Drain Plugs: Types, Pre-Launch Checks, Spares and Replacing a Garboard Drain

Take each plug out on land and look at it with the fitting.

• Expanding plugs: tighten until the rubber is firmly pressed against the tube and the plug won't turn or pull out by hand. Over-tightening can split the rubber.
• Threaded plugs: these seal on the threads or on an O-ring. Snug them with the right tool; don't use a pipe wrench or a cheater bar that can crack the fitting.
• Inside or outside? Some fittings take the plug from outside the hull, some from inside, depending on how the drain tube is mounted. Use the side the builder intended, so water pressure pushes the plug into its seat. Owners of boats kept on a lift sometimes want to move the plug inside so they can drain the bilge without wading or paddling to the stern. Whether that's safe depends on the fitting and plug design; ask the builder or a marine tech rather than forcing a plug into a tube it wasn't made for.

Mercury's pre-tow checklist says to keep the drain plug, the boat keys and other critical gear in the tow vehicle so they're not left at home. At the ramp:

• Install the plug in the staging area, before you back down, at the same time as you remove the transom straps. Sea-Doo's launch routine does exactly that: prep area, straps off, plug in, lanyard on.
• Have a second person check it, or touch it yourself one more time right before the trailer goes in.

As soon as the boat floats, open the bilge or engine hatch and look at the drain area. A steady trickle means the plug isn't seated. Don't idle away from the dock with a leak; it's much easier to fix at the trailer. Run the bilge pump briefly to confirm it works too.

If the boat is still on the trailer when you notice water coming in, BoatUS suggests pulling the whole rig back up the ramp and letting the water drain out while you find the plug. Be careful if a lot of water got in: the extra weight can exceed what your tow vehicle can safely pull up the ramp. In that case, pump it out first or get help.

Once the boat is on the trailer and out of the ramp lane:

• Remove the transom plug and let the bilge drain. Mercury's invasive-species tip says to make sure nothing blocks the flow to the drain, and to pull the plugs from livewells and bait wells as well.
• Many states require boats to be drained before leaving the ramp. Colorado's inspection stations, for example, ask boaters to arrive with the bilge plug removed and on hand to put back after the inspection. See our clean, drain and dry guide for state rules.
• Some states go further and make you drive with the plug out. Minnesota's invasive species law (statute 84D.10) says drain plugs and other draining devices for bilges, live wells and ballast tanks must be removed or opened while you transport the boat over land; only the short move in the loading and prep area is excepted. If you trailer across state lines, check each state's rules before you go.
• Put the plug straight back in the tow vehicle, in the same spot every time.

At least each season, and any time it leaks:

• Rubber plugs: look for cracks, hardening, flattening or tears. A rubber body that has taken a "set" may not expand enough to seal.
• Hardware: check the T-handle, lever, wing nut and bolt for corrosion and bends. A lever plug that no longer snaps firmly needs replacing.
• Threaded plugs: look for galled or flattened threads, and check the O-ring or seal if there is one.
• The drain tube or fitting: look for cracks, corrosion pits, a loose flange or old sealant that has pulled away. Brown streaks or wet wood around the drain can mean water is getting into the transom core.

Plugs are cheap. Replace a worn one rather than tightening harder.

A light film of waterproof marine grease on threads or an O-ring keeps a plug from seizing, unless your boat or plug maker calls for something else. Never put adhesive sealant on a removable plug. Owners who've grabbed a tube of sealant in a hurry report plugs that wouldn't come out again, sometimes forcing replacement of the whole fitting. Our marine sealants guide explains why permanent adhesives don't belong on anything you need to remove.

• Expanding plugs: loosen the handle fully, then wiggle and pull. If the rubber has stuck to the tube, work it gently with a plastic tool, not a screwdriver that can scratch the tube.
• Threaded plugs: use the correct wrench square on the head and steady pressure. Penetrating oil can help on threads that are just corroded.
• Stop if the whole fitting starts to turn in the transom, the flange cracks, or the tube moves. Forcing it can tear the seal between the fitting and the hull, and then you have a leak into the transom. That's a job for a pro or the next step.

If you're replacing a fitting or plug, Boating Magazine's guidance on garboard drains:

• Bronze is the traditional, time-tested choice below the waterline.
• Stainless steel looks good and is the better choice on aluminum boats, as long as it's properly isolated from the aluminum. Bronze and aluminum don't get along at all.
• Stainless is more prone to corrosion than bronze underwater.
• Plastic fittings aren't a good choice for a fitting that sits well below the waterline.

Match the plug to the fitting: plugs from different makers often don't seal in each other's fittings.

This is a moderate job on most trailer boats. Do it on land with the boat level and the bilge dry.

1. Measure the hole. Boating says garboard drains are made for holes from about 3/4 inch to 1-1/2 inches, with 1 inch the most common on today's recreational boats. Buy a fitting for your hole size.
2. Check the fit. The flange must have enough flat surface around the hole and must not hang below the bottom of the transom. The tube shouldn't stick out past the inner transom wall, or it will trap water instead of draining it.
3. Remove the old fitting. Take out the fasteners, cut the old sealant with a plastic scraper or knife, and push or tap the fitting out. Clean both surfaces back to bare, dry material.
4. Inspect the hole. If the transom has a wood or foam core, look at the cut edge. Wet, dark or crumbly wood means water has been getting in. Stop and get it assessed (see our transom rot guide) before sealing it up again.
5. Mount it solidly. Boating notes some fittings come with a backing plate for through-bolting, a good idea on any boat and especially one kept in the water. If yours uses screws through the flange, make sure they're long enough to bite solidly into the transom.
6. Bed it generously with a marine sealant rated for below the waterline, on the flange, in the hole and on the fasteners. Tighten evenly so a little sealant squeezes out all around. Let it cure for the full time on the tube before the boat goes in the water.
7. Test it. With the plug in, add a bucket of water to the bilge on the trailer and check outside for drips.

Garboard plugs are small and easy to lose, so Boating suggests having a spare or two. One product it mentions is a bright orange emergency plug that clips to the boat key ring. A practical setup:

• One spare on or with the boat keys
• One in the tow vehicle's glove box
• One aboard, in a spot you can reach quickly (many owners tape it near the drain)

Check that each spare actually fits your fitting.

Pick at least one:

• Keep the plug on the key ring, so the boat can't start without you holding it
• Hang a bright tag on the steering wheel whenever the plug is out
• Store the plug on the trailer winch handle or in the tow vehicle's cup holder, so you see it when you hitch up
• Put "PLUG" first on a laminated launch checklist

• Trailer and dry storage: Mercury's winter storage advice is to pull the drain plug(s) and keep the bow slightly raised so no water pools. Discover Boating's winterizing guide also says to remove drain plugs on dry land and make sure the boat is positioned to drain.
• Boat lifts: BoatUS describes a winterized boat left on a lift with the transom plug in; it filled with water and the lift collapsed. Rain alone can do the same in summer if the plugs are in or the drains are clogged.
• Personal watercraft: advice varies. Yamaha's post-operation checks say to keep the drain plug out while the craft is on land, and Sea-Doo's launch routine has you loosen the plugs after you pull out. Follow your manual.

Wherever the plug is out, the reminder from step 11 should be on the boat.

After storage, the plug goes in as the first step of spring commissioning or your next launch, not the last. Look at it and the fitting closely (step 5) before you do.