Had an interesting experience last week involving a siphoning loop that could have been a major issue. I think most of our boats were fitted stock with a bilge pump and a shower sump pump that share a common discharge thru hull. In order to prevent either pump from back filling the other sump, each pump is fitted with a check valve.
In a perfect world, this all works fine. But as the check valves age, they can fail partially open or closed. Since our discharge ports are close to the waterline, they can be underwater while under way, allowing water to enter the discharge thru hull, and even forced in by wave or water pressure if pounding or surfing thru waves. If the bilge pump fills up the other half of the hose, a siphon loop can be created if the check valve has failed. This allows water to enter the boat, and the bilge pump trying to keep up in a vicious circle.
This happened to me after anchoring after an especially rough passage. I heard the bilge pump going and saw water rushing into both the bilge and the shower sump. After a rather motivated check of all the thru hulls and shaft seal, and some consultation with a marine repair friend, it became more clear what was happening. I removed the shower sump pump and check valve so that there was only the bilge pump. That solved most of the back filling, but it turned out that the bilge pump check valve had also partially failed so there was still a siphon going on. Luckily I had a spare water shut off valve on board that I could insert in the loop to break the siphon.
So, there will be some new projects this fall to create a vented loop and anti-siphon valve, with no check valves! Ideally, the discharge should also be well above water line and in the transom, but Iām probably not going that far yet.
So check your check valves!
