The most common reasons for the engine cooling water to fail are weed in the intake or a problem with the pump impeller. If it happens the engine temperature alarm will go off – but not generally until the engine is close to being dangerously overheated.

The engine should be stopped immediately and not started for about an hour after the problem is fixed so that it does not suffer thermal shock from the sudden influx of cold water.
But there is a simple solution. All that is needed is a means of detecting the moment water stops flowing to the engine. You then have a few minutes of engine use before the engine seriously overheats. You can buy flow sensors for the intake pipe before the seawater pump, but my homemade solution – which gives both a visual and loud audible warning – is much cheaper.
A flow switch was inserted (the right way around) in the pipe immediately before the seawater pump. The pipe had to be warmed first in hot water to get it over the 15mm switch fittings. Jubilee clips also held it in place.

Unfortunately this switch closes its contact when there is flow, so we need to reverse this with a 100 ohm (Ω) resistor (of three watts minimum). What happens is the flow switch short circuits the buzzer and light emitting diode (LED) when there is flow, the 100Ω resistor limiting the current to a safe 0.12A (the flow switch can switch 1.0A). When flow stops and the flow switch opens, the buzzer and LED will operate at about 9V.

 The whole circuit can be assembled without the need for any soldering

The photo shows how I wired up the terminal block, which should be mounted close to the flow switch since its leads are only about 250mm (10in) long and it’s best to avoid extending them. The 12V supply should come from the ignition switch so that the sensor only works when the engine is on.

The alarm will sound between switching on and the engine firing up, which serves as a useful test.

What you’ll need:
SN 35600    Liquid flow switch    £14.39
SC 00990    12V DC red LED    £3.20
SR 00143    100dB buzzer    £2.38
CNO 7076    12-way terminal block    £0.78
REW 21100RJ    100Ω resistor (3W minimum)    £0.52
All parts available from CPC, tel: 08701 202530; www.cpc.co.uk

Send us your projects! We pay at least £30 for each one published.