Electric Bilge Pumps – all you need to know

Electric Bilge Pumps – Don’t Leave Home Without Them

The most important safety system on your boat, after the structural integrity of your hull, is your bilge pump system.

Why? Well, consider this – you’re offshore when a thru-hull fitting fails, or you lose your prop shaft. In either case you now have a 11/2 inch diameter hole in your hull about 3 feet below the waterline. This hole will let about 75 gallons of ocean into your boat each minute, or 4,500 gallons per hour.

Now, your bilge pump system is unlikely to be able to keep up with this inflow, but what it will do if it’s of an adequate size, is give you time to find the leak and plug it. And if you can find the leak and fix it, you don’t have to use all the other safety systems on your boat such as life jackets, life raft, EPIRB or flares.

Question: How many and how big?

Answer: At least 2 and as big as possible.

If all you’ve got in the bottom your boat is a little Rule 500 gallon pump, you don’t have a bilge pump system – all you’ve got is a way to pump out your bilge when your hatches leak.

A Rule of Thumb

A good rule of thumb is that a sailboat should have a minimum rated pump capacity of 100 gallons per hour for each foot of overall length. So a 30 footer should have a minimum total rated pump capacity of 3000 gallons per hour, and a 40 footer should have 4000 gallons per hour.

However, see the comment below on “Rated” capacity versus “Real” capacity. You may want to increase the sizes above for the real world.

And in every case there should be at least 2 pumps which together add up to the required capacity. For a 30 footer I would select a 1000 gallon pump as the primary pump with a 2000 gallon pump as the secondary pump, and for a 40 footer a 1500 gallon as the primary and 2500 gallon as the secondary.

The primary pump should be set at the bottom of the lowest part of the bilge, with its float switch at the same level. The secondary pump and its float switch should be set up couple of inches higher.

The primary pump, which uses less power, then handles the normal bilge pumping requirements and the secondary pump only operates if the primary pump has failed, or is not coping with the inflow of water. The secondary pump is also a little above the bottom of the bilge and will suffer less from clogging with bilge debris.

Each pump should be totally independent, ideally with its own electrical supply, but definitely with its own hoses, vented anti-siphon loop and thru-hull fitting, so that failure of one pump system does not affect the other.

Rated Pump Capacity

Bilge pump rating Manufacturers rate bilge pumps on an “open flow rating” which is the amount of water the pump will move horizontally, with no hoses on the either the inlet or the outlet side and the battery at 13.6 volts, i.e. fully charged.

When you introduce a more realistic battery charge of say 12.2 volts, a reasonable lift height of say 5 feet from your bilge to your thru-hull fitting, plus the resistance to the flow of the hoses and thru-hull, you very quickly reduce the ability of the pump to move water.

So another good rule of thumb is to assume that in the real world the capacity of any bilge pump is 50% of that indicated by the manufacturer. So a 1000 gallon per hour pump will pump approximately 500 gallons per hour out of your bilge.


In 2000, Practical Sailor conducted tests on 20 different electric bilge pumps. They recommended the Attwood V1250 followed by Whale Supersub, RuleMate and Lovett 1200.

You can find the test results here

Practical Boat Owner conducted tests on 12 bilge pumps in May 2008. They recommended the Attwood V900, Shureflo Piranha, Johnson L750 and Whale Supersub

In both tests, the Attwood brand scored well and the Whale Supersub was recommended as the best small pump.

Float switches

Each bilge pump should have it’s own separate float switch. These switches are very reliable and pump failure is usually due to failure in the electrical supply or blockage of the pump inlet rather than a failure of the float switch.

However they must be installed correctly.

Float switches should be installed fore and aft on the centerline at the lowest part of the bilge. They should be fitted with a protective cover that can be easily removed for inspection and cleaning. This cover protects the switch from surging water in the bilge that could lift the float backwards and break it off. It also stops debris lodging under the float and jamming it open, with the possibility of burning out the pump. However it will not stop sludge and small litter getting under the switch and interfering with its operation.

Hence the need for good access and regular inspection and cleaning.

Also you should ensure that the switch electrical leads are of marine tinned cable, large enough to limit the voltage drop from the battery to the pump to a minimum, (less than 3% – see below), and long enough to make the electrical connections above the floor boards, out of the bilge water.

Hoses and clamps

As indicated above, bilge pumps quickly lose their ability to move water when there is any restriction to the flow. So selecting the correct hoses for your system is important.

Firstly, the hoses should be at least the same diameter as the outlet on the pump. If the pump you choose has an outlet size that is not a standard hose size, fit the next largest bore hose. Note that fitting a larger bore hose does not increase the pump’s capacity.

