An undersized bilge pump isn’t an inconvenience — it’s the difference between a wet bilge and a boat that sinks at the dock. Most boaters find out their pump was too small only after it already failed to keep up. Here’s how to size it correctly before that happens.
GPH by boat length — the starting point
- Under 20 ft → 500–800 GPH
- 20–26 ft → 1,000–1,500 GPH
- 26–35 ft → 2,000–3,000 GPH
- 35+ ft → multiple pumps, 3,000+ GPH combined
Treat these as a floor, not a target — the number on the box is measured under ideal lab conditions, and real installs never match that.
The number on the box is not the number you’ll get
GPH ratings are measured with zero vertical lift and no hose restriction. Once you account for the actual lift height from bilge to through-hull and the length and bends in your discharge hose, real-world output typically drops 20–40% below the rated figure. Size up accordingly — a pump rated exactly at your minimum need will underperform once installed.
Always run two pumps, never just one
A primary pump handles routine water — rain, spray, minor seepage. A second, higher-capacity pump mounted a few inches higher kicks in only when water rises further, during real flooding. If your only pump fails or its float switch jams — and float switches do jam, especially with debris in an older bilge — a single-pump boat has zero backup. This is the setup that actually saves boats, not the pump rating alone.
Float switch vs electronic switch
- Float switches — mechanical, inexpensive, reliable, but prone to jamming on debris
- Electronic (solid-state) switches — no moving parts, far less likely to jam, moderately higher cost
If your bilge regularly collects sediment or debris, the electronic switch upgrade pays for itself the first time it doesn’t jam during an actual emergency.
Wiring: the part people skip and regret
- Use marine-tinned wire, not standard automotive wire — untinned wire corrodes fast in a wet bilge environment
- Fuse the circuit close to the battery, sized correctly to the pump’s amp draw
- Wire it to a dedicated always-on circuit, independent of the main battery switch — a bilge pump that only works when the main switch is on defeats the purpose if you’re away from the boat when it floods
Test it — don’t assume it works
Pour a bucket of water into the bilge periodically and confirm the pump kicks on and actually clears it. A pump that hasn’t run in months can seize, and a float switch buried under grime may never trigger. This five-minute check is the only way to know your setup actually works before you need it to.
Compare pumps sized correctly for your boat length in our marine pumps category.
