Seaworthy designs are always moored in reality
Posted on 30 May 2012
Written by Eric Sorensen
If the idea needed reinforcement, there’s been plenty of it in the news recently — big ship or small boat, there’s no room for complacency at sea.
The media attention devoted to the 100th anniversary of the April 1912 Titanic disaster reminds us again that the term “unsinkable,” at least as it applies to larger ships, is not an absolute.
Official findings on the Jan. 13 grounding of the Costa Concordia — on the first day of a Mediterranean cruise with 4,200 aboard — remain a long way off, but all indications are that human error, or hubris at the helm, will figure strongly in the final analysis. And the sinking of the 198-foot superyacht Yogi in the Aegean Sea in February remains a mystery. There have been crew references to “mechanical failure” on the yacht — described at the time of its 2011 launch as a “floating resort” — although I can’t think of a single mechanical failure that, by itself, could possibly sink a ship. (A 198-foot yacht is a ship.)
The latter two losses demonstrate that despite all of the advances in design, large-scale disaster at sea is not a phenomenon of the past and that putting to sea for any purpose is serious business.
Clearly, seaworthiness is a relative term that depends on the intended purpose or mission of the vessel. For me, the question to ask first is: what sort of damage would it take to sink this boat? For the typical 24- to 30-foot center console or express cruiser, the answer is not much. A single 2-inch hole just 6 inches below the waterline would do the trick — and quickly — flooding the hull at a rate of 55 gallons a minute or 3,300 gallons an hour. That is far beyond the capacity of the standard 500-gph bilge pump. With a typical 40 percent efficiency loss, such a pump would be able to put about 5 gallons a minute over the side.
As a first defense against sinking, well-designed small boats use foam injected into the bilge and self-bailing cockpits. Most boats 20 feet and longer lack sufficient foam to keep them from sinking. But there are exceptions, among them Grady-White, EdgeWater, Key West, Scout, Boston Whaler and others. The worst-case scenario if one of these unsinkable hulls is breached is that you’ll be waiting for a tow in a very large life raft.
The most basic approach to self-bailing cockpits in outboard fishing boats and some express cruisers is the use of scuppers and drain lines that channel incidental hose, bucket or rainwater over the side, rather than into the bilge.
Remember, boats that venture offshore must be able to get rid of large amounts of water that come aboard — and fast. They require that the deck be high enough to provide enough buoyancy to keep it above sea level, allowing water to drain. They also should have larger scuppers and drain lines — and more of them.
There are two factors that affect the effectiveness of scuppers and lines: clogging, and scupper or drain line size. If it’s less than 1.5 inches in diameter, it’s going to be susceptible to clogging with pine needles, fish scales, seaweed and other debris. In a 20- to 25-foot coastal fishing boat, my first choice is always big scuppers cut directly through the hull side or transom (with no plumbing), recessed slightly below deck level and at least 2-by-6 inches (12 square inches) in size.
After that, I’d be looking for a minimum of two 2-inch drain lines per side, in the aft corners and recessed slightly below deck level. You want them in the cockpit’s outboard corners because that’s where the water is going to collect when the boat is listing to one side, which is what it’s liable to be doing with green water on deck.
Ships rely on compartmentation, which divides them into multiple watertight sections separated by watertight bulkheads. These bulkheads should have no non-watertight penetrations below the point to which the vessel would settle when one, two or three compartments are completely flooded, depending on whether it is a one-, two- or three-compartment ship. A two-compartment ship, for example, is designed to stay afloat in a moderate sea — with adequate reserve buoyancy and stability — with two contiguous compartments completely flooded.
On many warships, experience drawn from World War II and subsequent conflicts has been used to create survivability standards. The USS Samuel B. Roberts, a 4,100-ton, 453-foot guided-missile frigate on which I served, is a good example. It has 11 watertight bulkheads that divide the ship into 12 watertight compartments. The Roberts and most other Navy ships over 300 feet are three-compartment ships. That means any three contiguous compartments, as much as 15 percent of the waterline length of the hull, could be opened to the sea and flood completely and the ship would remain afloat and upright in a moderate sea.
The Roberts nearly did flood three compartments in a row when it hit an Iranian mine in the Arabian Gulf in 1988. The 12-foot hole the mine strike created in the engine room actually flooded two compartments completely and one partially, allowing 1,000 tons of seawater to pour into the ship. If the third and largest compartment forward of the engine room had flooded, the ship would have taken on another 860 tons of seawater and likely would have broken in two, with the deck and hull weakened by the blast.
In any event, the designers of Navy ships mark a point on centerline 4 feet above the damaged waterline on each bulkhead and draw a 15-degree V up and out from this point. The 4-foot measure reflects wave action that causes the ship to heave up and down in relation to sea level. The 15 degrees is based on experience; if the ship lists more than 15 degrees in the first 10 or 15 minutes after damage, it almost certainly will capsize. There’s no sense worrying about keeping it watertight above this point. It’s time to abandon ship.
Below the V-line on a Navy ship, the bulkheads must be watertight with no doors or other penetrations that could allow progressive flooding from one space to the next. The commercial shipbuilding world uses margin lines, not V-lines, which are also based on the level of the damaged waterline and below which there should be no non-watertight penetrations in the bulkheads. However, these margin lines do not account for the heave action of a hull in a seaway, nor do they account for a real-world list of 15 degrees. They assume that a ship in extremis, with one compartment flooded, would be in a millpond. It’s a lot better than no compartmentation, but it is built to a lower-than-Navy standard.
