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Back to basics – course to steer

One of the most basic skills in navigation is working out a course to steer taking into account the tidal offset.

Get it wrong and frankly you look like a bit of a fool, particularly if you end up several miles down tide of your destination and then have to spend the next couple of hours explaining to your crew why you are punching into the tide as opposed to tucking into a hearty meal at the local yacht club.

Whether you’ve been navigating for years or are a total novice, Tim Bartlett’s RYA Navigation Handbook is a great way of brushing up your skills: This excerpt will provide an excellent starting point:

Course to steer for one hour or less

The first step is to draw in the intended track as a straight line from the point of departure to the intended destination and beyond.

The next is to draw another line, from the same starting point, to represent the tidal stream – pointing in the direction the stream is flowing, and with its length proportional to the speed of flow.

It doesn’t matter what scale you use: a knot could be represented by a centimetre, or by an inch, or even by some completely arbitrary unit.

The third step is to open a pair of drawing compasses or dividers to a distance equivalent to the boat speed, using the same scale as the tide speed.

Put one point of the dividers or compasses at the end of the tide vector, and use the other to strike an arc across the line representing the intended track.

Finally, draw a straight line to the point where the arc intersects the intended track. The direction of this line represents the course to steer. It does not matter if it seems to be starting from the wrong place (in the illustration, for instance, it is starting from Nimble Rock, rather than from the river entrance)

because it is only the direction of the line that is important. Nor does it matter if the lines drawn to calculate the course to steer pass over shallow patches, or hazards, or even cross the coastline: they are purely lines representing a mathematical construction: the only one that has any direct relationship to the movement of the boat across the surface of the real world is the one representing the intended track.

Tim said: “This is course to steer at its most basic level. Don’t forget that there are tricks you can use to enable your GPS to work it out.

“In addition to this, if you are trying to work out a course to steer for over an hour then this complicates matters. You also need to bear in mind any leeway, or sideways drift your boat may experience.”

For a full explanation of Course to Steer and many other aspects of navigation, you need to pick up a copy of Tim’s RYA Navigation Handbook.

Tim’s top tip: the one in sixty rule

“This is a little trick I picked up some time ago and can save you valuable time when working out a Course to steer.” Tim said.

“It all stems from the slightly obscure but very convenient trigonometric fact that the sine of 1° is about 1/60, and that for angles up to about 45°, dividing any angle by sixty gives a reasonable approximation of its sine.

Don’t worry if you forgot all about trigonometry and sines the day you left school, because it boils down to some very simple mental arithmetic, like this:-”

Tide speed x 60 = course correction
Boat speed

The ‘course correction’ is always up-tide. So if a vessel experienced a tidal stream of 2 knots, and had a boat speed of 5 knots

2 x 60 = 120 = 24°
5          5

“This is true if the tidal stream is roughly at right angles to the intended track. If it is ahead or astern, no offset is required. If it is at about 45° to the intended track, use two thirds of its rate in the formula instead of its full rate.”

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