Small Boat With Gps Radar Compass Funny

Whether it's to cope with electronics failure or simply to make the most of your surroundings, David Pugh shares some traditional pilotage tips to help you obtain and maintain a fix of your position

Fixing your position at sea has never been easier. Early GPS revolutionised navigation by giving a read-out of latitude and longitude, which you could plot on your paper chart.

Now, that position is displayed on highly detailed electronic charts and, providing the electronics keep working, you can arrive at your destination simply by placing a few waypoints and following the screen.

This technology has made the sea both safer and more accessible, but traditional navigation and pilotage techniques still have a place in modern sailing. A classic example is our recent cross-Channel voyage with Hantu Biru, during which we followed a compass course, allowing the ebb and flood tides to cancel each other out over the course of
the passage. This is significantly quicker than following the GPS track, by which you could easily sail many miles further than necessary.

Closer inshore, it's worth being aware of the different techniques available to help you gain and maintain a fix of your position without satellite assistance. The main reason is that it makes you independent of the electronics should they go wrong, but I would argue that using landmarks for navigation makes you more aware of your surroundings – surely a good thing in a sport that's all about enjoying the great outdoors – and adds an extra level of challenge which can be fun.

Getting a fix

The key to navigating without GPS is to be constantly on the lookout for opportunities to get a fix. On passage with GPS, we become used to being able to note down our lat and long in the log once an hour, regardless of the conditions. Without it, you may find that there are no suitable landmarks on the hour, or that the visibility has reduced. If you see the opportunity for a good fix, take it: knowledge is power.

That vigilance means being creative. There are a host of ways to use landmarks to give you a position, and the technique you use will vary according to the circumstances. The simpler the fix, the better – if it takes you an age to work out a bearing, chances are it's not that accurate.

The traditional navigator's toolbox

Must-haves: Dividers • Breton plotter/parallel rules • Ship's compass • Hand bearing compass • Log • Echo sounder/lead line • Charts • Almanac • Pair of compasses • Pencil

Nice-to-haves: Sextant (plastic is fine) • Station pointer • Pelorus • Radar

Eight traditional ways to get a fix

1: Charted Object

One of the easiest ways to fix your position is to sail up alongside a charted object. Buoys are the obvious choice, but remember that they can be relocated, especially in areas of shifting sand or mud banks. Keep your charts up to date, and where possible choose an object firmly fixed to the seabed. Grab your chart, make a cross next to the object, note the time and log reading, and you're done.

2: Transits

Transits are formed when two charted objects are in line. They are often found marked on charts, or in pilot books, to give a clearance from a danger or a safe entrance into port.

A single transit provides an accurate line of position and gets you halfway to an accurate fix – you just need to find where you are on the line.

3. Crossed transits

The best way to find your position on a transit is to find another one which crosses it, ideally at about 90°. This is a tremendously accurate way to fix your position, but you generally need to plan the fix beforehand, so it's often used to give an accurate fix for starting a passage.
The easiest way is to find charted transits, or objects on the chart which form suitable transits. Then, to get to the planned position, pick up one of the transits and sail along it until the other transit marks come into line.

4. Transit and bearing

A river entrance transit crossed with a bearing at roughly 90°

In the absence of another transit, a bearing on a fixed object provides a good fix. Again, try
to choose an object which lies at roughly 90° to the transit – shallow angles of intersection create greater errors.

5: Three-point fix

This is the classic classroom technique for fixing your position. Choose three charted objects, maximising the angles between them to minimise error. Lighthouses and buildings are the ideal choice as they don't move and are easy to sight through the compass. Headlands can be good, but shallow promontories are harder to accurately sight than steep cliffs. Buoys can be used, but remember that they move on the tide.

Take your bearings as quickly as possible, noting them down as you go and leaving the one most abeam until last – this bearing changes the fastest.

