In October 1998, a containership limped into Seattle. It was the victim of the largest ever loss of containers while on passage. The ship encountered a violent storm that lasted 12 hours. When it was over, a third of the 1,300 containers were lost overboard and another third were smashed beyond salvaging. What this ship had experienced was a phenomenon known as parametric roll resonance. Parametric roll first was recognized nearly 50 years ago but was believed to affect only small vessels of poor stability in following seas. The emergence of parametric roll in containerships has refocused attention on the problem. Studies are underway at the American Bureau of Shipping, the Society of Naval Architects & Marine Engineers, the International Maritime Organization and others.
There are, of course, less dramatic and more common reasons for roll damping – preventing seasickness being an obvious one. And there are other reasons to stop a power voyaging vessel from rolling: preventing injuries due to falls, making it easier to work on the boat, avoiding potential capsize and just plain old comfort while at sea.
There are four basic types of roll-damping systems: antiroll tanks, bilge keels, paravanes (flopper stoppers) and active vanes. The first three are passive systems – they have few to no moving parts and are mechanically simple, and therefore, are very reliable. They are not as effective underway as active vane systems but are more effective at low speeds and at anchor.
These are tanks fitted port and starboard on the boat. They function by allowing water (or, in some cases, mineral oil) to flow from the low side of the boat to the high side in opposition to the vessel’s direction of roll. In some sea conditions, with optimized tank/vessel design, roll reductions on the order of 50 to 60 percent have been achieved. In other sea conditions and less than optimized designs, the degree of roll reduction varies considerably.
Antiroll tanks generally are not well suited for smaller vessels due to their space and weight requirements and the faster roll rate of the smaller hull.
Also, they are relatively complex to design, install and fine tune, making them an expensive option.
In the bilge-keel system, ballast is carried within twin keels fitted under the turn of the bilge about midships. When the vessel rolls, the keels move large masses of water and create turbulence, which acts to reduce rolling amplitude and acceleration. Bilge keels have a further benefit in distributing the mass of the hull farther from the centerline, providing an inertial resistance to rolling. Roll reductions of between 40 and 65 percent are said to be achievable. However, according to Jim Leishman, vice president of Pacific Asian Enterprises, builder of the Nordhavn trawler yachts, “We’ve stayed away from these. In our experience, they’re only about 10 to 15 percent effective, and they’re very difficult to install without disturbing the flow of water along the hull.” He went on to explain, “If you don’t get them just right, you wipe out the hull’s efficiency.”
Known as flopper stoppers, these are aerodynamically shaped weights (often referred to as “birdsï¿½VbCrLf or “fishï¿½VbCrLf) attached to lines suspended from the ends of long booms. In rough seas, the booms swing out and the birds are lowered into the water. As the vessel rolls to port, the starboard bird rises. The resistance offered by the water to the wing surfaces of the bird slows the rolling motion of the boat. These systems are fairly effective with roll attenuations on the order of 40 percent or more. Primary advantages are that they are effective at slow speeds and while at anchor; they only drag when deployed; and they are easy to retrofit.
They can be a challenge to deploy and retrieve in heavy weather and can present some hazardous situations. For example, the loss of one paravane could impose some dangerous stresses on the vessel; the boom could dip into the water, tripping the boat and again setting up some serious stresses. Also, the bird could foul or snag on fixed fishing gear or debris, or if not towed at an appropriate depth, it could broach (break to the surface) and swing freely about the deck.
Active-fin stabilizers consist of a pair of hydraulically actuated fins – much like the dive planes on submarines – fitted port and starboard about midships below the waterline. They’re controlled by a gyroscope and motion sensors, which react automatically to counteract with precise speed and degree to the magnitude and speed of the rolling motion. “Active fins are especially suitable for boats over 50 feet, since paravanes can get pretty unwieldy on boats this size,ï¿½VbCrLf Leishman said. “Effectiveness is close to 60 to 70 percent, in our experience,ï¿½VbCrLf he added.
They are most effective at maximum vessel speed, less effective at lower speeds and generally ineffective when the vessel is making no headway; although both Naiad and Quantum offer models designed for zero-speed applications. Drag induced by the fins is always present; however, it is not a major factor, something on the order of 1 knot at 25 knots for an 80-foot vessel, according to active-fin manufacturer Vetus. However, on smaller boats, this drag – if compensated for by advancing the throttle – can represent a significant increase in fuel consumption, an important consideration for passagemakers.
Of all the systems discussed here, offshore voyagers typically opt for either paravanes or active-fin systems, depending on the size of the boat. For vessels under about 50 feet in length, paravanes are the system of choice; above that length, paravane gear gets pretty massive and difficult to handle, so active fins are more often selected. “The exception,ï¿½VbCrLf said Dan Streech, president of PAE, “is the owner who comes from a sailing background, comfortable with sail- and line-handling, reefing, furling, and sail-changing in heavy weather.ï¿½VbCrLf
Some owners choose both. Milt Baker is the former founder/owner of Bluewater Books & Charts and a principal organizer and manager of the Nordhavn Atlantic Rally 2004, in which 18 trawler yachts sailed 3,800 miles from Fort Lauderdale, Fla., to Gibraltar. After observing all the vessels in the fleet and talking to the skippers about their experiences, Baker decided to order a Nordhavn 47 – with both paravanes and active fins.