Bilge Pumps
by Kurt Küpper*

Bilge Pumps

Submersible centrifugal pumps are the most commonly used pumps for pumping out bilges. They are well suited to the task, because they can pump small contaminants in the bilge water and be run dry for limited periods.
Bilge pumps are rated at open flow, i.e. the flow rate specified will only be achieved when there is nothing connected to the outlet. The flow is also affected by the voltage supplied to the motor. Commonly manufacturers rate pumps at the flow achieved at 13.6Volts, which will in practice rarely be achieved unless the batteries are being charged at the time. The manufacturer’s ratings are thus totally unrealistic and serve only for comparative purposes.
A more detailed study is therefore required when selecting a bilge pump. Manufacturers publish performance curves for their pumps, indicating flow rates at varying head pressures. A study of various manufacturers’ curves will reveal significant differences.
Not all pumps with the same nominal open flow rating are born equal. Some will have significantly better performance at specific head pressures than others or, to put it differently, are able to pump water higher than others even though they have the same advertised open flow rating.
When determining the head pressure the pump must be able to pump against, take into account:
    •    Vertical lift. This is the height of the outlet above the pump. On sail boats in particular, it must be noted that this height can increase when the boat is heeled over.
    •    Resistance caused by the hose and any fittings and non-return valves fitted. This can be as much as the resistance caused by the pressure head.
As a simple rule of thumb, assume that the pressure head is double the heeled over vertical lift.
Even if a pump has been selected that is capable of overcoming significant flow resistance, it is good practice to minimise restriction. Use hoses with a smooth internal bore. Avoid using corrugated hoses if these have corrugated inner walls, as they significantly increase restriction.
If the hose outlet is to a through-hull fitting that can be below the water line when the boat is fully heeled over, steps must be taken to prevent water siphoning into the boat. Fitting a non-return valve will significantly increase flow resistance. It can also easily become blocked, rendering the bilge pump ineffective.
It is therefore better to route the hose from the pump to a high point above the highest possible heeled waterline level, with a vent to air as a siphon block at the highest point. The additional height will have to be taken into account when determining the pump size, but it will still be less resistive than a non-return valve. The hose should constantly run upward even at full heel, with no dips between the pump and outlet.
Another step to improve bilge pump performance is to ensure that they are run at the maximum possible voltage. Often the cable run to the bilge pump is long, and significant voltage drops can occur. The cable run is the length of the full circuit. That often includes the run to and from a switch and fuse mounted far away from both batteries and the pump, e.g. at the nav station. While it is ordinarily sufficient to use 10% voltage drop tables when sizing cable to run motors, use 3% tables for bilge pumps.
If a fairly large bilge pump with a commensurately thick hose is fitted, the water that runs back into the bilge when the pump is switched off can in some cases cause the automatic pump switch to turn the pump on again. This will result in the pump being turned on and off repeatedly. In such cases a good solution is to fit a second, smaller pump with a narrower outlet hose and an automatic switch positioned at a lower level than the one for the large pump.
It is essential to check that your bilge pumps are functioning properly on a regular basis. Flow can be reduced by inlet restriction caused by blocked strainers or strum boxes and physical restraints to the free rotation of the impeller such as hairs. Checking that the automatic pump switches are working should be part of your routine when leaving the boat.

*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455