One very important problem of voyaging in powerboats is controlling the vessel’s motions in waves, especially rolling. Long-range powerboats have fine, smooth hulls for maximum efficiency, and these characteristics exacerbate roll. Fortunately, there are a number of techniques to reduce roll motions. Each has its advantages and disadvantages, but one or a combination can be used to make powerboat voyaging safer and more comfortable.
A boat rolling in waves is a damped spring-mass system, like other physical systems from buildings in earthquakes to musical instruments to radio receivers. Similar physical principles govern all of these systems. The handiest such system is a simple pendulum. A plastic rod-type coat hanger makes a convenient pendulum. Suspend it by one corner over your finger so it is sideways. The spring of this system is the couple produced by the weight of the coat hanger acting down and the support of your finger acting up. Pull the coat hanger off vertical and let go. The weight and the support forces are no longer in line, and a sideways restoring force appears to bring the coat hanger back to vertical. However, the mass of the coat hanger acquires some velocity toward the vertical position, so when it reaches vertical, it overshoots. By the time it stops, it is well over on the other side, and the action repeats until the damping effect of friction against your finger and through the air takes away all of the initial energy. Notice that slight movements of your finger are sufficient to get the coat hanger swinging quite well, especially if you move your finger in time with the coat hanger’s frequency. Any mass-spring system has a well-defined natural frequency (or Eigenvalue, if you want to impress people). It is said to be in resonance with forces that happen to excite it at this frequency, and will actually magnify the exciting force, often by a factor of 10 or more. The magnification ratio is dependent on the damping. Tape a stiff paper card (a magazine blow-in card is good) to the hanger so that it catches the air as the hanger swings. Naturally the swing will die down quite quickly and it will swing much less for a given amount of movement.The boat as pendulumA boat acts like a pendulum, with its weight supported at the "metacenter," the point through which the buoyant forces appear to act. It rolls under the influence of wave action in exactly the same way as the pendulum with the couple between buoyancy and weight acting to restore the boat to vertical, overshooting the upright position each time. The boat has a characteristic roll period based on mass and stability. In fact, the roll period is a good measure of stability. Measure the roll period in various load conditions in calm water. Use a boat hook to push on the dock, time the elapsed period for several swings and divide by the number of swings. When the roll period increases, it is a warning that stability may be compromised due to flooding or increased topside weights such as icing.From our simple pendulum, we can see that adding damping is one way of minimizing roll, and the oldest method is like the card: steadying sails. A small sail can add a great deal of air drag, which will significantly cut down on roll. Fishermen have long used steadying sails. In order to prevent flogging, a sail should be cut dead flat with a hollow leach (which also makes it inexpensive) and be rigged boomless. One reason sails work well is that fluid dynamic forces go up with the square of speed. The portion of the sail near the top of the mast is traveling through the air very fast due to the long distance from the roll center (roughly the waterline) so the forces generated by it are large. These forces also produce a large moment, again because the distance to the roll center is large.Sails can be inconvenient and cause other problems, especially with steering. Water is 400 times denser than air, so underwater appendages can be much smaller than sails. Most ships use bilge keels: shallow, long fins welded to the hull at the turn of the bilge. Typically they stick out a foot or so from the hull of a large ship over about half the length of the ship. Bilge keels typically reduce roll by 50% or more. Bilge keels are a good first step toward controlling roll and should be considered, especially on round hulls. Though their effectiveness is limited because they can’t be very far from the roll center, they are easy to fit, cheap, require no maintenance, and only add a very small amount of drag. Active fins
Some yachts and ships, especially fast military vessels, use active fins for roll control. These fins usually retract into the hull and are wider than bilge keels, but their big advantage is that they are automatically controlled by gyros and rotate to oppose roll by providing lift. Passive damping devices must allow a fair amount of roll to produce damping, but such active devices actually can eliminate all but the small amount of roll required to activate the control system. The chief disadvantage of these devices is their cost and maintenance, the amount of interior space they require, and the fact that they only work when the boat is going fast enough for the fins to develop lift (they also produce a small drag penalty).
A small yacht could use moveable appendages such as bilge keels or fins. I am currently designing a shallow-draft, narrow vessel for inland waterway cruising. It has a simple pair of leeboards fastened to pivots at the sheer strake, port and starboard. The boards extend three feet deeper than the bottom of the boat when down and will damp roll considerably. In shallow water, they will either pivot up or be removed altogether.
