Pumps: Understanding what it all means.

💧 Pump Flow Rate

• Definition: The volume of fluid a pump moves over a certain time.

• Measured in:

  • Litres per minute (L/min)

  • Gallons per minute (GPM)

  • Cubic meters per hour (m³/h)

Key Points:

• Describes how much fluid is being moved.

• Important for tasks like filling tanks, circulating water, or ensuring enough water reaches fixtures.

• Higher flow rate = More water (or fluid) moved per minute.

Example:

• A freshwater pump with a flow rate of 10 L/min can fill a 100-liter tank in 10 minutes.

⚙️ Pump Pressure

• Definition: The force the pump uses to move the fluid through a system.

• Measured in:

  • Bar

  • PSI (pounds per square inch)

  • kPa (kilopascals)

Key Points:

• Describes how hard the pump pushes the fluid.

• Important for overcoming resistance: long pipes, vertical lifts, or restrictive fixtures.

• Higher pressure = Fluid can travel further or uphill, or push through tight plumbing.

Example:

• A pump with 3 bar pressure can push water vertically up to around 30 meters (theoretically).

How They Work Together

• Flow rate and pressure are connected:

  • If you increase pressure (resistance), flow rate usually decreases (and vice versa).

• Think of a garden hose:

  • With no nozzle: high flow, low pressure.

  • With your thumb over the end: pressure increases, flow becomes a jet (less volume, more force).

⚓ On a Boat Example:

• A washdown pump might have high pressure to blast water off the deck, but it doesn’t need much flow.

• A freshwater supply pump might need moderate pressure to reach all faucets, but higher flow to fill sinks and tanks quickly.

Diaphragm pumps: Commonly used in fresh domestic water systems providing the water to your faucets, shower and in the case of a fresh water flushed toilet, your toilet too! Or also used in waste pump applications, these use a diaphragm that ‘squishes’ the water in a circular motion to force the water out. They can be run dry with no adverse effect, usually hard wearing and have adjustable cut in pressures and flow rates. However, some downsides: Can pulsate if not installed with the correct ancillaries (such as an accumulator tank), not suitable for high temperature applications and although adjustable, cannot output very high flow rates.

Bilge Pumps: Speak for themselves! Have the task of removing water from the bilge. Some are classed as submersible and have an inbuilt sensor or float, that will activate the pump automatically. Others can be rubber impeller pumps similar to what you have on your engine that activate from a separate float. There are a vast array of variants in this subject, alone. But generally, they tend to have a plastic impeller that sits under the pump and are usually classed as a centrifugal pump.

Clutch Pumps: Very similar to the engine raw water pump, commonly using a rubber impeller, these are attached to an electrically operated clutch that can be activated when demand is required. For instance, in the case of a washdown pump, you may not need it all the time. But once a button is switched, you can activate the pump, thus reducing working times when not necessary and prolonging the life of the components.

Circulation pumps: From your coolant pump on your engine, to the circulation pumps sometimes used in heating systems, the job of these pumps isn’t so much to produce pressure, but to gently push the fluids around to enable the fluid to either distribute, or shed, the heat. Generally fitted with a solid impeller and also classed as a centrifugal pump.

Engine Raw Water pumps: Largely the most common pump aboard a boat. Although a few manufacturers exist, they all tend to be built around the same design; a rubber (synthetic) impeller on a shaft, that spins. Inside the housing, a cam profile or body cast with one, a lip seal, usually two bearings and a wear plate sitting behind the impeller. The end cover plate is generally fitted with small bolts/screws and has a gasket or O ring to seal it. The act of having flexible blades means they can be squeezed to reduce the size of the space in between the blades. Then as the blade passes the incoming port, opens up to suck in water, as the blade approaches the outlet, it is squeezed to push the water out, and the cycle continues.

Macerator Pumps: The pump that does the job of breaking down all of your toilet waste. There are a few variations but generally tend to have a solid ‘chopping’ blade followed by a rubber impeller to push the waste out and into the holding tank/outlet.