Renaissance Explorers – How to make a Chip Log to Estimate Ship Speed

A Chip Log was a crude and early form of speedometer and the only means the Tudor sailors had of telling the approximate speed they were travelling at:

Source

During ancient times, sailors measured ship speed by something called ‘Heaving the Log’  Heaving the Log consisted of throwing a wood log into the water and watching how fast it moved away from the ship.

It was during  the 1500s that the ‘Chip Log‘ method was invented.  The chip log was a rope which was knotted at equal intervals and weighted at one end.  This was used alongside a time measuring device (usually a half minute sand glass)

Sailors would throw the weighted end of the rope off the rear end of the ship, allowing it to unwind off the reel as quickly as the boat was moving.  The faster the ship was moving forward the faster the rope would unwind. The sand glass was turned and each knot counted as it unwound, until the sand had emptied into the bottom chamber. By measuring the number of knots which disappeared into the sea over a specific time (measured by the sand clock) the sailors could tell the ship’s speed. The nautical measurement of the Knot originated from this practice.

Making our own Chip Log

The children were all involved in this exercise.  L12 was in charge of the rope, tying the knots at 5 foot intervals.  C12 would be in charge of data collecting and calculations, whilst T13 would be in charge of timings.  Although we would be making our own half-hour sand glass, for ease of use we used a modern day timer:

L weighted her rope down to the plant pot.  T set off the timer and L began to walk away from the plant pot, unraveling the rope and counting knots as she went:

When the rope ran out she shouted stop to T and he stopped the timer.  C12 made a note of the number of knots and the time it took for them to unravel.  She then carried out the calculations which would allow us to estimate at what speed L12 was travelling.  The calculations were as followed:

L was travelling at a speed of 1.2 miles per hour.

This was a really good exercise for the children to experience how speed was calculated without the modern day speedometers and GPS.

Making our own Sand Glass

Sailors would have used a half minute Sand Glass to count the knots.  

A sand glass is made of two glass bulbs attached together via a thin tube.  Inside there is enough sand in one bulb that when the sand glass is inverted the sand travels from one bulb to another in a specific amount of time.

We had hoped to make a sand glass to use alongside out Chip Log above but as we made a hour hour sand glass as apposed to a half minute one we quickly realised we would not have enough rope to keep going for half an hour.  I mean, L12 would have ended up in Timbuktu!

Although we didn’t actually use a sand glass for our chip log I did think it was important for the children to understand how sailors knew when a certain amount of time had passed.

To make our hour hour glass we collected the following items:

• Two empty, dried 500ml bottles
• Card to make a disc
• Scissors
• Gaffa tape
• Salt

We filled one bottle half full with salt:

Using the card we cut a disc and made a small hole in it using a pair of scissors:

We attached the two bottles together with the disc in between and secured them using the gaffa tape:

We timed how long it took for the salt to travel from one bottle to the other through the hole in the disc:

The hole can be made larger or smaller depending on the timing you desire.  We wanted half an hour so made ours a little bigger.  We tweaked until it took exactly half an hour for the salt to fall from one bottle to the next.

We had made our Half Hour Sand Glass:

Fun times!