Chip Logs

[RIPP Tar]

Does anyone know how to calculate the knots on the line of a Chip Log. I remember reading them somewhere but cannot find them again. Any help would be appreciated.

Thanks

[D B Couper]

The line is marked with tags tied 47 feet 3 inches apart, with each tag having one additional knot. The chip is dropped overboard, and timed with a 28 second timer. The number of knots to go through the counters hands is the number of knots you are going.

47 feet 3 inches is calculated by dividing the earths diameter at the equator (21,600 miles) by 360 (degrees). 1 degree equals 60 miles. 1 minute of longitude at the equator equals 1 nautical mile. 21,600 miles divided by 360 (degrees), divided by 60 (minutes) = 1 mile.

[Capt. Trueblood]

Although Master Couper is correct, 47' 3" is the traditional distance between marks with a 28 sec interval, I must ask if your intention is traditional or practical. If, in fact, you wish to create a chip log for practical use, you may consider a smaller time interval. A good interval for a modern 20'-30' boat would be 10 sec. For this, once you have decided on your interval, multiply said interval by 1.6875 to get the distance between marks. Therefore your marks would be 16.875' apart.

[rusty spike]

While DB is correct about the distance between the knots if you wish tomake use of the chip log with modern chart if you want a historically correct instrument tjhe distance was actually seven fathoms or 42 feet. and there are traditional markings every so many knots for more info see the book latitude hooks and azmith ringsnad there are several web sights that relatethe building of navigation instruments.

[RIPP Tar]

Thanks guys, the boat we are looking to make this for is the Formidable, http://pirateshipcharters.com/Welcome_Aboard.html . Several of our crew has worked on her over the last year and we are talking with her Captain about possibly sailing her down to our homeport using some traditional navigational methods. It’s not a long haul but we thought it would be cool to try and get some hands on experience in this.

[Fox]

Actually, any standardization in the distance between knots occurred after the GAoP. For GAoP and earlier the chip log and glass had to be a matched pair, and the knots in the log were tied at the correct points relative to the duration of the glass. You also have the problem that until 1929 there was no single internationally recognised definition of a nautical mile.

The maths for this is fairly easy with a calculator. 1 knot is equal to one nautical mile per hour, therefore the ratio between your knot-length and one nautical mile needs to be the same as the ratio between your glass and one hour. If, for example, your glass runs for 15 seconds, that is 1/240 of an hour, simply divide whichever nautical mile you've decided to use by 240 to get the distance between knots.

[RIPP Tar]

Thank you again gentlemen this extremely helpful