Flooding and Damage Control
Water coming in faster than it goes out ends voyages. Knowing where to look and how to stop it buys you time.
Finding the Source
When you discover water in the bilge at a rate that concerns you, the priority sequence is: identify the source, reduce the inflow, then remove what has already entered. Pumping while the source is unrestricted is a losing battle.
Common flooding sources:
- Seacocks: Failing ball valves, cracked housings, or hose failures at the through-hull connection
- Stuffing box / shaft seal: The fitting where the propeller shaft exits the hull. A slight drip is normal and deliberately maintained for lubrication. A steady stream is not normal — the packing gland needs immediate tightening.
- Keel bolts: In a grounding or collision, keel bolts can work loose, allowing water entry. Often difficult to access, but a wet area around the keel sump is the indicator.
- Hose connections: The hose connecting a seacock to a head, sink, or engine intake can fail at the clamp or split along its length. Inspect all hose connections annually.
- Hull damage: Collision or grounding damage can stave a section of hull. In fibreglass boats this is usually localised; in steel or aluminium it may be a crumpled plate.
Triage: Get below with a torch. Move systematically from bow to stern. A calm, systematic search locates most leaks within two minutes. The person checking the bilge must report clearly — rate (drip vs stream vs flood) and location.
Before every offshore passage, physically operate every seacock. They must move freely — a seized seacock that cannot be closed provides no protection. Annual lubrication with waterproof grease prevents seizure.
You discover a steady stream (not a drip) from the stuffing box at the propeller shaft. What is the correct response?
Seacocks and Wooden Bungs
Every through-hull fitting below the waterline must have a seacock — a ball valve that can close the opening from inside the boat. This is not a recommendation; it is fundamental to the boat's damage control capability.
How a seacock works: A bronze or polymer ball valve with a lever handle. The lever inline with the pipe = open; perpendicular = closed. Know which direction is which before you need it.
The wooden bung: A tapered wooden plug, one per seacock, attached to the seacock body with a lanyard. If the seacock fails — the housing cracks, the ball corrodes stuck, the hose attachment fails — the wooden bung is driven into the through-hull opening from inside, blocking the water flow. It must be immediately accessible and sized to fit.
Seacock locations on a typical sailboat: Engine raw water intake; engine raw water discharge; head intake and discharge (often two or three fittings); cockpit drain (one or two); galley sink drain; depth transducer housing; speed log impeller; livewells or other deck drains. Count them. Know where they all are.
Annual maintenance: Exercise every seacock quarterly at minimum. Lubricate the ball valve with waterproof grease annually. Bronze seacocks last decades with maintenance; neglected ones seize and are useless in an emergency. Composite (glass-reinforced nylon) seacocks are increasingly common — they don't corrode but can crack.
Closing all seacocks in port: If leaving the boat unattended for extended periods, closing all seacocks is prudent. A failed hose connection below the waterline, undetected for days, can sink a boat.
Label every seacock with a marine label (permanent marker on stainless tag or laminated card) indicating what it serves. In a flooding emergency, crew who are not familiar with the boat can close the correct seacock immediately without guessing.
The seacock serving the head intake has cracked and water is entering faster than the ball valve can stop it. What is the immediate action?
Pumps and Emergency Plugging
Once the source is identified and seacocks are closed, pumping begins. Understanding pump capacity versus flooding rate determines whether you have time to repair or must call for help.
Pump capacities: A quality manual bilge pump (Whale Gusher, Henderson) moves approximately 15–25 litres per minute under sustained effort. An electric bilge pump (Rule 1500, typical installed size) moves approximately 25–40 litres per minute. A high-capacity electric pump (Rule 3700) moves approximately 60 litres per minute.
Flooding rates: The rate of inflow through a breach depends on the opening area and the depth below the waterline (water pressure). A 25mm diameter opening at 0.5m depth admits approximately 40 litres per minute. A 50mm opening at 0.5m admits approximately 160 litres per minute — more than three standard electric pumps can remove. You must stop the flooding, not just pump around it.
