[sailingdog]

Smacky—

Again, it would help if you would learn to read and would read what has been already written. Since you can't find this information via the links I've provided in the past, I'll post it here.

The real problem with breaking waves is not the fact that they're breaking, it is the fact that they usually end up accelerating the boat to the point where it hits the wave in front of the breaking one.

From Don Jordan's website:

From physical considerations it is virtually impossible for a breaking storm wave to approach from a significantly different direction. Breaking waves are formed by the wind and by the addition of the energy of the smaller waves that they overtake. If a wave moved across a series of smaller waves it would lose all its energy in turbulence. We have many aerial views of the sea surface in the Sydney Hobart storm. If a large wave had moved across the smaller waves we would see a white streak running across all the other streaks. There is no such a streak. What actually happens is that if the boat is lying at some angle to the prevailing sea as the breaking wave approaches, the action of this wave yaws the boat until it is abeam. This yawing motion is not observed by the skipper and he thinks the wave direction has changed, whereas it is the boat that has moved. It is true, however, that the waves that caused the damage were "exceptional"

In gale force winds most of the waves can have breaking crests, but the speed and height of the waves are such that they do not constitute much of a threat.. However, in hurricane force winds the sea is generally blown "flat", but from time to time very large and dangerous waves appear, often moving in pairs or groups. A boat can ride for hours without encountering such a wave and then be destroyed in seconds.

Another optical illusion is that it is possible in a survival storm to reduce the hazard by running off before the waves and, by skillful seamanship, to out maneuver a dangerous wave. This is a particularly unfortunate choice. The waves are moving faster than the boat can go. A 40 ft .breaking wave will be moving at a speed of approximate 23 knots. The breaking wave is completely random. Furthermore, by far the most important concern is that, if the boat is moving through the water, the chance of being caught by the wave and surfing to a dangerously high speed is greatly augmented.

In the modern world we have an understanding of storm waves. There is no more mystery. Such terms as "rogue waves" serve to confuse the issue. For over a hundred years we have had an engineering grasp on non-breaking waves but it is only in the last 30 years or so that we have been able to determine the position, velocity and acceleration of every water particle in a dangerous breaking wave.

A final misconception is the belief that a breaking wave "strikes" the boat and that the moving water in the crest does the damage. Actually, the boat is lifted by the forward face of the wave with no impact. When it reaches the breaking crest the boat velocity is close to the wave velocity. The crest water is aerated and has little damage potential. Damage to the boat is incurred when the boat is thrown ahead of the wave and impacts the green water in the trough. The leeward side and the deck are struck. A careful reading of "Fastnet Force Ten" and "Fatal Storm" will confirm this conclusion.

The Jordan Series Drogue works by pulling the boat through the crest of the "breaking" wave, preventing it from accelerating the boat to wave velocity and preventing the impact against the trough.

Also from Don Jordan's website:

Worst Case Breaking Wave Strike
I have chosen the case of the Winston Churchill in the 1998 Sydney Hobart race as an example of a worst case breaking wave. The Churchill was a classic wooden sloop of 25 tons displacement and 55 ft. LOA. Of the experienced crew of 9, two perished in the accident.

From "Fatal Storm' by Mundle. "A sea came out of nowhere", said Stanley, " I could feel it from where I was in the aft coach house. It picked the boat up and rolled it down its face - 25 tons of boat- into the trough at a 45 degree angle. It was like hitting a brick wall when we hit the bottom". A crewman below reports that a sudden motion of the ship picked him up and threw him 7 ft. He observed that 8 ft of the heavy timber bulwark and planking had been torn off near the leeward shrouds, and the ribs were exposed. . The boat filled rapidly and sank in a matter of minutes.

This is an unusual type of accident. Although there are records of many storm casualties, I am aware of no documented instance of a well found yacht of the size and reputation of the Churchill and crewed by an ample group of expert sailors, suffering such catastrophic structural damage that it sank in a matter of minutes. How could this possibly happen? The severity or the storm was extreme but by no means unprecedented. There are numerous reports of large sailing yachts surviving hurricanes of the same general magnitude. Although yachts have been lost in such storms I have been able to find no record of comparable structural damage.

