GPS is not infallible, so it makes sense to know the basics of shaping a rough ‘course to steer’ on a paper chart, says Dick Everitt.

To keep things simple let’s say we want to go north 000° with a boat speed of 5 knots and there’s a 2-knot tide trying to push us to the east 090°.

We draw a long line due north, and at the departure point an hour’s worth of tide due east (see diagram 1).

Shaping a course - We want to go north, and the tide is pushing us east

From the end of the tide line set a pair of compasses to an hour’s worth of boat speed and strike off an arc to cross the vertical line. Connect those two points and our course would be 340°, if we were motoring and there was no wind (diagram 2).

Shaping a course - This would be our course if motoring, without wind
With any wind we have to allow for leeway, which is how much it will blow us sideways (explained in a previous Nav in a nutshell article). Let’s say the wind is blowing with the tide, so we compensate by heading up into the wind another 5°. Our ‘course to steer’ is now 335° (diagram 3).

Shaping a course - Factoring in leeway gives the course we need to steer

The RYA teaches navigation chart work in degrees ‘true’ (°T) and then converts to ‘magnetic’ or ‘compass’ at the end. Charted tidal diamonds and bearings, such as leading marks and sector lights etc, are given in degrees true because True North never changes, whereas magnetic variation is shown on the chart to move a bit each year.

But I prefer to work directly in degrees magnetic, because it’s fast and you can forget the old ‘East is least and west is best’ mnemonic for converting from true to magnetic (it reminds you to ‘subtract east variation and add west variation’. If, for example, your true heading is 110° and the variation is 5° west, add 5° to the true heading to get a magnetic heading of 115°).

Working in magnetic allows you to work directly with your ship’s and handbearing compasses, and most GPS units can also be set to work in magnetic.

Set the compass rose to 115°– we’re lining it up with the 5° west variation

If you use a Breton Plotter for chart work you can mark the amount of magnetic variation and use that as zero (see photo, how 110°T automatically reads 115°M).

This Nav in a Nutshell was published in the February 2012 issue of PBO. For more useful archive articles explore the PBO copy shop.