How to Specify the Right Bilge Pump for your Vessel

Did you know that more vessels sink whilst tied up alongside their marina berth than ever sink at sea!  And the most common factors involved in these sinkings are the improper sizing, installation, and maintenance of the vessels bilge pumping systems.

Here’s how to get it right.

Why do you Need a Bilge Pump?
Whether it’s waves, rain, deck-wash or leakage – few bilges remain dry for very long.  When a bilge is pumped out routinely, the flow rate is not usually critical. The pump just needs to be efficient, reliable and easy to use.
However, in an emergency the bilge pump can be a lifesaver. A fast flow rate can buy valuable time allowing you to implement damage control measures and slow or even stop the ingress of water completely.  This is when bilge pump performance and reliability really are vital. Here we look at what you need to consider.

Firstly, you should decide whether to install:
• an electric pump
• an engine-driven pump
• a manually operated pump
…. or – if appropriate – all three.
And then you need to get the specification right for your vessel.  Here’s what you should look for.
1. Pump Performance – Flow Rates
Liquid flow rate and ‘head’ are the two basic measures of bilge pump performance.
Flow rates are commonly measured in gallons per hour (GPH) or litres per hour (LPH)
1 GPH = 3.78 LPH
Note: The volumes quoted on most electric bilge pumps relate to US gallons because most of pumps are manufactured in the US based. (To compare: 1 x UK gallon = 4.54 litres)
Although a small electric bilge pump rated at 500 GPH may sound quite powerful, these ratings can sometimes be misleading. The manufacturers don’t design these pumps to actually pump out 500 gallons of water in one hit.
If you had 500 gallons of water leak into your boat in under an hour, then most small boats will probably have already sunk. The rating on these pumps is best used to understand how quickly they can pump out a small amount of water, such as two or three buckets of rainwater.
A 500 GPH bilge pump will pump 1,890 litres in an hour which equates to 31 litres in a minute. Your average bucket can hold 9 litres of water, so a 500 GPH bilge pump is probably going to be no quicker at pumping out three buckets of water than you manually scooping them out by hand!
If we then look at how quickly water will flow through a hole into your boat we can see that for damage control purposes, we would want a much higher flow rate.

Water Flowing through a Circular Hole:
Example A
Hole diameter   1 inch
Depth below water line  12 inches
Water flow into the boat  2,700 Litres per hour
Example B
Hole diameter   2 inch
Depth below water line  12 inches
Water flow into the boat  11,000 Litres per Hour
We can see that for a small hole just below the waterline a small pump of approximately 800 GPH (3,024 LPH) should keep up with the in-flow of water.
However if the hole is bigger, such as a 2 inch (38mm) seacock failing, then we would need a pump of at least a 3,000 GPH (11,340 LPH) to keep up with the in-flow.
If we now move the hole lower down to 36 inches (1m) underwater – where most collision damage typically occurs – then the flow rate becomes much more serious.
Example C
Hole diameter   2 inch
Depth below water line  36 inches (1m)
Water flow into the boat  20,000 Litres per Hour
So, for a serious hole caused by a collision, the failure of a seacock on an engine water intake, or a paddlewheel skin fitting, we would ideally want a pumping capacity of over 5,500
GPH (20,790 LPH)

2. Pump Performance – The Impact of Vertical Height on Flow Rates
In many types of liquid transfer pump the performance can sometimes be expressed as the working pressure of the pump measured in PSI (pounds per square inch) or in BAR.
However, in the boating world ‘head’ is often used as a more convenient alternative to pressure, and therefore as a way of determining a pump’s performance.
Commonly expressed in metres or feet, head is the ‘vertical height difference between the liquid supply and the discharge level’.
A pump capable of delivering water from ground level to an elevation of 2m is therefore said to be working against a ‘head' of 2m.
It’s much easier to measure the height to which you need to pump water – as this is much more straightforward than trying to calculate the pressure required to lift the water in PSI or BAR.

