US Coast Guard Foolishness
Years ago, the Coast Guard only evaluated the stability of sailboats that needed a certificate to carry passengers as if they were powerboats. Tour and sailing cruise boats simple had to meet the same requirements with their sails down as powerboats. Sail area wasn't regulated at all. This may seem silly but it made a whole lot more sense than what came later. There are still a few of these fortunate old timers around that don't have their sail area drastically restricted.
In the 1960's, the loss of the Brigantine Albatross created concern about the capsizing of large sailing vessels. About the same time, a vessel was constructed for the cruise trade along the lines of a sailing Coast Guard cutter that the records showed had been considered dangerous by the service during her years of operation. The Coast Guard decided to establish stability criteria for sailing vessels.
The guiding philosophy was that sailing vessels should be regulated to survive the worst case scenario. This would be a motionless vessel in calm wind suddenly struck broadside by a gust with the sails sheeted flat. This is indeed one of the worst case stability scenarios but it is also one of the least likely. By deciding to regulate the improbable, they firmly set the course of sailing vessel regulation away from the plausible and towards a rosy horizon of irrelevance.
The basic methodology used by the Coast Guard regulations is based on the concept of dynamic energy balance. Here is the theory:
When the vessel is struck by a gust of wind of a speed the corresponds to the wind heel curve shown in red, the amount of "work" done by the wind is measured by the area under the wind heel curve. This has to be resisted by the righting arm energy, thus (in this la la land anyway) the vessel will heel until the areas under the two curves are equal and then come back to the point where the curves cross. In the case shown above, the vessel will heel to about 49 degrees and then come back to adopt a steady heel angle of 20. There is so much wrong with this idea when applied to sailing vessels that I could go on for pages but I won't bother trying to explain why boats don't act the way you've never seen one act. If you've sailed, you already know this is crazy.
I was saying in the early 1980's that I had never seen a sailboat act this way. A quarter century later, I still haven't. Wind tunnel tests conducted at the Wolfson Unit in England in the early 1990's proved me right. Sailing vessels with their sails up behave statically, i.e. when the gust hits, they heel over until the two curves cross and then stop. The Wolfson experiments did show some dynamic heeling on sailing vessels under bare poles and this behavior has been observed in power craft. USCG rules us static methods for power boats and dynamic methods for sailboats. Go figure. The Coast Guard's obsession with the idea that boats can momentarily heel twice as far as simple static methods would predict has created paranoia at headquarters that has colored every decision made about certificated sailing vessels. Worse, it has so muddied the waters that it has obscured some important insights that can be gained from simple static analysis.
"Windjammer" cruise vessels that do not go more than 20 miles from shore or harbor of safe refuge are permitted to have a range of stability of as little as 70 degrees. Conventional wisdom viewed through the dynamic stability lens says that the vessel can therefore be heeled up to 70 degrees and still recover as long as the wind is increasing slowly and the worst case dynamic situation the rules are aimed at does not occur. Look however at this graph for such a vessel.
The wind heeling curve for a wind strong enough to heel the vessel to 50 degrees just touches the righting arm curve at about 50 degrees and then is above the righting arm curve at larger heel angles. Whenever the wind heel curve is above the righting arm curve, the vessel will continue heeling. Thus, if a gradually rising wind heels this vessel to just beyond 50 degrees, it will be committed to capsizing with no further increase in wind velocity. The effective range of stability under influence of wind is 20 degrees less than what would conventionally be considered the limit of stability. As far as I know, I am the only one to have ever pointed out this fairly obvious fact which has been obscured by the obsession with dynamics.
A variation of this, which has been an important component of a couple of well known large vessel capsizes is the effect of bulwarks. Many vessels have freeing ports with flaps. These function much like one way valves, the water can flow out but can only flow in slowly. When the vessel heels, the bulwarks extend the freeboard and the buoyancy of the water in the deck well adds to the righting arm curve. When the rail cap goes under and water flows over it however, this buoyancy is lost and the righting arm curve suddenly drops back to that of the hull alone. Look at what this means for this vessel when the effects of the bulwarks are included in the righting arm curve.
