Trolling Speed

Not sure if this will help, but I use a handheld gps.  It seems that most of the newer smart phones have a gps app that will work as well.  We used something similar to this years ago.  
 

The gps seems to be accurate for me.  I have paced alongside friends boats and we've had nearly identical readings.  I've heard that gps' are accurate to within ~3', and the unit must report some kind of average for the speed (if only the past 5 seconds) or the speed would continually fluctuate.         
 
It's fairly accurate in the car.  I use it when one of my kids or the wife is driving and it keeps me from straining to look at the dash.  i can apply that imaginary brake pedal without looking over, lol.

Hey thanks for the advice.  Dawned on me I have an old Garmin Etrex   Tried it out today in my vehicle and it was pretty accurate.  I'd say on the lake at a steady trolling pace it would be pretty much dead on.   Thought about the Iphone app thing, but that would burn the battery in no time just like the navionics app.  Maybe I need a cigarette lighter in the dash so I can charge my phone......hmmmm......

opsman, i think what he was saying isyou cannot rely ONLY on your t.m. to give you the same speed at all times since wind ,n, waves affect your boat. in other words, 3rd will not always be 2.1 mph, 2nd won,t always yield 1.7 mph. you will need another source of measurement. but since you already have the etrec, problem solved.