What causes a Boat to Capsize

Donald J.Jordan makes some

interesting observations on

the vulnerability to capsize of

different hull and rig

configurations from tests

using simulated open ocean

breaking waves.

are the New York 32 design of tlie '30s (left) and tlie more modern Standfast design (right). Small models are scaled to represent a 32-foot boat; larger models a similar 43-footboat.

feet

The author's representation of a deep-water breaking wave. The time between waves is y2 second; frames A-C are f r o m computer simulation data, w h i l e frames D-F are the author's approximations based on rough calculations

Anyone who goes offshore, whether cruising or racing, Icnows that in a severe storm there is always the possibility of being struck by a brealdng wave. The question then is, what are the best tactics to avoid damage or capsize? The storm that hit the 1979 Fastnet Race fleet produced a large number of breaking waves, and over one-third of the fleet was knocked down until their masts were parallel to the water's surface. Twenty-five percent of the fleet capsized, and some were rolled 180 degrees. However, even in a storm far less severe, it is not unusual to hear of a boat that has been struck by a rogue wave and knocked down.

For a long time I have had an interest in the dynamic behaviour of small boats at sea. Although I have sailed for many years I have never been in a really severe storm in a small boat — and I hope I never will be. However, I have spent much of iny working life designing aircraft and aircraft engines to survive the worst that nature offers. I am convinced that if we gain a better understanding of what causes a capsize, we have a good chance of developing equipment and tactics.

One of the basic tools that the aircraft industry uses is a small-scale model test. After a number of false starts I have developed a method of investigating the behaviour of a boat model when it is struck by a simulated breaking wave. M y results

24

are somewhat surprising. For example, my tests suggest that streaming a sea anchor may prevent a capsize even though the boat is lying abeam to an approaching sea. To date the experiments do not provide conclusive proof, but they do contribute to an understanding of the problem and point the way to some useful full-scale testing.

The dynamic behaviour of regular waves has been understood for many years. I t has been established that a symmetric ocean wave, with a wave length of 250 feet, travels at a speed of about 21 knots and can grow to a height of 25 feet from trough to crest. As the wave moves horizontally the surface water particles move through a circular path with a forward velocity of about 6.5 knots at the crest and similar backward speed at the trough. The maximum slope at the crest is less than 30 degrees to the horizontal, and someone riding over the crest in a small boat will experience a downward acceleration of approximately 0.5g: in other words his apparent weight will be reduced by about one-half. This kind of wave should pose no serious threat to a small boat.

However, when waves intersect, the energy of two or more waves adds to form a new wave that can be much steeper than a 'regular' wave, and it may also be unstable. If this wave is unstable it becomes a breaking wave and a dramatic change occurs. The forward face steepens and a mass of water builds up on the

forward crest. This water mass moves essentially in a horizontal direction and accelerates to a speed greater than the speed of the wave. I t is this fast-moving mass of water that poses the major threat to any small boat.

Until recently no one had made a mathematical model of a breaking wave. However, with the help of a computer, the shape of a breaking wave and the velocities of the water particles have been calculated to the point where the wave starts to curl over at the crest. I have taken these data, apphed them to a 25-foot wave, and then estimated how the wave would continue to break (see diagram). Frames A, B, and C come from the computer simulation, and frames D, E, and F were drawn by me using rough calculations and what I hope is good judgment. I believe this figure represents a reasonable deep-water breaking wave.

In a storm the sea can be completely disorderly. However, the shape of each individual wave must be consistent with the laws of dynainics, and I think it is probable that the dangerous characteristics Of the waves shown in the diagram are typical of the dangerous characteristics of most breaking ocean waves.

Anytime before frame A a small boat should not be in trouble. However, at frame B (V2 second before the wave breaks) the wave is steep and nasty.

A 30' boat being

strucli by such a wave, using rough dynamic calculations would be carried along by the wave and be severely rolled but the boat will probably not be damaged.

But in frames C though F in the diagi-am, the situation is much worse. The boat is struck by a very fast moving mass of water that is discharged from the crest of the wave at a velocity greater than wave speed. This water picks up additional velocity as i t falls down the face of the wave, and the boat, when struck by it, capsizes.

