How to Calculate Actual Bilge Pump Capacity

A 1-inch hole below the waterline of your boat could allow over 1,000 gallons of water per hour to pour into the hull. So if a thru-hull fitting fails, you could have a fatal amount of water entering your boat.

The rated pumping capacity of a bilge pump and its actual pumping capacity are two different things. Several different parts make up a bilge system, and they all reduce the pumping capability of the bilge pump. Before you buy a bilge pump, it’s important you calculate its actual pumping capacity and not just go by its rated capacity.

What Lowers the Pumping Capacity of a Bilge Pump?

There are three main factors that combine to reduce the actual capacity of a bilge pump:

Pump Head. This is the maximum height a pump can raise water to. When the flow of water from a pump is not being lifted, all the pump’s energy is being used to move the water, and the rate of flow will be at its maximum. As the flow of water is raised, the pump has to use some of its energy to lift the water. This results in the flow of water decreasing as the rise increases. Eventually, the pump uses all its energy to lift the water and none to move it, and at that point the flow of water stops (the maximum pump head).

Friction Resistance. For every foot or inch that water flows through a hose or pipe, it encounters resistance in the form of friction. The bilge pump has to use more of its energy to combat this resistance, leaving less energy to move the water, which creates a reduced flow rate. The uneven surfaces of corrugated hoses or pipes generate much more resistance than smooth hoses/pipes. Bends and turns in the pipework also generate a lot of resistance, as do any valves and thru-hull fittings the water travels through.

Electrical Loss. A bilge pump uses electrical energy and converts it to physical movement that pumps water. If the pump doesn’t receive the maximum amount of electrical energy, it won’t generate the maximum amount of movement/pumping. Unless the batteries on a boat are fully charged, they don’t deliver 100% of the power required by the bilge pump. Also, some of the electrical current is lost between the batteries and the bilge pump due to the electrical resistance in the wiring, and corrosion and imperfections at every terminal and connection. Incorrect gauge wiring also reduces the current passing from the battery to the bilge pump.

Calculating Bilge Pump Capacity Loss

Example - A typical bilge pump rated at 1,100 GPH (gallons per hour) operating on 12 volts:

Calculating Pump Head Loss. The GPH rating given to a bilge pump by the manufacturer is based on its capacity to pump water horizontally and doesn’t account for head. However, bilge pumps are located at the bottom of the bilge and the discharge pipe outlet is well above the waterline, so the pump has to use some of its energy lifting the water up.

The loss of pumping capacity from pump head varies from pump to pump, but typically a rise of 3 feet in head reduces a pump’s flow rate by 20%-30%.

• A 25% loss of flow with a 1,100 GPH bilge pump equals 275 GPH.

Calculating Friction Resistance Loss. A bilge pump manufacturer’s GPH rating doesn’t account for resistance and frictional losses. The exact amount of flow lost varies with the diameter and types of hoses and pipes in the bilge system; the number and the angles of bends in the pipework; and the number and diameter of any fittings the water has to pass through.

Each 90-degree bend in a pipe can reduce water flow by up to 10%; corrugated hoses or pipes can reduce flow by 20% or more; and valves and thru-hull fittings can reduce flow by 2% each. The total reduction in flow rate from resistance losses for an average bilge system can be 30% 

• A 30% loss of flow with a 1,100 GPH bilge pump equals 330 GPH.

Calculating Electrical Loss. The longer the wiring circuit between the batteries and bilge pump, the greater the electrical current loss. Also, the more terminals and connectors the current has to pass through, the more current will be lost. If the electrical circuitry is well maintained, the electrical loss will be minimal. However, incorrect wire gauges and badly corroded terminals can cost a bilge pump 5%-10% flow rate.

• A 5% loss of flow with a 1,100 GPH bilge pump equals 55 GPH.

Actual Bilge Pump Capacity. Using the losses from above, a typical 1,100 GPH bilge pump loses a total of 660 GPH flow rate, which means its actual operating capacity is well below 50% of its manufacturer rating.

The actual capacity of a bilge pump can be less than half its rated capacity, so always buy the largest capacity bilge pump you can. Boat builders often install lower-capacity bilge pumps just to keep costs down. However, a lower capacity pump might be fine for removing nuisance water, but may not be capable enough to keep your boat afloat during an emergency.