A smooth bore hose is required. Don’t use reinforced ribbed hose as it causes far too much resistance. Also, un-reinforced PVC hose is not strong enough and will tend to collapse. The best solution is heavy duty water hose or reinforced heavy duty vinyl hose. A lower cost, lower quality solution is reinforced PVC hose.

The hoses must be run to the thru-hull by the shortest route, with the least number of bends. Avoid 90 degree bends if possible, two 45 degree bends will cause less flow resistance.

To avoid airlocks in the system, hoses should rise continuously from the pump to the anti-siphon vent, with no drooping.

All hose connections should be double clamped with non-perforated stainless steel hose clamps. These clamps should be checked each year by loosening the screw and checking for corrosion under the screw. Any sign of corrosion and you should replace the clamp.

Anti-siphon loops

Theoretically, all thru-hulls should be above the waterline. On a sailboat this means above the heeled waterline – and the heeled waterline on a sailboat is the waterline when the rail is under. So realistically you’re probably going to have the outlet below the heeled waterline, in which case you’ll need a vented loop in the hose just before the thru-hull to avoid siphoning water back into the boat when the outlet goes underwater.

The anti-siphon valve must be installed so it’s above the heeled waterline at all times. Make sure this vented loop is easily accessible as you’ll need to check the valve regularly to make sure it’s not clogged, which would allow the siphon to operate.

Anti-siphon loops can be purchased at most chandleries.

Batteries and electrics

The size and quality of your battery system will determine how well your bilge pump system will perform under extreme conditions. Unless you have relatively new, fully charged, heavy duty, deep cycle marine batteries in your boat, you’re going to run out of battery power long before you can find and plug a serious leak.

Ideally, each pump would be powered by a different house battery, so that if one battery fails, one pump will still operate.

A typical 1000 gallon pump will draw between 3 and 5 amps, while a 2000 gallon pump can draw between 8 and 12 amps. Larger 4000 gallon pumps can draw up to 20 amps.

Another rule of thumb states that total battery capacity on a sailboat should be about 4 times the anticipated daily electrical demand. This assumes that the batteries are proper deep cycle marine batteries, are at 80% of full capacity and are not discharged below 50% capacity.

So if you have a well equiped small sailboat set up for coastal cruising with a daily electrical demand of 100 amp hours, you may well have a house battery capacity of 400 amp hours – more than enough to run a pair of 2000 gallon pumps for a couple of hours while you find and plug the leak.

However, if you have a sailboat not set up for off-shore cruising, your battery capacity may be well below that required to run a pair of pumps for more than one hour. One new 120 AH deep cycle marine battery could run a pair of 2000 gallon pumps drawing 12 amps each for less than 2 hours before it is 50% discharged.

Wiring your bilge pumps

All wiring on board your boat should be tinned marine cable. To check the wire size you need for your boat go to:

Voltage Drop Calculator

Select the 12 volt supply, enter the length of the run and pump power draw in amps and select a gauge of wire. The result will be the voltage drop and percentage voltage drop for those criteria. You can then alter the wire gauge and recalculate to find the right size for your installation.

Wiring size should be sufficient to ensure that the voltage drop from the battery to the pump is less than 3%

Don’t forget to check on the table on the right on the above site to confirm that the wire gauge you have selected will carry the amperage you need.

All electrical connections should be crimped and soldered with ring, not spade end connectors. End to end wire connections are particularly liable to failure.

Many pumps come with electrical leads 6 foot or longer which should allow connection between the pumps and the pump switches to be made on a terminal block well above the bilge.

The bilge pumps should be wired directly to the batteries, or to the back of the terminals of the battery shut-off switch, not through the shut-off switch, to ensure they are not disconnected inadvertently.

To check if your pumps are correctly wired, turn off all power at the battery shut-off switch, set the bilge pump switch to auto and then lift the float on the float switch. If the pump doesn’t start, you’ve wired it incorrectly.

The pump circuit should be protected by an in-line fuse which should be installed in a waterproof fuse holder in an easily accessible position. You need to be able to inspect this fuse regularly to ensure it is not corroded which could cause pump failure.

To round off your bilge pump installation you should consider installing “bilge pump on” indicator lights on your bulkhead for all pumps and also a high water alarm that sounds when the secondary pump activates. Without an audible high water alarm you might not notice when the secondary pump starts. In which case you may not notice until the floor boards are afloat – which may be much too late.

You’ll also have to decide if a seacock should be fitted. See the article on seacocks