Bilge pumps: what size, and how to wire one properly

What GPH actually means

Centrifugal bilge pumps are rated in gallons per hour at open flow: no hose, no fittings, no vertical lift, and a full 12 volts at the pump. Every manufacturer rates them this way, which makes the numbers comparable to each other and optimistic compared to your installation.

Once installed, output drops. Independent testing has found pumps delivering roughly 15 to 33 percent below their rated capacity, and in many real installations closer to 60 percent of the rating once hose, fittings and lift are accounted for.

The dominant factor is head, the vertical distance the water has to be lifted to the discharge. As a working figure, expect to lose something like 8 to 10 percent of the rated flow per foot of lift. A pump rated at 2,000 GPH may be moving well under 1,500 by the time the water leaves the boat.

So the practical rule is simple: size up. A pump that looks generous on paper is about right in the boat.

Choosing a pump, and how many

Think in terms of jobs rather than one number. A primary pump sits at the lowest point of the bilge and deals with the ordinary business of rain, spray and packing drips. A second, higher-mounted pump with more capacity is there for the day something is genuinely wrong, and its switch sitting higher means it only runs when the first pump is losing.

Automatic pumps contain their own water sensing and switch themselves on. Non-automatic pumps need an external float switch, or a manual switch at the helm, or both. Kits that pair a pump with a float switch exist for exactly this reason.

A manual pump is worth having as well as, not instead of, an electric one. It works when the battery does not, which is precisely the scenario where you most want to move water.

None of this substitutes for finding the leak. A bilge pump buys time and manages nuisance water; it is not a repair.

Wiring it so it works when you are not there

The point of an automatic bilge pump is that it runs with nobody aboard, which shapes how it is wired. The pump circuit is normally fed so that it stays live independently of the master switch, protected by its own fuse or breaker sized to the wire, not to the pump.

Use tinned marine wire, sized for the current and the length of the run so voltage drop does not starve the pump when it matters. A pump at the far end of undersized wire turns slowly and moves much less water than its label suggests.

Make the connections properly. Heat-shrink terminals seal the joint; bare crimps in a bilge corrode and fail. Keep splices as high as the run allows, because the one place you do not want a connection is underwater.

A manual override switch at the helm lets you run the pump when you want to, independently of the float or sensor. Many owners also fit a counter or indicator light so a pump that is cycling more than usual becomes visible before it becomes a story.

The discharge, and the mistake to avoid

The discharge fitting should be above the waterline, and ideally high enough that it stays there when the boat is loaded, heeled or squatting under power.

The failure to avoid is siphoning: a discharge that ends up below the water with a hose that runs downhill can let water flow back into the boat, which is the opposite of the job. Routing the hose up to a high point before it exits is the usual defence.

Use smooth-bore hose of the full diameter of the pump outlet where you can. Corrugated hose and step-down fittings both cost flow, and flow is the whole point.

Keep the strainer clear. The most common reason a working pump does not move water is debris at the intake, and it is the cheapest thing on this page to check.

Common questions

What size bilge pump do I need?

More than the rating suggests. Pumps are rated at open flow with no hose and no lift, and installed output is commonly 15 to 33 percent lower, sometimes nearer 60 percent of the rating. Expect to lose roughly 8 to 10 percent of the flow per foot of vertical lift, and size up accordingly.

Why does my bilge pump move less water than its rating?

Because the rating is measured at open flow, at zero vertical lift, with no hose or fittings and full voltage at the pump. Your installation has all of those things, and head is the biggest single loss.

What is the difference between an automatic and a non-automatic bilge pump?

An automatic pump senses water and switches itself on. A non-automatic pump needs an external float switch, a manual switch, or both.

Should a bilge pump be wired to stay on all the time?

An automatic pump only does its job if it can run with nobody aboard, so its circuit is normally fed independently of the master switch and protected by its own fuse or breaker sized to the wire. A manual override at the helm is worth having as well.

Where should a bilge pump discharge?

Above the waterline, high enough to stay there when the boat is loaded or heeled, with the hose routed up to a high point before it exits so water cannot siphon back in.

Do I need two bilge pumps?

Many owners fit a smaller primary pump at the lowest point for everyday water, plus a larger pump mounted higher for a genuine problem. A manual pump is also worth having, because it works when the battery does not.