Travel time made simple
David Roulston’s ‘Screw-Gap’ device can help you estimate time of arrival
When sailing on my Peter Duck ketch on the South Coast, or across the Channel, I often want to know how long it will take before we reach the same point as a ship heading towards us, or estimate the time to a significant stationary object without recourse to determining our position on the chart.
I realised that a simple screw set in a three-edge device would solve this problem.
The dimensions of the wood structure that I created are 3cm x 3cm x 5cm but could be larger.
I used a standard 6mm diameter bolt size with a 1mm thread held in a nut embedded with Araldite to one surface.
The other inside surface is at a distance slightly exceeding 20mm.
I created a dial on a word processor (but it could be carefully drawn), then marked in numbers 0 to 9. I glued and varnished the dial to the top surface. Each complete turn of the screw corresponds to 1mm (the thread size), therefore each notch on the dial is 0.1mm.
The knob with the pointer was a standard electronic control switch.
How to use it
Hold the device at arm’s length and adjust the screw to line up the distant object as precisely as possible between the flat surface at the bottom and the base of the screw. Measure the gap by noting the dial value (eg 3), which is tenths of a millimetre. Then count the number of full rotations (eg 7) until the gap is closed. In this example G = 7.3 mm.
To estimate your time of arrival or intersection, carry out step one, above, and note the time (time A). Then turn the screw anticlockwise to set the gap to twice this value (in millimetres: eg 14.6mm). Observe the object with the device still at arm’s length until the object fills this new gap. Note the time (time B). At constant speed, the
time difference will be the same as the time remaining before reaching the object. In other words if the difference between time A and time B was 20 minutes, you have 20 minutes until arrival or intersection.
This simple gadget is small enough for the helm to carry in a jacket pocket and may help to inform appropriate course and/or speed alterations.
Further uses
This screw-gap device can also be used to measure the thickness of ropes or wood or metal sheet before ordering replacements. The device gives accuracy to better than 0.1mm. The device can also be used to estimate angles subtended by distant objects, eg lighthouses. Held at a distance L from the eye of the user, the angle in degrees subtended is equal to (G/L)x 57.3. If the screw is held at a typical arm’s length of 570mm, a screw gap G of 10mm is approximately a subtended angle of 1°, or about 2° when held against an elbow. The angle range can be increased from 2° to 10° (at arm’s length) by bolting on a strip of wood with pins at exactly 20 mm intervals extending above or below the zero gap surface, as illustrated below.