A bilge pump is one of the most important safety features on board. Rainwater, splash water, condensation, or a small leak can accumulate in the bilge, the lowest point of the boat. A good electric bilge pump automatically or manually drains this water, keeping your boat drier and safer. But how much capacity do you actually need? A pump that is too small cannot drain water quickly enough, while a pump that is too large is not always necessary for smaller boats. In this article, we explain what to look for when choosing a bilge pump, how capacity is measured, and which model suits your boat.
1. What does a bilge pump do?
A bilge pump pumps water from the bilge overboard via a discharge hose and thru-hull fitting. This water can be caused by rain, spray, condensation, cleaning water, or small leaks. Especially for open boats, sports boats, and boats that are in the water for extended periods, a reliable bilge pump is important.
A bilge pump is intended as an aid to drain unwanted water. It is not a substitute for proper maintenance or fixing leaks. Is there regularly water in the bilge? Then it is important to find the cause.
Check out our collection of electric bilge pumps for this.
2. How is capacity measured?
The capacity of a bilge pump is usually indicated in liters per hour or GPH. GPH stands for gallons per hour. The higher this number, the more water the pump can theoretically move.
Please note: the stated capacity is often measured under ideal conditions. In practice, the actual output may be lower due to:
- height difference between pump and discharge
- long or narrow discharge hoses
- bends in the hose
- clogging in the pump filter
- voltage drop in the wiring
Therefore, it is better to choose a pump with some extra margin than one that is precisely at the minimum capacity.
3. Guidelines per boat length
The correct bilge pump capacity depends on the size and use of your boat. The guideline below helps with an initial choice:
| Boat length | Recommended capacity | Suitable for |
|---|---|---|
| Up to approx. 5 meters | 500 - 800 GPH | Small open boats, tenders, and sloops |
| Approx. 5 to 7 meters | 800 - 1100 GPH | Sports boats, console boats, and larger sloops |
| Approx. 7 to 9 meters | 1100 - 1500 GPH | Larger motorboats and cabin cruisers |
| From approx. 9 meters | 1500+ GPH or multiple pumps | Heavier boats, cabin cruisers, and intensive use |
These values are general guidelines. Does your boat have multiple sealed compartments or a deep bilge? Then it may be wise to install multiple pumps.
4. Factors influencing pump choice
Not only the length of the boat determines which bilge pump is suitable. The type of boat and the conditions in which you sail also play an important role.
Open boat or cabin cruiser
An open boat is more likely to take on rain and splash water. A cabin cruiser often has more enclosed spaces where water can accumulate unnoticed.
Use on open water
Do you often sail on open water, at higher speeds, or in rougher weather? Then extra pumping capacity is advisable.
Number of bilge compartments
Some boats have multiple low points where water collects. In that case, one pump is not always sufficient.
Hose diameter and discharge
A powerful pump only works well if the discharge hose is large enough. A hose that is too thin restricts flow and reduces actual capacity.
5. Automatic or manual switching?
An electric bilge pump can be operated manually with a switch, but automatic operation is often wiser. For this, you use a float switch or built-in automatic sensor.
Manual bilge pump operation
With manual operation, you turn on the pump yourself when you see water in the bilge. This is simple, but only works if you are on board and notice the problem in time.
Automatic bilge pump
An automatic bilge pump switches on automatically when the water level rises. This is especially useful when your boat is in the water for a long time or when rainwater can accumulate.
For extra safety, you can combine an automatic pump with a manual switch at the dashboard.
6. Installation and maintenance
A bilge pump must be placed at the lowest point of the bilge so that water is pumped out as effectively as possible. Ensure that the pump is securely fastened and remains easily accessible for cleaning and inspection.
Installation tips
- Place the pump as low as possible in the bilge
- Use the correct hose diameter
- Avoid sharp bends in the discharge hose
- Place the discharge above the waterline
- Use appropriate fuses and marine-grade wiring
- Test the pump regularly with clean water
Maintenance
Regularly check that dirt, leaves, oil, or sand do not block the pump. Clean the filter and test if the float switch is working properly. A bilge pump that is only needed in an emergency must function reliably immediately.
Frequently asked questions about bilge pumps
How much capacity should my bilge pump have?
For small boats, 500 to 800 GPH is often sufficient. Medium-sized boats usually use 800 to 1100 GPH, while larger boats often need 1500 GPH or multiple pumps.
Do I need an automatic bilge pump?
An automatic bilge pump is highly recommended if your boat remains in the water or if rainwater can enter the bilge.
Why does my bilge pump pump slower than indicated?
The actual capacity can be lower due to hose length, height difference, bends, contamination, or voltage drop in the wiring.