Learning from Failure: The Case of That Sinking Feeling

Published July 29, 2022
By: Christopher D. Barry, PE

A recent marine movie, All is Lost, touched on a subject that at least the long-distance sail or power cruiser should consider.  In the movie, Robert Redford is a single hand sailor crossing the Indian Ocean in a sailboat.  He strikes a semi-submerged container and various problems ensue. Unfortunately, his yacht did not have a watertight subdivision, which might have eased some of his problems.

The most obvious, though fortunately not most common, hazard to a boat is sinking. If the basic marine rule that the water should be kept on the outside is violated, most recreational boats longer than 20 feet will eventually fill with water and leave the surface. In the U.S., small powerboats and most manually propelled boats are subject to 33 CFR 183.101 – 183.335 (and possibly ABYC H-8) which requires permanently installed foam floatation that meets certain requirements for resistance to soaking and deterioration by gasoline, diesel, other oil, and bilge cleaning chemicals. These rules cover boats up to 20 feet in overall length. When these boats are freely flooded, they are supposed to float with either some portion of the boat above water (basic flotation) or approximately level with adequate stability to provide a safe refuge for the crew (level flotation), depending on the type of boat. ISO 12217-1 (power boats over 6 meters) requires flotation (either foam or specified types of air reservoirs) for boats over 20 feet certified under Category B, Option 3, and Category C and D Option 4. ISO 12217-3 (all boats under 6 meters) also has options requiring flotation, though not for all boat types required to have flotation under 33 CFR (so some ISO 12217-3 boats might not be allowed for sale to the public in the U.S.).

However, a boat much larger than 20 feet might require a very large amount of foam and the foam would begin to occupy otherwise useful spaces. This is even more true of ships, so ships are subdivided by transverse bulkheads spaced sufficiently closely so that if one compartment (or more, depending on the applicable rule) is flooded, the remaining intact spaces will prevent sinking.


Following a series of sinkings, one resulting in fatalities, a study was done to evaluate watertight subdivision for a small “weekender” keel boat. This showed that if bulkheads were placed about 7 feet forward and aft of midships (on a 27-foot boat, so the bulkheads were 14 feet apart), the boat would not sink if any one of the three compartments was flooded. The other two would float the boat high enough that the deck edge would be at least three inches above the water (this is the “margin line” used for standard floodable length calculations). The weekender had low freeboard and a narrow beam, but fairly long overhangs, in the tradition of a Dragon or Knarr, and a small cabin. Placing the bulkheads so close to the ends only cost space that was largely useless for accommodations because they were narrow and low. The spaces forward and aft of the cabin were mainly for sails and other stowage. The boat was never built but was the subject of a short article about subdivision for the local sailing magazine.

About the same time, Charles Curtze (a retired Navy admiral) wrote an article about the design of his yacht Thule that also had subdivision, though in this case, two compartment subdivision (any two adjacent compartments could be flooded without sinking the yacht). Again, the subdivision did not result in an unreasonable design.

Watertight subdivision is standard for most commercial vessels and is not particularly burdensome for most boats and small ships. In the 90s, a series of studies to determine the impact of requiring flotation for commercial fishing vessels through a group of naval architects involved in fishing vessel safety was done. This study found that it again was a reasonable requirement. NVIC 5-86 encouraged subdivision in fishing vessels, and finally 46 CFR 28.500 requires it for new vessels over 79 feet.

Watertight subdivision works better for recreational boats than commercial ships because they generally have a lot of internal volume for their weight. They are usually carrying people and thus their interiors are mostly air (vice iron ore, for instance). This means they need relatively few bulkheads.


One problem with subdivision for small craft is that the traditional floodable length calculation is unfamiliar to a lot of builders, often not supported by stability software and fairly confusing; the diagram that results is, for a given weight, the location of bulkheads that will prevent sinking centered about a given point. An alternative method is easier to understand and use. As is often the case, the location of the bulkheads is dictated by the boat arrangements. Then a space between bulkheads is considered to be flooded (and hence not providing flotation) with the boat barely floating at some trim, the whole deck edge is at least three inches above the water with the lowest at exactly three inches. The calculated buoyancy of the remaining intact compartments is the greatest weight at that longitudinal center that the intact spaces will support. Change the “barely floating” position to another where a different point on the deck is lowest and repeat the calculation.

Finally, a series of these calculations will produce a curve of limiting longitudinal center of gravity versus weight for flooding that compartment. This curve (and those for the other compartments) defines the maximum weight and center of the intact boat to be safe from sinking. These weights and centers can be used to load the intact boat to determine the limits of forward and aft draft that prevent sinking as well, which can be marked on the hull. There are not a lot of small crafts that sink, fortunately, but subdivision could be considered for craft that go well offshore (like Robert Redford’s Cal 39).


Another related point is also important, and much easier to implement. Ships, particularly military craft, have posted damage control documents that detail which valves, vents, and watertight doors should be closed in case of flooding, or as a preventive measure in conditions that may involve damage or flooding. This is rare in recreational boats, which is especially a problem because most flooding in small crafts does not involve hull damage, but failure of an internal system. In a recent incident, someone’s small boat was sinking in the middle of the river, so he had run it ashore on a community beach. The owner had no idea what had happened, nor any idea what hull penetrations his boat had. When the community harbormaster was able to pump out the boat, it was found that a cooling water line had slipped off, through its hull. If the owner had a valve on the inlet, or even a wood plug, he would have been able to prevent an expensive insurance claim. He didn’t have a valve, or a plug, but most important, he didn’t know about the hull penetration.

It is actually quite common for boats to flood through some sort of intentional penetration, often the head plumbing or engine cooling lines, with propeller shaft seals a distant third. It is not very expensive to at least post a list of the hull penetrations, their locations, and how they should be set, especially when the boat is left at the dock – a lot of boats have been found one morning with just their masts sticking out of the water. A few wood plugs and a couple of rolls of duct tape would be nice too. This will not only prevent sinking, but problems like running the engine with the cooling water shut off or trying to pump a sewage tank against a closed valve and rupturing the tank or a line.


Christopher D. Barry, PE, s an experienced naval engineer who has, over the years, gained extensive experience in the maintenance, overhaul, acquisition, design and construction of commercial and military ships and boats, offshore oil platforms and other floating equipment. He is well versed in the areas of hydrostatic, structural, hydrodynamic and mechanical engineering analyses of resistance, motion RAOs in waves, mooring systems and more. See Christopher’s full CV here.


A short item on this subject in the Professional Boat Builder archives, http://www.proboat.com/2013/03/the-unsinkable/

ABYC, “Buoyancy in the Event of Swamping”, Project H-8, Standards and Technical Information Reports For Small Craft, www.abycinc.org

The Code of Federal Regulations, on line: https://www.gpo.gov/fdsys/browse/collectionCfr.action?collectionCode=CFR)

Curtze, Charles A. “A Cruising Boat” Chesapeake Sailing Yacht Symposium, 1975, Marine Technology, Oct 1976, Society of Naval Architects and Marine Engineers, Jersey City, NJ

ISO, Small craft — Stability and buoyancy assessment and categorization — Part 1: Non-sailing boats of hull length greater than or equal to 6 m, International Standard ISO 12217-1, 1 Mar. 2013

ISO, Small craft — Stability and buoyancy assessment and categorization — Part 2: Sailing boats of hull length greater than or equal to 6 m, International Standard ISO 12217-1,  23 July 2017

ISO, Small craft — Stability and buoyancy assessment and categorization — Part 3: Boats of hull length less than 6 m, International Standard ISO 12217-1, Second Edition, 1 Mar. 2013