History shows that the probability of a yacht being capsized and damaged by a large breaking wave is strongly influenced by the displacement of the vessel. Yachts under 35 ft. have a poor history while yachts over 50 ft are rarely capsized and damaged.
The nature and extent of the damage incurred by the Churchill is also most unusual. The vessel was designed by Sparkman and Stevens and was maintained to the highest standard. Yet the heavy timber bulwark was shattered, the planking gone and the ribs exposed.

There is no question of the fact that the leeward bow of the boat was driven into solid green water at an extremely high velocity, far higher than would be expected in a simple contact with a breaking wave. We now have a technical understanding of how such a destructive force can be generated. Observations from many experienced sailors on a number of the SH yachts provide data which permit a sound engineering analysis of the performance of the waves and the boats in the race.

Water forces are applied to the hull of a yacht by two means, buoyancy forces and dynamic forces. Buoyancy forces are the familiar pressure forces which keep the boat afloat. They never reach sufficient magnitude to damage a well found yacht.

Dynamic forces result from the motion of the boat relative to the water, either as a result of the boat velocity or the water velocity due to wave motion. A speeding power boat can be destroyed by striking solid water. Similarly, a sailing yacht can be destroyed if it is accelerated up to a high speed by a breaking wave strike and then impacts solid green water in the preceding trough. This is the fate that befell the Churchill.

To understand this phenomenon we must consider the concept of energy. A moving car or boat has energy. This form of energy is called kinetic energy. Kinetic energy is measured in foot-pounds. Kinetic energy can be calculated by the formula KE=1/2 (w/g) times (v²). Where w is the weight of the car or boat, g is the acceleration of gravity (32.2 ft/sec²) and v is the velocity in ft./sec.

Thus a 3,000 lb. weight traveling at 30 mph (44 ft./sec.) would have a kinetic energy of 90,000 foot pounds. Now...and this is very important to our understanding of the Churchill disaster...if the moving vehicle strikes an object, the kinetic energy determines the severity of the collision and the extent of the damage.

In addition to energy due to motion, a vehicle can possess energy due to height. This type of energy, also measured in foot pounds, is calculated simply as the height times the weight. A 3000 lb car hoisted to a height of 50 ft. would have 150,000 foot pounds of energy. If dropped from 50 ft to a solid surface, the car would dissipate this energy in damage. If the car was compressed by 2 ft. the average force during the impact would be 75,000 pounds. If it landed on its top and compressed four feet (because it was softer) the average force would be 37,500 lbs. .These numbers (compression and force) are not precise but the product must be the same to satisfy the energy balance.

Since a car accident is a more familiar event than a wave strike I will continue with this analogy since it is technically identical to the Churchill event.

Fig.1 shows a car being dropped from 50 ft. It will impact the ground at 57 ft/sec (39 mph) and will have a collision energy of 150,000 ft. pounds. It will sustain the appropriate damage.



Fig. 2 shows the car on a ramp 50 ft. high. The car rolls freely down the ramp and strikes a tree. The velocity at the bottom of the ramp will be the same as if the car had been dropped vertically, that is 57 ft/sec. Thus the collision damage will be comparable to that of the vertical drop.

Now we come to the key element in our study of storm damage. This explains why the crew of the Churchill felt an impact similar to that of striking another boat.

Fig. 3 Here we assume that the entire ramp is mounted on wheels and is propelled toward the tree at 30 m.p.h. The moving ramp simulates the front face of a large breaking storm wave. The car is released from the top of the ramp and is permitted to roll down the face. The " increase" in speed while descending the ramp is the same as when the ramp is stationery, 39 m.p.h.. Thus the final speed of the car as it leaves the ramp and strikes the tree is 30 plus 39 or 69 m.p.h..

However since the kinetic energy (collision energy) varies as the square of the speed, the kinetic energy (collision energy} is 480,000 foot lbs or over 3 times as much as if the ramp had been stationary. There is no other wave - boat interaction which can generate such destructive loads.

The wave in this event acts as a sling shot, hurling the vehicle, car or boat forward at a high velocity. This is the mechanism which destroyed the Churchill and the same mechanical system that David used to destroy Goliath.

With this understanding we can design a simple system to decelerate the boat before it strikes the solid water in the trough.

I'd also point out that most boats aren't ever hit by a breaking wave, and that a breaking wave strike directly on a boat is a rather rare event.