Navigation and Chart work - Instruments
Navigation instruments
There are numerous instruments available to assist the navigator, but the important ones are the basics that have been around a long time. If you have all the modern aids aboard, it is still a good idea to have some means of navigating if everything fails. Even if it only happens to you once in your sailing career, you will be glad you had a back up system in place.
• Clock.
A reliable clock that can be easily read when the logbook is being filled in 
is still essential. It is then the standard time for everyone on the boat, and this saves people digging into their waterproofs to find their watch, if they have them sealed up in bad weather.
If the clock has an alarm, this will help to get the crew moving in the morning and make missing the weather forecasts less likely.
• Barometer.
A reliable barometer is essential, again it must be easily accessible to someone filling in the logbook, or they will not bother. The barometer should be adjusted occasionally; there are times when the air pressure should be obvious from the information given with a forecast. If you are in the vicinity of a Coastguard Station they may be prepared to give the current air pressure at their station if they are not too busy.
• Echo sounder.
I personally feel that a reliable echo sounder is the most important 
instrument on the boat. When you are not sure of your position, a good watch on the changes in depth may indicate where you are, and will probably keep you in safe water!
The echo sounder must be backed up, by carrying a lead line. A lead line can be used to:
• Check the calibration of the main echo sounder (do this every time you pick up an unfamiliar vessel).
• Used from the dinghy to sound the way in to shallow water ahead of the yacht.
• Act as a reserve if the main system fails.
• Can be useful to sound all the way around the boat when anchoring or mooring, to make sure there is an adequate depth.
There are limits to the operation of echo sounders. Most will only operate to about 100m, in most situations this is adequate, but if you are entering somewhere like Las Palmas in Grand Canaria the first depth reading will be when you are only about 100m from the harbour!
• Interference.
All echo sounders will suffer from interference from turbulent water, the most common cause of this is passing close astern of a ship, and it can be very disturbing to have the shallow alarm go off in mid channel! This disturbance may also be caused by turbulence in overfalls or in rough weather-especially in shallow water.
Occasionally, when over very soft mud, the return echo is lost and the instrument becomes unreliable.
• Alarms.
The shallow and deep alarms found on most sets are very useful for navigation. Obviously, a shallow alarm will assist you in avoiding running aground, but when either are set to a suitable depth they will give a position line. When sailing from Salcombe to Guernsey, a vessel crosses the Hurd Deep (not on the RYA training charts); this depth increase is very sudden and is a clear indication of the distance off the island.
The other alarm is an anchor alarm, this can be set for a range from the minimum to a maximum depth, if the anchor drags, the depth will change and the alarm sound, just remember to allow for changes of tide height!
• Log instruments
Modern logs may have many different functions; it is worth spending some
time with the manuals to learn what all of them do (photocopy the manual as the pages of the original often stick together in a marine environment).
• Distance.
The distance function usually has two parts, a log and a trip log.
The Log displays the total since the instrument was first fitted, and the trip log can be set back to zero, most trip logs will reset each time the power is turned off.
If you use the trip log, be aware that it may zero itself when the engine is started or the battery voltage falls.
• Speed.
This function may offer:
Speed through the water.
Speed over the ground (only if interfaced with GPS).
Average speed since the log was switched on.
VMG. VMG stands for Velocity Made Good. This is the speed directly towards the wind or directly down wind, if you are beating or broad reaching downwind, and can be used to decide the optimum angle relative to the wind in which to sail.
• Calibration.
All logs will require calibration when first fitted. During the season, they will require re-calibration because the water flow over the hull changes as the bottom of the boat grows weed.
Many sailing boats will display different speed on port and starboard tack. This may be because:
(i) The impeller leaves the water when heeled to one side. A solution is to have two impellers fitted, one on each side.
(ii) The water flow is uneven across the hull.
You need to have a good idea of how accurate your log is and if there are any differences from one tack to the other. Remember that in rough weather, the log may under read because of the turbulence and the impeller leaving the water. It may be prudent to work out what the errors are and to get in to the habit of applying a correction.
This may result in the situation where the vessel is closer to a landfall than expected; a potentially dangerous situation.
This last point is especially relevant to high-speed motor vessels. The log must be positioned where it remains in the water at all speeds.
Log types.
There are essentially two types of log:
Towed and through hull.
• Towed logs.
These are impellers that are towed behind the vessel; they may be 
electronic or mechanical.
Advantages.
(i) They are the most accurate (no interference from the hull).
(ii) The require no electricity to run (mechanical ones only).
(iii) Easy to clear is they become fouled.
Disadvantages.
(i) They are awkward to use, especially in busy waters.
(ii) They can be caught around the propeller (easy to do at night when entering harbour).
(iii) They are easily fouled on floating debris.
• Through hull logs.
These are impellers that are inserted through the hull, and may be 
mechanical or electronic.
Advantages.
(i) They are out of the way, and require no effort to use.
(ii) If you need it, they have advanced functions and can be interfaced with the rest of the vessel's electronics.
Disadvantages.
(i) If they are fouled, they require removing from the hull (easily done with a little practice). If the boat is left for any time, the impeller should be removed and a plug put in to replace it, this will reduce the build up of marine growth on the impeller. If you do not do this you will soon find the log does not work when you next take the boat out.
(ii) On a long passage, they use electricity.
• Satellite navigator.
Virtually all yachts have a GPS (Global Positioning System) receiver
aboard now. Many even have two or three, as the costs have fallen dramatically.
They may be a main unit built in to the vessel or a hand held device.
Having both can be a great back up for safety. If the main power fails, the
aerial is damaged or you have to abandoned ship, a portable unit can be very useful. If you have a hand held unit aboard, ensure you have an adequate supply of spare batteries; some can use a considerable number!
As with all electronics, the best approach is to spend some time reading the manual and pushing the buttons until you are happy with all the functions available.
• Chart plotters.
Many vessels are fitting a chart plotter that displays an electronic chart,
interfaced with the data from the GPS (and all the other instruments aboard). These make life very simple for the navigator, especially when there is a repeater fitted at the steering point in the cockpit.
GPS navigators are one of the most useful pieces of equipment to have aboard, but it is important to maintain a written log and to have a system of checks to ensure the GPS receiver is working correctly.
It must also be remembered that electronic systems do not predict accurately where you will be in the future; the navigator still needs to be observing any changes to boat speed and course then planning as necessary.
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