Here is a posting on GPS accuracy I though useful
|Al Bowers||Deputy Director of Research (acting)|
|firstname.lastname@example.org||NASA Dryden Flight Research Center|
“It is my belief that flight is possible.” – Wilbur Wright, 03 Sep 1900
Okay, As for the “dither” the GPS has. That has been turned OFF for a couple of years now. The uncertainty on the location is actually a product of the number of satellites your reciever is seeing (more satellites, better accuracy). Commercial units are within a few feet (we use a technique called integer wave counting where we can nail it down within a couple of inches, a common instrumentation technique we use). The speed from the GPS is REALLY good, I mean REALLY well nailed down. We’ve run LOTS of tests to try and “spoof” the GPS and (as long as the local USAF and Navy are not trying to run their GPS jamming gear) it’s REALLY good. The speed is determined by Doppler shift (not differentiating the location signal) so if your reciever uses a good timing crystal (and most of them are pretty dang good) the speed will be pretty rock solid.
BUT, GPS doesn’t give an instantaneous speed. It AVERAGES the time over a long period (the shortest time is 1 second). So max speeds may be “filtered” (an engineering word for “averaged”) down to a lower level. And as has been pointed out, current will change the readings between GPS and Speedmate.
Okay, another problem with Speedmate. The boat hull deflects the water passing aroound it. The result is the water is flowing past the hull FASTER than the boat is moving through the water. Water, being incompressible, has to flow around the hull faster due to the depression the hull makes in the water. Similar to the wing of an airplane (air flows faster over the top than the bottom because of the shape, which is what causes the lift to develop). So Speedmate, even if perfectly calibrated, would read higher than GPS (which measures filtered REAL speed). We spend a LOT of time and money calibrating this part out (called the installation error) in airspeed systems on aircraft. It’s an artform in and of itself. As for laminar v turbulent flow over the hull, if the impeller is near the cockpit or forward of it, it shouldn’t be a problem. I don’t know about aft , though the impeller migt be far enough out into “free water” that it might not matter (I dont’ think there is much separated flow on the hulls we use now, if there were, the boat woud probably be pretty slow).
More than anyone really wanted to know. It’s probably best if we all just go paddle and not worry about it too much.