It is not uncommon for boater's who venture offshore to discuss the merits of redundancy. Twin engines, two VHF's, two chartplotter/MFD's, etc., can add up in costs, but two is always better than one. If not already equipped, one of the cheapest options to add redundancy is the addition of a second bilge pump. There are two main considerations that drive the decision to add a second pump.
The most obvious driver is to provide a backup should the primary pump or any pump system component fail. The second pump should be wired separately to a different switch, float, and battery than the primary and use a dedicated discharge hose.
A less obvious reason is capacity. If you have ever had to use your bilge pump to empty the bilge of any significant water, you may be shocked by how long the process takes. Although pumps are labeled with a GPH (Gallons per Hour) rating, they usually perform between 40-50% of their rated flow. Even when installed properly, the physics of "lifting" water to a higher level and pumping through a hose overboard negatively impacts the performance of a bilge pump system. This can be further compounded by undersized wiring or hose selection.
Once you decide to add a second pump, your first step should be to determine the pump sizing, location and configuration. There is no reason to limit the size of the pump based on the specifications of the primary pump. In fact, if you feel it is undersized, this is a good time to upgrade the primary pump. For a typical outboard powered boat that is used offshore, you will generally find a primary pump ranging from 1000 to 2000 GPH. A second pump should at least be 2000 GPH or larger.
For an outboard powered boat, generally the stern will be where water will accumulate and where the primary pump will be located. The second pump should be located in the same area and generally at the same height as the primary however, a distance of two feet should separate the pumps in order to avoid disrupting the performance of either pump when both are in operation.
The discharge hose diameter will range from 1 1/8" to 1 1/2" and is dependent on the pump brand and specification and will require an appropriately sized through-hull fitting. The discharge through-hull should be located at a height above the water line similar to the primary pump. The discharge hose should have a smooth internal bore and not be corrugated which will negatively impact pump performance. Unfortunately, corrugated "bilge hose" is often advertised as ideal for bilge pump application but should be avoided.
The second pump should employ the use of a float switch and be powered by a different battery than the primary. The float switch for the second pump should be mounted higher than the primary by a few inches. This enables the primary pump to switch on first should water be present in the bilge until it reaches a more critical level.
The wiring for the second bilge pump will generally need to run from the battery to the console mounted switch and then to the pump. The pump manufacturer will provide a recommended wiring gauge based on the wire run length and breaker/fuse size. It's best to use a "triplex" wire that will include positive lead wires for both the float and direct pump and one negative lead combined in one housing.
All bilge wiring connections should be made with waterproof heat shrink connectors. It's a good idea to check with the boat manufacturer as they may have provided an option for a second pump and can provide a matching console mounted switch and label. If not available or if room is not available on the console switch panel, a standalone bilge switch can be purchased.
So how does it all work? In our 28 Contender test boat, a 2000 GPH secondary pump was added to the existing 1500 GPH primary pump. The installation included smooth bore 1 1/8" discharge hoses, 10 gauge wiring, and a Blue Seas/Carling 3-position switch. 25 gallons of water was added to the bilge and both pumps were turned on. It took 54 seconds to clear the bilge which calculates to approximately 1700 GPH.