Bilge Blower and Engine Compartment Ventilation on Gas Inboards and Sterndrives

Locate the blower switch at the helm, the warning label, the blower itself (a round fan housing in a hose, or a flange-mounted unit), each duct hose, and the matching vents outside. Follow each hose from the bilge to its vent. Note the blower's size and airflow rating (cubic feet per minute) from its label.

Open the engine hatch and smell the bilge before you touch any switch. BoatUS calls your nose one of the most sensitive vapor detectors aboard. The blower can clear vapor, but it cannot clear spilled fuel, which keeps giving off more vapor.

Turn the blower on. You should hear it clearly. Go to the exhaust vent on the outside of the boat and hold a tissue or a light strip of plastic at the opening: it should blow outward steadily. If you hear the motor but feel little or no air, the hose is probably disconnected, crushed or blocked.

Run the blower for at least 4 minutes before every start, after every fueling, and whenever the boat has sat with the hatches closed. Discover Boating stresses running it after refueling. Do a final sniff before you turn the key.

With the blower off, check each hose for cracks, crushed sections, kinks and loose ends. A duct that has slid off the blower or the vent fitting moves air around inside the compartment instead of out of it. Keep hoses away from the exhaust manifolds and other hot parts, and secure them so they cannot fall into bilge water. The blower's pickup end should sit low (lower third) but above the water line in the bilge.

Look into each vent and cowl. The BoatUS Foundation notes birds and insects love to nest in ducts. Clear out nests, leaves and wasp nests. Make sure a cover, cushion or snap-on canvas does not block a vent when the boat is in use.

The blower should be screwed down firmly and its housing should not be cracked. Spin the fan by hand with the power off: it should turn freely without grinding. A seized or noisy blower is near the end of its life.

Look for green corrosion, loose connectors, electrical tape splices and wires lying in bilge water. With the switch on, check for battery voltage at the blower's wires. Voltage there but no fan means a failed motor. No voltage means a fuse, switch or wiring problem upstream. Replace a blown fuse with the same rating only; if it blows again, find the short before going further.

BoatUS points out that most bilge blowers are made for short runs, not continuous use; running them for hours can overheat them. If you want to ventilate while idling or cruising slowly, make sure your blower is rated for continuous duty. Natural vents work much better when the boat is moving than when it is sitting still.

If the blower is dead, buy a marine bilge blower labeled ignition protected, with at least the airflow of the old one and the same hose diameter. Ignition-protected parts are built so they cannot spark into fuel vapor. Never use a household, automotive or computer fan. Choose the same mounting style (in-line or flange) so the ducts fit without stretching. The federal standard sets the minimum blower rating from the volume of the engine compartment, so never downsize; two smaller blowers can be combined, but have a pro check the numbers if the engine, the compartment or the ducts were changed.

Disconnect the battery first and confirm there is no fuel smell. The blower must pull air from the bilge and push it out through the exhaust vent: check the arrow on the housing. Clamp both hoses securely. Use heat-shrink connectors and tinned wire, keep the splices high and dry, and keep the fuse the size the blower maker specifies.

Reconnect the battery, run the blower and repeat the tissue test at the outside vent. Record the date in BoatMend. Check airflow again at the start of each season and after any work in the engine compartment.