Roll stabilization

Displacement powerboats, more commonly known as trawlers, ocean motorboats, or passagemakers, have the long voyaging range and speed of fast sailboats combined with the convenience of operation and comfortable living quarters of a powerboat. These vessels, although well done by a number of naval architects, are still plagued with the basic problem of all monohulls: rolling. The scourge of monohulls, rolling has been a prime target in development efforts aimed at improving the ride of displacement powerboats. No amount of hull shape improvements has been able to eliminate the rolling problem. Early developments in roll control of displacement powerboats concentrated on passive anti-roll flopper-stoppers pioneered by fishermen and adapted to pleasure craft by power voyaging guru Capt. Robert Beebe (see his book Voyaging Under Power, recently updated by Jim Leishman, published by International Marine). On a fishing boat already carrying a mast, boom, and sundry other superstructure rigging, this was a natural addition that provided the fisherman with a more stable working platform, enabling him to endure a longer stay at offshore fishing grounds and to operate in rougher seas. For the conservative fisherman it also added the advantage of not being another mechanical/electric device that needed to be maintained along the way. The same advantages were quickly picked up by Beebe and other early passagemaker enthusiasts, and those advantages have stood the test of time. A number of other passive stabilization means such as bilge keels, endplates on regular keels, more ballast, and steadying sails have also been tried but found wanting in capability. The last may come as a surprise to former sailors who are well acquainted with the effectiveness of sails for damping out the roll of their boats in most seas. They must, however, also remember the relative size of those sails and the complex of spars, rigging, and crew demands that went with them. High-tech mechanical stabilization systems designed around gyroscopes, electronics, hydraulics, and pneumatics have also taken their place alongside flopper-stoppers as today’s state of the art. We have grown so accustomed to complexity in our everyday lives that we tend to accept it as being a better way of life in spite of the premium price tags and maintenance demands that go along with it. Active fin stabilization is a well-developed bit of technology and has become very popular on contemporary passagemakers, especially the larger ones. Much of the popularity of fin stabilization can be attributed to the fact it operates underwater and remains hidden, which is unlike the visible assemblage of poles and rigging required for using flopper-stoppers. Cosmetics aside, it is another option to make the displacement powerboat more comfortable on long passages. Displacement powerboats that spend their active time in casual cruising along inland waterways, on lakes and sheltered bays, or for short coastal cruising do not really need roll stabilization. This is particularly true of the smaller displacement powerboatssay, under 40 feethaving hard chines. The discomfort of moderate rolling can be tolerated by most persons for limited periods of time, especially when there is fun to be had on the water.Flopper-stopper design The key to flopper-stopper roll stabilization is a pair of submersible stabilizers (often called fish and sometimes improperly called paravanes) suspended from inclined stabilizer poles on both sides of the boat. The stabilizers are towed 10 to 15 feet underwater, the depth being such that they will not surface in a rough sea. As the boat tends into a roll, the stabilizer on one side will move downward while that on the opposite side will be pulled upward. A downward moving stabilizer is designed to nose-dive with little resistance because of its heavily weighted nose and large tail plane. The stabilizer being pulled upward becomes horizontally oriented, providing maximum flat plate upward resistance to the roll. As soon as the boat starts to roll in the opposite direction, the stabilizers reverse their action to resist the roll. Simply put, the downward roll of the boat causes the one stabilizer to dive without resistance while the other stabilizer flattens and resists the roll. There is never any slack in the hanging spring line, hence, no jerking at roll reversal. The stabilizer is a unique design developed over years of experimentation. It is shaped like a modern delta-winged airplane and ballasted to plane at a range of speeds. The delta-shaped stabilizer varies in size from 1 1/3 square feet per unit for a 40-foot boat to five square feet for a 110-foot yacht. The only adjustment involved is to shackle the spring line from the pole to a predetermined speed hole in the stabilizer plate compatible with the boat’s chosen cruising speed. From there on, everything becomes automatic. The rigging of the flopper-stopper should be old hat to the person coming off of a sailboat, although the elements may be far less exotic. Poles may be galvanized pipe with sturdy welded end fittings mating with strongly reinforced deck fittings. Lighter, extruded aluminum, spar-type poles from the sailing industry make handling somewhat easier. Swivel joints allow for some misalignment as the poles are raised and lowered not unlike the spinnaker pole problem. The running rigging is a combination of nylon