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Hi Terry,

I just experienced a similar issue with an air conditioning raw water outlet. Mind you, this happened just two days before leaving the Caribbean to sail non-stop to Newport, RI.

The seal on the circulation pump had gone and was leaking water coming from the seawater inlet even when switched off as the whole installation is below the waterline. I closed the through-hull to prevent any further leaking until I could replace the seal. We cast off the lines and set sail without any further thought about danger.

On the first day out I noticed the bilge pump was cycling continuously and checked the bilge to find that we had taken aboard approximately 60-80 gallons of sea water, probably more considering the cycling of the bilge pump. The water was leaking from the broken seal. What transpired was the sea water outlet (located 6 inches above the waterline) at rest, was now on the windward side and given our heeling angle was about 4 feet above the waterline. Our installation did not include an anti-siphon loop between the outlet and the air conditioning circulation pump. Thus, the waves were splashing into the outlet, filling the pipe to the pump and then draining into the bilge through the broken seal.

So . . . Yes, you could install a non-return valve, but my concern is that these valves can easily get stuck in the open position with a piece of debris or fail all together. Also, placing a valve in the line may restrict pump flow. Without a shadow of a doubt, I would immediately install an anti-siphon loop. Take the outlet pipe from the pump to a point on the boat much higher than the waterline in a loop before it continues down to the outlet. Further to this, at the highest point of the loop you should install a breather valve to allow air in, but no water out; this will prevent a back siphon. Installing the anti-siphon loop should be pretty easy to do yourself and the various parts are inexpensive and can be found in most chandleries.

Good job on noticing this one Terry - - it could ruin you whole day.

Best regards,

Captain Ian Fagg