I've had one intractable problem with my CPF390i: The SOG and COG vary so wildly that both numbers are unuseable in ocean swells.
The cause is the lack of dampening (electronic averaging) to compensate for sway and surge.
is the linear vertical (up/down) motion
is the linear lateral (side-to-side) motion
is the linear longitudinal (front/back) motion
speed (velocity) and direction are derived by comparing the present position with the most recent previous position. Sway can be substantial, causing a boat
to move sideways 20 feet or more on each crest and trough several times per minute (depending on the wave height and periodicity) at velocities greater than the boat's forward motion through the water
When a boat
sways in ocean swells, the GPS-derived direction is distorted in the direction of the sway, and the speed is distorted by the surge. The result: the boat appears to change direction (yaw) by a large factor - I've seen apparent changes in direction of over 60 degrees. With a swell period of 10 seconds, the boat will appear to turn 12 times per minute. And surge causes the speed to continually fluctuate by several knots. The changes in SOG and COG are so wild at times that they can't even be read.
I reported the problem to Standard Horizon several times, and while the response times in SOG/COG can be adjusted slightly, the adjustment is inadequate. Their final suggestion is that I use the boat's compass
instead (!!), and they had no answer to my follow-on question: "How am I supposed to determine course-over-ground from my compass
- without reverting to 19th century technology?" One doesn't buy a chart plotter that requires using ancient navigation
methods for accuracy.
I compensated for the problem by installing a Garmin
72H receiver in the cockpit
next to the CPF390i. The GPS 72H allows the user to set a long averaging time on course and speed. I set that time to 20 seconds, which is longer than the usual periodicity of swells in my region. The Garmin
SOG/COG display is rock solid and usable for navigation
, where the CPF390i SOG/COG display is not usable on anything but glassy lakes.