Replacement blanking plug for Airmar ST800 speed/temperature sensor. Fits P120/B120 thru-hulls; self-closing valve lets you swap inserts cleanly afloat. Read more.
Designed for Airmar’s ST800 Smart™ speed-and-temperature sensor, this plug lets you pull the paddle-wheel insert without hauling the boat. Slide it into the same P120 (plastic) or B120 (bronze) thru-hull sleeve from inside the hull; a spring-loaded flap in the housing snaps shut as the insert comes out, reducing the flow to a small trickle so you can work safely.
Purpose & timing: Use the blanking plug whenever the boat will sit in salt-water for more than a week, when the vessel is hauled, or if fouling is suspected because the log readings look wrong.
Essential hardware: The internal flap valve restricts flow, but it is not a watertight seal. Always secure the plug with the cap-nut and lash it with the safety wire after installation.
O-ring care: Inspect the twin O-rings each time; replace damaged rings and lubricate both with silicone grease or petroleum jelly before insertion to guarantee a tight seat. Never install the plug dry.
Housing compatibility: Direct replacement for ST800 sensors mounted in low-profile P120 plastic or B120 bronze housings; stem diameter and key-way match the original insert. (Part number confirmed in Airmar’s parts list.)
Winterising: After haul-out, remove the plug to drain any water, then refit it, this prevents trapped water from freezing and cracking the plastic.
Anti-fouling tip: If the plug will stay in place for weeks, coat the section below the lower O-ring with a water-based antifouling paint to stop marine growth.
TIP: Keep the plug in a labelled bag next to the thru-hull so it’s always at hand when maintenance calls.
