What makes a boat truly unsinkable? The following all come into play:

  1. Flooding Rate - the rate at which water enters the boat.

  2. Pumping Rate - the rate at which water exits the boat.

  3. Watertight Bulkheads - Bulkheads creating watertight compartments that will remain buyant when other watertight compartments of the boat flood.

  4. Foam Floatation - Either built into the boat as a core or added to areas of dead space, to add floatation value.

  5. Float Bags - Placed in comaprtments where they can be remotely inflated with CO2.

  6. Margin Lines - A boat's margin line indicated the critical point at which, when flooded beyond, it will no longer float.

When seawater starts gushing into a hull, lacking certain technical modifications to the boat, the question will not be how to stop the flow of water, but rather how to deploy the life raft and bailout bag and save the crew. A professional mariner tells how you might avoid this fate.

coast guard boat

Are any boats truly unsinkable? Photo courtesy of the US Coast Guard.

Venturing across the world's oceans has an allure unmatched by few other experiences, but this positive exhilaration can be quickly replaced by a sense of peril if a boat begins to flood. Sinking is probably every sailor's worst fear. Early one morning last year, the crew of a 60-foot sailboat crossing the Atlantic was stunned when it found water rising rapidly inside. St. John's, Newfoundland was the closest harbor of refuge, but it was several hundred miles away, so the crew worked quickly to find the source of flooding. There were only a few through-hulls to check, each of which felt intact, yet the water level inside the boat continued to rise mysteriously.

Not being able to find and stop the flooding, the boat called for assistance and, providentially, its distress signal was heard by a passing merchant ship, which provided a large diesel salvage pump. Fortunately, the pump started, lowering the water level and exposing the flooding's cause. An unsecured battery had knocked against a through-hull fitting and dislodged its hose.

This recently built sailboat, equipped with all the necessary safety gear and built with several watertight bulkheads, was on its first ocean passage, and yet it had come close to sinking or, at the very least, being abandoned. Later, the crew remarked that a combination of inability to slow or stop the ingress of cold water and crew exhaustion would surely have led to the yacht sinking had not a merchant ship with a pump been nearby.

Many similar incidents have been documented in which a boat has been lost or abandoned in the face of flooding. In the 1994-95 BOC Challenge singlehanded around-the-world race (now called the Around Alone), a 60-foot aluminum cutter hit an object that penetrated two of its watertight compartments, resulting in massive flooding that sank the boat in a matter of hours. The boat's skipper, an experienced and able sailor, was unable to stem the flow of water, but - again fortunately - he was rescued by a fellow competitor.

Flooding Rates

Can a boat be made unsinkable? Yes, but it takes some planning and critical attention to engineering details. Boats sink because they fill with water. And given the smallest opening, water will enter and fill a boat at a mind-boggling rate. Because water can fill and sink boat extremely quickly, procedures for handling flooding must be fast and simple.

A quick review of water pressure and flooding are in order. Flooding rates for various size holes at selected depths are calculated using this formula:
Q = 20 x d x square-root of h
Q = flooding rate in gpm
d = diameter of hole in inches
h = depth of hole underwater in feet

Water exerts about a half-pound of pressure per square-inch for each foot of depth. At a depth of three feet - the distance, for example, over an engine intake - water pressure equals 1.5 pounds per square inch (1.5 psi). What does this tell us? It indicates that a hole just 1.0" in diameter at this depth will pass 34 gallons of water in a single minute. Over an hour, that amounts to 2,040 gallons.

Consider a standard 40-foot cruising boat: Its internal volume is approximately 1,400 cubic feet - (40' by 10' by 5') x 70% = 1,400 cubic feet. (The 70% is a rough approximation of the hull shape's share of the 40-foot-long cube.) If water starts flooding this boat through a 2.0" diameter opening (an engine through-hull for example), two feet below the waterline, the flow rate will be approximately 110 gallons per minute. A gallon of water occupies .1337 cubic feet, so 110 gallons per minute is equal to 14 cubic feet per minute. So 1,400 cubic feet of boat volume will be filled in 100 minutes, or just over an hour and a half.

