Bookmark and Share Email this page Email Print this page Print

Staying afloat

Jan 1, 2003

Thanks to the popularity of the movie Titanic, most people are aware of watertight subdivision. The disaster of Titanic notwithstanding, watertight subdivision has saved many ships, both commercial and military, from sinking. It is a technique that could be effectively employed in voyaging yachts.

Small Navy and Coast Guard boats are required to be at least "one-compartment ships"flooding any one compartment will not sink themand small passenger vessels are required to have watertight subdivision depending on construction and the number of persons embarked. Most cargo vessels also have watertight subdivision, though current international rules use a somewhat more complicated method of describing the level of subdivision.

Unfortunately, while watertight subdivision has been demonstrated to work, it is almost unknown in yachts. The reasons generally given are that the calculations are too complicated, that the bulkheads would divide the interior up too much and that watertight doors are too heavy, expensive, and complicated. Finally, relatively few yachts are lost by sinking, so most builders and designers don't think the marginal increase in safety is worth it.

Each of these points has some merit, but the potential for greatly increased safety at relatively little cost is worth at least an examination of the possibilities.First, the calculations are not taught in most yacht design courses and are tedious if done entirely by hand. However, they can be readily automated with most electronic spreadsheets, and professional naval architecture computer programs (such as GHS or SHCP) include modules for all the necessary calculations, so that performing the required analyses takes only a day or so for an experienced naval architect. It is also worth noting that yachts have relatively few loading conditions and combinations of possible damage, so many fewer cases have to be examined than for a commercial ship. Second, the notion that watertight subdivision requires many closely spaced bulkheads is based on cargo ships carrying heavy loads. A fully loaded cargo ship is mostly below the water, having relatively little freeboard (distance from the deck down to the water) compared to its draft. Yachts, on the other hand, are mostly empty space. A 40-foot sailboat might have five feet of freeboard with less than two feet of draft to the bottom of the hull. This gives the yacht tremendous reserve buoyancy compared to its weightthe total enclosed volume below the watertight deck in a yacht might be six or seven times its displacement, so only two bulkheads are typically required.

Watertight doors on ships are pretty impressive with their multiple dogs and heavy steel construction. But these doors are designed to a standard that allows them to be installed anywhere aboard. This means that they must resist more than 30 feet of water head, so they have to hold a load on the order of a ton per square foot. They are also, incidentally, required to resist a fire for 60 minutes without leaking any smoke. A watertight door in a yacht might not have to even be watertight all the way around, since the top would generally be above the deck, and any fire in a small yacht that burned for an hour would create more problems than smoke passing through a door.Few iceberg collisions

Finally, it is true that relatively few yachts sink due to being holed in collisions (especially with icebergs), but the number that are flooded due to failures of through-hulls or shaft tubes is much larger. During a single month in 1985, I salvaged two sailboats in a single marina that sank through their toilet fittings (I guess this can be called going down by the head).

In the 1998 Sydney/Hobart race and, of course, the 1979 Fastnet, boats were lost or flooded, to the point of being abandoned, by seas entering through the hatches and windows. In fact, water entering from above the deck causes some 20% of sinkings. Finally, in recent racing yacht disasters, a number of boats that appeared to be sinking in fact did not. The crews were lost when they abandoned their boats for their life rafts. If these boats had had watertight subdivision, the crews would have had the confidence to stay with the boat and might even have been able to pump out the flooding water.

Let's look at real numbers. In 1976, I designed a cold-molded wood weekend racer for a friend in San Francisco. The boat was 27 feet 6 inches long and displaced about 3,000 pounds. That summer a small offshore racer had sunk off Santa Cruz with loss of two lives, and another small yacht had sunk, fortunately without loss of life, in the bay. We decided that the boat should have positive flotation, but since it was wood, we were concerned that it would rot if spaces were foam filled.