3. What is Open Flow?
When manufacturers specify the performance of their pumps, they often use a flow rate such as 500 GPH or 1890 LPH stated on the box. However, you need to be aware that these figures usually refer to the performance of the pump at ‘open-flow’.
Open-flow is when the pump is operating at a zero head, ie when it is NOT pumping the water upwards – just horizontally.
All manufacturers will normally give you the performance of their pumps at varying levels of ‘head’ in the instruction booklet or in the technical specifications, so check carefully to ensure that the pump you are considering is capable of delivering the flow rate you require and to the height you need.
A typical 800 GPH rated electric bilge pump operating at 12v DC might have the following performance:
800 GPH  @ open-flow
660 GPH @ 1m head
510 GPH @ 2m head
Using the calculations from Example A above, you can see that if you have to pump the water up to a ‘head’ of 1m, to discharge it over the side, then the 800 GPH rated pump would probably not keep up with the in-flow from a 1 inch hole 12 inches below the waterline, and therefore a higher capacity pump would actually be required.

4. The Impact of Hose Friction
Water flowing through a hose loses energy through friction with the hose wall.
Hose friction losses will depend on flow rate, hose length, internal diameter and the hose configuration – such as any bends, connections, valves or other obstructions.
To minimise hose friction losses, you should keep inlet and discharge hoses as short and as straight as possible. As a general rule, the hose internal diameter should be at least as large as the diameter of the ports on the pump. If the hose is very long or has a convoluted routing, use hose of a larger internal diameter.

5. How to get your Pumping Mix Right
Marine surveyors often comment on the inadequacy of the bilge pumping systems in many of the vessels they inspect.  Too often pumps are not the right size, or are not correctly installed or maintained.
When considering your bilge pumping requirements, give serious thought to:
• the water capacity your bilge pumping system may have to handle
• how many pumps you might need
• how you intend to power your pumps.

6. Capacity and Number of Pumps
If we take the scenario of a 2 inch skin fitting failing as the most common source of a major flood, we know that a pumping capacity of 5,500 GPH (20,790 LPH) would be needed to keep up with in-flow of water whilst repairs are undertaken.
However, fitting a single bilge pump of this size may be impractical, especially on smaller vessels with limited space for installation.
Many boat owners would not want to rely on just one pump anyway, and with most modern sailing boats and motor cruisers being fitted as standard with a manually-operated cockpit bilge pump, as well as an electric bilge pump – the issue of redundancy has already started to be addressed.
Unfortunately, many boat owners rarely check the capacity of the bilge pumps fitted by the manufacturer – and in most circumstances the combined capacity of the small electric bilge pump and the small cockpit mounted bilge pump might not be enough to handle a serious intake of water.

7. Using Electricity with Water
If an electric bilge pump is going to be your primary pump for clearing large volumes of water, then special attention should be given to the wiring, location and security of the power source.
Batteries are often located low down in the bilge areas, so it would be wise to ensure that they are protected from the effects of any major flood.
Siting batteries higher up, away from areas where flood water would collect should be considered – or even better, locate them in a separate watertight compartment.
All electrical wiring to and from the pump and power supply should have watertight connections and if possible, the circuit for activating the bilge pump should be isolated from the main switch panel or other electrical circuits so if the main fuse panel blows, the bilge pump can still be operated.

8. What about Diesel-operated Pumps?
Very few leisure boat owners take advantage of the power from their main engine when it comes to bilge pumping.
Diesel engines need no electrical supply to keep them going (ideal in a flood situation) and usually have ample power to propel the boat whilst driving a substantial bilge pump.
Engine-driven clutch operated bilge pumps are tough, fully serviceable belt-driven units able to self-prime rapidly and operate continuously over a wide speed range.
Installed on many fishing and work boats, clutch pumps are becoming very popular on larger leisure vessels, and should be given serious consideration by more leisure boat owners with smaller vessels.

9. Muscle Power!
Don’t be mislead into thinking that a large cockpit bilge pump with a high capacity will be an adequate substitute for installing a high capacity electric or engine-powered bilge pump.
In an emergency situation with fear and adrenalin pumping, few crew members will be able to work a hand pump for very long before they tire and bilge pump output rapidly falls.
A manually-operated bilge pump is best used for managing residual water once the in-flow has been stemmed or completely stopped.
In a real emergency, you and your crew’s time is better spent stopping the in-flow of water, than working a manual bilge pump.
To see the range bilge pumps available for leisure and commercial vessels, click here or for further technical advice, contact the Safety Marine technical team on or call +44 (0)2380 226300. 


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