Bilge Pump & Battery Sizing
What your pump really moves once you account for lift and hose, how long the battery runs it, and how badly a hole out-floods it.
A bilge pump handles nuisance water, not damage. Packing gland drips, rain, spray and the odd wave over the transom. No pump you can fit will out-pump a real hole — the section further down puts numbers on that, and they are not close.
For a boat this size
A hole in the boat
The physics is Torricelli's: water leaves at the speed something would reach falling from that depth, so flow goes as the square of the diameter and the square root of the depth. Doubling the hole quadruples the water. Going twice as deep only increases it by forty per cent — depth matters far less than size, which is the opposite of most people's intuition.
The practical conclusion is a soft wooden plug tied to every skin fitting. Stopping the water costs nothing; pumping it costs more than any boat can carry.
What lift costs you
| Total head | Fraction of rated flow |
|---|---|
| 0 ft — how it is advertised | 100% |
| 3.3 ft | 75% |
| 6.6 ft | 55% |
| 10 ft | 38% |
| 13 ft | 22% |
Typical for a centrifugal bilge pump against total head including hose and fitting losses. Your pump's own curve is better if the manufacturer publishes one.
Things worth knowing
- A bilge pump is for nuisance water — packing gland drips, rain, spray. It is not a damage-control device, and no pump you can fit will out-pump a hole of any size.
- Rated capacity is open flow. By the time the water has gone up a hose, round a bend and through a skin fitting, a pump is doing well to move two thirds of the number on the box.
- Wire it heavily. A bilge pump on undersized wire loses voltage, spins slower and moves far less water, and it does that quietly for years until the day it matters.
- The float switch fails more often than the pump. Test it by lifting it, not by tipping water in, and do it on the same schedule as the flares.
- Two pumps at different heights beats one big one. The high pump is an alarm as much as a pump: if it ever runs, the low one is losing.
How to use
- Enter boat length, the pump rating on the box, and the total lift including hose losses.
- Add the battery size and type for continuous running time and a minimum wire gauge.
- Use the hole section to see what the pump is actually up against.
Frequently asked questions
Why does my pump move less than its rating?
Ratings are open flow — no hose, no lift, no fittings. Once the water has to go up a hose, round a bend and out through a skin fitting, a typical installation delivers around sixty to seventy-five per cent of the number on the box.
What size bilge pump do I need?
Common practice runs from about 800 gph on a small open boat to 4,000 and up past 35 feet. But the size matters less than the arrangement: two pumps at different heights on separate circuits beats one big one, because the high pump doubles as an alarm.
Can a bilge pump keep up with a hole?
Almost never. A two-inch hole two feet below the waterline admits roughly 4,100 gallons an hour, which is more than the biggest pump commonly fitted actually delivers. A soft wooden plug tied to every skin fitting is worth more than any amount of pump capacity.
How fast does water come in through a hole?
Flow goes as the square of the diameter and only the square root of the depth. Doubling the hole quadruples the water; going twice as deep adds only forty per cent. Size matters far more than depth, which is the opposite of what most people assume.
What wire size does a bilge pump need?
Enough to hold voltage drop under three per cent, because a pump on undersized wire spins slower and moves far less water — quietly, for years, until the day it matters. An eight-amp pump on a fifteen-foot run needs 10 AWG.
How long will my battery run the pump?
Usable amp-hours divided by the draw. A flooded lead-acid battery gives you half its rating before you damage it, AGM about eighty per cent, lithium ninety. A 100 Ah flooded battery runs a 2,000 gph pump for a little over six hours flat out.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.