I was playing with some math today figuring out the best way to use my E6-B flight computer for marine applications. Somehow my mind went to log lines and sand timers.
I got the idea that with todays watches, all of which incorporate a second hand, working a log contrary to the old way, which took counting knots to determine speed after a certain time, typically 28 or 30 seconds, a predetermined length of log line might easily be timed to determine speed.
The following is a list reflecting time in seconds vs speed in knots for a log line extending 60ft:
1 kt 35.5 sec
2 17.8
3 11.8
4 8.9
5 7.1
6 5.9
7 5.1
8 4.4
So you take a fishing line 60 feet long on a cuban rig fishing spool. Attach a log device to throw into the water which pulls the line. Throw the log into the sea and start your timer. When the line pulls taught stop the timer. Determine your speed over the water using the chart above. If the spool is held any more than 4 or 5 feet off the water accuracy may suffer. With high cockpits a longer line would be better; just refigure the knots vs time table.
The math is thus:
1knot = 6076ft ÷ 3600s = 1.6878ft/s
You could have a log line 6076 feet long and spool it off for 3600 seconds which would tell you your speed is 1 knot. But there's a better, faster way.
60ft of line ÷ multiples of 1.6878 gives time in seconds
Ex:
60 ÷ 1.6878 = 35.5s for 1 knot
60 ÷ (2 × 1.6878) = 17.8s for 2 knots ... etc.
You can help me out here and let me know if I got the figures wrong. Any length of line would do beyond a certain minimum. I took 60 because in an emergency 10 arms spread would about equal 60ft. You can kinda sense here that line length doesnt get critical until you reach the higher speeds.
Have fun and give it a try next time you're out on the lake and dont forget to tell us what happened.
Cheers
I got the idea that with todays watches, all of which incorporate a second hand, working a log contrary to the old way, which took counting knots to determine speed after a certain time, typically 28 or 30 seconds, a predetermined length of log line might easily be timed to determine speed.
The following is a list reflecting time in seconds vs speed in knots for a log line extending 60ft:
1 kt 35.5 sec
2 17.8
3 11.8
4 8.9
5 7.1
6 5.9
7 5.1
8 4.4
So you take a fishing line 60 feet long on a cuban rig fishing spool. Attach a log device to throw into the water which pulls the line. Throw the log into the sea and start your timer. When the line pulls taught stop the timer. Determine your speed over the water using the chart above. If the spool is held any more than 4 or 5 feet off the water accuracy may suffer. With high cockpits a longer line would be better; just refigure the knots vs time table.
The math is thus:
1knot = 6076ft ÷ 3600s = 1.6878ft/s
You could have a log line 6076 feet long and spool it off for 3600 seconds which would tell you your speed is 1 knot. But there's a better, faster way.
60ft of line ÷ multiples of 1.6878 gives time in seconds
Ex:
60 ÷ 1.6878 = 35.5s for 1 knot
60 ÷ (2 × 1.6878) = 17.8s for 2 knots ... etc.
You can help me out here and let me know if I got the figures wrong. Any length of line would do beyond a certain minimum. I took 60 because in an emergency 10 arms spread would about equal 60ft. You can kinda sense here that line length doesnt get critical until you reach the higher speeds.
Have fun and give it a try next time you're out on the lake and dont forget to tell us what happened.
Cheers