Note: If the water entered the boat at a constant rate, the boat would, in theory, sink faster than that - perhaps in half that time. However, the flooding rate of a boat will slow down considerably when the water level rises above the leak. In this case, the "h" in the flooding formula becomes the difference in level between the water inside the hull and the water outside it. Additionally, any air in the cabin will provide back-pressure to further slow the flow. For this reason, it's best (and safer, too) to close all hatches and stay on deck aboard a sinking boat.

If a crew becomes aware of the leak at the precise moment the flooding begins - if everyone has his wits about him, and the incident does not occur in the middle of the night or in cold water - then stopping the inflow may be possible. But what if the flow starts without a witness? What if flooding is only noticed when water floats the cabin-sole floorboards? Then what? Where do you start looking? What do you do when through-hull fittings are farther below the flooding water's surface than your arm is long? What if it is night and none of your waterproof flashlights work? What if the water is so cold that your extremities become numb?

Use of sails, plugs, collision mats, and the like are all legitimate methods of stopping a flood only if certain initial conditions are present:

  • The crew is trained, and has practiced rigging and using damage-control equipment. I have only seen such training performed in the Navy, Coast Guard, and Merchant Marine.

  • Flooding is witnessed by a crew and immediate action is taken.

  • Environmental conditions are friendly: The water is warm; it is daylight; the seas are benign. Imagine attempting to find a source of flooding while waist deep in 50? water during a dark night when a boat is pitching and yawing.

  • A crew is of sufficient size to attack the problem. It is difficult to put out a four-alarm fire with a couple of firefighters.

If you cannot see the flooding and are not certain where to look, all the damage-control gear in the world is not going to be of much help. So what are possible solutions to this ever-present concern about flooding and sinking? They are a combination of four design considerations: (1) flotation bags, (2) watertight bulkheads (vertical and horizontal), (3) rigid foam, and (4) pumping.

Since there are many examples of boats having one or more of these features and still sinking, what is the problem or common thread? Usually, upon close examination, such sinkings have simple explanations - failure to immediately locate and stop flooding, watertight doors left open, too few watertight bulkheads, little or no rigid foam, no inflatable flotation bags, and deficient pumping capacity for the existing flooding rate.

When flooding is detected, actions to stop water ingress need to be immediate and decisive. Actions that are less than immediate and decisive result in the volume of incoming water quickly overwhelming any subsequent actions.


A boat's pumps are installed only to remove accumulated water once the source of that water has stopped. Pumps are not designed to keep ahead of a steady flood. A comparison of flooding rates versus pumping rates quickly reveals that no reasonable small-boat pump can stay ahead of serious flooding. Though many reliable pumps are available for keeping bilges dry and removing unwanted water from shower sumps and anchor wells, aside from large diesel-powered salvage pumps no on-board marine pumps can stay ahead of water gushing through a ruptured through-hull fitting or stuffing box.

Bilge pumps are rated on their ability to pump water horizontally, not vertically. For example, a pump rated at 50 gallons per minute (gpm) indicates a horizontal movement of 50 gallons. Any vertical lift reduces a pump's capacity dramatically. Note that many pumps rate their capacity in gallons per hour, not minute. Remember that a two-inch opening two feet below the waterline will let in 110 gallons per minute, which equals 6,600 gallons an hour! A high-capacity 50-gallon-per-minute pump is going to move 3,000 gallons per hour - only half the incoming volume. The largest hand pumps on the market move one gallon per stroke and are rated at between 30 and 42 gallons per minute. Try pumping for an hour.

Pacer Pumps sells a 380-gpm 3"-diameter "trash pump" that is lightweight (about 112 pounds), extremely robust, and relatively inexpensive. It can run at this capacity pulling from a six-foot head using only 8 HP. This pump has been used extensively in single-handed Vende-Globe Challenge and BOC/Around Alone boats for years as a water-ballast pump. The Pacer is powerful enough that, when used with insufficient venting on the tank, it has been known to literally blow the tank off the hull. These pumps can be belt-driven off the main engine, using a standard air-conditioning clutch from any American car. However, it needs the main engine to be functional in order to work. Simply stated, if the main engine is flooded, the pump can't run. Also, if such a pump is activated via a solenoid, flooding may short-circuit the electrical connection, rendering the pump inoperable. And these pumps are not fast-activating, for access to the engine must be gained to engage the clutch.

dewatering pump

Dewatering pumps, like this one carried by the US Coast Guard, can help in a sinking situation - but aren't usually the ultimate answer. Photo courtesy of the US Coast Guard.

