If you’re installing new bilge pumps in your boat, read this first! This weekend I put an additional set of pumps into Writeaway, and was reminded of the fact that many people plumb a bilge pump line directly to the through-hull fitting – which can lead to disaster. If the fitting ever dips below the surface as the boat rolls or sits low due to load, water can back-flow into the bilges.
What’s the solution? Many people think there should be a check-valve in the pump line, to prevent water from flowing the wrong way. But these valves are notoriously unreliable. A far, far better way to ensure no back-flow is to install an anti-siphon loop in the line. That’s what the ABYC (American Boat & Yacht Council) recommends, and that’s what most manufacturers do. But not all of them – a few years back I was on a 26′ cuddy boat that almost sunk, after we loaded up on tuna fish. 350 additional pounds (plus the crew’s weight) on one side of the boat was enough to put the through-hull in the water, and the builder had failed to install an anti-siphon loop. We discovered the problem when the bilges flooded high enough to short-circuit the batteries. 40 miles from the inlet with no electricity, no pumps, and a flooding bilge is what we in the boating business term “not good.” So if you’re putting pumps into your boat, always remember the anti-siphon loop.
Here’s how it works: simply make a loop in the pump’s discharge line, and secure it as high as possible in the boat. Then water can’t back-flow through the hose unless the loop itself is lower then the waterline. Here’s a crude illustration: