NavList:

Bill:

Were you using the GPS's indication of CMG or bearing to a distant mark?

Hopefully you've seen all the subsequent posts, but basically we've all
said CMG is NOT a reliable instantaneous heading indicator, but bearing
to a distant waypoint is (assuming you're pointing at it).

Lu Abel

Bill wrote:
> 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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