This device provides a mechanical way of finding what course to steer in order to make good a certain given course, allowing for tidal current.

The graphical (pencil and paper) method is this :-

Draw a straight line AB, of indefinite length. A is the starting point, and the direction of AB represents the required course (CMG).

Then draw the line AC, of a length which is proportionate to the tidal rate (perhaps 1” = 1 knot), and at an angle to AB equal to the tidal direction minus CMG.

From C, draw an arc whose radius represents the speed of the boat (same scale as AC).

This arc intersects AB at D. The length of CD gives the speed made good (over the ground) and the angle ADC is the adjustment to be made to the required course (CMG) to give the course to steer.

It will be obvious that this method, though easy to perform on a chart table, is virtually impossible in a dinghy with the chart in a plastic bag on one’s knees!

Hence the mechanical method, which is based on exactly the same principles and calculations, is more convenient, and can be carried out with the device about to be described (which can be constructed at very little cost, and no great skill — just reasonable care).

CONSTRUCTION

The materials required are two pieces of ⅛” clear Perspex, 6” by 4” and 2½” by 2¼”, together with a couple of lengths of 1/16” wire (16 s.w.g.), copper. These sizes produce a device which is convenient for handling, using and stowing. It can be scaled up or down, or the scales of speed extended, to meet particular requirements.

The following dimensions assume a boat speed of up to 3½ knots, and tidal streams up to 3 knots. If you sail near Hurst Point or Le Havre, or have an “E-type” boat, you will have to double or treble the scale markings (on both the boat-speed and the tide-speed scales).

Working on a scale of ½” = 1 knot, scribe a circle (radius 1½”) near one end of the larger piece of Perspex; and then, concentrically within that circle, two other circles with a radius of 1” and ½” respectively. Then mark in the long axis, through the centre of the circles. Using this as the reference line, scribe radial lines every 10°, between the ½” and the 1½” circles; you can omit the 180° line.

Then drill 1/16” holes at every intersection of the circles and radial lines, except that on the inner circle, holes are only on alternate radii, every 20° from the reference line. It is not necessary to drill the centre of the circle, nor on the 180 radius (which should have been omitted!) Finish off this piece by drilling several 1/16” holes close together along part of the reference line; break these holes into each other, and then use a hacksaw blade to elongate this slot, so that it extends 3” from the ½” circle. A nail file will be found useful to smooth the sides of this slot.

Scale Marking

Mark every 20° from the reference line, engraving or scratching the figures; it may be helpful to fill the scratches with pigment for better legibility. Mark the three circles ‘1’, ‘2’ and ‘3’ to show the tide speed in knots (the space between 160° and 200° can be used for this). Make short transverse lines across the reference line, every ½” along the slot, and calibrate from 1 to 7 (speed made good) knots.

Now take the smaller piece of Perspex and mark a central reference line. Mark a series of points along this line, starting about ¼” from the edge, and drill 1/16”. These holes represent boat-speed at ½-knot intervals, so eight holes will be needed for 0 to 3½ knots. Each of the holes should be slightly countersunk to aid the insertion of the 1/16” pins, which should not be a sloppy fit. To permit fitting one pin permanently, the end hole should be countersunk at both ends and the lightly-tinned pin secured with a dab of solder each side of the Perspex. The loose pin has a head of solder and a length of thread slip-knotted to it; with care, the solder can be made to retain the thread.

To Use the Calculator

Given: (a) the course to make good (CMG) (b) the tidal direction and speed (S and R) (c) the speed of the boat (K)

Steps: Subtract CMG from S (not vice versa); if the answer is negative, add 360° to S and then subtract CMG. Set the moveable pin in the hole corresponding to K. Arrange the small piece of Perspex over the larger piece, so that the moveable peg passes not only through the correct K hole, but also through the right hole for R and (S—CMG). Read the speed over the ground from the calibrated slot. From the degrees scale on the smaller piece and the slot on the larger, read off the correction to be made to CMG, plus or minus, according to which side of the degrees scale is over the slot.

Scale marking of the smaller piece.

To reduce confusion with two sets of lines and figures superimposed, these scale markings should be in a different colour from those on the other piece.

(1) Mark the seven free holes from ½ to 3½ knots. (2) Scribe lines every 10° from the line of holes, and calibrate from 10° to 40°. (3) Mark the RIGHT side of the piece +, and the LEFT side — (minus).

EXAMPLE

Tide sets 1½ knots, 105° Course to make good 025° Thus the tidal angle (angle DAC) is S—CMG or 105—025, which is 80°

Alternatively, Tide sets 1½ knots, 050° Course to make good 330° In this case, S—CMG is negative (050 minus 330), so we add 360° to S and proceed as before: 050 + 360 = 410; subtract 330; answer: 080°.

Set the moveable pin at 3 knots for boat speed, and pass it also through the 1 knot (inner circle) 080° hole. Then read off the course adjustment — 025° — and speed over the ground, about 3 knots plus. The 25° reading was on the left side of the midline, and therefore MINUS, so the course to steer is 305° in the second example, or 000° in the first.

Accuracy

The speed of small boats is usually only estimated, perhaps no better than ± 25%. You will only rarely be near one of those Admiralty diamonds for which tidal information is at all accurate. And it is very difficult to sail a dinghy on a compass course to within 5 or 10 degrees. So the ten and twenty degree intervals and one-knot steps are sufficiently accurate for most purposes. Just don’t look for Christchurch harbour in thick fog!