Emergency plugging materials: Tapered wooden bungs (pre-sized to each through-hull); underwater-setting epoxy putty (works in contact with water); closed-cell foam; rags and a wedge mallet; underwater repair tape (temporary). The philosophy is layers: a bung stops the immediate flow; epoxy or foam backing holds it; traditional damage control timber wedged against it provides final support.
External collision mat: A piece of heavy canvas or sail material deployed over the hull exterior, held in position by lines run under the bow and aft — called a collision mat or fother. Water pressure presses it against the hull, reducing inflow. Used when internal plugging cannot be reached or when the breach is on the outside of a structural member. Effective for moderate breaches in calm to moderate conditions.
Pump capacity calculations show that a 50mm breach through-hull at depth exceeds the capacity of all but the largest commercial dewatering pumps. If flooding is faster than pumps can manage, stop the flooding physically or call MAYDAY — you are not pumping your way out of a large uncontrolled breach.
Your electric bilge pump (40 L/min capacity) cannot keep pace with flooding. What does this tell you?
Collision and Grounding Damage
Collision and grounding introduce different challenges to a simple through-hull failure because the breach may be in an inaccessible location, the damage may be structural, and refloating a grounded vessel may worsen the flooding.
Collision holing: Identify the breach location from inside. If the hole is accessible, internal plugging is the priority. If above the waterline, the boat may be heeled toward the damaged side to lift the breach clear — this trades stability for damage control. A collision mat deployed externally can reduce inflow for a breach that cannot be reached from inside.
Grounding: In a grounding, the keel or hull has impacted the seabed. Assess the structural damage before attempting to reflot. A boat that is holed and grounded in shoal water is effectively plugged by the seabed — refloating it may open the breach to rapid flooding. Confirm the integrity of the hull before powering off the ground.
The heeling trick: On a monohull, heeling the boat using crew weight, a halyard taken to an anchor, or the spinnaker halyard attached to the masthead can lift a breach above the waterline. Even 10–15 degrees of heel can change the depth of a breach from 0.3m to above the waterline — reducing inflow from 'serious' to 'manageable'.
Calling MAYDAY timing: Call MAYDAY as soon as you assess that the flooding is not immediately controllable. Do not wait until the situation is critical — SAR response takes time. An early MAYDAY call with a strong control position gives you options. A late MAYDAY call from a sinking boat gives you fewer.
Carry underwater repair epoxy putty aboard. It sets in contact with water and is effective for sealing small cracks and failed hose connections from inside the boat. A 500g pack is small, cheap, and has saved boats that wooden bungs alone could not.
A boat grounds in shallow water and the hull is breached. The crew wants to power off the ground immediately. What risk does this create?
Summary
Identify the flooding source before pumping — pumping without stopping the inflow is ineffective for large breaches.
Every through-hull must have an operable seacock and an attached wooden bung sized for that fitting.
Know the location of every seacock before departure — label them and exercise them quarterly.
Pump capacity typically cannot overcome large uncontrolled breaches — physical plugging is the solution.
On a grounded vessel with a hull breach, the seabed may be plugging the hole — assess before refloating.
Key Terms
- Seacock
- A ball valve on every through-hull fitting below the waterline, allowing the opening to be closed from inside
- Wooden bung
- A tapered wooden plug, attached to each seacock, used to block the through-hull opening if the seacock fails
- Stuffing box
- The fitting where the propeller shaft exits the hull — deliberately maintained at a slight drip for lubrication; a stream indicates failure
- Collision mat
- Heavy canvas or sail cloth deployed externally over a hull breach — water pressure holds it against the hull, reducing inflow
- Through-hull
- A fitting that penetrates the hull below the waterline to allow water intake or discharge for engine cooling, heads, or drains
Flooding and Damage Control Quiz
Water is entering the bilge at an increasing rate. What is the correct priority sequence?
What is the purpose of a wooden bung attached to each seacock?
Your single electric bilge pump can move 40 L/min. The flooding rate is estimated at 120 L/min. What should you do?
A crew member reports water around the keel sump after a hard grounding. What might this indicate?
You have a small breach (25mm) just below the waterline that cannot be reached internally. What technique might reduce the inflow?
References & Resources
Related Links
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BoatUS Foundation — Damage Control
Practical guidance on bilge pumps, seacocks, and emergency flooding management