I ran a floodable length calculation on the hull at the full load weight and came up with the floodable length curve (see diagram above). The curve indicates the length of the vessel that can be flooded without sinking enough to immerse the margin line (a line three inches below the deck edge) at every point along the hull. Since the curve is based on the center of the flooded space, an isosceles triangle bounded by the two watertight bulkheads indicates if a given length of flooding will sink the boat by whether or not the point of the triangle is above the floodable length curve.

In this case, all three compartments result in triangles lower than the curve, so, despite flooding any one of these spaces, the deck will be at least three inches above the water. The curve assumes that 100% of the interior volume of the boat can fill with water, which is called 100% permeability. This is a somewhat conservative calculation, as most regulations assume about 5% of the volume of berthing spaces is unavailable for flooding due to the volume occupied by furnishings and so on, resulting in 95% permeability. Nonetheless, the main compartment in this boat is 13 feet from bulkhead to bulkhead. Most of the space lost was in the extreme bow or under the cockpit and was useless as accommodation. The forward bulkhead could even have been moved another foot forward, if desired.Flooding and sinkage

The way the curve is high in the middle and dips down at both ends is typical of all ships. Midships flooding mainly causes sinkage, when the boat gets deeper in the water but remains level, because the lost buoyancy is near the center of gravity (which is above the intact center of buoyancy). When an end floods, the lost buoyancy is away from the center of gravity, so there is a moment that also causes the end to trim down, in addition to sinkage. Considering flooding in the center compartment, it's easy to see that the lost buoyancy in the middle causes the whole boat to sink straight down. The waterline rises, and this adds buoyancy in the forward and aft intact compartments. When these two compartments have as much buoyancy as was lost in the middle compartment, the sinking stops.

Flooding one end, for example, the forward end, not only causes downward sinking, but causes the forward end to trim down, often enough to lift up the intact compartment at the aft end, causing still more loss of buoyancy. The intact middle compartment must not only make up the lost buoyancy, but it must produce enough bow-up trimming moment to hold up the flooded end. This trimming moment comes from the forward end of the middle compartment trimming deeper than the aft end. This, plus possible loss in buoyancy in the aft end due to its rising out of the water, shifts the center of buoyancy forward to make up the lost buoyancy forward and thereby support the flooded end. It is worth noting that raised forecastles and poop decks provide more volume in the ends and raise the deck edge in the ends as well, enhancing floodable length in the ends. This is one reason why these features are often seen in small commercial vessels.

The slight upward rise of the extreme ends is caused by the fact that the ends of the boat have relatively little volumenote that the rise forward, where the bow comes to a point, is larger than the rise aft, with the larger transom.

The cold-molded boat has relatively long ends and is light, both of which favor floodable length, but it also has very low freeboard and narrow beam, which do not. Though very few floodable length curves have been published for sailing yachts, typical bulkhead placement of small Navy and Coast Guard boats and small passenger vessels (which also have considerable above-deck volume for their weight) suggest that a midships floodable length of about half the overall length is reasonable. Also, at the 1975 Chesapeake Sailing Yacht Symposium, Charles Curtze, a retired Navy admiral, published a floodable length curve for his double-ended 43-foot cruiser Thule. The basic characteristics of the curve are similar and the midships floodable length is also on the order of half the boat's length despite having much less buoyancy in the ends. Thule has a displacement/length ratio of nearly 400, whereas the weekender is more nearly 150. These two boats represent the extremes in characteristics for watertight subdivision, so it is probably reasonable to assume that a typical sailing yacht can sustain flooding of a space about half its length. Motor yachts are probably a bit better because they don't have lots of ballast and generally have even more freeboard and beam for their weight.