Though every boat should have several large pumps aboard, when their pumping rate is compared to flooding rates, from even the smallest openings, it becomes apparent that pumps are not the only answer to staying afloat during a serious flooding situation. Pumps will slow flooding and will remove flood water once a flood has been stopped, which is their proper application, but they are not intended to stay ahead of an inflow of flood water.

Watertight Bulkheads

Effective watertight bulkheads need to be ruggedly built and sufficient in number and spacing. Ideally, they are installed during construction and designed with a minimum of wire and hose penetrations. Necessary penetrations should be positioned as high as possible on the bulkhead to reduce water pressure on these openings should flooding occur. Access through watertight bulkheads should be provided by hatches that are truly watertight when closed, and able to withstand pressure from either side. Watertight bulkheads are a valuable addition to a boat for both strength and survivability, and they reduce noise and contain heat in living areas when sailing in cool waters and climates.

Spacing of watertight bulkheads is critical; it is the size of the spaces sectioned off that matters most in their performance. For example, if a vessel is built with two watertight bulkheads, dividing the hull into thirds, the size of each compartment should be nearly equal. In this way, if any one of the three is completely flooded, the other two will remain intact and be of sufficient volume to keep the vessel afloat. An example of an inadequate design might be a small forward compartment and large aft compartment separated by a watertight bulkhead. Flooding of the large aft compartment could sink the boat, so a watertight bulkhead, in this case, is no indication of seaworthiness.

In reality, a moderately-sized cruising boat needs four watertight bulkheads to keep it afloat under extreme conditions. Four bulkheads would divide an interior into five separate watertight volumes, with the smallest being a collision area in the bow. If each space has a volume equaled by adjoining spaces, then complete flooding in any one space will be balanced by the adjacent compartments. Not to be discounted is the tremendous pressure a head of water can impose on bulkheads and hatches.

watertight bulkhead

Watertight bulkheads, like the one in the engine room of this Kadey Krogen trawler, significantly improve a boat's ability to remain afloat.

Retrofitting true watertight bulkheads on a production boat is a difficult job, as a watertight bulkhead needs to be made part of the boat's structure, bonded or welded to the entire hull and deck. Bulkheads need to withstand pressure from both sides, so tabbing or welding needs to be rated for pressure exerted by a fully-flooded compartment on either side of the bulkhead.

An example of a well-designed and constructed boat with proper watertight compartments is Northern Light, a 40-foot, 14-ton double-ended steel ketch designed by Jean Knocker of France. This is the design Moitessier sailed around the world. This boat has five watertight compartments, separating the interior into a (1) lazerette, (2) aft cabin, (3) cockpit/engine-room, (4) amidships living area, nav-station and head, and (5) forepeak.

Horizontal watertight bulkheads - creating, in effect, a double hull - are also used to make a hull watertight. A watertight cabin floor will restrain flooding in the event of a hole in the boat's bottom. Water will rise only to the level of the cabin sole or watertight flooring. Horizontal watertight bulkheads often make more sense than vertical bulkheads, since holing and flooding occurs below the waterline, not above.

Additionally, integral water and fuel tanks will add to a vessel's watertightness, but only if tanks and their fittings are built to withstand the pressures and forces imposed, for example, when filled with air and acting as buoyancy for the boat. Most important with integral tanks is how the fills and vents are attached to a tank. Can they be closed quickly and easily? A tank cannot be considered as reserve buoyancy unless it can be completely closed, sealed, and held in place.

