The number on the box is a bench figure
Bilge pump ratings are open-flow figures: the pump running at its rated voltage with no hose and nothing to lift against. That condition does not exist on a boat. In an installation there is a static lift from the water in the bilge up to the through-hull, usually with a loop above it, and there is a hose run with bends and fittings, and both of them are head the pump has to overcome. A pump rated at 1,100 gallons an hour delivering somewhere around eight hundred through a dozen feet of inch-and-an-eighth hose at four feet of lift is behaving normally, not badly.
Two figures drive the estimate here. The rated open flow sets the top of the pump curve, and the shutoff head — the height at which flow stops entirely — sets where it reaches zero. Between them the curve is approximated as a parabola, which is the standard first approximation for a centrifugal pump. If your pump has a published curve, that curve beats this estimate.
Where the head comes from
Static lift is the honest, unavoidable part: the vertical distance from the water surface in the bilge to the discharge, including the anti-siphon loop, which is often higher than the through-hull itself. Friction is the part people leave out. It rises steeply with flow and falls steeply with diameter — roughly with the fifth power of the bore, which is why going from three-quarter inch hose to an inch and a quarter is not a marginal improvement.
Corrugated hose, which is what most bilge installations use because it resists collapse and follows a curve, is considerably rougher inside than smooth hose. So is a check valve, and so is every tight elbow. The fittings allowance on this page exists to fold all of that into an equivalent extra length of straight hose, which is a crude but serviceable way to handle it. Set it high rather than low.
Wire is part of the pump
A twelve volt pump on undersized wire over a long run does not get twelve volts. It gets eleven, or ten and a half, and its output falls with the voltage while its current rises. This is one of the most common reasons an installed pump underperforms its own curve, and nothing in the plumbing will fix it. The run is from the battery to the pump and back, and the return path counts. The wire size calculator and the voltage drop calculator cover the arithmetic; the point here is only that a pump problem is sometimes a wiring problem.
Nuisance water and the other kind
Bilge pumps handle nuisance water: rain, spray, a stuffing box doing what a stuffing box does, condensation, the slow accumulation of a boat sitting on a mooring. For that they are excellent, and sizing the pump against a measured ingress rate is a reasonable exercise. The output here against your ingress figure tells you whether the pump gains or loses ground, and how fast.
What no bilge pump does is keep a holed boat afloat. A one inch hole a foot below the waterline admits water at a rate that a large bilge pump does not approach, and the rate increases as the boat settles and the hole goes deeper. That is not a criticism of pumps, it is arithmetic about pressure and area. The response to a serious breach is not a bigger pump, it is stopping the water, getting into shallow water, and calling for help early rather than late. A pump that keeps up buys minutes to do those things.
What this model does not know
It does not know your pump curve, which is the single biggest source of error, because the shutoff head is an input and most people will be guessing it. It does not model a partly blocked strainer, a float switch that cycles the pump on and off, a discharge that submerges when the boat is down by the stern, or a hose that has collapsed on a tight radius. It also assumes the pump is actually running at its rated voltage. Treat the output as an estimate of the right order rather than a measurement — the useful part is the shape of the answer, which is that the label is optimistic by a wide margin and that hose diameter matters more than people think. For the same head-versus-flow problem in a house, the sump pump sizing calculator works the pit end of it and the water flow and pressure drop calculator handles pipe friction on its own.
Questions people ask
Why does my 1100 GPH bilge pump not move 1100 gallons an hour?
Because that rating is measured at zero head with no hose attached, which is a bench condition rather than an installation. In a boat the pump lifts water several feet to the discharge, usually over an anti-siphon loop, and pushes it through hose with bends and fittings, and every foot of that is head that reduces flow. Delivering sixty to seventy-five percent of the rated figure in a typical installation is normal. Add undersized wiring, a partly blocked strainer or a check valve and it drops further. The ratings are not dishonest, they are just measured somewhere your boat is not.
Does hose diameter really matter that much?
Yes, more than almost anything else you can change. Friction loss falls with roughly the fifth power of the internal diameter, so moving from three-quarter inch to one and a quarter inch hose on the same run cuts friction to a small fraction of what it was. That is why an undersized discharge can cost a third of a pump output while an oversized one costs nothing but a little money and a slightly larger hole. The other half of the same point is that the hose has to be the size the pump outlet wants — necking a pump down to a smaller hose because that is what was on the shelf is the most common self-inflicted flow problem in a bilge.
How big a bilge pump do I need?
Size it against the nuisance water you actually get, which you can measure by watching how often the pump cycles and how much the bilge holds between switch points, rather than against an imagined emergency. Bigger pumps are cheap insurance up to a point, but a very large pump on a small battery and small wire delivers less than a properly wired moderate one, and it will not save a holed boat regardless. Most people are better served by a correctly wired pump with an adequate discharge, a working float switch, a second pump higher up as a backup on its own circuit, and the habit of checking that the strainer is clear. What is required as equipment is set by federal and state rules that change, so ask your state boating authority rather than sizing to a number from a page.
Will a bilge pump keep up with a hole in the hull?
No, and it is important to be blunt about it. Water enters a hole under pressure that increases with depth, and even a modest breach below the waterline admits water at rates that dwarf what a recreational bilge pump moves. The boat settling makes it worse, because the hole gets deeper and the flow increases. Bilge pumps are for nuisance water and for buying time. In a real ingress the useful actions are slowing the water at the source, heading for shallow water where the boat can ground rather than sink, getting everyone into life jackets before it is difficult to put them on, and calling for help at the start rather than when you are sure.
Does a check valve in the discharge help?
It stops water in the hose draining back into the bilge each time the pump shuts off, which reduces short-cycling, and that is a genuine benefit on a long or high discharge run. It costs flow, because the valve is a restriction and it is one more thing that can jam partly open on a piece of debris — and a check valve stuck shut is a pump that runs and moves nothing while sounding entirely normal. Opinions differ sharply and the tradeoff depends on the installation. If you fit one, it becomes something to inspect rather than something to forget, and the fittings allowance on this page should go up to account for it.