Hi all,
I've used to have the one they advertise in Murray's called speedmate. Yes the is a line that dangles in the water and though it's not weighted, is somehow always stayed about 4 inches underwater. It was relatively accurate i guess but it has many short falls.

1. If you mount the unit towards the front, sometimes the cord would get tangled in the rudder and then it's not always easily visible from the back of the tramp.

2. If you mount it somewhere towards the back, you end up snagging it onthe mainsheet, tiller, and occasionaly yourself.

3. The bay i sail through is merky and has lots of crap floating around, so sometimes something would hit the propeller and jam it. No damage to it, you just have to reach down and pull the cable out of the water to clear it.

If you really want something to accuratly measure speed, distance, etc. I would get a GPS. If you get one that is WAAS enabled, then you get an incredibly accurate reading. Even now, most GPS's come with some sort of mapping data. Garmin has a select few units with what they call blue chart capability (basically marine maps on your gps) So though you won't get a direct reading of your depth, you can look to see the contour lines you are between. I never leave home without mine (Garmin 76s). I velcro it to my fore arm and at a quick glance can see where i am on the lake, my speed, and my direction. Extreemly helpful if you need to call for emergency help because you can rattle off the lat and long for quicker rescue. To me, i didn't like the fact of having something dangling behind me and then having to be in a certain spot to read what it said. GPS is far more useful. Just my few cents

~Reid
Lake Superior
N5.2