The truth about unsinkable boats (2023)

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A professional seafarer explains how to avoid sinking.

from the staff

December 20, 2016

What makes a boat truly unsinkable? Here's what all comes into play:

  1. Flood rate - the rate at which water is entering the boat.

  2. Pump Rate - The rate at which water leaves the boat.

  3. Watertight Bulkheads - Bulkheads that create watertight compartments that remain intact if other watertight compartments in the vessel are flooded.

  4. Foam Floats - Built into the boat as a core or added in dead space areas to increase buoyancy.

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

  6. Ledge Lines - A boat's ledge line indicates the critical point at which it will become unflootable if flooded.

When seawater enters a hull without any engineering changes to the boat, the question is not how to stop the flow of water, but how to open the life raft and life bag and rescue the crew. A professional seafarer explains how you can avoid this fate.

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

Venturing across the world's oceans has an allure unmatched by any other experience, but that positive euphoria can quickly be replaced by a sense of danger when a boat starts to flood. Sinking is probably every sailor's greatest fear. One morning last year, the crew of a 60-foot sailboat crossing the Atlantic was surprised to discover that water was rapidly rising in its interior. St. John's, Newfoundland was the closest refuge, but it was several hundred miles away, so the team worked quickly to find the source of the flood. There were only a few continuous hulls left to check, each one appearing to be intact, but the water level inside the boat mysteriously continued to rise.

Unable to find and stop the flood, the boat called for help, and luckily her distress signal was heard by a passing merchant ship, which provided a large diesel rescue pump. Luckily the pump went off, lowering the water level and revealing the cause of the flooding. An unprotected battery hit a through-hull fitting and released its hose.

This recently built sailboat, equipped with all the necessary safety equipment and built with several watertight bulkheads, was making her first ocean voyage and, however, was on the verge of sinking or at least being abandoned. The crew later commented that a combination of the inability to slow or stop the entry of cold water and the crew's exhaustion would certainly have resulted in the yacht sinking had it not been for a merchant ship with a bomb nearby.

Many similar incidents have been documented where a boat was lost or abandoned due to flooding. On the one-handed circumnavigation of the 1994-95 BOC Challenge (now called "Around Alone"), a 60-foot aluminum cutter struck an object that penetrated two of its watertight compartments, causing massive flooding that sent the boat into the Sink brought from hours . The boat's captain, an experienced and capable sailor, was unable to stop the current, but was – again fortunately – saved by a competitor.

flood rates

Can you make an unsinkable boat? Yes, but it requires some planning and critical attention to technical details. Boats sink because they fill with water. And with the slightest opening, water will rush in and fill a boat at incredible speed. Since water can fill and sink boats extremely quickly, procedures for dealing with flooding must be quick and simple.

A quick water pressure and flood check is in order. Flood rates for different sized holes at selected depths are calculated using this formula:
Q = 20 x d x Quadratwurzel von h
Q = flood rate in gpm
d = hole diameter in inches
h = depth of hole under water in feet

Water exerts about a pound of pressure per square inch for every foot of depth. At a depth of three feet - the distance above the engine inlet, for example - the water pressure is equivalent to 1.5 pounds per square inch (1.5 psi). What does this tell us? This indicates that a hole just 1.0 inches in diameter at that depth can pass 34 gallons of water in one minute. Over an hour that makes 2040 gallons.

Consider a typical 40-foot cruise ship: Its internal volume is approximately 1400 cubic feet - (40 feet x 10 feet x 5 feet) x 70% = 1400 cubic feet. (The 70% is a rough approximation of the hull shape's contribution to the 40-foot cube). feet, so 110 gallons per minute equals 14 cubic feet per minute, so 1,400 cubic feet of boat volume is filled in 100 minutes, or just over an hour and a half.

Note: If water entered the boat at a constant rate, the boat would theoretically sink faster - perhaps in half that time. However, a boat's flood rate decreases significantly when the water level rises above the leak. In this case, the "h" in the flood formula becomes the difference in level between the water inside the hull and the water outside. Also, any air in the cabin will create back pressure to further decrease flow. For this reason, it is best (and also safest) to close all hatches and remain on deck aboard a sinking boat.

If a crew is aware of the leak as soon as the flood starts - if everyone is sane and the incident is not happening in the middle of the night or in cold water - it may be possible to stop the flow. But what if the flow starts without a witness? What if the flooding is only noticed when there is water floating on the cabin boards? So what? where do you start looking What do you do if the hull penetrations are further below the surface of the flood water than arm's length? What if it's night and none of your waterproof flashlights work? What if the water is so cold your extremities feel numb?

The use of sails, plugs, baffles and the like are legitimate methods of stopping a tide only when certain initial conditions are met:

  • The crew is trained and has practiced updating and using damage control equipment. I've only seen this training in the Navy, Coast Guard and Merchant Marine.

