How do you do the GPS log, if you don't mind me asking? I would love to do something similar when I'm out on the water.
My GPS performance logs are derived from the .gpx database generated by my Garmin Montana 650 GPS. After completing a rowing session, I save the GPS track on the Garmin and upload it (USB cable) onto my computer using the (free) Garmin program called "Base Camp". That displays a map of my rowing track and a database can be uploaded into MS Excel for additional processing. I chose speed vs. distance for additional processing. Usually the longer I row, the slower I go, but today, my return home was downwind, therefore faster by about 0.5 mph at the end (from mile 3.5 to mile 5.0).
The graph is created in MS Excel. The image inserts are from the GPS built-in camera on the left, screen capture from the GPS in the middle, and on the right side is the GPS track displayed on Google Earth which is done using Garmin Base Camp (which interfaces with Google Earth).
I would like to add a cadence counter to the GPS, but the only available cadence counters are designed for bicycles and would be difficult to adapt to the larger range of motion of the oars and oarlocks. So I count the strokes per mile and determine that going into the wind and waves, I average 13 feet per stroke. In calm water I average 16 feet per stroke. With a strong wind on the stern, I can get 17 feet per stroke.
For those who are concerned about physical fitness, a wireless heart rate monitor can be easily added to the Garmin 650.