Although i t is difficult to simulate a breaking wave in model tests, after considerable experimentation I devised an apparatus. A horizontal jet of water is discharged at the model to simulate the conditions shown on frames D, E, and F in diagram 1. For a boat model I used my own 30-foot sloop scaled down so that Vn inch equals 1 foot. The model is 11:23 inches long and is made of balsa, hollowed out and weighted with lead to provide the correct dynamic characteristics.

My first goal was to study the behaviour of the boat when it was struck abeam with a force of water sufficient to roll the mast to the water. I gradually increased the velocity and mass of the simulated wave crest by increasing the amount of

discharged water. When the simulated wave crest velocity reached 6-7 feet per second (20 knots at full scale), the boat rolled 90 degrees and put the mast in the water.

When struck abeam the boat does not roll down as you might think. Instead it is thrown violently sideways. The keel rolls up out of the water, and the boat fhes freely for about 40 feet (full scale) before finally falling heavily back into the water on its

M A Y / J U N E 1983

side.

From stroboscopic pictures of this event is is possible to determine the trajectory, the instantaneous velocity, and

acceleration, as well as an approximate estimate of the loads during a knockdown.

The initial impact is very sharp. A crewman sitting on the leeward bunk would be thi-own across the cabin and would strike the windward side with a velocity

equivalent to a free fall of 8 feet. The boat would move to leeward at a speed of 13 knots and roll 60 degi-ees per second. When the boat falls back into the water, this sideways motion is stopped and objects in the cabin would be hurled to the overhead.

I t appears to me that the buoyancy forces that normally provide stability to a boat are not relevant in a knockdown of this type. Rather i t is the dynamic forces, both inertial and hydrodynamic, that are generated by the sidewise motion of the boat that predominate. The water force has an upward component strong enough to h f t the hull to the surface.

How realistic is this model simulation, and how similar to it were the Fastnet capsizings? I have searched the literature, and I find some evidence that my models tests may be quite similar to the real world.

I n many instances of a capsize the crew has reported hearing a roar before the event. I n some cases the crew has seen water cascading like a waterfall before the wave struck. I n the diagram, the wave shape in the first three frames would not produce a roar. But a capsizing wave with a shape similar to the last three frames would make such a noise.

How about a boat being thrown 40 feet and landing on her side? K Adlard Coles in his classic. Heavy Weatlier Sailing, says, 'When a yacht suffers damage in a gale it is

usually because it is struck by a sea and literally thrown down in the trough so the doghouse, or coachroof, is stove in on the lee side.'

One very rough check I made was to compare the wave crest velocity that is necessary to roll the model down with full-scale experience. I t required speeds of 6-7 feet per second to knock down the model. This corresponds to a full-scale velocity of 20-24 knots. I t takes a breaking wave that is about 20 feet high to generate such a speed. I have not heard of a 30-foot boat being capsized by a wave less than 20 feet, so I think I am in the right ballpark.

I also investigated the effects of a wave striking the boat at various angles from bow-on to stern-on. using a wave velocity that would cause a knockdown if the boat were hit abeam. I found that if the wave hits the boat bow-on, even though the boat pitches, the loads are low and all is well. I f the wave hits the boat stern-on, the motion is simular, but the wave does sweep the cockpit and high-velocity water strikes the companionway. A rough calculation to full scale indicates that a crewman held in the cockpit by his lifeline could experience a drag force as high as 1,500 pounds from such a wave.

But when the wave direction differs from bow-on or stern-on by as httle as 15-20 degrees, the boat is picked up and thrown violently on its side. Someone on a boat experiencing this action would probably think the boat is being pitchpoled. This test suggests that lying abeam may be as good a procedure as lying in a quartering sea.

I t must be remembered that these tests simulate a situation where the boat is either dead in the water or moving at only a few knots, that is lying a-hull. I f the boats were moving rapidly down the waves.

Photo 4: Three models representing over 50 years of yacht design include (left) Tally Ho. the winner of the 1927 Fastnet; the traditional hull design ofthe New York 32 (centre); and the fin keel Standfast design (right). All three models are scaled to equivalent of 32 feet.

Photo 8: Once weight is added to the mast and to the keel, the smaller Standfast model rolls over to 45" and then begins to recover.

