JohnB, we were in a 5-mile wide stretch of Chesapeake Bay - not a narrow river or channel. The SOG on the Lowrance GPS is extremely accurate, and I checked the tidal currents during the same time frame and the maximum tidal current was less than 1 knot, so that wouldn't account for it either. I knew there would be skeptics - that's why I photographed the GPS.
The Lowrance HDS7 GPS/Plotter is a pretty amazing device, and over the years I've found the Lowrance systems to be extremely accurate, much more so than the Garmin and Raymarine systems many of my friends own. Part of this may have something to do with the number of satellites being tracked at any given time. Obviously, the more satellites tracked, the greater the degree of accuracy. The few times I've looked at the satellite tracking information on the display I've noted that there seemed to be at least 5 to 7 in range that were identified. As for the sampling rate, I've never had a reason to look at it, but I know it is adjustable with just a few button pushes and menus.
I talked with the local, Lowrance rep about boat movement affecting speed accuracy and he agreed that if the boat was rocking in any direction it would have some affect, and the violently the boat's fore to aft movement, the greater the degree of inaccuracy. The boat speed at one point ranged from 9.9 to 10.1 for a period of about 20 minutes. During that period the wind was fairly consistent, very few puffs, and at the end of about 3 hours, like almost every day during the summer, it died off to 5 to 10, at which point the black flies moved in for a feast and the sun began to set.
While I realize those wonderful hull speed formulas were created by some fairly smart folks, for some unexplained reason, they just never seem to be all that accurate. To unequivocally state that the Morgan OI will sail at a certain speed because the math doesn't allow it, well, like I said at the outset - that's why I took the photos. And the guy that was with me on the boat, who was at the helm at the time, is a 25-year veteran sailing instructor, was amazed as well. He verified the speed on his hand-held GPS - just to confirm the Lowrance readings. His handheld GPS was reading 10.4, which I guess is within the margin of error. Maybe that ice cold water that is more dense has a bearing on your sailing speeds in Canadian waters.