# Bilge Pumps and Float Switches: Sizing, Wiring and Testing **By Sawley's Auto and Marine** · 2026-09-16 A bilge pump is the one piece of equipment on a boat that is bought to be forgotten. It sits in the lowest, wettest, dirtiest part of the hull, it is expected to work automatically after months of neglect, and the first time most owners think about it is the morning they find the boat sitting lower on the mooring than it was the night before. ## How big a bilge pump do you need? Bigger than you think, and probably two of them. Pumps are rated in litres per hour, and the figure on the box is measured at the pump outlet with no hose attached. Add a metre of head — the vertical lift from the bilge to the outlet — and some hose, and a pump delivers perhaps half its rated flow. A pump advertised at a few thousand litres an hour is moving something well short of that by the time the water is over the side. For a trailer boat up to five or six metres, a single automatic pump in the middle of the range is the usual fit and it is adequate for rain and spray. For anything moored, or anything larger, the sensible arrangement is two: a smaller automatic pump low in the bilge that handles the day-to-day, and a larger one mounted slightly higher that only runs when the first one is overwhelmed. The second pump is the one that buys you time. Pumps, strainers and the fittings to plumb them are in [boat pumps and fittings](/collections/boat-pumps-fittings-water-pumps-hose-fittings-plumbing-parts). One honest note. A bilge pump is for nuisance water. If the boat has a real leak, no pump you can run off a battery will keep up with it for long; the pump exists to get you to the ramp or the marina, not to make a holed boat float. ### Float switches The switch is what makes the pump automatic, and it is the part that fails. The traditional **mechanical float switch** is a hinged arm that rises with the water and closes a contact. Simple and cheap, and it jams: a bit of debris, a stray cable tie, a wet leaf, and the float either sticks down and the pump never runs, or sticks up and the pump runs the battery flat. Mount it where nothing can fall on it, and fit the guard. **Electronic switches** sense water without moving parts — a field sensor, or a pair of probes — and have nothing to jam. They cost more and they are the better choice in a bilge that collects rubbish. Some pumps have the sensing built in and switch themselves. Whichever type, **test it before every trip**. Lift the float, or pour a bucket in, and listen for the pump. Thirty seconds, and it is the whole difference between a system that works and one you assume works. ### Wiring it correctly Two circuits are the standard: the automatic circuit runs from the battery through a fuse to the float switch and the pump, and stays live all the time; the manual circuit runs from a dash switch to the pump so you can run it regardless of the float. The three-position bilge switch on most panels — auto, off, manual — does exactly this. - **Fuse it at the battery.** The automatic circuit bypasses the main switch by design, which means it is live when everything else is off. It must have its own fuse close to the battery. - **Size the cable for the run.** Pumps draw more than people expect and thin cable over a long run starves them. The voltage drop shows up as a pump that is slower than its rating. - **Joints above the water.** Every connection in the bilge is a connection under water at some point. Use adhesive-lined heat shrink on crimped joints and route them as high as the cable allows. Tinned cable only; plain copper wicks water up the strands and corrodes from the inside. [Tinned marine cable](/collections/electrical-cable-tinned-copper-battery-wiring-cable) and the connectors are in our electrical range. - **A battery that can do the job.** On a moored boat the pump is drawing from a battery that nobody is charging. Our guide to [wiring a second battery](/blogs/guides/wiring-a-second-battery-vsr-dc-dc) covers keeping a house battery topped up, which is what keeps the bilge pump alive over a fortnight of rain. ### The hose and the outlet Smooth-bore hose, as short and straight as it can be, with no low loops where water sits and no reduction in diameter along the run. Every bend and every metre costs flow. The outlet goes through the hull well above the waterline, and on a boat that heels or sits low, a loop up to the deck level with an anti-siphon vent stops the sea coming back in through the outlet, which is a real way for a boat to fill. Bilge and discharge hose in the correct grades is in [water and bilge hose](/collections/hose-water-bilge-marine-pump-bilge-discharge-hose). ### Keep the bilge clean Everything above assumes the bilge holds water and not much else. Fuel, oil, leaves, fishing line and the small hardware that ends up down there will block a strainer, jam a float and shorten a pump's life, and a bilge with oil in it should not be pumped over the side at all. Keep it clean and dry between trips and the pump will spend its life waiting, which is the job. We fit and rewire bilge systems — pumps, switches, hose and the panel wiring — through [device and accessory fitting](/pages/device-accessory-fitting) at Kadina. If the boat is moored and you are not sure what is down there or whether it works, ring [0412 532 984](tel:+61412532984) and we will look. **Tags:** Electrical, Guides --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/bilge-pumps-and-float-switches)