Bilge Pump Wiring

Common bilge pump switch
Common bilge pump switch

Like most boats, you’ve probably got a switch for your bilge pump(s) something like this. It’s got three positions, but you would typically leave it in the ‘AUTO’ position, which is activated by a float switch in the bilge. Usually the ‘MAN’ position is spring-loaded, so it needs to be held down to operate. This switch type is commonly written ‘(On) – Off – On’. Some boats even have a simple on/off switch.

One failing with this type of switch is that it’s very easy to accidentally brush against the switch face and toggle it to ‘OFF’. At the sailing school where I work, it’s common for students to turn off all switches on the panel after a sail – and they frequently turn off the bilge pumps as well! The result is often a bilge full of water the next day; luckily nothing worse (yet).

It’s common to wire bilge pumps directly to the battery, bypassing the house master switch. But inserting this switch in the circuit cancels that out.

This is a simplified diagram of how a switch like this is commonly wired. It’s even how the manual tells you to do it. What’s never made sense to me is why you would ever want to turn your bilge pump permanently off. So why is it so easy to do it? I’m assuming the intention is to avoid draining the house batteries.

But think about this: which would you prefer – to have your boat sink, or to drain your battery? The latter is easy to fix, but a sunken boat is pretty expensive. Obviously a bilge pump is not going to solve a serious leak, but it might give you time to fix the problem.

I’ve been meaning to rewire my bilge pumps for a while now, but other projects always seemed to take priority. But this recently popped right to the top of the list when I was away on a two week boat delivery. At some point – we don’t know when – the primary bilge pump switch got toggled to the ‘OFF’ position. When I got back, I happened to be looking in the bilge for another wiring project when I noticed the water level was way up! Luckily my boat has a very deep bilge, or it could have been much worse.

So it was time to trace wires and figure out how to fix this problem. Turns out my setup was even worse than the above diagram. The bilge pumps are fed from the main house panel, not directly to the battery, and also have two breakers. That means, if another random circuit blows the main breaker, or if someone turns off the house panel manually, I’ve got no bilge pumps!

The solution was to run new wire from as close to the battery as possible, then add a manual switch on the panel. This switch can only turn the pump on – there is no ‘OFF’ option. Here’s the schematic of how it works.

The reality is a little more complicated, but this is essentially how it works. The float switch is fed directly from the house battery bank (fused), and a switch on the main panel allows me to turn the pump on manually if needed. This switch is spring-loaded, so it needs to be held down to operate; I can’t accidentally leave it running.

This project also removed several redundant wiring runs from behind my panel, freed up a breaker, and cleaned up some messy wire splices in the bilge area.

I’ve seen some boats where it would be problematic to do a complete rewire of the pumps. Fortunately there’s a very easy way to upgrade the existing switch to make it behave like the above. All you need to do is add a jumper wire from the positive feed to the ‘AUTO’ terminal of the switch.

This accomplishes two things: The ‘AUTO’ position is permanently enabled, no matter how the switch is set, and there is no way to select the ‘OFF’ position. (This assumes your switch is wired directly to the battery, unlike mine). This makes it idiot-proof and accident-proof. No need to run any new wire or tear up the cabinetry.

The next step in this project will be to add a second backup bilge pump, similarly wired, and a high-water alarm.