What is the best bilge pump setup and redundancy system?
I am refitting a 35-foot sailing yacht and currently only have one ancient Rule 500 pump down in the sump. I want to upgrade to a proper, redundant system that will actually keep me afloat if a seacock fails or I take a wave. What is the best setup, and how should I wire and plumb it?
Answer
The ideal bilge pump setup uses a tiered system of a small automatic pump mounted at the lowest point to handle nuisance water, backed up by a high-capacity emergency pump mounted higher and a manual diaphragm pump operable from the cockpit. Wiring the electric pumps to separate batteries with independent float switches and plumbing them to individual skin fittings ensures a single point of failure will not sink your boat.
The Primary Nuisance Pump
Your first line of defence is a low-capacity pump sitting at the very bottom of the bilge sump. Its job is to handle rainwater, shaft gland drips and condensation. A Rule 500 or Johnson Pump 750 GPH (gallons per hour) centrifugal pump works well here, costing around £40 to £60. You want a smaller unit for this role because it will cycle frequently. A massive pump trying to clear half an inch of water will suck air, burn out or constantly run back down the discharge hose. Pair this with a solid-state switch like a Water Witch 101 rather than a mechanical flapper switch that can jam on a stray cable tie or clump of hair.
High-Capacity Emergency Pumps
If a 38mm (1.5 inch) seacock fails, it lets in 50 gallons of water per minute. Your nuisance pump will be overwhelmed instantly. You need a secondary emergency pump like a Rule 2000 or 3700 GPH mounted 10 to 15 cm above the primary pump. This pump sits dry most of the time to preserve its lifespan. Because it draws 15 to 20 amps, wire it directly to your engine start battery, completely independent of the house bank running the primary pump. The float switch for this secondary pump must connect to a high-water audible alarm at the helm. If this pump activates, you need to know immediately that the hull is taking on water.
Plumbing Without Compromise
Never tee bilge pump discharge lines together. If you connect the primary and emergency pumps to a single hose, the high-capacity pump will blast water backward through the smaller unit and into the bilge. Each pump must have a dedicated smooth-bore hose routing to its own skin fitting. Corrugated hose is cheaper and easier to bend, but internal friction restricts flow by up to 30 percent.
Always route the discharge hoses through a vented loop mounted well above the static heeled waterline to prevent siphoning. Avoid using non-return valves (check valves) in bilge plumbing. They are notorious for getting wedged open by debris. If a centrifugal pump is air-locked by a closed non-return valve, it will spin endlessly without moving a drop of water.