  • The flood is observed by a team and immediate action is taken.

  • Environmental conditions are benign: the water is warm; it is day; The seas are benign. Imagine trying to find a flood source while you're waist-deep at 50? Water during a dark night, when a boat rocks and bounces.

  • A team is big enough to solve the problem. It's hard to put out a four-alarm fire with few firefighters.

If you can't see the flood and aren't sure where to look, all the damage control devices in the world aren't going to help much. So what are possible solutions to this ever-present concern about flooding and subsidence? They are a combination of four design considerations: (1) buoyancy bags, (2) watertight bulkheads (vertical and horizontal), (3) rigid foam, and (4) pumps.

As there are many examples of boats exhibiting one or more of these characteristics and still sinking, what is the problem or common denominator? On closer inspection, such sags often have simple explanations - failure to locate and stop the flood immediately, watertight doors open, insufficient watertight bulkheads, little or no rigid foam, no inflatable buoyancy bag, and insufficient pumping capacity for the rate of flood in question.

When flooding is detected, measures must be taken to stop water intrusion immediately and decisively. Actions that are not immediate and decisive result in the amount of water inflow quickly outweighing all subsequent actions.


A boat's pumps are only installed to remove accumulated water after the source of that water has dried up. Pumps are not designed to withstand a constant tide. Quick comparison of flood rates with pumping rates shows that no reasonable small boat pump can prevent severe flooding. While many reliable pumps are available to keep bilges dry and remove unwanted water from showerheads and mooring wells, in addition to the large diesel-powered rescue pumps, no onboard pump can prevent water from leaking through a pipeline or Supply box ruptured in the hull.

Bilge pumps are classified by their ability to pump water horizontally rather than vertically. For example, a pump rated at 50 gallons per minute (gpm) will indicate 50 gallons of horizontal movement. Each vertical hit drastically reduces the bomb's capacity. Note that many pumps list their capacity in gallons per hour, not minutes. Remember, a two-inch opening two feet below the waterline will let in 110 gallons per minute, which equals 6,600 gallons per hour! A high capacity 50 gallons per minute pump moves 3,000 gallons per hour - just half the inlet volume. The largest hand pumps on the market move one gallon per stroke and are rated between 30 and 42 gallons per minute. Try pumping for an hour.

the pacemaker pumpsells a 3-inch diameter, 380 gpm "garbage bomb" that is lightweight (about 112 pounds), extremely durable, and relatively inexpensive. She can run at that capacity by extracting only 8 horsepower from a 6-foot head. This pump has been used extensively on Vende Globe Challenge and BOC/Around Alone boats for years as a ballast water pump. The Pacer is powerful enough to literally explode if used with insufficient ventilation in the tank away from the hull. These pumps can be belt driven from the main engine using a standard air conditioning clutch found on any American car. However, the main engine must be operational to function. Simply put, if the main engine is running, the pump cannot work if the engine is flooded. If such a pump is activated via a solenoid, flooding can also short circuit the electrical connection, rendering the pump inoperable. And these pumps don't activate quickly to gain access to the engine and must be squeezed to engage the clutch.

Drain pumps, like this one carried by the US Coast Guard, can help in a sinking situation - but they're usually not the ultimate solution. Photo courtesy of the US Coast Guard.

While every boat should have several large pumps onboard, when comparing their pumping performance to the flood rate of even the smallest of openings, it is clear that pumps are not the only answer to staying afloat during a severe flooding situation. The pumps delay floods and remove floodwater once the flood ceases, which is their real use, but they are not designed to stay ahead of an inflow of floodwater.

anteparas ponds

Effective watertight bulkheads must be robustly constructed and of sufficient number and spacing. Ideally, they are installed during construction and are constructed with a minimum of wire and pipe penetrations. Required penetrations should be positioned as high as possible in the bulkhead to reduce water pressure in these openings in the event of flooding. Access through watertight bulkheads must be provided by hatches which, when closed, are truly watertight and capable of withstanding pressure from both sides. Watertight bulkheads are a valuable addition to a boat in terms of strength and survivability, reducing noise and retaining heat in living areas when sailing in cold waters and climates.

The spacing of watertight bulkheads is critical; It is the size of the partitioned spaces that is most important to its performance. For example, if a ship is built with two watertight bulkheads dividing the hull into thirds, the size of each compartment should be nearly the same. That way, when one of the three is fully submerged, the other two remain intact and have enough volume to keep the ship afloat. An example of poor design might be a small front compartment and a large rear compartment separated by a watertight bulkhead. Flooding the large aft compartment could sink the boat, so a watertight bulkhead is not an indication of seaworthiness in this case.

