Enter the size of a hole and how far below the waterline it is, and the calculator gives the inflow. Add the boat's pump capacity to see whether the pumps keep up once real-world losses are counted.
Water coming in
Good Old Boat explains the physics: water flows through a hole at the speed an object would reach falling from the depth of the hole. Flow = area of the hole × √(2 × g × depth). For a 1-inch hole 2 ft down that's 28 gallons a minute, about 1,680 an hour. Doubling the diameter quadruples the flow. The calculator makes no allowance for the edge of the hole slowing the water, so it errs on the high side.
A failed 1½ in through-hull 2 ft down lets in about 63 gallons a minute, close to 3,800 GPH.
What the pumps really move
The figure on a pump is measured with no lift. Good Old Boat says head and hose friction bring the output down to about 40% of the rating, and the calculator uses that. Practical Sailor adds two points: some makers rate at 13.6 volts where 12.2 volts is more realistic, and a 90° bend in 1 in hose adds as much resistance as 3 ft of hose.
Capacity guides
| Source | Guide |
|---|---|
| Practical Sailor | 1,000 to 1,500 GPH primary and about 2,000 GPH backup on a 30 to 35 ft boat |
| Steve D'Antonio | At least 100 GPH per foot of length in total, with a pump in each bilge compartment |
| American Bureau of Shipping, as Practical Sailor quotes it | One 24 GPM pump plus one 12 GPM pump under 65 ft |
On the survey
D'Antonio's point is that a pump that runs doesn't necessarily pump water: the only real test is filling the bilge and watching. Check each pump's rating plate, the float switch and the high-water alarm, which he says should sit no more than 2 inches above the float switch. See bilge pump and float switch.
Citing it in a report
"Bilge pumping: one automatic pump rated 1,100 GPH, about 440 GPH in service, on a 38 ft vessel. A failed 1½ in through-hull 2 ft below the waterline admits about 3,800 GPH. Recommend a second automatic pump of at least 2,000 GPH and a high-water alarm." Give what's fitted against what a single failure lets in.