fix, no matter
where you might be headed.  In practice, I still plot my DR and steer my
course by the box compass. I have fallen into the habit of taking my
speed from the GPS and using that to plot my DR every 15 minutes.  Then
every so often I'll plot my bearing and distance off from the reference
waypoint and compare this fix to my DR.  In most cases, I'll restart the
DR from the fix, but by comparing DR to the fix I get a good idea of set
and drift.  My goal is to be ready at any time to loose my GPS and still
have all I need to continue with traditional DR without missing a beat.

-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Rodney
Myrvaagnes
Sent: Friday, October 17, 2003 1:01 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: DR plotting techniques

On Thu, 16 Oct 2003 11:14:47 -0700, Royer, Doug wrote:

>I would like to ask the members of the list who sail or pilot power
vessels
>how or what techniques they use to find their DR pos. while underway.I
am
>only interested in what methods you use and don't wish to get into a
>discussion of accuracy as yet.

My most frequent forays out of sight of land (except for fog) are
annual trips to Maine. After Cape Cod, I generally take a rhumbline
course from Race Point to Frenchboro Long Island, south of Mount Desert
Island. The first time, ca 1978, was without Loran or radar. I used
textbook DR, checking with twilight shots of two planets in the
evening.

We were aware of tidal currents, but assumed they would cancel out over
the 50-hour trip in a 30-foot sailboat. They did. We came in as
expected, but could see the coastal lights at Matinicus Rack and Mount
Desert Rock far enough so it didn't matter. The Main coast is a big
target over 180 miles.

We did use a TI 59 calculator with navigation software for sight
reduction. On a later trip with friends between Nova Scotia and Maine
my wife programmed a spreadsheet for sight reduction in a 1985 DOS
laptop.


Nowadays we mark hourly gps fixes on chart 13260. If motoring, we leave
the radar running. If sailing, we leave it on standby and scan every 10
minutes. We keep a Loran running also as a check against any weirdness.

The 13260 is overdue for replacement, and many years fixes make a swath
across it.

We don't use navigation software or chartplotters.

If the Loran dies we will probably use two gps receivers. We do carry a
handheld GPS but don't run it normally.

My wife and I are both trained in formal DR plotting, and we have used
it before the electronic devices were available or affordable. We keep
two hand-bearing compasses and a compass binocular for visual piloting.
We use them most often when tacking to windward near land. We plot
danger bearings on headlands and use them to decide when to tack. For
example, coming down Penobscot Bay.

With two people on a 36-foot sailboat, we don't come close to the
procedures of a Navy bridge as described in Dutton's, but we do keep
track of where we are.


>1.Do you use paper charts,rules and dividers?
>2.Do you use mathematical methods such as the following:Lat=Lf + -
>e[S/60(cos T*)] ; Lon.=Lon.f + - e[S/60(sin T*/cos Lf)]
>3.Do you use chatplotters and gps?
>4.Do you use a computer or laptop with navigation software and gps?
>5.Do you use gps only?
>6.Any other method not listed above or a combination of the above.
>Steven,thanks for telling me your training.In future conversations I
now
>have a baseline and know how to frame my questions or discussions with
>you.By the way,thanks for the work you did on the Lunar files.I hope to
make
>some use of them.
>Jared and Dave,I'll get the brand name of the LED nav. lights when I
get to
>the yard this week to work on the skiff and post that info.




Rodney Myrvaagnes J36   Opinionated old geezer

Eagles may soar, but weasels don't get sucked into jet engines.

   

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