Bilge Pumps

Fitting adequate bilge pumps is next in importance after ensuring the integrity of your hull.

Do not wait to take the matter seriously until you are woken in the night by chilly buttocks and you see your shoes sailing in formation past your bunk.

A boat does not have to be in a collision or storm for it to sink, it is quite capable of doing it all on its own.

All those modern 'necessities' such as the heads, engine cooling intakes, stern glands etc, mean that many boats have more holes in their bottoms than a Dutch cheese.

Any of those through-hull fittings can leak even if the seacocks are closed.

The essence of good seamanship is safety .

How many?

How many pumps and what capacity for your boat and how to check them and plumb them correctly.

Even the smallest open boat should have some means of bailing.

A frightened boater can move a lot of water with a bucket, but only if he can get at it and is able to jettison it quickly.

A bilge pump is the answer for any enclosed spaces which are not easy to get that bucket into.

Even canoes can benefit from having a small pump fitted.

How many pumps your boat should have will depend on several criteria.

Every hull is different however, as a rough, starting guide, a boat from 16ft to 26 ft / 5M to 8M in length should have at least two pumps, 26 ft to 35 ft / 8M to 10M, at least three and anything above that should have at least four pumps.

It is worth remembering that even on a small boat that one bilge pump is only adequate if it is working.

And the more bilge pumps you have the more of your crew will be able to help with the pumping.

By the same criteria, larger boats should have at least one more than the minimum if only as a back up.

Next thing to consider is where the pumps are to be operated from.

A cabin cruiser, for instance should have one which can be operated from the cockpit and another from within the cabin and again I'm referring to minimums.

Where the water accumulates is another criteria to consider.

Any sealed compartments which do not drain to the bilges should have their own dedicated pump.

Even if these compartments will not hold much water it is worth having a small pump to drain them if only to prevent water damage.




The next problem is deciding what size or pumping capacity your pumps should be.

The table I have drawn up below gives an indication of the capacities you should be aiming for however, there are several other factors to take into consideration.

First do they all need to be the same size?

The bilge pump used for everyday needs, providing you do not have any serious leaks will not require a large capacity.

And a smaller pump will require less energy to drive it whether its manual energy or battery power.

Save your largest pump for emergencies.

On the subject of emergencies, which I sincerely hope none of us has to cope with, a large capacity manual bilge pump could be operated by the stronger members of the crew while weaker members could help by pumping the lower capacity ones.

The next thing is how high the water has to be raised.

The height the water has to be pumped vertically up before it can discharge will reduce the amount the pump will be able to cope with.

This is something sail boaters need to consider, a pump which discharges on the windward side will have to lift the water higher than one emptying on the leeward but the leeward outlet could be underwater when the boat is well heeled.

The discharge rate will also be affected by the size of the piping, using a smaller bore of pipe might reduce the flow-back but it will dramatically reduce the out flow.

When it comes to buying pumps I guess the best advice is to buy the best you can afford.

The efficiency and standard of the better know brands is exceptionally high, unfortunately many manufactures complicate the process of choosing by quoting the capacities in different configurations.

A quotation in gallons or litters per hour sounds very impressive but how many of us could pump manually for an hour and how many of our batteries would go flat in that time.

And the capacities quoted are rarely related to the height the water has to be raised.

When choosing, it is best to err on the safe side.

This table below is a very rough guide to the minimum pumping capacities you should be aiming for.


Boat length 16ft to 26 ft/5M to 8M 26 ft to 35 ft/8M to 10M
US gal. per hour 2500 to 3500 3500 to 4500
US gal. per min 40 to 60 60 to 75
UK gal per hour 2090 to 2910 2910 to 3750
UK gal per min 35 to 50 50 to 63
Litres per hour 10000 to 13000 13000 to 17000
Litres per min 140 to 227 227 to 284

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What type?

When it comes to choosing between electric, manual or engine driven bilge pumps I believe it is a good idea to hedge your bets

Personally, I would feel very vulnerable without at least one manually operated pump.

I know that pumping is hard work and few people can keep it up for long but at least I do not have to worry about the electricity being swamped.

Another advantage to the modern diaphragm bilge pump is that it is self priming.

And they can also be used to pump any stale air or more importantly gas out of the bilges, most have very few metal parts so the risk of causing a spark is minimal.

Be sure to choose one which can be repaired and serviced easily .

On the other hand an electric pump with a float switch is perfect for maintaining peace of mind when the boat is left unattended, providing the switch works and the battery system up to running it for the necessary period of time.

