Practical_Boat_Owner_-_November_2015_

Seamanship

Light sector cut-off

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.

Light sectors have cut-offs which make good

position lines. A cross bearing gives a fix

Dipping distances are easily

calculated with almanac tables

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.

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.

Dipping distance crossed with bearing

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.

7. Running fix 8. Double the angle on the bow

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.

The pelorus is used to take relative bearings,
The running fix is not particularly accurate, but provides a useful fix if landmarks are scarce useful for methods such as doubling the angle

Eight traditional ways to get a fix continued...

316°T

1410

125nm

090°T

Bearing 1:

043°T

1100

151nm

Bearing 2:

312°T

1200

156nm

RFIX

1200

156nm

EP

1200

156nm

Bearing 1,

transferred

043°T