Use this calculator to estimate bilge pump size from flooded compartment volume, target drain time, static head, hose length, fittings, and reserve margin.
It helps compare ideal flow, installation losses, and practical pump ratings before choosing a nominal marine pump capacity.
Calculator Inputs
Plotly Graph
The chart compares estimated delivered flow from the recommended nominal pump against discharge head. A dashed target line marks the adjusted required flow.
Formula Used
1) Flooded volume: Volume = compartment length × compartment width × water depth × flooded fraction.
2) Gallons to remove: Gallons = cubic feet × 7.48052.
3) Base flow requirement: Base GPH = gallons to remove ÷ drain time × 60.
4) Adjusted flow requirement: Adjusted GPH = ((base GPH + ingress GPH) × safety factor × loss factor) ÷ pump efficiency.
5) Loss factor: This calculator estimates installation penalty from static head, hose length, hose diameter, and fittings.
6) Recommended nominal pump: The result rounds upward to the next common pump rating, because published pump ratings usually reflect ideal test conditions.
How to Use This Calculator
- Enter the flooded compartment length and width.
- Add the estimated water depth inside the bilge.
- Set how much of the compartment is flooded.
- Choose the target time for dewatering.
- Enter discharge head, hose length, diameter, and fittings.
- Add safety factor and any incoming water allowance.
- Press the calculate button.
- Review gallons, required flow, delivered flow, and suggested nominal pump size.
- Use the chart and exports for reports or comparison notes.
Example Data Table
| Scenario | Compartment (ft) | Depth (in) | Drain Time (min) | Head (ft) | Adjusted Need (GPH) | Suggested Pump (GPH) |
|---|---|---|---|---|---|---|
| Small runabout | 6 × 3 | 4 | 6 | 3 | 540 | 800 |
| Center console | 8 × 4 | 6 | 5 | 4 | 1120 | 1500 |
| Work skiff | 10 × 5 | 7 | 4 | 5 | 1960 | 2000 |
| Cabin boat | 12 × 5 | 8 | 4 | 6 | 2890 | 3000 |
FAQs
1. Why is the suggested pump larger than the base flow result?
Nominal pump ratings are usually measured at ideal conditions. Real installations lose flow through lift, hose friction, bends, wiring limits, and dirty strainers. A larger nominal rating helps preserve usable delivered flow on the boat.
2. Does this calculator replace naval architecture review?
No. It is a practical sizing aid for bilge dewatering estimates. It does not replace vessel-specific safety rules, builder guidance, classification requirements, electrical design review, or emergency damage-control planning.
3. What drain time should I choose?
Choose a time that matches your risk tolerance and vessel use. Shorter drain times require larger pumps. Small recreational craft often target fast removal for nuisance water, while workboats may add more reserve capacity.
4. How should I estimate flooded percentage?
Use 100% if the compartment footprint is fully covered. Use lower values when only a section holds water because of stringers, tanks, or geometry. Conservative assumptions usually produce safer pump selections.
5. Why does hose diameter matter so much?
Smaller hose raises friction loss and cuts delivered flow quickly. Keeping discharge hose appropriately sized, short, and smooth can improve real pump performance without changing the nominal pump rating.
6. Should I include water ingress allowance?
Yes, when ongoing seepage, spray, or leakage may continue during pumping. Adding ingress gives a more realistic requirement because the pump must overcome both stored water and incoming water at the same time.
7. Is one pump enough for every boat?
No. Many boats use multiple pumps placed at different bilge levels or compartments. Separate automatic and emergency pumps can improve redundancy, maintenance flexibility, and damage-control response.
8. Can I use this for saltwater and freshwater equally?
Yes for basic flow estimation. However, material compatibility, corrosion resistance, wiring protection, strainer maintenance, and marine service rating remain important when selecting the final pump model and installation hardware.