Then plot them on the chart. Unless you have a stationary boat and extraordinarily steady hands the lines will not intersect perfectly, instead coming together in a triangle called a 'cocked hat'. Your true position could be anywhere within this triangle, so the smaller it is, the more accurate your fix.

6: Sounding crossed with bearing

A depth contour may be the first useful postion line available when making landfall

The echo-sounder has of late become simply a tool to prevent you running aground, but in conjunction with the chart it can be a useful source of position information.

To use the echo-sounder for position, you need to know the current height of tide.
For coastal passages at low speeds it's enough to choose a port close to your planned route and work out the height of tide for each hour as a quick reference. Armed with this information, you can pick up depth contours as you sail towards the coast. To pick up the 5m contour, for example, add 5m to your predicted height of tide, then sail in until the echo-sounder reaches the required depth.

Once you are on a depth contour, take a bearing of a charted object: your approximate position will be where the bearing and the contour intersect. A depth contour is unlikely to be completely accurate, especially if the chart is based on an old survey, so treat positions obtained this way with care. If possible, another intersecting bearing will add accuracy.

7: Running Fix

The running fix is not particularly accurate, but provides a useful fix if landmarks are scarce

If there's only one suitable charted object in sight, and you can't determine your range using a depth contour, given time you can find your position using a running fix. This is often the case at night, when your choice of objects from which to take bearings are limited to those that are lit.
This method is most appropriate when you are sailing more or less parallel to the coast. Take a bearing on the object when it is approximately 45° on the bow. Plot the bearing on the chart, and mark it with the time and the log reading.

Sail a constant compass course for a time, typically until the object is around 45° astern. Take another bearing, and plot it on the chart with the time and log reading. Then, from any point on the first position line, follow the procedure for working out an estimated position (EP), using your logged distance and the predicted tidal set and rate. From your EP, draw a line parallel to the first bearing. The point where the two bearings intersect should be your current position.

8: Double the angle on the bow

The pelorus is used to take relative bearings, useful for methods such as doubling the angle

A special case of the running fix, this method relies on the fact that an isosceles triangle has two sides the same length, with the larger angle being double the smaller ones.

To use this, pick an object which is less than or equal to 45° off the bow. (A pelorus is a great help with this technique, as it accurately measures angles relative to the boat's heading.) Note the object's relative bearing and its compass bearing. Plot the compass bearing on the chart, together with the time and the log reading. Sail on a constant course until the angle off the bow has doubled. Note the new compass bearing and the log reading, and plot them on the chart. Your position is on the position line drawn, and your range from the object is exactly the same as the distance logged.

Navigating at night

The techniques above can also be used at night providing the landmarks are lit, but there are a couple of techniques using lighthouses which are only valid at night.

Dipping distance crossed with bearing

Dipping distances are easily calculated with almanac tables

On a clear night, the distance at which a lighthouse beam first becomes visible can provide a surprisingly accurate calculation for your distance off. To work it out, you need to estimate the height of your eye above sea level, calculate the height of tide and find the height of the lighthouse.

The latter is easy. On the chart, a lighthouse will have a legend something like Fl(3).15s13m20M, telling us that this a white light which emits a group of three flashes every 15 seconds, has a height above Mean High Water (MHW) of 13m and is visible for 20 miles in a straight line in clear conditions. This latter figure is of no practical use in navigation other than giving an idea when you should begin to see the light when approaching the shore, as it takes no account of the Earth's curvature. The figure of interest is the height of the light above MHW: 13m.

To calculate your distance off, calculate or estimate the height of tide, subtract it from the value for MHW (available for the nearest port by consulting the almanac), and add the result to the charted height for the light. Estimate the height of your eye above sea level. Then, find the table for calculating dipping distance in your almanac and cross-reference your height of eye with the (adjusted) height of the light. This gives your distance from the light.

Take a bearing on the light, draw it on the chart and strike off the dipping distance with your compasses or dividers. The advantage of this method is that gives a fix from a single object. However, it can only be used in good visibility.

