If your boat gets holed and starts to fill with water, you’ll quickly turn on your bilge pump and hope it can pump out the water quickly whilst you plug the hole. But how fast will it pump? Are the manufacturers misleading you with their numbers? Read on to find out one unexpected thing that can boost pumping performance by about 25%.

This article focuses on the Rule 1100gph Bilge Pump. The manufacturer claims it will shift 1100 gallons per hour (1.4 litres per second). Does it actually deliver that performance? A simple bathroom test gives the answer:

  • Fill up a big tub of water and weigh it using bathroom scales.
  • Time how long it takes to pump out the water from the basin until the pump runs dry.
  • Weigh any leftover water.

This lets us calculate the pumping rate in litres per second (1l of water weighs 1kg).

I performed this test four times: using 25mm and 29mm hoses, and with/without the one-way valve:

  • 29mm hose, no valve: 0.94 l/s
  • 29mm hose, with valve: 0.76 l/s
  • 25mm hose, no valve: 0.92 l/s
  • 25mm hose, with valve: 0.70 l/s

It’s not surprising that the bigger hose is a little better, but the big surprise is that the one-way valve massively degrades the pumping performance.

Why are one-way valves needed? If an automatic bilge pump detects sufficient water, it’ll run the pump until the water level drops. When the pump stops, the pipe is still full of water. If this water then drains back into the bilge under gravity, it can cause the pump to reactivate. This can repeat again and again, eventually flattening your battery. Using a one-way valve is one way to prevent this happening.

However, if you set up your bilge pump to run manually – ie. you only plan to use it for a “boat filling with water” emergency – then this isn’t a problem. Consequently, you can live without the one-way valve and enjoy a 25% higher rate of pumping!

Our best result (0.94l/s) is still well short of the advertised 1100gph (1.4l/s). There’s a few factors that aren’t included in the “advertised” performance:

  • My test used a 2m length of smooth tubing, coiled to fit into my bath. It’s best to have a straight run to reduce friction. I didn’t do a test with a straight tube, so I don’t know what magnitude of difference this makes.
  • I was pumping the water up and over the 60cm high edge of my bath. This seems at the easy end of what you might find in a boat (eg. bottom of bilge up to above the waterline). But the higher you lift the water, the more work the pump has to do. The chart here shows the performance dropping linearly up to 4m, so a lift of 0.6m would mean my 1100gph pump is only going to deliver 935gph (1.18l/s)

Overall, I’m still left with a figure of 0.94 l/s, or 745gph. That’s 68% of the “headline” pumping rate, and 80% of the amount you’d expect for a 0.6m lift. Not amazing. But, the difference between the best and worst setup was a whopping +34% pumping rate which was pretty surprising!

There are many variables in a test like this:

  • Power supply: I tried several power sources. Firstly, a bench power supply, but it was capped to 3A whereas the pump draws 3.6A, and this was the bottleneck. Then I changed to using a relatively new car battery (12.4v open circuit, 12.2v under load, drawing 3.60A) for the above tests. I also tried using an older motorcycle battery which was not fully charged (12.v open circuit, dropping to 11.8v under load) and found a +6% pump rate improvement once I’d charged it up.
  • Pump position: The pump must be flat on the bottom of the bucket to pump properly. If you let it fall onto its side, the pump rate drops a lot because it can’t suck water in from all sides.
  • Hose primed or not: I drained the hose before each test. This means the pump had to run for a few seconds before water came out the other side, which is representative of what’d happen in a boat.
  • Hose draining back or not: After each test, I immediately lifted the pump out of the water to stop the pipe contents from flowing back into the bucket. This gives a truer “rate of pumping” for my artificially small experiment.
  • Measurement accuracy: I used bathroom scales to measure the water before (around 30kg) and after (around 4kg) and the scales claim to be accurate to within 0.1kg. I used a stopwatch to time the pumping, from when I turned the pump on to the instant the pump started running dry. I rounded to the nearest second, so there’s probably an error of 0.5s here. Since typically my runs took 30s, that’s about a 2% error