Skip Links

Facing A Flooding Emergency

Uncontrolled flooding is one of the more insidious emergencies a boater can face. Here's how to both improve your odds and give your boat a fighting chance.

Dramatic hull breach

This dramatic hull breach is beyond what can be managed with ­standard damage control. (Photo: Scott Croft)

Many boat owners purchase equipment and train to save themselves or those onboard, however, aside from a few wooden plugs and a vague plan of action, very few are adequately prepared to mount a credible attempt to save the boat itself should flooding due to hull breach occur.

Boats And Holes

My first introduction to "Damage Control Basics" was as a newly minted Coast Guard Seaman attending training aboard the Navy's "USS Buttercup" (look it up!). It was an eye-opening experience that continues to guide my own damage-control philosophy some 30 years later.

As with any emergency situation, advance preparation is the key to dealing with a hull breach. It should include everything from planning for possible causes of flooding to assembly of a proper damage-control kit.

Sinking powerboat off the coast of Hawaii

Three people were rescued from this sinking 45-foot boat off the coast of Hawaii. (Photo: U.S. Coast Guard)

Damage control (DC) itself can be categorized into a number of steps, from prevention and preparation to damage assessment, control, and temporary repairs. Topics such as proper and timely distress signaling, temporary repairs, emergency pumps, and rescue are also important. But the focus here is to find, access, and stop (or at least mitigate) the source of uncontrolled water entry into the hull. The focus is on fiberglass hulls, although many of the techniques can also be applied to other vessels.

Discover A Leak Or Breach?

Do these first:

  • Stop forward way. Shift weight to elevate leak if this is safe and practical, and you know where it is.
  • Listen carefully and quickly inspect. Take needed safety precautions including donning life jackets and making a distress call or signal.

Time Is Of The Essence

Early detection is crucial when dealing with flooding. The more time you have to address the problem, the better chances your boat (and possibly you) will survive. Precautions such as a high-water bilge alarm system and bilge pump "ON" indicators at each helm position will provide the maximum benefit in this regard, however, those should be part of your preparedness strategy. As mentioned earlier, our purpose here is to highlight actions to consider when a hull breach occurs.

The first goal in a flooding situation is to find the leak and stop or at least bring it under control. Without firsthand experience, it's almost impossible for the average boater to imagine how quickly flooding can occur, even during a relatively small leak (such as a ruptured hose or damaged thru-hull fitting). More water in the boat can make a leak harder to find in the first place, another reason why time is so critical.

It's also important to note that the added weight of flood water (each gallon of saltwater weighs approximately 8.5 pounds) can negatively affect vessel stability and operation. This additional weight may also give you an early sign that you are taking on water, as the boat becomes more sluggish, harder to steer, slower, or behaves differently.

Boat Flooding Rate In Gallon Per Minute

Depth of hole
below waterline
Diameter of opening or hole
1 in 1.5 in 2 in 2.5 in 3 in 3.5 in 4 in
1 ft 19.4 43.8 77.9 121.7 175.3 238.6 311.6
2 ft 27.8 62.5 111.1 173.6 250 340.2 444.4
3 ft 33.9 76.3 135.7 212 305.3 415.6 542.8
4 ft 39.3 88.4 157.1 245.3 353.5 481.2 628.4
Source: U.S. Coast Guard Auxiliary, National Response Directorate

Hull Breaches

Hull breaches can be divided into two broad categories: structural breaches and equipment failure below the waterline. A structural breach of the hull typically occurs as a result of impact damage from grounding or collision with an object, although failure of the hull due to other reasons (severe delamination, broken stringers, inadequate design) can also happen.

Equipment failure below the waterline

Equipment failure below the waterline (such as this dripless shaft seal) can sink a boat in minutes.  (Photo: BoatUS)

Flooding due to equipment failure can include anything from a broken seacock to a ruptured hose. While equipment failure can often be traced to improper installation or lack of maintenance, there can also be some crossover between the two, such as rudder loss or sterndrive damage due to grounding.

Assembling A DC Kit

The two things you need to tackle a hull breach are a cool head and a well-thought-out DC kit. From shower curtains and beer koozies to T-shirts or throw pillows held in place with your foot, most every boat will have something onboard that can be used for damage control in a pinch. While such repairs on the fly can be effective, why risk being unable to MacGyver a solution in an emergency when you can easily buy or assemble a proper DC kit beforehand?

Your kit should be tailored to meet the needs of your particular boat. A small runabout will obviously have neither the space nor need to carry the same kit as an offshore cruising vessel. All boats should carry some form of DC kit however, which should be stored in a readily accessible, dedicated bag or container labeled "Damage Control Kit." There's no hard and fast rule of what it should contain, however, the time to think about it is now. The list below is a good starting point for assembling your own kit.

