I discovered a rather dangerous situation today in our bilge. The bilge has always seemed to have a fair amount of water in it. I strive for a dry bilge, but this is an old boat and water makes its way into the bilge; in the end, I would rather go cruising than stress about a few drops here or there.
Regardless, I check the bilge a few times a day and pump it out when it looks full. My go-to pump is the electric rule pump, as it only takes the flip of a switch to drain the bilge. I also have a manual bilge pump in the cockpit and a massive manual bilge pump that is operated from inside the cabin.
Our water tanks in the bow are a little low, about 80 gallons shy and we are anchored with 100 feet of chain out. This is a lot of weight removed from the bow (640 pounds of water missing, 165 pounds of ground tackle missing) so the bow of the boat is sitting a bit higher than normal. We are also loaded up to go cruising so our waterline is a bit higher than normal.
Since the bow is high and the rest of the boat is low, the through hulls in our stern are now awash. Most importantly, the through hull to the bilge pump is now submerged! When I ran the bilge pump, I drained the bilge and primed the line to create a syphon to fill the bilge back up as soon as I turned off the pump. Thankfully I was watching and saw about 8 inches of water fill the bilge in a few minutes! I turned the pump on again and the water vanished, only to return again when the pump stopped.
I realized what was happening and needed a way to break the syphon! The cockpit manual bilge pump is teed into the electric bilge pump discharge hose, so in theory, pumping it when the bilge was dry would draw air into the line and break the syphon! It worked and the bilge stayed dry after the pump shut off.
It is very important to check your bilge and to make sure that your through hulls are above the waterline, otherwise your bilge pump could turn into your water intake and sink your boat! Check valves are nice to have, but they are prone to clogging and stopping up your much needed bilge pump, so the best defense is to keep the through hulls mounted high and dry, where a syphon is not a problem.