Paravanes and flopper-stoppers are another removable damping appendage. Since the length of the arm from the damping device to the roll center substantially increases the damping effect, setting relatively small fin or plate dampers well outboard hanging off an outrigger can be very effective. Paravanes are usually steel plates shaped roughly like delta-winged jets. They are balanced and rigged so that when the supporting wire goes slack they dive rapidly. When pulled up by the wire they assume an angle to the water that produces a large downward force on the rigging, suppressing roll. Like active fins, they require that the vessel have some speed ahead to work well, and are sized for a particular range of speed.
Flopper-stoppers are generally some sort of disk, plate, or sometimes even a leaky bag. They use pure drag (or in the case of the bag, the weight of the entrained water). They are meant for use when the boat is stopped. These devices are very effective at roll control and are often used by research vessels and fishermen as well as by yachts. It is worth remarking that the forces on these devices are not trivial and that the outriggers and other rigging must be strong. The disadvantages of these devices includes the difficulty of handling them, the drag they produce, and the narrow range of useful speed (unless the paravanes are changed).Anti-roll tanks
In addition, D.W. Bass of the Memorial University in Newfoundland recently discovered one more important problem: In 1990, the Canadian F/V Straits Pride II capsized with loss of several lives. Bass determined that this occurred when one paravane was carried away in bad weather. Due to bad conditions, the crew couldn’t bring in the other paravane, on the lee side, and it acted to increase, rather than reduce, the roll of the fishing vessel. Other fishing vessel losses are suspected to have occurred in the same way, though no survivors are available to confirm this. Basically, this happens because, as the vessel rises and rolls away from a crest, the paravane on the lee side is pulled upward, producing a strong downwave moment. There are other effects involved in this as well, but the key point is this: If one paravane is lost, it is imperative that the other be either recovered or cut free. Anyone using paravanes should make sure that it is possible to release the paravanes no matter what.In order to eliminate this danger, to provide roll control at a range of speeds, and to eliminate the drag of the paravanes, Bass decided to experiment with a common roll-control device for large ships: anti-roll tanks. Anti-roll tanks depend on the effect of water sloshing in a tank to partially counteract the forces of roll. The tank is designed so that it is nearly resonant but out of phase with the boat’s roll.
As the boat rolls one way the water starts to slosh to the low side, but it is a bit late. By the time the water reaches the low side, the low side has become the high side, and the weight of the water acts to counteract the roll, as does the inertial force of the rushing water. The tanks also reduce the stability of the boat slightly and thereby shift the resonant period.
Bass installed tanks on several trawlers out of Newfoundland and found that the tanks were actually more effective than the paravanes.
We can use our coat hanger experiment to demonstrate this system. This time, hang a wire coat hanger (with the hook twisted so it is in the same plane as the plastic one) to the lower corner of the plastic hanger. Notice that, when you move your finger, the plastic hanger moves much less (and the wire one moves wildly). Energy is being transferred to the wire hanger. In the same way, roll energy is transferred to the water sloshing in the tank. By adding baffles or nozzles, the energy in the tank can be damped.
Large ships often used tanks in the form of a U-shaped tube, but a trawler yacht tank would be a more modern form comprising a pair of tanks connected by a narrow, full-depth trough. This design has a wider resonant range and is faster, better matching boat-roll periods. The period can also be tuned by setting the depth. The tank would contain about 1.5% to 2.5% of the weight of the boat and reduce metacentric height about 20%. The fluid could be sea water, fresh water, or even fuel. The main disadvantage of anti-roll tanks is the space they take up, but, especially if they are designed in from the start, this doesn’t have to be a problem.
The design of anti-roll tanks should be done by naval architects with experience in ship motions, but computer programs for optimizing tank design are well proven, and, if done by experienced engineers, their design is routine and quite reliable.
Paravanes and active fins are common on power yachts, but bilge keels, movable appendages, steadying sails, and anti-roll tanks all can have a place in power voyager design and should be carefully considered as well.
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Christopher D. Barry’s experience includes commercial ship, ferry, fishing vessel, tug, and workboat design with consultancies and shipyards in the U.K., California, and Washington state. Barry now works for the U.S. Coast Guard and is licensed in both naval architecture/marine engineering and mechanical engineering in Washington and in mechanical engineering in California. The opinions expressed herein are those of the author and do not represent official policy of the Coast Guard.
Christopher Barry can be contacted at cdbarry@hotmail.com.