Damage Control Kit

  • Two or three waterproof flashlights (a mix of handheld and head lamps)
  • Hatchet or small ax (to chop away interior furnishings or split wood wedges)
  • Heavy hammer or maul
  • Folding hand saw
  • Utility knife
  • Pry bar
  • Marine caulk (such as Boat-Life Life Caulk) that cures fast and bonds underwater
  • Tapered soft wooden plugs (various sizes)
  • Soft wooden wedges (various sizes)
  • Foam emergency plug (various sizes)
  • Screwdrivers (various sizes)
  • Stainless steel mousing (seizing) wire
  • Duct tape. Is any type of kit really complete without it? The Gorilla brand works well
  • Butyl tape
  • Self-fusing silicone tape (e.g., Rescue Tape®)
  • Grease tape
  • Can of expanding spray foam (great for filling voids and gaps around wood patches)
  • Sheets of rubber or rubberized cloth, various sized squares (old raincoat sleeves and pant legs, or cut tire inner tube works well)
  • Hose clamps (various sizes)
  • C-clamps
  • Duct Seal Compound (aka Monkey Poop)
  • Wax toilet bowl ring (works well to seal gaps around shafts and glands)
  • Twine and/or paracord
  • Wire ties (assorted sizes and lengths)
  • A couple two-by-fours
  • One sheet of thin (1/2- or 5/16-inch) plywood, precut to various sized squares. Slightly larger, precut squares of thick felt or thin foam rubber gaskets will help wood patches seal against the hull
  • Nails
  • Square-drive, self-tapping or self-drilling, rust-resistant screws along with a matching hand driver or battery-operated drill with driver extension (to secure wood patches to the hull)
  • Utility snips
  • Underwater epoxy putty or fiberglass repair kit*

*Note: While they have their place in a DC kit, epoxy compounds or fiberglass repair kits will be of no use for an immediate, stop-the-flooding solution. For starters, just because they cure underwater doesn't mean they'll bond. Most will not stick to a wet surface (especially a hole that's under pressure). The surfaces to be bonded must also be completely immobile (epoxy can't bond surfaces that are flexing or moving in relationship to each other). Cure time can also be greatly affected by temperature (colder water can add days to advertised cure time).

Access Is Crucial

In a perfect world, a hull breach would never happen. But if it did, it would be in an easily accessible location. With adequate hull access, a hole can more readily be found and hopefully stopped by something as simple as a plug, plywood patch, or whatever method resourcefulness provides.

Regardless of the training, equipment, and ingenuity a boat owner possesses, it all amounts to nothing if a leak can't be found and accessed. Fiberglass boats can be notoriously bad when it comes to providing even minimal internal access to the hull. The use of one-piece pans and liners almost guarantees a crew dealing with a structural hull breach will be faced with water gushing from multiple holes for wire runs, piping, or ducting and no clear idea of just where the damage is located.

The best option in such cases will likely be battlefield-style surgery to remove internal furnishings and cut access holes into the liner in efforts to expose the hull and find the breach — one reason having the proper tools (saws, pry bars, axes) readily available when needed is imperative.

Despite your best efforts, access to a leak may be impossible, due to either location or a leak being already underwater by the time a problem is discovered. In such cases, isolating the space to prevent flooding of the entire vessel is a preferred option, although only for boats with watertight bulkheads (something not found on the vast majority of recreational vessels).

Another possible option might be gently beaching the boat, assuming the hull configuration is amenable to it, it is safe to do so, and there is a nearby soft (sand or mud) bottom. Just keep in mind that beaching the boat may increase incoming waterflow, something that must be considered against the benefits of a successful beaching.

Sinkings By Cause

We delved into the GEICO claims files to find out the top causes of sinkings in 2021. Here's what we learned:

Boat sinking by cause pie chart

Realistic Assessment Of Damage

Once the leak is found, next up is a realistic assessment of the damage or nature of flooding, as well as the odds of successfully addressing it. In some cases, flooding may be stopped by simply closing a seacock. Others (such as a structural breach) will likely not be as easy. While the hope when tackling a hull breach is that it can be completely sealed, the reduction of flooding to a manageable leak is the most likely scenario. The difference between the two is that a leak can be managed by bilge or emergency pumps, while flooding cannot and will eventually lead to sinking unless addressed.

In addition to ascertaining the extent of damage, another part of this initial assessment is danger to the crew should they attempt a repair. Damage rarely occurs under ideal conditions, which means other factors (darkness, heavy weather, loss of power, poor access, rate of flooding) must also be considered when making the decision to attempt repairs or make preparations for abandoning ship.

