Here's a trick question: how many gallons per hour will a 360-gallon-per-hour bilge pump actually move? As you may have guessed, the answer is not 360 gallons. Not in the real world.

We all see the units neatly packaged in the marine stores with the capacity clearly marked in gallons per hour (GPH). One would assume that would be how much water the pump can move in any given hour – and one would be wrong.

To properly understand how bilge pumps actually perform, we need to look at how those pumps are rated and what those figures really mean.Glen JusticeA pump's capacity when installed can be a fraction of its rating.

The GPH rating is commonly achieved by testing the pump in conditions that are seldom achieved when installed on a boat. For example, that rating is often achieved by running the pump at 13.6 volts, which provides favorable performance. In the real world, your battery is more likely to provide closer to 12 volts.

The rating on the label is also achieved at zero head or open flow. The term "head" is the distance above the pump that it has to push the water. Obviously, the higher the distance, the lower the GPH performance that is achieved.

The American Boat & Yacht Council (ABYC) standard for bilge pumps requires that they be rated at zero head, at 1 meter (3.35 feet), and at 2 meters (6.7 feet), all done at a design voltage that is 113 percent of the nominal voltage. For a 12 volt system, this equates to 13.6 volts. These additional ratings are usually found in the detailed instructions provided with the pump.

An extensive series of tests performed by Phil Cowley, a product tester at West Marine, indicated that each additional foot of head in a bilge pump installation costs 5 percent or more of the pump's capacity.

Here is an example of how a pump's performance can vary directly with the conditions of installation. Not to pick on Rule pumps – I happen to like them – but here are some figures contained in the specification sheet for the Model 24, which a 360 GPH pump that is among the most widely used.

GPH at open flow and 13.6 volts: 360
GPH at open flow and 12.0 volts: 330
GPH at 1 meter lift and 13.6 volts: 265
GPH at 1 meter lift and 12.0 volts: 240
GPH at 2 meter lift and 13.6 volts: 190
GPH at 2 meter lift and 12.0 volts: 150

If you are running this pump on 12 volts and pumping water just more than six feet, the capacity is less than half of the 360 GPH rating.

Pump output is also affected by the very things that help convey the water out of the boat: hoses. Ideally, a bilge pump discharge hose should have a smooth interior. The corrugated hose commonly seen in bilge pump installations may inhibit cost pump performance due to increased turbulence in the water flow.

The restrictions encountered in thru-hulls and other fittings can also cause a loss of pump performance. In fact, the restriction of one thru-hull fitting can add the equivalent resistance of several feet of hose.

The lesson here is clear: evaluate pumps properly, have realistic expectations about how much water they can pump out of your boat and pay careful attention to the installation.