How to Clear a Clogged Pickup or Drain Thru-Hull

My washdown and livewell pickups lost suction — how do I clear a clogged thru-hull?

When a washdown or livewell pump suddenly loses suction, the blockage is almost always at the very start of the water path — the thru-hull scoop or its strainer — not the pump itself. Weed, a plastic bag, barnacles, or a slug of mud/silt caps the opening so the pump just sucks air. The fix is to find and clear that opening (and the inline strainer), confirm the seacock works, then prime the pump. Below the waterline this is a sinking-risk job, so treat any open seacock with respect and keep your matching soft-wood bung/plug handy.

ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.

💵 $0-$40 DIY (new strainer O-ring, hose clamps, a length of wire-reinforced marine hose); $90-$250 at a marine shop for diagnosis and clearing; $150-$400+ if a haul-out or diver is needed to reach a below-waterline thru-hull. ⏱ 30-60 minutes for a strainer/scoop cleanout and re-prime; add time if you back-flush hoses or need a haul-out. ● Use caution

Safety: Any thru-hull below the waterline is a sinking hazard: opening a hose or strainer with the seacock open can flood the boat fast, so keep the seacock closed while working inside and have the matching soft-wood bung/plug at hand. Only open the seacock with a hose disconnected when the boat is hauled or on the trailer. Don't run pumps dry. If you go in the water to clear a scoop, never enter the water near AC shore power — electric shock drowning (ESD) can incapacitate a swimmer; cut shore power at the dock pedestal and have a spotter. In engine and fuel spaces, keep to ignition-protected components and avoid creating sparks. After the job, double-clamp below-waterline connections and confirm no leaks before leaving the boat unattended.

Common causes

Debris over the thru-hull scoop or intake — eelgrass, a plastic bag, leaves, or sand/mud sucked against the opening when running shallow or beaching (most common)
Quick check:
Clogged inline raw-water strainer (sea strainer) basket between the thru-hull and the pump (common)
Quick check:
Marine growth — barnacles, slime, or hard scale partially blocking the thru-hull and scoop, especially on a boat kept in the water (common)
Quick check:
Air leak on the suction side (loose hose clamp, cracked hose, bad strainer-lid O-ring) that mimics a clog by letting the pump lose prime (less common)
Quick check:
Closed or seized seacock, or a collapsed/kinked suction hose (less common)
Quick check:

How to fix it

Confirm the symptom and protect against sinking first. If the intake is below the waterline, locate the seacock and its emergency bung/plug before you open anything. Work at the dock or on the trailer, not while running. Turn the pump off so you are not running it dry.
Check the easy stuff before touching the thru-hull: is the seacock actually open (handle/lever in line with the hose = open, crossways = closed), is the suction hose kinked or collapsed, and are the hose clamps tight? A loose clamp or cracked hose on the suction side lets the pump pull air and feels exactly like a clog.
Clean the inline raw-water strainer. Close the seacock first, then unscrew the clear bowl or lift the basket, pull out weed/debris, rinse the basket, inspect the lid O-ring or gasket, and reassemble. A dried-out or pinched O-ring causes an air leak — replace it with the correct marine part if it looks bad. Reopen the seacock and watch for leaks before you walk away.
Clear the thru-hull opening from outside. With the boat on the trailer or hauled, look at the scoop/intake. Pick out weed and debris by hand, and gently ream the opening with a soft plastic rod, zip tie, or wood dowel — do not gouge bronze/Marelon with metal. Scrape off barnacles and growth around the scoop. Avoid forcing a stiff wire deep into the hose where it can puncture it.
Back-flush the line — do this with the boat hauled or on the trailer, where an open thru-hull cannot flood the boat. Disconnect the suction hose at the strainer or pump, open the seacock, and push fresh water (garden hose) or low-pressure air backward through the seacock so debris blows out the thru-hull. Keep pressure low so you don't blow a hose off or split it. Close the seacock again before relaunching, then reconnect and clamp the hose.
If the boat is in the water and you cannot haul it, do NOT back-flush through an open seacock from inside — with the hose disconnected an open seacock will flood the boat fast. Clear the scoop by hand from the dock if you can safely reach it, or call a diver / plan a haul-out rather than poking blindly below the waterline.
Reassemble, then prime the pump. Open the seacock, make sure the strainer bowl and all clamps are tight, and prime per your pump's manual — washdown diaphragm pumps are typically self-priming only over a short lift, and submersible livewell/aerator pumps must be flooded (mounted below the water level), so keep the suction run short and flooded for faster prime. Run the pump and confirm steady flow with no air spitting.
Verify and re-check for leaks. With water flowing, inspect every joint you opened for drips, confirm double clamps on below-waterline hose ends where the barb is long enough (ABYC H-27 practice), and make sure the seacock operates smoothly. If suction still drops off, suspect a worn pump impeller or a hidden suction-side air leak rather than the thru-hull.
Use only marine-rated parts. Replace any suspect suction hose with wire-reinforced (helix) marine raw-water hose rated for below-waterline suction so it won't collapse under vacuum — do not use wet-exhaust hose for an intake. Use corrosion-resistant 300-series stainless clamps (two per below-waterline joint where the hose end allows), and keep bronze or Marelon thru-hull/seacock hardware matched — never mix dissimilar metals or substitute a gate valve for a proper seacock.

DIY or call a pro?

Clearing weed from a strainer or scoop and re-priming is a straightforward DIY job for a competent owner with the boat on a trailer or at the dock. Call a pro (or a diver/haul-out) when the thru-hull is below the waterline and you can't clear it from above, when the seacock is seized or weeping, or when you suspect a cracked thru-hull, failing pump, or a leak you can't stop.

Tools & parts

Soft plastic rod, wood dowel, or zip ties for clearing the scoop (no metal probes into hoses)
Replacement strainer lid O-ring/gasket (correct size for your sea strainer)
Corrosion-resistant 300-series stainless steel hose clamps (two per below-waterline joint where the hose end allows)
Wire-reinforced (helix) marine raw-water suction hose to match diameter if a hose is cracked or collapsed
Screwdriver and nut driver for clamps and strainer bowl
Garden hose or low-pressure air for back-flushing (boat hauled/on trailer only)
Plastic scraper or putty knife for barnacles/growth
Emergency soft-wood bung/plug sized to the thru-hull (keep aboard regardless)
Bucket and rags
Pump manufacturer's manual for priming/lift specs

Keep a record of every fix you make — what broke, what it cost, how you solved it.

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Based on: BoatUS / BoatUS Foundation; ABYC (American Boat & Yacht Council) — H-27 seacocks/thru-hulls; USCG / USCG Auxiliary; NMMA (National Marine Manufacturers Association); Engine and pump maker service guidance (e.g., Mercury, Yamaha, Volvo Penta, Jabsco/Johnson Pump)

General marine-maintenance guidance, not a substitute for a qualified marine technician or surveyor. Boats and conditions vary; for fuel, electrical, fire, or structural issues — or anything safety-critical — consult a professional. Always follow your engine and equipment manuals.