Foam Floatation

A vessel can be kept afloat using solid installed foam. Recall the Boston Whaler commercials where a tender is cut into thirds and remains afloat with a person in each section? However, the drawback to using installed foam in a cruising boat is quantity or volume of foam needed to keep a flooded displacement hull afloat. A 30,000-pound boat, (a common displacement for a 45-foot to 50- foot passagemaker) would need nearly 500 cubic feet of solid foam within the hull to keep it from foundering when flooded. Fitting 500 cubic feet of foam into a yacht's hull would be a feat, requiring every nook and cranny to be filled since 500 cubic feet equals a space nearly 20' long, eight feet wide and three feet high. To keep such a boat afloat, foam three inches thick inside the entire hull would be needed, and that would only support the boat to its gunwales.

boston whaler cut in half

As this classic Boston Whaler ad demonstrated, foam floatation can be used to keep small boats afloat through just about anything - including being cut in half.

Using installed foam is beneficial in some areas. For example, foam inside a forward collision space would keep water from filling that space if the hull is breached, and foam used along the inside of a hull will provide thermal and noise insulation, as well. Since 1970, Etap Yachts in Belgium has built a line of sailboats that are designed to be "unsinkable." Etap can say this with some authority, for its boats are the only line in the world to be granted a certificate of unsinkability by the French Merchant Marine, the only body in the world authorized to issue such pedigrees. Etap achieved this status by incorporating a double-hull construction into its designs. Between each "hull," closed-cell foam is injected that will ensure that the boat not only will stay afloat even when seriously damaged, but also will be able to be sailed.

etap 46

The Etap 46, with its double hull construction, is an excellent example of an unsinkable boat.

Caution should be exercised when using some foams. Some types are combustible and give off poisonous gas when exposed to flames. These should not be used in galley or engine-room areas where fires are most likely to occur. A list of non-toxic foams - ones approved for use on passenger-carrying commercial vessels - is available from the U.S. Coast Guard. Foam often comes as a two-part liquid that is mixed and then poured into voids, or sprayed into place. Poured foam will expand when mixed and should be put in areas where expansion will not cause damage. Although foam can be installed by an owner, it is best done by a qualified yard.

Generally the quantity of foam needed to prevent a holed vessel from sinking consumes too much of a vessel's internal volume to make its use as the sole form of flotation practical on boats larger than runabouts. However, remember that foam-core or balsa-core boats already have a significant volume of flotation built in. For instance, in a lightweight 40-foot performance cruiser with foam core, there will be about 2,400 pounds of flotation, not including deck, bulkheads and furniture, which also have flotation value. So the basic boat, in many cases, already has a head start on a flotation safety margin.

Flotation Bags

Inflatable flotation bags can be fitted to existing boats and take up less space than solid foam. Bags are custom built to fit odd shaped spaces, maximizing use of a boat's internal volume, and they take up little space when in their normal deflated state. Often constructed using inner and outer bladders, bags are held in place with nylon webbing and substantial bolts, which makes them resistant to chafe and ensures that they stay in place when inflated.

To compute the number of bags necessary to support a particular boat its displacement and hull materials are considered to determine the vessel's immersed weight. For example, a 30,000-pound fiberglass boat would require 10 (5' x 4' x 3') bags to remain afloat, based on the following computations: fiberglass supports 30% of itself in seawater, so only 70% of a 30,000-pound hull, or 21,000 pounds, needs support. Compensation for error in a boat's weight and quantity of supplies carried aboard should be considered when determining the required number of flotation bags. Ten inflation bags might appear to be a large number to mount inside a boat, but in their deflated state their dimensions are greatly reduced.

Steve Dashew, co-author of the Offshore Cruising Encyclopedia, notes in his commentary on the (no longer produced) Yachtsaver float bag system: "The most important advantage of this system is psychological. Knowing your vessel will not sink under you should add an enormous amount of security to your cruising. And, heaven forbid, if you ever find your self in a situation where a life raft might otherwise seem inviting, knowing you have buoyancy bags would tend to keep you with the boat. In almost all cases a crew which abandoned their vessel would have been better off staying with their vessel than taking to a raft."

Most flotation bag manufacturers recommend arranging and mounting bags so that when they are inflated, they keep the cockpit or stern area higher than the bow. This allows a boat's interior to be bailed by dumping water into the cockpit and provides a secure place for the crew to gather while water is removed, damage repaired, and freeboard regained.