Suspect Areas

Unless you've struck something, run aground, or know where the breach is located, the first places to consider are all hull penetrations below the waterline, such as seacocks or glands for the shaft or rudder. Other "weak" areas to check include shaft log hoses (the section of hose joining the shaft log to the shaft gland or stuffing box), hose clamps, as well as hoses located in out-of-the-way areas (toilet intake, sanitation discharge hoses, cockpit drains, and the like) that may not get looked at on a regular basis (though they should be).

Extensive hull damage

 The extent of hull damage will dictate your DC response. (Photo: Bob Stefanowicz)

If you have struck an object or grounded, then that would dictate where the initial search would begin. Pull up floorboards, open hatches, and clear lockers in efforts to determine where the leak is coming from.

Flooding Fun Facts

Well, they're not really fun, but they are facts you should know.

  • The volume of water that enters a boat is proportional to the square root of the depth of the hole and the square of the hole's radius.
  • A 2-inch hole that is 1 foot below the waterline can fill a 55-gallon drum in 42.4 seconds.
  • A 1-inch hole that is 2 feet below the water line is forcing your boat's weight to increase at 240 pounds per minute or 4 pounds per second.
  • As the boat fills with water, the effective hole depth increases, forcing faster water ingress.

Controlling The Flood

Once the damage has been evaluated and a determination made that it's safe to proceed with repairs, now's the time to apply your training, preparation, and ingenuity. Impact damage to a fiberglass hull will likely result in a jagged, uneven split (rather than a nice, round hole) so don't be afraid to use your imagination — that baking sheet slathered with caulk and screwed into place may be the perfect patch.

You also don't need to worry about neatness or how pretty the repair looks. It's only temporary and just needs to be effective at stopping the leak. Once the flooding is stopped, you can always go back and improve the strength of the repair if needed.

Foam or expansion-style plugs can be jammed into smaller holes. Stuffing rubber sheeting, towels or rags coated with marine caulk, or similar items into them can also be effective. These can also be wrapped around wooden plugs or wedges before driving them into place, which helps fill the hole, provides a better seal, and keeps the plug or wedge in place.

Bag of various sizes of soft wooden plugs and wedges

DC kit basics include various sizes of soft wooden plugs and wedges. (Photo: Frank Lanier)

Wooden plugs and wedges should be made from soft woods, such as pine or fir. Soft wood wedges can be more easily split to fit different hole sizes and more readily conform to irregular hole shapes. Soft wood also compresses when driven into a breach and will absorb water and swell, improving the seal. You can also use multiple plugs or wedges to seal large or irregularly shaped holes.

Items such as a cushion or mattress can be used to plug larger holes, as can plywood patches, which can be nailed or screwed in place. As water will be entering the hull under pressure, plugs and patches may also need to be shored or braced into place. That's the purpose of the aforementioned two-by-fours. However, free your mind and think outside the box when the need arises — a boat hook, table leg, or even a sturdy shower curtain rod could also be used in a pinch.

In some cases, a collision mat, sail, piece of canvas, or even a heavy trash bag (if strong enough not to be sucked in) can be lowered over the side and held into place by straps and water pressure. While this type of emergency repair may be worth a shot in certain cases (a large breach or one that can't be located for example) my personal preference is to concentrate on repairs that can be attempted from inside the hull. The effectiveness of collision mats or similar items can be hampered by hull shape, heavy seas, or even location of the breach itself. Another downside is that effective deployment will also likely require that someone enter the water to place and secure the mat into position, an undesirable option under the best of circumstances.

A split or ruptured hose or pipe can be wrapped with tape or rubber strips secured in place with hose clamps, twine, or wire ties, while a failed seacock or thru-hull can be sealed with a plug.

It's also good to remember that sink drains and other thru-hulls above the waterline can become submerged due to flooding, possibly allowing water to back siphon into the hull if not plugged or otherwise secured.

As with all safety training, the time to learn damage control techniques is before they're needed. Planning and preparation beforehand will help ensure both you and your boat have a fighting chance should the need arise.

Related Articles


Click to explore related articles

safety and prevention boat safety


Frank Lanier

Contributing Editor, BoatUS Magazine

Capt. Frank Lanier is a SAMS Accredited Marine Surveyor with more than 40 years of experience in the marine and diving industries. He’s also an author, public speaker, and multiple award-winning journalist whose articles on boat maintenance, repair, and seamanship appear regularly in numerous marine publications worldwide. He can be reached via his YouTube channel “Everything Boats with Capt. Frank Lanier” and website