In fact, a medium-sized cruiser needs four watertight bulkheads to stay afloat in extreme conditions. Four bulkheads would divide an interior space into five separate watertight volumes, the smallest being a collision area at the bow. If each space has a volume equal to the adjacent spaces, then the total flooding in any space will be offset by the adjacent areas. The enormous pressure that a column of water can exert on bulkheads and hatches should not be underestimated.

Watertight bulkheads, like those in the engine room of this Kadey Krogen trawler, greatly improve the boat's ability to stay afloat.

Fitting true watertight bulkheads to a production boat is a difficult task, as a watertight bulkhead must be part of the boat's structure, glued or welded to the entire hull and deck. Bulkheads must withstand pressure from both sides, therefore any straps or welds must be designed to withstand the pressure exerted by a fully flooded compartment on both sides of the bulkhead.

An example of a well designed and built boat with proper watertight compartments isAurora boreal, a 40-foot, 14-ton double steel ketch designed by Jean Knocker of France. With this project, Moitessier sailed around the world. This boat has five watertight compartments dividing the interior into (1) leisureette, (2) aft cabin, (3) cockpit/engine room, (4) lounge area amidships, navigation station and bow and (5) ) bow peak.

Horizontal watertight bulkheads, effectively forming a double hull, are also used to waterproof a hull. The cabin's watertight floor prevents flooding in the event of a hole in the bottom of the boat. Water only rises to the level of the cabin floor or waterproof floor. Horizontal watertight bulkheads often make more sense than vertical bulkheads because holes and flooding occur below the waterline, not above.

Furthermore, integrated water and fuel tanks contribute to a ship's watertightness, but only if the tanks and their accessories are built to withstand the pressures and forces encountered, for example when they are filled with air and used to supply buoyancy to the boat. The most important thing with integrated tanks is how the fillers and vents are connected to a tank. Do they close quickly and easily? A tank cannot be considered a reserve of buoyancy unless it can be fully enclosed, sealed and held in place.

foam swimming

A ship can be kept afloat with permanently installed foam. remember thoseBoston WalfängerCommercials where an offer is divided into thirds and stays afloat with one person in each section? However, the downside to using installed foam on a cruising boat is the amount or volume of foam required to keep a flooded displacement hull afloat. A 30,000-pound boat (a common displacement for a 45- to 50-foot passenger boat) would need nearly 500 cubic feet of solid foam in the hull to keep it from sinking in floods. Fitting 500 cubic feet of foam into a yacht's hull would be a feat, filling every corner, as 500 cubic feet equals a space nearly 20 feet long, 8 feet wide, and 3 feet high. Keeping such a boat afloat would require three inches of foam across the hull, supporting the boat only up to the rail.

As this classic Boston Whaler commercial demonstrates, foam buoyancy can be used to keep small boats afloat in just about anything - including bisection.

Using built-in foam is beneficial in some areas. For example, foam in a head-on collision space would prevent water from filling that space if the hull were to breach, and foam used along the inside of a hull also provides thermal and acoustic insulation. Since 1970,Etap Yachtsin Belgium built several "unsinkable" sailboats. Etap can say this with some authority, as its boats are the only line in the world to receive a certificate of unsinkability from the French Merchant Marine, the only body in the world authorized to issue such pedigrees. Etap achieved this status by incorporating double hull construction into its designs. Closed-cell foam is injected between each "hull", which ensures that the boat not only stays afloat, but can also sail, even in case of severe damage.

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

Care must be taken when using some foams. Some types are combustible and emit toxic fumes when exposed to flame. They should not be used in kitchens or machinery areas where fires are more likely. A list of non-toxic foams – those approved for use on passenger ships – is available from the US Coast Guard. Foam usually comes as a two-part liquid that is mixed and then poured into cavities or sprayed in place. Spilled foam will expand when mixed and should be placed in areas where expansion will not cause damage. Although the foam can be installed by a homeowner, it is best to have it done by a qualified yard.

In general, the amount of foam needed to keep a punctured vessel from sinking consumes too much of a vessel's internal volume to make its practical use as the only form of buoyancy in boats larger than runabouts. However, keep in mind that foam or balsa core boats already have significant buoyancy. For example, in a 40-foot foam-core performance light cruiser, there is about 2,400 pounds of buoyancy, minus the deck. Bulkheads and furniture that also have buoyancy. Therefore, the base boat already has a head start on a buoyancy safety margin in many cases.

swimming bags

Inflatable buoyancy bags can be attached to existing boats and take up less space than rigid foam. Bags are custom designed to fit irregularly shaped spaces, maximizing a boat's internal volume, and take up little space when in their normal empty state. The bags, which usually consist of inner and outer bladders, are held in place with nylon straps and heavy-duty screws, making them resistant to abrasion and ensuring they stay in place when inflated.

