NavList:

Robert Eno wrote:

Anyone out there ever use their GPS to swing a steering compass.
I'd like to hear about your experiences.

Robert:

That is a problem I am currently working on. Lake Michigan, 350 Catalina
(friend's boat).

There is leeway and current (albeit small) to contend with.  To minimize the
effect we tried it on a low-wind (below 4 kt true) drifter day (dying 1-3 ft
swells) under power. One GPS with mast-head-mounted antenna. One hand held
GPS in the cockpit.  A hockey puck compass placed on the cockpit coaming
visually aligned at the onset.

Getting any observation to match within +/-5d of the boat's compass was hit
or miss.  When the helmsman reported the binnacle compass was settled in he
would shout "now" and report the heading. The GPS watchers would record the
GPS course, binnacle compass reading and time, as would the observer
watching the hockey-puck compass.  Getting any two to match within +/-8d was
problematic. The owner wanted to keep the speed low (approx 3.5 kt), to what
end I do not know.  My take, at 3.5 kt the swing of the bow was a problem,
combined  with roll.

Considering the GPS is looking at COG 3-4 ft above the waterline, and it is
moving quickly sidewise as well as forward, a fool's game as I read it.
(What the heck, if you want to increase a sailboat's speed, just have
someone run to the foredeck with a hand-held GPS ;-)

I did enter the data into Excel, and plotted the information on a Napier
Diagram, looking hard at sanity checks when the GPS units were within +/- 2d
of the ship's compass, and found some opposed hot spots around 120d and 300d
(repeated the 120d and 300d test 2 more times each on the water when they
popped up on my mental radar). Plotting later, the points on the diagram
were offset by 2d west.  Also looked at the the lake-currents web site at
the time of observations, set and drift 225d/0.4 kt and factored that in for
a second look see, but nothing remarkable to report about the overall trend
at 120d and 300d.

In the end, the hockey puck on the cockpit coaming was within 0-1d of
reality when there was a visual target, and about the only instrument we
could trust. 

I learned a lot from the experiment. Utmost, if using a GPS to compare with
the ships compass on anything but glass, get the GPS as close to the pivot
point (keel in this case) as possible.  Ideally, although not likely, the
antenna would be at water level.

Bill    






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