Photo 7: When the mast is removed on the smaller Standfast model, the boat quickly rolls into an inverted position when struck by the breaking wave.

the dynamic situation would be quite different.

My next research was to attempt to determine what design features could make a boat vulnerable to a capsize from a breaking wave. For these tests I defined a capsize as a roll that puts the mast in the water.

I tested first to see what effect the size of a boat has on its vulnerability to capsize. 1 used the models shown in Photo 1. Photos 2 and 3 show the two Standfast models being struck by identical waves. The smaller boat is capsized but the larger one is not. I got similar results using the two New York 32 models. From tests such as this using a variety of models, it became apparent that simple similarity

relationships could be apphed to the capsize behaviour. 1 use similarity here in the technical sense, where, by the application of known physical laws, we can predict the performance of objects of different size or scale. For example, if a 25-foot wave will capsize a 30-foot boat, then a similar 50-foot wave will capsize a similar 60-foot boat. Similar, in this case, means precisely scaled in all dimensions.

In a breaking wave capsize, it is not the height of the wave but the mass and velocity, or kinetic energy, in the wave crest that causes the capsize. I f the boat is driven to a high enough velocity (both sideways and rolling) by the striking wave, it will capsize. I f i t remains below this critical velocity, it will not be rolled all the way down. I f we apply the similarity relationships to a breaking wave, we find that the kinetic energy necessary to capsize any specific design will vary as the fourth power of the boat's length (L''). Thus a 60-iooter must receive sixteen times as much kinetic energy from a moving wave

26

Photo 9: When an optimum-sized sea anchor or drogue is properly deployed from the stem, it turns the boat through 90° and the hull lines up moving comfortably with the wave crest. crest as a 30-footer in order to capsize.

When the sea builds up in a storm, the energy in the breaking crests also increases A t any given time there should be many more breaking wave crests capable of capsizing a 30-foot boat than there are crests capable of capsizing a 60-footer.

Thus far I have assumed that all boats are of the same design and are merely scaled up or down in size. What effect does design have on capsize vulnerabihty? Are modern racing sloops, for example, more vulnerable than more traditional sailing craft? To investigate design factors, I used the three models shown in Photo 4. For these tests, I scaled all three boats to the same length, in this case 32 feet, to eliminate the size effect, although in reality Tally Ho is 47.5 feet overall, the New York 32 is 44 feet, and the Standfast design is 43 feet.

Photos 5 and 6 show Tally Ho and the New York 32 struck abeam by an identical wave (Photo 2 shows the Standfast). To my surprise, all three boats displayed about tlie same capsize performance. Tally Ho rolled over at a somewhat slower rate, but the difference is not very important. After this unexpected result, I set about to t r y and evaluate the effect of specific design features.

My analysis of boat trajectories shows that about 95 percent of the kinetic energy imparted to a boat by a wave goes into the sideways motion of the boat. Only 5 percent or so goes into the roUing motion, further, any structural mass above the centre of gravity of the boat provides an inertial torque that resists the roll. (This torque is proportional to the square of the distance of the mass from the centre of gravity.) The mast, of course, is far above the centre of gravity, and you would expect

it to have a large effect on the rolling behaviour. The keel, on the other hand, even though it is very heavy, is located quite close to the centre of gravity and therefore has a much smaller effect.

Photo 7 shows the smaller Standfast model struck by a wave identical to that shown in Photo 3. For this test the mast has been removed. The result is that the boat is quickly rolled to the inverted position. In the 1979 Fastnet Race, the sloop Ariadne was dismasted, and later, when struck again, she snap-rolled through 360 degrees. The mast provides about 50 percent of the moment of inertia about the roll axis, and clearly is an important factor in reducing capsize vulnerability.

If removing the mast makes things worse, in terms of capsizing, adding more weight to the top of the mast should make things better. Photo 8 shows the smaller Standfast model again struck by an identical wave. But for this test 250 pounds have been added to the top of the mast and 2,500 pounds have been added to the keel to retain the same stabihty. The boat's resistance to capsize is greatly improved, though this design solution is obviously not practical in the real world. However, it does ihustrate why large boats are better able to withstand a breaking wave. They have a greater movement of inertia about the roll axis. This test also suggests to me that a modern taU rig is an advantage over a shorter gaff rig in breaking seas.