To calculate the number of bags needed to transport a given boat, its displacement and hull materials are taken into account to determine the vessel's immersion weight. For example, a 30,000 pound fiberglass boat would require 10 bags (5' x 4' x 3') to stay afloat based on the following calculations: Fiberglass retains 30% of itself in seawater, therefore, only 70% of a 30,000-pound or 21,000-pound hull needs support. When determining the number of buoyancy bags needed, errors related to the weight of the boat and the amount of supplies carried onboard must be taken into account. Ten inflation bags might seem like a lot to fit on a boat, but when deflated, their dimensions are greatly reduced.

Steve Dashew, coautor doEncyclopedia of Offshore Cruises, commenting on the (no longer produced) Yachtsaver Floatbag system, notes: “The main benefit of this system is psychological. Knowing that your ship isn't sinking below you should add tremendous confidence to your sailing. And, God forbid, if you're in a situation where a lifeboat might look inviting, knowing you have buoyancy bags would tend to keep you on the boat. In almost all cases, the crew that abandoned ship would have been better off staying with the ship than going on a raft."

Most buoyancy bag manufacturers recommend arranging and mounting buoyancy bags so that, when inflated, they keep the cockpit or aft area higher than the bow. This allows the interior of the boat to be hollowed out draining water into the cockpit and provides the crew with a safe place to gather while the water is removed, damage repaired and the freeboard restored.

Bags are mounted to the bottom of a hull, under floorboards, berths and holds, where they take up minimal space when deflated and provide maximum buoyancy when inflated with CO2. On boats that also carry SCUBA cylinders for diving, an auxiliary hose can be connected to a floating bag system so that a cylinder can be used to fill the bags.

Swim bags come in a variety of shapes and sizes to fit in a boat's compartments.

Calculation of border lines

A very important question to ask when a flood occurs and the flow of water is not stopped is: how high do you allow the water to rise before abandoning ship? When do you reach the point where the amount of water in the boat exceeds the buoyancy of the boat and it sinks?

Knowing this point or level will help you assess your progress (or lack of progress) and see if you're holding on and hopefully riding the tide. And if so, you don't need to leave the ship. However, if the water approaches a level where the boat starts to float, it is best to gather food and water and leave.

Every boat has a certain amount of internal volume called a buoyancy reserve. This is a measure of the space on a boat that can be flooded without the boat sinking. This buoyancy reserve volume can be visualized by floating a container in a bathtub and slowly pouring water into it. The container gets lower and lower in the water until it reaches a point where it suddenly sinks. A marginal line is the height of the water in the tank when it sank. Edge lines can be calculated using the following formula:

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

Using these values, we calculate a value for H of 1.4'. You can now locate this point on the inside of the hull and measure 1.4 feet from the waterline. This limit line indicates how high the boat can tolerate flooding, and you and your crew now have a physical gauge to watch for in the event of flooding.

As Van Dorn says in his bookoceanography and seamanship: "By drawing a ledge line on the inside of your boat (recommended on the mast or push post for masts with stepped decks), you'll know when to stop pumping and start jumping."

In addition to the technology and physics to keep a boat afloat, there is the human factor. What can you realistically expect from a team in an emergency? Can an untrained traveling family handle hundreds of gallons per minute of cold water entering their boat through a cracked hull during a storm or at night? We must not forget about training and mental preparation, and who other than commandos and marines trains for such unlikely events? We must not allow the sense of security provided by technology to diminish the fundamental truth that it is our skills and resolve that count in emergencies.

The conclusions of the BWS

Given the extensive safety precautions we all go through before setting off on a voyage - checking flares, EPIRBs, fire extinguishers, life rafts and man overboard equipment - it's also worth considering taking action when our ship closes, starts to flood or sink. (Although you might want to watch it first.Browsing Tips: Three Safety Tips). A boat equipped with a well-trained crew, watertight bulkheads, some firm foam, several pumps and good buckets and buoyancy bags can probably avoid sinking and the last step in a lifeboat.

What would make a blue water boat unsinkable? We dare to combine the following offers:

  • trained staff

  • inflation bag

  • Vertical and horizontal watertight bulkheads

  • bombs and buckets

  • foam

  • Integrated tanks (fuel and water)

  • Absolute minimum hull penetrations (use sea box)

Staying afloat is like pulling a man overboard. The best technique for recovering from a man overboard is to not fall overboard in the first place, and the best way to stay afloat is to not let the water in in the first place.

Editor's note; This article was last updated in December 2016.

Former coastguard Michael Carr, master of steam and power vessels of no more than 1,600 tons, is president of Ocean Strategies, a weather and routing service in Peak's Island, Maine, which consulted participants in the Around Alone Race . The principles and formulas presented were reviewed and verified by Stephen Baker, N.A., who entered two of his projects in Around Alone.

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