Light sector cut-off

Light sectors have cut-offs which make good position lines. A cross bearing gives a fix

Sectored lights are common on the approaches to harbours, or on headlands where rocks project in a particular direction. The cut-offs between the coloured sectors are marked on the chart, and when viewed from the sea are very abrupt, making them a good position line for navigation.
The cut-off line needs to be crossed with another range or position line to give a fix. A bearing on another light is ideal, or if you are on the dipping range for one of the sectors that will give a range. Another option is to use your echo sounder to find the depth, correct it for tide and cross the depth contour with the light sector cut-off to find your position.

How to fix your position using radar

If you have radar on board, it can be a powerful tool for positioning as well as collision avoidance.

Using the VRM/EBL

All radars are equipped with a variable range marker (VRM) and an electronic bearing line (EBL). These allow you to obtain an accurate fix from a single object. Placing the electronic bearing line on an object will give you its bearing, relative to the ship's head, and the VRM
will give its range.

By adding or subtracting the bearing from the compass course, depending on whether to object
is to starboard or port, and correcting for deviation and variation of the ship's compass, the bearing can be drawn on the chart and intersected with the range to give an accurate position by day or night.

Crossed radar ranges

Intersecting ranges give two positions, but it's usually clear which one is valid

Another way to obtain your position by radar is to use the VRM to find the range of two objects (Racons are ideal). This avoids any calculation at all: simply find the two ranges and draw them on the chart with a pair of compasses, using the charted object as the centre.
The resulting circles will intersect in two places, giving two possible positions. However, it's usually obvious which is correct – the false one is often on land! If there is any doubt, a rough bearing will clear up the ambiguity.

Using a sextant

The sextant is usually thought of as only suitable for astro-navigation, but in fact it is simply a highly accurate means of measuring angles. As nearly all pilotage involves plotting angles, a sextant can be a powerful tool to help obtain an accurate fix.

Distance off by vertical sextant angle

The sextant gives a range, which can be crossed with a bearing

Similar to the technique for finding your position using a dipping distance, you can calculate your position by measuring the angle between sea level and the top of a charted object of known height, such as a lighthouse. Using the sextant, bring the top of the lighthouse down until it is touching the sea, and read off the angle.

In this case, it's acceptable to approximate your eye as being at sea level, and therefore the distance off is the stated height of the object above sea level, corrected for tide, divided by the tangent of the sextant angle. Divide by 1852 to get the distance in nautical miles. A useful approximation is 1.856 x (Elevation +/- water height)/Sextant angle in minutes. Some almanacs have tables to assist this calculation. Once obtained, this range can be crossed with the light's bearing to give a fix.

Horizontal sextant angles

A more accurate variation of the three-point fix, in this method you use the sextant to measure the two angles between three onshore objects.

You then plot these on the chart. The easiest method is to use a station pointer, which has three arms – one set at zero, and scales to set the other two at relative angles either side. With the pointer set up and laid on the chart, there is only one orientation at which it fits, so the centre of the scale gives your position.

This method is more tolerant of small angles than a hand-bearing compass.

PBO Conclusion

The above methods are not the only techniques available, and often it pays to be creative in their use. For instance, a single position line, while not a fix, can still be useful as it can be transferred using your distance run to help find a hard-to-spot object onshore, such as the mouth of a river.

Using these techniques challenges your seamanship and hones your skills in recognising and using landmarks. It puts you more in touch with the sea and reduces your dependence on electronics. Above all, it can be fun. I'm not suggesting that you should ignore the GPS, and in conditions of reduced visibility many of these techniques become impossible. But the art of sailing a boat is more than getting from A to B; it's the art of making best use of what nature is offering in terms of winds, waves, tide and landmarks, and that's what traditional navigation is all about.

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Source: https://www.pbo.co.uk/seamanship/pilotage-14-ways-to-get-a-fix-27588