Choosing a bilge pump for your boat seems a simple enough task, doesn't it ?
However, many bilge pump makers publish performance figures for their products which are under ideal laboratory conditions.
When they are needed to perform in a boat that is taking water, the situation is often far from ideal !!
Manufacturers assume that the electrical supply is from a fully charged battery. Their figures are usually based on the output from the pump without any piping attached or any vertical height to be overcome, which in practice is obviously necessary to get the water discharged from the bottom of the bilge, to a height above the waterline.
A 50mm hole in a boat will admit 300 Litres per minute to your bilge. A combination of not having a fully charged battery and having to pump water from the bottom of the bilge to a hull discharge outlet 3 feet above, can reduce the stated performance of the pump by more than 50%.
It is also crucial that the discharge pipe diameter is no smaller than the pump outlet diameter and avoid the use of corrugated piping.
Smaller piping and bends or corrugations in the pipe wall can impede water flow and down-rate their performance further.
Many faults on bilge pumps can be attributed to their electrical installation.
Ensure that tinned wiring is used, with a correctly sized cross sectional area.
Ensure that the fuse size is correct to allow for increases in current caused by start-up and partial blockages - if in doubt follow the manufacturers instructions.
Ensure that the cable that comes with the bilge pump (usually moulded into the casing) is connected to the boats wiring, above the waterline and using a watertight junction box with ring terminals or adhesive lined crimp terminals. Avoid the use of terminal strips as these can corrode in the damp environment.
At a low voltage like 12V, it doesn't take much in the way of corrosion or a bad connection to create a high resistance joint, which will affect the performance of your bilge pump and its ability to clear your bilge.
Most bilge pumps on smaller boats are the submersible centrifugal type. The better ones use ball bearings to allow them to run longer than the budget ones.
Finally, always fit a riser loop above the discharge point, to prevent water flowing back.