Bags are mounted low within a hull, under floorboards and bunks and in bilges, where they take up minimal space when deflated and provide maximum lift when pumped full of CO2. On boats that also happen to carry SCUBA tanks for diving, an auxiliary hose can be fitted to a flotation bag system, allowing use of a tank to top-off bags.

float bags

Float bags can be found in different shapes and sizes, to match a boat's compartments.

Calculating Margin Lines

A most important question to ask when flooding occurs, and the inflow of water is not being stopped, is: How far do you let the water rise before abandoning ship? When do you reach the point where the amount of water inside the boat overwhelms the boat's buoyancy and it sinks?

Knowing this point, or level, enables you to gauge your progress (or lack of progress) and determine whether or not you are holding your own and, hopefully, gaining on the flood. And if you are, there is no need to abandon ship. But if water is approaching a level at which the boat will flounder, it is best to gather food and water and depart.

Every boat has a certain amount of internal volume called reserve buoyancy. This is a measure of the amount of space inside a boat that can be flooded without the boat sinking. This reserve buoyancy volume can be visualized by floating a container in a bathtub and then slowly pouring in water. The container sits lower and lower in the water until it reaches a point at which it suddenly sinks. A margin line is the height of water inside the container when it sank. Margin lines can be calculated using the following formula:

H = F-45W (B x LOA - A)
H is height of margin line above normal W.L.
F is minimum freeboard in feet
W is displacement in tons in feet
B is maximum beam in feet
LOA is length overall in feet
A is cockpit area below deck level
On our 40-foot voyaging boat we can use the following figures:
F = 3'
W = 14 tons
B = 10'
LOA = 40'
A = 8 cubic feet (4' by 2' by 1'), the approximate area of cockpit below deck level

Using these values, we calculate a value for H of 1.4'. You can now locate this point on the inside of the hull, measuring up 1.4' from the waterline. This margin line indicates the height to which the boat can tolerate flood water, and you and your crew now have a physical gauge to watch should flooding occur.

As Van Dorn says in his book Oceanography and Seamanship: "By painting a margin line on the inside of your boat (recommend on the mast, or compression post for deck stepped masts), you will know when to stop pumping and start jumping."

Beyond the technology and physics of keeping a boat afloat, there exists the human factor. What can be realistically expected from a crew in times of an emergency? Can an untrained voyaging family deal with hundreds of gallons per minute of cold water entering their boat through a ruptured through-hull during a gale or at night? We must not forget training and mental preparation, and who beside commandos and marines train for such unlikely events? We cannot let the perceived security gained from technology distract from the fundamental truth that it is our skills and resolve upon which we rely in times of emergency.

The BWS Conclusions

In view of the extensive safety preparations we all put ourselves through before departure on a voyage - checking flares, EPIRBs, fire extinguishers, life raft, and man-overboard gear - it is also well worth considering actions to take if our vessel begins to flood or sink. (Though first, you may want to watch Boating Tips: Three Safety Suggestions). A boat equipped with a well-trained crew, watertight bulkheads, some solid foam, several good pumps and buckets, and flotation bags can probably avoid a sinking and that final step into a life raft.

What would make a bluewater boat unsinkable? We venture to offer a combination of the following:

  • Trained crew

  • Inflation bags

  • Vertical and horizontal watertight bulkheads

  • Pumps and buckets

  • Foam

  • Integral tanks (fuel and water)

  • Absolute minimum of through-hull fittings (use a sea-chest)

Staying afloat is like man-overboard recovery. The best technique for man-overboard recovery is to not fall overboard to begin with, and the best way to stay afloat is to not let the water in in the first place.

Editor's note; this article was updated in December of 2016.

Ex-Coast Guardsman Michael Carr, Master of Ocean Steam or Motor Vessels of not more than 1,600 tons, is president of Ocean Strategies, a weather and routing service on Peak's Island, Maine that has consulted with Around Alone Race entries. The principles and formulas presented have been reviewed and verified by Stephen Baker, N.A., who has two of his designs entered in the Around Alone.