Let's look at a medium/light-displacement aluminum hard-chine voyaging boat 45 feet long. It has a modern aft cabin, aft cockpit arrangement, with what is normally the owner's cabin partly tucked under the cockpit and bridge deck. The bulkheads on either side of the head would have watertight doors. The aft cabin could be directly accessed from the cockpit, or it could have a watertight door to the main cabin. This boat is partly a two-compartment shipif either of the head bulkheads is breached, it still floats. Watertight doors

Watertight doors on a yacht don't have to be big thick steel slabs. A watertight door on a yacht like the one described will only have to hold a few feet of water head at the mostabout two psi or so at a sill five feet below the deck. This could be made of plywood or single-skin fiberglass, or possibly even sheet-molded plastic with some corrugations. The relatively small pressure would not require heavy dogsreadily available quarter-turn marine hatch catches are rated for a hundred pounds or so, and high gasket compression is not required at such low pressures, either. It may even be possible to arrange the door so flooding water pressure will help hold it closed.

Since the tops of the doors will generally be above the deck, they only need to be splash resistant at the top. A door could be dropped in, either as one piece or as separate slats like a conventional cockpit hatch. The opening would get narrower towards the bottom, providing the wedging action for a gasket. One scheme uses a wedge-shaped door on a loose hinge. To make the door tight, it is lifted slightly before closing, then pushed down and latched. This also solves the shipboard problem of wiring, ductwork, and piping. Many of the penetrations can be above deck, in the raised portion of the deckhouse, so they don't need to be absolutely watertight. Yachts also simply don't have the many piping and electrical systems of a commercial vessel.

Maintaining subdivision can also be made easier because there are varying degrees of flooding hazard depending on conditions. Watertight doors may only need to be secured in heavy weather or at night. Otherwise they could be left open or latched but not dogged, as long as they are ready to be closed in case of trouble. Military craft set "damage control conditions." Each door and valve is designated by a letter that corresponds to one of three conditions, X, Y, or Z, of increasing hazard. Doors designated X are closed in most conditions, Y in more severe conditions, and Z in the worst conditions. A variant of this scheme is worthwhile even without watertight subdivision. Designating and marking the engine-cooling water valves and similar functional items might ensure that the boat is left tied up in the safest possible condition, but no one ever tries to run the engine dry or floods the sink. (Valves involving sewage can be especially important. Though they may not cause flooding, the results of accidents can be quite unpleasant.)

It is also worth noting that there is a small risk of capsizing after flooding. The boat may lose enough buoyancy so that it is no longer stable enough to resist the overturning effect of wind and waves. There are various standards for stability following flooding depending on the type of craft, but it would not be very useful if the boat were to float upright, then turn over, so this should be checked. Fortunately, this is rarely a problem for ballasted sailing yachts and even motor yachts, provided there are no bulkheads running fore and aft that would cause flooding on one side only. Such bulkheads are occasionally seen on commercial or naval craft, but they are often also provided with automatic means of cross flooding. Large ships also flood slowly, so the crew can act to control stability, and they are generally trained in damage control. A smaller vessel might flood too quickly for such methods to be effective in time.

If you decide to explore watertight subdivision for your boat, consult a naval architect, preferably one with commercial experience. Watertight subdivision studies are routine, and they will have the requisite software.

As a first step, you may want to do your own rough calculations. There is an approximate method for placing bulkheads in the small passenger vessel regulations (46 CFR, Subchapter T), which can be obtained over the Internet. It is possible to do floodable length calculations by hand from a naval architecture text, but it's tedious and difficult to explain. Calculating allowable weight and center is easier and works well with an existing arrangement.

Watertight subdivision is feasible for most yacht designs. It can be a fairly difficult retrofit, but is easy to design in from scratch. Foam flotation is another alternative but is much more expensive and consumes a great deal of otherwise useful volume. Either alternative can greatly improve safety, not only by preventing sinking but also by giving the crew the confidence and time to stay with the boat. It's worth considering.

Christopher D. Barry has designed commercial ships, ferries, fishing vessel, tugs, and workboats. He has consulted with shipyards in the U.K., California and Washington state and now works for the Coast Guard. He is licensed in both naval architecture/marine engineering and mechanical engineering in Washington. The opinions expressed herein are those of the author and do not represent official policy of the Coast Guard.