Navigation Notes I - May 3, 2002
From Navigator John Fishbeck
Over the past week I've spent time developing and refining navigation techniques
and tools that I believe may be useful in the NOOD racing effort. This is a
summary of what I've come up with to date.
First, I borrowed Doug Carlson's Magellan hand-held GPS after our meeting Saturday.
I've used this device before, but it's been a couple of years, and I wanted
to re-familiarize myself with it. In reviewing and working with it, I discovered
a capability that this unit has that will make setting a layline destination
waypoint quicker and more accurate. Turns out that I can 'project' a new waypoint
location from a position known to the GPS, either current position or a stored
waypoint, by entering a bearing and distance. So computation of a waypoint at
the layline, given the heading of the current tack, goes like this:
1) Store current position on GPS
2) Note heading on present tack
3) Plot GPS boat position on to chart
4) Plot layline to windward mark on chart
5) Extend boat course from GPS position to intersection with layline
6) Measure distance along course from GPS posn to layline intersect
7) Using GPS position, recorded current heading, and the measured distance to
the layline, create projected GPS waypoint at layline
8) Activate GPS to 'goto' layline waypoint
This ability of the GPS to derive the 'projected' waypoint reduces the time
required to determine the layline destination waypoint, and increases the accuracy
of the GPS layline waypoint.
In trials so far I can complete the above steps in just under 5 minutes. I will
continue to practice this, but believe that it's reasonable to assume that I
can't get this procedure down in under much less than 4 minutes. And, mind you,
this is working on my dining room table, not on a chart table below decks onboard
a boat in active water. But this is probably fine for offshore racing in a 33'
boat - it's not like we're gonna be tacking this boat around in wild abandon
like we do small boats in very shifty inland lake winds.
And here's another thing that I've worked through that I think may be helpful.
The procedure above will give a roughly accurate fix at the layline given the
present course. This information will provide us with a roughly accurate estimate
of how much time and distance we have before reaching the layline. But in addition
to this, we may be able to use the GPS to tell us almost exactly when we're
on the layline. How? By activating a 'goto' to the windward mark as we approach
the layline. The GPS displays bearing to the currently active waypoint, and,
hence, will let us know precisely (well, give or take) when we're at the layline.
Let me work through an example of how this will work. First, we have the location
of the windward mark stored as a waypoint in the GPS (presumably we'll be given
positions of the marks at the skippers meeting). Then assume that we've determined
previously that in the current wind and water conditions the boat tacks through
an angle of 92 degrees. We're on starboard tack with a current heading of 53
degrees. So the layline to the windward mark will be on bearing 321 (53 (curr
hdg) - 92 (tacking angle) + 360 (correct for wrapping backwards through 360).
As we sail to the layline we have the GPS set to the layline waypoint as the
active waypoint, providing us with info on distance and time remaining to the
layline. A couple of minutes or so before arriving at the layline, the active
goto waypoint on the GPS is changed to the windward mark. Now we have bearing
information from the current boat position to the windward mark. Initially,
in this example, it'll be something like 330, for example. But as we approach
the layline, we see the bearing to the layline displayed by the GPS approach
the bearing we know we need to sail (321). When the displayed bearing to the
windward mark is 321, boom - we tack, right on the layline! Well, maybe a degree
or two beyond, for margin. Also note that in this last few minutes as we approach
the layline it's easy to accommodate wind shifts, as the arithmetic to compute
the bearing to the windward mark given current heading and known tacking angle
is trivial and can be accomplished in moments.
It should be clear that both of the above techniques (storing a waypoint at
the layline, and using the bearing to the windward mark waypoint to call the
tack at the layline) depend upon knowing, with reasonable accuracy, the boat's
tacking angle. Therefore, one of things that needs to happen during our practice
sessions are some tacks back and forth in a short time frame (reduce wind shift
variations) in varying wind and water conditions so we can collect the necessary
data regarding the boat's tacking angle.
