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Bilge Pump Capacity: Do The Math

Just how much water can your bilge pump move? Probably a lot less than you think.

Bilge pump math illustration

If you have any boat larger than a small runabout, chances are you have an electric bilge pump somewhere aboard. That bilge pump is not intended to keep your boat afloat in the event of a hull breach, but simply to remove "nuisance" water — including, among other things, small amounts of spray or rainwater that come in when the companionway is open, drips from the stuffing box, or leakage or seepage from a deck fitting or port light that's not quite watertight. Though those bilge pumps are not intended for hard duty, the BoatUS Marine Insurance claim files show that in many cases, sinkings have been delayed and even avoided because a bilge pump kept up with a small leak and allowed time to fix it before it became a large leak. Conversely, many sinkings involve a bilge pump that has been ignored for weeks or months as it cycled increasingly frequently, or one that has run the battery flat in a relatively short period of time when the volume of water entering the boat led to continuous running of the pump.

A standard electric, submersible centrifugal pump that can move 500 gallons per hour (gph) sounds as if it should be able to handle nuisance water easily, and even be capable of dealing with bigger problems. After all, 500 gph is just over 8 gallons per minute (gpm). That's the equivalent of just over one-and-a-half jerry cans of water every minute. But chances are, that pump is not even moving half that volume. To get an estimate of your pump's real-world pumping ability, figure out how much each of the following might be reducing its rated capacity on your boat.

-20% Voltage Drop

Most manufacturers rate their bilge pumps while running off 13.6 volts of DC power. But bilge pumps will only rarely see 13.6 volts in the wild. Batteries will only deliver that kind of power while being charged through shore power or with the engine running; they'll deliver considerably less when they're not being actively charged. In addition, the voltage at the battery will not be what reaches the bilge pump six or a dozen feet away in the bilge. Bilge pump wires are sometimes undersized, and connections are often corroded (it is the bilge, after all). All of this can decrease the voltage actually reaching the pump by 20 percent or more, and can easily result in a 20-percent drop in capacity. To maximize voltage to the pump, clean all connections regularly, and check the wire-gage recommendations for your pump size. If that wire's undersized, up-size. And if you decide to install a higher-capacity pump after reading this article, make sure to put in a new, larger wire.


Bigger isn't always better when it comes to fuses. Use the fuse size recommended by the manufacturer; otherwise increasing resistance in the wires or the pump may not trip the breaker, allowing heat to build up and potentially start a fire.

-30% Head Height

Most manufacturers also rate their bilge pumps when pumping water horizontally. But bilge pumps are located in the bilge — below the waterline. Water is going to have to be pumped UP to reach the outlet, and it will have to go even higher if the outlet hose includes an anti-siphon loop above the waterline, as it should on most boats. Therefore, in the wild, all bilge pumps have to lift the water some amount of vertical distance — called head — from the bilge to the top of the anti-siphon loop or to the outlet. Head is expensive in terms of pumping capacity. Three feet of head height, as might be found on larger powerboats, decreases the capacity of that 500-gph pump by about 30 percent. Six feet, as could be necessary on a large sailboat, decreases capacity by more than 40 percent. Assuming the anti-siphon loop is appropriate for your boat, there's no way around that loss of capacity. If you want to move a lot more gallons per minute than you are right now, you'll have to up-size the pump.

What Size Bilge Pump Do I Need On My Boat?

-20% Hose Resistance

Anything that increases the amount of energy required to move the water through the hose will also decrease the pump's effective capacity. That includes corrugated hoses, which create more friction than smooth-bore hoses, long hose runs, and runs with sharp bends or with dips where water can pool. Some people install a check valve in the outlet hose to keep the last of the water in the hose from flowing back into the bilge. Not only can check valves jam closed, but according to Rule, who makes bilge pumps, they can reduce a bilge pump's performance by as much as 50 percent. Worse, if the valve is close to the pump, the bilge pump may not be able to overcome the resistance of the valve at all when it is being held in position by the weight of the water in the hose.

Corrugated hose alone can reduce pumping capacity by 20 percent. If your installation has all of these ingredients, you could be losing more than half of your pump's rated capacity just to hose issues. To increase capacity, get rid of the check valve, switch to smooth-bore hose, and create as straight and smooth a hose run as possible.

= Actual Capacity

Having gone through this exercise, you may be wondering how much your pump will really move in a minute or an hour. One easy way to find out is to pour five gallons of water into the bilge and start your stopwatch when the bilge pump kicks in. A 500-gph pump should empty the bilge in 35 seconds if it's working at its rated capacity. A 1,500-gph pump should be able to do it in 12 seconds. Of course, given all of the above, it's more likely that 500-gph pump will take longer than a minute to get rid of the water. Eighty to 90 seconds wouldn't be unheard of, and the 1,500-gph pump will take 25 seconds or more. Do the math to see how your bilge pump stacks up against the writing on its side.

Table 1. Recommendations For Minimal Bilge Pump Sizing By Boat Length

Boat Length Pump Output – Gallons Per Hour
Source: West Marine
To 18' 300 to 500
18'-22' 450 to 700
22'-26' 600 to 1,200
26'-30' 800 to 1,000
30'-35' 1,000 to 1,600
35'-40' 1,600 to 2,200
40'-45' 2,000 to 4,000

Then consider whether or not you have enough pumping capacity. When it comes to pumps, more is (almost) always better, but Table 1 shows West Marine's recommendations by boat size. How does your boat measure up? Increasing the pumping capacity you have aboard by installing a larger bilge pump is an easy and inexpensive way to give you — and your boat — time when something starts to go wrong, time that could make the difference between being on the water and in it.

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Beth Leonard

Contributor, BoatUS Magazine

Beth Leonard is the former technical editor of BoatUS Magazine.