For moving large amounts of water quickly and easily an engine driven pump has got to be the best option but they are expensive to install and not a lot of use if you can not start the engine.

They also need to be mounted at the front of the engine which, for small boats is likely to be an issue due to lack of space.

Using the engine cooling intake is not really an option for small engines as the amount of water drawn by the impeller is not very great.

And it is not worth the risk of burning out the impeller and wrecking the engine if it runs dry or it becomes blocked with debris from the bilges.

For small boats and canoes there is nothing wrong with the small plastic stirrup pumps.

However, few can be stripped down easily so, keep that bucket or cut down milk or oil carton handy as well.

Electric Bilge Pumps.

The main problem with electrical bilge pumps is not the pump but the power supply.

The modern pump is pretty rugged and reliable.

When they fail it is usually the fault of the power supply.

A big pump drawing a lot of amps can quickly flatten a battery.

For larger boats with several big batteries this is less of a problem than for small boats. However, all boats have a problem where electricity is concerned; water and especially salt water!

Any corrosion in wiring connections will increase resistance resulting in voltage drop.

While minimizing the risks is important for any wiring it is especially important for emergency systems.

So, make sure that any connections are kept to a minimum and are as high above the bilge as possible.

Tin the wire ends and use terminal blocks with brass terminals and place the blocks in a junction box with the entry hole for the wire at the bottom.

Do not try to skimp on wire size and support the wire every so often with cable clamps or ties.

Keep the wiring runs as short as possible but do not be tempted to tap into the supply for other equipment.

The pumping circuit should be kept separate from the other electrics independent of the main shut off switch and fuses.

And while it is good practice to protect any circuit with a fuse or circuit breaker it might be worth making an exception in the case of the submersible pump with a float switch.

Whether or not you fuse the float switch check it regularly by just lifting the switch, it should work regardless of whether the main switch is on or off.

Unfortunately float switches are notorious for failing so, check them regularly.

There are several other forms of switch for detecting water levels available none of which I have any experience with so cannot comment upon.

As far as I'm concerned the float switch is cheap, simple and easy to check, clean and replace.

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Fitting Bilge Pumps

There are a number of things to bear in mind when fitting pumps.

  • Where does the water collect when the boat is at rest.
  • And where does it collect when under way.
  • Inlet hoses in the bilge need a strainer to keep out any debris, hairs, sludge and the like.
  • Float switches need to be kept free, clean, and be protected from surge
  • Hoses need to be secured so they do not move or kink.
  • Hoses should be smooth bored.
  • Emergency float switches should be set high to minimize use and thus wear but low enough to prevent water damage to the cabin sole.
  • For everyday pumping the inlet needs to be as low as possible.
  • Locate bilge pumps where they can be easily accessed for maintenance.
  • Keep pipe runs as short as possible.
  • The discharge outlet needs to be above the waterline.
  • Insert a riser loop in the outlet pipe to prevent back siphoning should the outlet become submerged.
  • Always use the size of hose recommended by the pump manufacturer and that includes the size of the outlet fitting.
  • If joining two outlet pipes use a Y connector rather than a T junction.Where two outlets are connected the subsequent pipe must have an increased bore.

It may seem obvious to place the pump inlet where the water collects, on a low powered boat this will tend to be at the same low point both at rest and underway.

However, on more powerful motorboats the water may accumulate in the mid section while at rest but run aft when underway. So, pumps will be needed in both locations.

Float switches should always be fitted with the '╦ťflapper' facing aft to minimize surge damage.

Surge can also be reduced by sighting the switch aft of a bulkhead.

It is fairly common practice to place the outlet fitting low down to reduce the amount of staining by dirty bilge water on the outside of the hull.

However, this can lead to back siphoning unless there is a riser loop before the outlet.

The top of the loop must remain well above the water line whatever the angle of heel.

This can be a problem for sailing boats in which case the outlet is perhaps better sighted in the transom.

My own bilge pump outlets drain into the self draining cockpit, the possibility of any dirty water in the cockpit is, as far as I'm concerned, preferable to the risks of back siphoning.

You can fit siphon breaks and non-return valves but these have a tendency clog, better to have that outlet well above the waterline.

Having fitted the pumps the next job is to keep those bilges clean.

No system no matter how good will be able to cope for long if the bilges are full sludge and debris.

And make sure that the limber holes are kept clear so any water can drain towards the bilge pump inlets.

And finally make sure that nothing, especially those wires and hoses, can interfere with the float switch.

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