Practical_Boat_Owner_-_November_2015_

Nab 7.85nm

Boniface 3.1nm

PBO conclusion

Traditional navigation methods

T

he 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.

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.

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.

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
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.

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.

Distance off by vertical sextant angle

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

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

1520

187nm

Sextant

angle 0°45’

019°T

A station

pointer plots

angle pairs

easily