Cruising Helmsman - July 2018

24

Cruising Helmsman July 2018

PRACTICAL

ELECTRICAL

Most LED torches and headlamps have

a red light so this will also help as long

as the light is not too bright.

For many years sailors and other outdoor

people have preferred using Polaroid

sunglasses rather than simple dark lenses.

Light generated by most natural objects

is unpolarised; that means that the light

waves radiate in all directions. Polarising

filters (i.e. Polaroid lenses) remove all

light except those radiating in only

one direction: near vertical.

Light ref lected from highly ref lective

horizontal surfaces like the sea, is

ref lected horizontally.

Polarised lenses work by filtering out

the horizontal part of the light, allowing

only vertical light rays to pass through.

This minimises the glare and allows us to

see through the surface making shallow

areas stand out more.

A simple check to confirm that our

sunglasses are actually polarised is to

get a second pair and rotate them while

looking through them; if what you are

looking at goes dark they are polarised.

Often computer and especially liquid

crystal displays use polarising effects so

displays can go to black when looked at

through polarising lenses. If you notice

this simply rotate the screen a little and

the image magically returns.

Of course none of this is relevant if you

never venture out after dark, but then you

are missing out on some beautiful time

on the water!

DESCRIPTION FOR CIRCUIT 1

The switch uses a simple single throw,

double pole switch fed from fuse 2. This

has a centre-off position, sometimes

called a triple throw.

Throwing to internal lights allows only

the white lights in the cabin to go on.

When the switch is turned to navigation

on, the white lights are turned off and

the internal red lights turn on with the

navigation lights.

The cabin red and white lights can be

placed in the same light fitting but need

dedicated red and white globes (or LEDs).

If you need a white light for a particular

place then you will need to run a separate

circuit from fuse 1.

DESCRIPTION FOR CIRCUIT 2

Like circuit one, the switch has a central-off

position where all lights are off. When it

is turned to white cabin lights the current

will f low only through the white lights; the

diodes in the other lights are reverse-biased

and prevent any current from f lowing.

When the switch is turned to the

navigation position, the diodes are

forward-biased and so the red cabin lights

and the navigation lights turn on; the diode

in the circuit for the white lights prevents

current f lowing and they now turn off.

This works for ordinary incandescent

lights and LED fittings as most modern LED

fittings have internal electronics that make

them insensitive to the battery polarity; that

means it does not matter which way they

are plugged into the battery supply.

The voltage dropped across this

additional diode is trivial and will

not affect the lights operation. If you

are using LED lights that are polarity

sensitive then you can simply omit the

additional diodes. ≈

John is a retired electrical engineer

who resides in the Blue Mountains

west of Sydney. He and his wife Patsy, cruise the

east coast of Australia when they can. John has

built and repaired the electrics on his Duncanson

35 Ard Righ as well as on many other yachts.

JOHN TYLOR

+ Bus bar

CIRCUIT 2 Separate globes with shared wiring

Switch Down

for white only cabin lights

3 position Switch

Up for red & Nav. lights

*Centre position is off

Cabin lights

Navigation

lights

DPDT*

• Bus bar

Fuse

+ Bus bar

CIRCUIT 1

Separate globes with individual wiring

*Centre off position

Switch

SPST

3 position

Switch

SPDT*

Red cabin

lights

White cabin

lights

Navigation

lights

• Bus bar

Fuse 1

Fuse 2

GLOSSARY

Switches come in different combinations

of operation, here are the codes.

SPST: single pole single throw. This is

the basic on-off switch with one contact.

DPST: double pole single throw. This is

the basic on-off switch but with two sets

of contacts, in effect two switches in one

that turn on at the same time.

SPDT: single pole double throw. A basic

switch that will turn one circuit on when

operated in one direction and another

separate circuit when turned in the

opposite direction, in effect a changeover

contact. When in the middle position no

contacts are made; these are sometimes

called triple throw and is the type used in

these circuits.

DPDT: double pole double throw. Same

as the SPDT but with two separate

sets of contacts that operate at the

same time.

ABOVE: This circuit uses individual globes

and separate wiring for the cabin lights.

ABOVE: This circuit uses individual globes

with shared wiring for the cabin lights.