rope, stainless steel cable, and some chain. The latter is inserted in the spring line to minimize the humming sound transferred to the hull. Rigging and derigging of the flopper-stopper is enough of a chore that it should be done only once for each passage, barring unforeseen problems arising en route. Handling of the flopper-stopper usually requires two persons, although it can be specially rigged to be handled by one. It is awkward and should be rigged/unrigged only when the boat is still in quiet waters. If there is room in an anchorage to leave it rigged during a layover, leave it rigged and replace the stabilizers with harbor anti-rolling devices.Active fin stabilizers Many persons object to the ungainly rigging of a passive flopper-stopper and the rigging problems associated with it, so they choose to install active fin stabilizers for roll-damping underway. The fins (usually twoone on each side) are mounted to the hull well under the waterline and so do not change the appearance of the vessel. Fin stabilizers act much like the ailerons on an airplane that make it roll or not roll as the input command designates. On a boat the objective is to prevent rolling, a simpler taskand the stabilization system is designed to automatically react to a wave-induced roll, deflecting the fins in opposite directions to oppose the rolling motion. The system does not demand crew attention except to switch it on and off and make such gain settings as will produce the desired level of anti-roll comfort for the seas being encountered. Fin stabilizers are able to resist up to 90% of the rolling motion induced by the sea while underway. A fin stabilization system consists of three elements: a pair of fins, an actuating system, and a roll sensor. Fins are hydrodynamically shaped for minimum drag and are of low aspect ratio because of the 1) high bending moments they produce on their axle support and 2) to minimize the possibility of damage from floating debris and contact with pilings when docking. The size of a fin is matched to a particular vessel by length and displacement with additional allowances for a particular hull shape. This is a job for the naval architect working with the stabilizer manufacturer. The fins are not small. They range from three square feet in area (each fin) for a 40-foot boat up to 10 square feet for a 110-foot yacht. While most fin installations are made as an opposing pair, two pair can be installed to minimize their size and reduce their exposure to damage. Mounting of the fins must be done by a competent boatyard or by the boat builder. The reason is the high loads that develop in use. Each fin is cantilever-supported by a shaft, and the bending stresses induced in the local hull skin are very high requiring a broad distribution of the load. This is done either with a substantial internal hull reinforcement or an external metal plate. Not only must the loads of normal operation be accommodated, but shock loads from the fin striking a fixed underwater object can be severe. Extreme loads caused by striking a reef, for instance, must not cause the fin to break into the hull and leave a hole. It is preferable that the fin bend back or even break away before the hull is overstressed. Fin damage is somewhat minimized by the fact that the fins are usually located inside the limiting lines of the waterline beam and the keel draft, reducing their exposure to unknown fixed hazards. Lesser hazards, like becoming entangled with surface seaweed, serves only to reduce their efficiency.Hydraulic and pneumatic Two forms of operating muscle are used to actuate the fins: hydraulic and pneumatic, well-developed technologies from the world of machinery. Both are precise and powerful and can be easily controlled. Hydraulic power has the advantage that it can be integrated with other hydraulic systems on the boat such as side thrusters and a windlass. The pneumatic system has the advantage of operating at a low pressure and without an environmentally messy fluid. The argument prevails whether fluid viscosity and compressibility play significant roles in the performance of the competing actuation systems. Interestingly, air is the action fluid in a pneumatic system but is the bugaboo in a hydraulic system. Manufacturers build their cases around their chosen fluid and optimize their system designs so that significant differences seem to disappear. The third element of the fin stabilization system is the sensor or brain of the unit. All systems in operation today use a mechanical gyroscope to measure the onset of boat rolling. The Naiad system uses an unusual hydraulically driven gyroscope that directs signal fluid to servovalves on the actuator units. No electrics/electronics are involved, so electrical failure will not terminate fin operation, but, of course, hydraulic power must still be available. The electrical gyroscope used in other models has been highly developed for both ship and airplane use. As part of an electronic control system, it can be programmed to match a variety of sea conditions. In operation, active fin stabilizers are very effective throughout a range of speeds from about four knots up to the top displacement cruising speed of the vessel. Vessels operating at higher planing speeds (sometimes called semi-displacement boats) can position the fins at a minimum drag angle and rely on hull stability at the higher speeds for roll control.