High Water alarm Systems

There are several ways to create a safety backup system for your bilge pumping system and never enough can be said about having good bilge pumps and enough capacity to cover all situations. There are times where all that planning just won't keep the boat afloat.
What we're concerned with, in this section, is when a boat is taking on more water than the pumping system can handle by itself.  The reasons for this could be pump failure, auto switch failure,  loss of 12 volts to the pump, debris around the pump, and more water coming in than the system was designed to pump out.
Since one of the causes for pump failure is the loss of 12 volts to the pumping system, the High water warning system we are recommending is both 12 volt and 120 volt. This can be approached in several ways,  a single system, (12v /120v), two separate systems, (one 12v and one 120v), or a tie in to an existing burglar alarm system.( which are usually 12/120 volt)
  Parts required: 1- float switch (2 preferred), 1- 120v to 12v converter power supply, 2- 12v blocking diodes (if using one float switch) (Radio Shack cat. No. 276-1114)
Optional, but recommended, 1- small marine boat horn.
The selection of the 12 volt (battery) power supply should not be the same source as the bilge pumps. It also should be direct wired to the battery so as to prevent other system failures from affecting the alarm system. Be sure to follow wiring and fusing codes.
For the 120 volt power to the alarm system, you will need a  120 to 12 volt power adapter that has an continuous output capacity of about 5 to 7 amps. This adapter is similar to the ones used to power cordless appliances and 12 volt TVs. Radio Shack has several models available.
If your going to use one float switch for both 12 volt power sources, you must install a blocking diode in each power output + wire, to prevent back-feed into the rest of the 12 volt electrical system. (also available at Radio Shack). If using two float switches , and only one horn for this system, it is still a good idea to use the blocking diodes.   
Since the idea is for people on the dock to hear this warning, I recommend using  the installed boat horn or adding an additional horn just for this system. There are also buzzers available, similar to the over heat buzzer used on many engines, but often they are not loud enough to grab peoples attention. The horn should be located in such a spot as to make it audible while standing on the dock.  It is also a good idea to have a sticker made up, to put in the window, that would alert people to the fact that you have this alarm and that it is not just a short circuit in the horn.

Pre - Existing Burglar Alarm System 
If you have a burglar alarm system in the boat already, it is easy to add high water protection.
Since most alarm systems use a looped zone design, you can add a "break" to activate sensor, for this purpose, by hanging a bilge pump auto-float switch up-side down. (A ball type auto-switch  will not work for this purpose) As the water rises the auto switch will "break" the circuit and the burglar alarm system will activate. Most alarm systems are sensitive to the type of wire used for each loop, so you'll need to make the original auto-switch wires as short as possible, and many systems require a small resister be shunted into the loop, so take a look at the rest of the loops and see if you  need to duplicate these items. If there is a zone loop close by, often you can tap into that zone by just putting your switch in series with the other sensor.( You won't need an additional resister shunt if you are tapping into a pre-existing zone loop.
One note here is that burglar alarms are often ignored by others so, although this might be easy to install, it would be a good idea to let the marina operator know that your alarm also is a high water alarm.

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