Low Water Pressure Alarm Switch

How long will a Raw Water Cooled motor last running along on plane if the raw water supply is suddenly cut off (by a rag over intake, thrown belt on pump, broken hose, etc)?

Thirty seconds!

Furthermore, the temperature sensors and alarm switches don’t work properly when not covered with water, so you can’t count on the gage or alarm system to save your motor. Obviously, another system is needed.

The system I developed (which is similar to those used on some modern outboards) is to monitor the raw water pressure. You’d be surprises how low this pressure is: At the exhaust manifold, at idle, it’s only 1 psi. The pressure rises to about 5 psi at 2,000 rpms—still low but still usable for a pressure switch.

You’ll need one adjustable pressure per motor. I found some switches on eBay that were adjustable from 1 to 6 psi—-perfect. I also located some 12 volt buzzers for the alarm. A trip to Radio Shack supplied the toggle switches. The wiring diagram (see sketch) is fairly simple: 12 volts goes to the switch, then to the buzzer, then to the pressure switches, with other side of the switch to ground. The box with the buzzers and the switches was then mounted near the helm. Install a tee in the intake manifold and screw in a barbed fitting (available at a hardware store). Run a hose from there to the pressure switch, and be sure to use a clamp at each end.

Note: The following is for raw water cooled engines ONLY.

Note: Mount the pressure switches close by and on the same level as the motor if possible. The bulkhead works well