Certain modern design features might contribute to capsize vulnerabihty: they include hght displacement, high freeboard, a f i n keel, and the combination of large beam and high centre of gravity. I attempted to evaluate each of these features, using the breaking wave, and I arrived at the following conclusions.

Light displacement

Modern boats are lighter because of improved structural design and materials. If a hull and mast are lightened

proportionately, a lighter boat is more vulnerable to capsize because a given amount of kinetic energy transferred from the wave will accelerate the boat to a higher velocity. I found that, by dynamic analysis, it is possible to relate the effect of weight to boat size or scale. For example, if we say that modern boats are 30 percent lighter than older boats, then a modern boat must be 20 percent longer than an older design to retain the same

\-ulnerabiIity to capsizing. In other words, a -40-foot lightweight modern boat would have the same capsize vulnerability as a 33-foot older type design, all other design features being the same.

Freeboard

Greater freeboard increases capsize vulnerability because more surface is exposed to the striking wave. Again it is possible, by dynamic analysis, to relate this feature to the effect of size or scale. I f a modern boat has 20 percent more freeboard, for example, it must be 13 percent longer to have the same capsize vulnerability. Thus, a contemporary -10-footer would be equivalent in \ ulnerabilty to a 35-foot older boat. Fin Keel

1 tested the models using a range of underwater shapes, from a full-length keel to no keel. I n general I found that the less underwater area a boat has, the less capsize vulnerability it has. The reason is that a large keel tends to trip the boat. A fin keel is better, according to my tests, than the

old-fashioned long keel, but the difference is not very great.

A large beam and high centre of gravity produce effects whose characteristics are more complex, and I do not feel 1 can draw any generalised conclusions about these features based on my tests. Raising the centre of gr'avity did cause the test boats to roll less when struck by a wave, which is favourable.

However, if a boat did roll to 90 degrees or beyond, its recovery was never as good as the boats with a low centre of gravity. Also a beamy high centre of gravity design can be expected to have poor recovery characteristics from an inverted position. However, much more test work remains before we can really understand these design tradeoffs.

The fact is that the three designs shown in Photo 4 had about the same capsize performance, even though each has certain undesirable design features. Tally Ho has a full-length keel, a large topside area, and a short rig. The New York 32 has a hght displacement (relative to Tally Ho) and half-length keel. The Standfast has light weight and more topside area than the New York 32. I n my opinion, all these various design features seem to have, more or less, cancelled each other out. In any case, one thing seems clear. A modern design does not show markedly inferior performance to an older design when it is struck abeam by a breaking wave crest.

Of course, it is a fact that more small boats go to sea these days. With good modern fibreglass construction it should be possible to survive a Fastnet-type storm in a small boat.

I have also been studying the effect of using a sea anchor or drogue on capsizing in breaking seas. This device shows much

promise, but as you might expect, the subject turns out to be relatively complex. However, photo 9 shows the Standfast model with an optimum-sized drogue deployed from the stern, and the boat reacts remarkably well. Unfortunately, there are problems. For this test I used a rode with no elasticity or stretch. When I simulated the elasticity of a realistic rode, the drogue lost most of its effectiveness because the rode had too much stretch.

However, a drogue or sea anchor can have three different functions. The first is to hold the bow, or stern, towards the wind and sea. The second is to reduce drift and prevent surfing. The third is to act in a powerful and immediate manner to align a boat with the fast-moving water of a breaking wave crest. I t is the third function that my tests simulated and that show the drogue with the elastic rode to be not effective. In K. Adlard Cole's classic Heavy Weather Sailing, he describes a case whei-e a boat with a well-built drogue deployed from the stern was struck by a breaking wave and badly damaged. My tests suggest that in that case the drogue was too small.

1 am continuing to study this aspect of the capsizing problem.

I will only add that all my observations are based on small-scale testing and that the test method does represent a simplification of what is a very complex subject. Nevertheless, I beheve that this work does constitute a framework on which we can build our understanding of the behaviour of small boats in breaking seas. Don Jordan, a lifelong sailor is a retired engineering manager of Pratt & Witney Aircraft, a senior lecturer at the

Massachusetts Institute of Technology and a regular contributor to Sail Magazine.

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