Bilge Management · A buyer's reference

1. Why two stages

The single most common bilge-pump failure pattern we see is a single high-flow centrifugal pump installed as the only bilge pump on the boat. The pump is sized for emergency removal of large quantities of water — useful in the case of a hull breach, but mismatched to the actual day-to-day bilge water that accumulates on most boats, which is a few cups to a few gallons of stuffing-box drip, rain ingress, and condensation. The centrifugal pump does not draw down well below its inlet, so the boat sits with a quarter-inch of water in the bilge all the time, the bilge smells, the wood rots, the wire chases corrode.

The fix is a two-stage system: a high-volume primary for the emergency case, and a low-volume diaphragm secondary that draws the bilge dry. Every boat we work on gets both. The two-stage architecture is the single most consequential design decision in bilge management, and it costs about $300 in additional hardware over the single-stage install.

2. Primary pumps

The primary bilge pump is sized for emergency removal of large quantities of water at low head. The standard recommendation is centrifugal — typically a Rule 1500 GPH or a Rule 2000 GPH, depending on the boat's size — installed at the lowest point in the bilge, with a discharge through the topsides via a 1-1/8" or 1-1/2" reinforced PVC hose to an above-waterline through-hull with a vented loop.¹

The Rule pumps (now under the Xylem umbrella) are the industry standard for the recreational market. The competing options — Johnson Pumps, Attwood, SeaSense — are functionally interchangeable. The decision is less about the brand than about the sizing: a 1500 GPH centrifugal pump moves about 1100 GPH at typical lift heads (the 1500 is rated at zero head); a 2000 GPH moves about 1600 GPH at typical heads. The right size is the one that exceeds the worst-case ingress rate you can plausibly imagine on the boat. For most cruising sailboats and small trawlers, the 2000 GPH is the right answer.

3. Secondary pumps — the Whale Gulper case

The secondary bilge pump is a low-volume, low-head diaphragm pump designed to draw the bilge dry. Our standing recommendation is the Whale Gulper 220, a 60-GPM diaphragm pump that runs at low draw and is self-priming. The Whale Gulper has been on the market in its current form since 2003; it is dry-running tolerant, runs cool on lithium banks, and is rebuildable in the field.

The Gulper's discharge runs to its own through-hull (separate from the primary), or — on smaller boats — tees into the primary's discharge after the primary's check valve. The Gulper is controlled by a float switch (Ultra Safety Systems Senior, in our standing recommendation) wired through a dedicated breaker on the DC panel, labeled SECONDARY BILGE. The primary breaker is labeled BILGE PUMP.

4. High-water alarms

Every bilge system gets a high-water alarm. The alarm is not the pump; the alarm tells you the pump is failing or the water is winning. We install a separate float switch (Aqualarm or equivalent) at a level approximately 4" above the bilge floor (or above the primary pump's float switch trigger). The alarm wires to a panel-mounted 85-dB piezoelectric buzzer in the cabin, with a remote-monitoring tap into the N2K bus where available.

The alarm float switch is independent of the pump floats and is tested at every haul-out. The single failure mode worth naming: an alarm float that has corroded its actuating arm and reads "always wet" or "always dry." The annual function test catches this; the alternative is a sinking boat at the dock and an angry owner.

5. Oily-water compliance

Federal law (33 CFR 155 implementing MARPOL Annex I) prohibits the discharge of oily bilge water from any vessel into US waters at concentrations exceeding 15 ppm of oil.² The rule applies, with various carve-outs, to vessels over 26 feet. The practical compliance for a recreational sailboat or small trawler is: install an oil-absorbent bilge sock (PIG, Cabot, etc.) below the primary pump's pickup; replace the sock when saturated; bring the saturated sock to a hazardous-waste recovery point (the Port of Port Townsend operates a dockside collection point at Building 32 on the boatyard).

For commercial vessels above the 26-foot threshold that carry hydraulic systems or fuel-handling equipment subject to MARPOL inspection (most working fishing vessels, commercial charter boats, harbor service vessels), the compliance is more involved: an oily-water separator unit, an oil-content monitor, an oily-bilge alarm, and a logbook entry for every discharge. We do not install OWS units; for commercial vessels we refer customers to the engine-side trades who specialize in MARPOL hardware. The shop's MARPOL work is limited to the absorbent-sock-and-collection compliance pattern that fits the recreational fleet.

The other ongoing oily-water exposure point is the engine-room stuffing box, which is the source of about half of the oil that ends up in a sailboat's bilge. The standing recommendation is a drip pan under the stuffing box, draining to a small (1-quart) collection bottle that is changed quarterly. Cheap, easy, MARPOL-friendly.

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