The triangular peak at the top of the righting arm curve shows the effect of the bulwarks. What this graph is telling us is this: If the wind is steadily increasing and the heel angle is allowed to increase, the water pouring over the bulwarks will have enough effect on the vessels stability to capsize it immediately with no further increase in wind speed unless quick action is taken with the helm or the sails. This relationship between wind heel and righting arm curves has been present in more than one of the large sailing vessel accidents but also missed due to the obsession with dynamic analysis.
The Coast Guard rules for passenger vessels have led to some really spectacular miscarriages of the regulatory process. A cruise schooner in the Rockland fleet capsized, fortunately with no loss of life. Her range of stability was only about as shown above. Her hull is basically similar to a large cat boat and she passes the Coast Guard regulations by having a very small sail plan. She is not a vessel that was ever intended for deepwater work. Meanwhile, the vessel that John Alden designed for General Patton to go around the world, as seaworthy a vessel of her period as could be found, is unable to carry passengers under the same stability rules. This despite that the fact that she can heel twice as far before water can enter an open hatch and has a range of stability well beyond 90 degrees.
I worked with the Coast Guard in the 1980's to develop stability rules for sailing school vessels that were an improvement but it was still tinkering around the edges of a flawed system. General Patton's boat can be a sailing school vessel but the whole idea of regulating sail area has a basic conceptual flaw.
This basic problem is that the rules are based on evaluating the vessel with the maximum amount of sail area that can be set. They also let you bring a poor and dangerous hull into compliance and obtain a certificate by reducing the maximum sail area. This makes the boat safer in light winds but, as soon as wind increases and the vessel would have been reefed or had sails struck, the rules become irrelevant. You're stuck with a dangerous hull and the rules have become pointless just as the wind has risen to the point where stability regulation might be needed.
Consider this thought experiment. The basic result of a safety regulation governing the design of something like a sailing vessel should be a physical difference between vessel that are permitted to operate and vessels that are not. Two absolutely identical sister ships are built by two brothers. One brother decides that he will put his boat into USCG Certificated passenger carrying operation so he has the stability tests performed and submits the plans and calculations. The naval architect discovers that the boat has too much sail according to the rules. However, if he does not carry one of the topsails and reefs the mainsail, the boat will pass. He therefor puts the topsail ashore and has the main cut down at the first set of reefpoints. The Coast Guard issues him a certificate to carry passengers.
The two vessels set out in company in light winds, each carrying their full sail plan. It is true that the certificated passenger vessel has greater margin of safety against sudden wind increases so, the rules are making the passengers safer. Now the wind increases to about 18 knots. The uncertificated vessel strikes it's topsail and reefs the main. The two vessel now have identical sail plans and identical margin against unexpected wind increases. The only physical difference between them is that one has an extra sailbag on deck and a roll of cloth tied up along the boom. This is not a significant physical difference and has no bearing on vessel safety yet one vessel is considered "safe" by the Coast Guard and the other is not. As wind continues to increase and the two vessels continue to reduce their sail plans, their actual safety and margin against gust remains identical. As soon as the wind gets to the velocities where stability accidents are likely, the USCG stability rules become useless.
The rules can be worse than irrelevant. They can be counter productive. Suppose our two vessel are going on a long passage together to something like a tall ships event. The winds are light for the first portion of the trip so our "dangerous" uncertificated vessel uses its extra sail area to get well ahead. Towards the end of the trip, there are strong winds. Most vessels carry their food and water low so that stability decreases as the supplies are used up. Both our vessels now have exactly the same sail plan but the slower vessel has used more of her fuel and water so the physical difference between the two ships when they reach the same point is that the "safe" USCG approved vessel has LESS stability. Not quite, I think what the rule makers had in mind. Finally, when the two vessels are close to the date they need to fulfill their event commitments and one is a few hundred miles close to port because of its extra sail area and light wind performance, which one do you think is going to be sailed more conservatively?
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