Ups and downs
The problem with building a yacht to the watertight integrity criteria incorporated by the Navy is that the V-line standard creates a significant inconvenience for crew and passengers. On the Roberts, we couldn’t just waltz through a door from one main space into the next. We had to go up to the DC deck (the deck to which the watertight bulkheads extend) and back down again. It was a nuisance, but it’s precisely this high design standard that saved the Roberts from sinking when it hit the mine.
Any watertight doors below the margin or V-lines that must be closed to prevent progressive flooding represent a substantial compromise in comfort and convenience. Brokers contend that they could never sell such a vessel, which may well be true.
In any event, bulkhead penetrations below the margin lines (the point to which the bulkheads should be watertight) constitute a substantial compromise in terms of seaworthiness and passive survivability. It’s one thing if you’re talking about coastal vessels that venture no more than a few hours from protected waters. The problem is that 200-foot yachts are capable of crossing oceans and are marketed as such.
Judging from the online video and photos, both Yogi and the cruise ship Costa Concordia listed heavily to one side and remained afloat in that position for a while before sinking in the one case and settling on rocks in the other.
One objective for the naval architect is to design a ship so that when partially flooded it will settle and sink on an even keel so it can be evacuated in an orderly way, with the lifeboats remaining functional. If stability is so marginal that once the hull is partially flooded it becomes more stable on its side than its bottom, then you end up with passengers who are unable to move around at all, let alone evacuate in lifeboats.
This is one of my biggest objections to modern cruise ship design; if it does not stay nearly on an even keel for a prolonged period after damage, it physically cannot be evacuated.
When ships start looking out of balance to the practiced eye, they probably are.
When you open a yachting magazine, you read about things such as how little the hull draws, the commodious superstructure, the great view from the upper- and lower-deck staterooms, the large swimming pool on the top deck and the large tender garage at the stern. This tender garage usually leads through a watertight door — one that’s supposed to be kept closed by the crew when the ship is at sea — to the interior of the vessel. But when is the last time you read about the stability of these yachts?
All of the features mentioned here may sell yachts, but in the extreme they can work counter to seaworthiness. A well-found yacht strikes the proper balance between accommodations and comfort and seaworthiness.
Pushing the envelope
Every time you incorporate a feature that diminishes seaworthiness, including stability and resistance to passive flooding, you compromise safety. Think of these design elements as part of a risk management policy, as checks and balances against the unforgiving sea.
It’s unreasonable to expect a yacht to be built like a Navy ship in terms of subdivision simply because Navy ships are designed to absorb mine blasts and torpedo hits, and yachts are not. However, a seagoing yacht should be every bit as stable and resistant to capsize in the intact and the damaged (partially flooded) condition to which it is designed. So the one-compartment yacht, when it loses its engine room to flooding, should be as stable as the warship that loses three compartments to mines.
In my view, vessels that push the stability envelope — from small craft to cruise ships — simply do not have the righting energy to keep the ship upright when the free surface effect of water in the bilge diminishes what stability there is.
You hear cruise company executives and captains talking about how light the superstructures on these ships are, that the heavy weights — machinery, fuel and water tanks — are down low, and that center of gravity is what matters. This is true if the ship is sitting still in a millpond. The problem is that the dynamic effect of wind, especially a beam wind with the sail area created by those enormous superstructures, and the gyradius (think of a flywheel) created by the distribution of mass when the ship is rolling pushes stability to the limits. In other words, it’s not just the vertical center of gravity that matters; the distribution of weight and the sail area of hull and superstructure also affect dynamic stability.
Going for a dip
When I see a 14-deck-high cruise ship with the superstructure literally reaching from the forecastle to the transom, and on the top deck is a 40-foot swimming pool, I can only marvel at the folly of these designs and hope the ships stay within a short sprint of a lee shore.
Although stability calculations no doubt take into account the free-surface effect of the water in the pools, this is one more design element that is contraindicated (if I can borrow that word from medicine) for the mariner.
The officers assigned to these ships must be well-versed in steering around moderate weather and avoiding putting wind and seas on the beam because of the heeling moment created by those colossal superstructures. And what happens if the active fin stabilizers, which when operational reduce roll by 70 or 80 percent, fail in heavy weather?
Hubris trumps design
The Costa Concordia accident, of course, never should have happened. Even the best designs cannot survive extreme damage. If the Navy captain of a three-compartment warship were to buzz an island as if he were on a ski boat and rip a 160-foot gash in the side of his 820-foot (at the waterline) ship — 20 percent of its waterline length — then it, too, would have sunk. Even a Navy ship could not have survived opening four or more compartments to the sea.
The laws of physics describing the effects of inertia, mass, gravity and buoyancy have not changed in the past 50 years, but the key assumptions behind yacht and ship design sure have. Compare the Queen Mary, built in the 1930s, or Aristotle Onassis’ yacht Christina O, built in the 1940s as a Canadian frigate, with a modern oceangoing cruise ship or yacht, and ask yourself which era’s product you would rather put to sea in.
If naval engineers and professional mariners put aside their training and judgment, relying more on computerized hydrostatics programs than on common sense and at-sea experience, they must have perfected the art of career-driven rationalization.
And then there’s the human element. Even the most seaworthy designs can’t compensate for the extreme acts of men or nature, and it’s obvious that we need to get back to basics and reverse these design trends.
What can you do? Vote with your wallet, of course. Altering consumer demand is the surest way to seaworthy designs safely moored to reality rather than fantasy afloat.
This article originally appeared in the June 2012 issue.