Boat instrumentation

Boat instruments provide basic information about the conditions under which you are sailing. Entering regular readings into a log will provide you with a record

your voyage and, over time, may help to predict changes in the weather or interpret local conditions in a new environment.

Many essential yacht instruments are repeating, with one set in the cockpit where the helmsman can read them, and another set down below at the navigation station.

Depth sounders

Depth sounders, or echosounders indicate the amount of water under your hull. It is normally part of a set of instruments and it can be combined with your log. A simple instrument, it doesn't give much trouble, but is invaluable when coming into shallow waters or looking for a safe anchorage.

Heavy swells can easily create false readings, leading you to believe you have a wider safety margin than is the case, so take care when sailing over a reef, particularly if the swell is big. Depth sounders can also be used to follow a contour line on a chart, which is very helpful in fog when you are close inshore.

Barometers

While not essential for navigation, it is important to have one on board. Low (falling) barometric pressure indicates that bad weather is on the way, so if you log the barometric pressure every hour you will get an indication of what is occurring.

GPS receivers

GPS (global positioning system) receivers are devices that display latitude and longitude at any given time. GPS operates off 24 earth-orbiting satellites which transmit radio signals that can be detected by anyone with GPS receivers.

Position is determined by readings from at least four satellites.

The combined readings give a fix (a specific set of coordinates that indicate your current position). There are always enough satellites in sight for the GPS to get a fix that is accurate to about 30m (100ft).

It is a good practice to log this data every hour so that if anything goes wrong, you always have a definite fix on where you were 60 minutes previously.

Yacht GPS works best for marine navigation when the latitude and longitude positions are plotted on an upto-date chart which is compatible with GPS.

Data on older charts was probably compiled from sights and bearings taken before the advent of electronics, and may include errors, such as features placed fractions of a nautical mile out of position, sometimes more, so slavishly following GPS information could put you aground.

Electronic charts

Electronic charts plot your position and will give early warning if the track you have plotted differs from the course you are sailing.

When connected to a GPS and laptop computer, they give a bird's-eye view of your boat superimposed on a nautical chart, in real time.

You can see your boat moving over an area and can check the course you have made from a previous port or waypoint, as well as your progress on a new route. You can also judge how far away you are from any known dangers.

A number of reputable manufacturers produce electronic charts, most of which are Nimea-compatible and also accept input from other systems. (Nimea is an international standard and all Nimea-compatible instruments connect to each other.)

Electronic charts and chartplotters are great, but don't forget that computer-based systems can fail, so make sure you always have paper charts as a back up.