In addition to this, I'm intending to bring along my Garmin eMap GPS as well.
While not suited to over water navigation (it doesn't display compass bearing
information, for instance), it does have mapping and excellent historical tracking
capabilities. This means 1) the eMap can be used to assist in navigation when
returning to harbor in reduced visibility conditions, as the mapping function
provides addition visual information regarding location, and 2) with the saved
tracks of the sailed races, this information can be available for later post-race
reviews and analysis (i.e., which side did we go to in the first beat of the
first race, how was that different than the last beat of the third race, etc.?).
Given the above, I've come to the conclusion that there's really no need to
take up Terry's offer of his plotting GPS, or of his laptop with the charting
software. I think I have everything I need with what I have at hand. This reduces
complexity of figuring out what devices and power are needed, where things are
going to fit, worrying about equipment damage (a concern I had with the laptop
sliding off in a sea), and spending additional money on extra equipment (e.g.,
a remote antenna for my eMap GPS).
One last item - again from the above narrative, it should be obvious that I'll
need an effective chart table. It need not be fancy, but it does need to be
1) flat, 2) stable, 3) accessible, 4) relatively smooth, and 5) big enough.
Big enough is... well, the full sized chart for Lake St. Clair is nearly 3'
by 3' (35" x 35"). However, assuming that we're not going to be sailing
in the northern part of the lake (probably a safe assumption), then I can trim
the chart down to 21" tall by 31" wide.
That's it. There's some food for thought for you. I'll be looking forward to
our practice sessions to work out these techniques on the water.
Regards,
John
Navigation Notes II - May 5, 2002
NOTE from Pat Dolan: In response to some questions from me (about storage issues
for instruments and tools as well as what we might expect in the way of mark
location info), John sent the following:
Glad you found my navigation procedures update interesting; I thought that you
might be intrigued.
As to mark positions - it's been a while, but I have sailed in off-shore small
one-design events (Flying Dutchman, Lightning), and my recollection is that
they provided at least minimal information on mark locations. They really have
to, as the course itself is well out away from shore, and the course marks are
not visible from the start of a leg. At the very least my recollection is that
they provide compass bearing info for the legs, by which you can at least navigate
around the course. And in this day and age, with the widespread availability
of inexpensive GPS receivers, I wouldn't be surprised if the RC actually posts
lat and long coordinates on the RC boat prior to the start.
But you're absolutely right - worst case is we fumble our way around the course
first time, and set mark waypoints as we round them. From that point forward
we'll actual, accurate fixes to work from.
By all means, bring along your hand bearing compass. I'm a believer that more
navigation equipment than you need is a far better thing than not enough.
The Garmin eMap is a compact, hand-held unit, designed specifically for automotive
use as an 'electronic road map'. If you're interested, see Garmin web page at
http://www.garmin.com/products/emap/ for pic and info. I can actually carry
the thing around in my pocket, except, of course, it needs 'visibility' to the
sky overhead to fix on GPS satellites. I actually have a very slick mount for
this, that I use with my motorcycle (the reason I acquired it in the first place),
with a ball-and-socket articulated arm and a U-bolt type clamp that could easily
attach to a stanchion or other piece of tube somewhere.
My preference for holding my stuff would be at minimum an open box to contain
and keep together the various implements. A cigar box, for instance, would be
ideal, except 1) probably wouldn?t hold up well under high moisture conditions,
and 2) it wouldn't be long enough to hold my parallel plotter (~ 12" wide
by 3" high). Beyond that, a lid that would close and secure to keep the
stuff in the box when not being used in wave conditions would be a desirable
plus.
Inventory of stuff I'll be using, and have on board, is:
* parallel plotter
* dividers
* compass (drawing, not navigation, type)
* pencils
* stick eraser
* and, naturally, the chart
That's it. Looks like you guys had a fine day for the Lightning regatta yesterday,
hope it was a great event.
Regards,
John