The watchword is to stabilize. Until a few years ago, only megayachts had appendages that came out of the hull and dampened unwanted movements. Today, however, many systems are available to offer the advantages of a stabilizer even to those who have a 10-meter boat. Especially the so-called zero- speed solutions that work even at berth. Or those that (much less onerously) dampen movements and adjust speed when sailing.
Stabilizers: six models, how they work and price
Among gyro stabilizers, those made by Seakeeper are maybe the best known. They have a rotating part kept under vacuum to increase the rotation speed, thus reducing the necessary mass. The basic model, the Seakeeper2 (rpm: 9000; torque: 5.2 kNm; weight/mass: 188 kg, price 19 thousand euros) is suitable for 8-meter boats up to 8 tons; the biggest model, the Seakeeper 35 (rev/rpm: 5150; torque: 73 kNm; weight/mass: 1778 kg; price around 200 thousand euros), instead, is designed for yachts over 85 feet.
The MC2X line includes nine models of stabilizers from the MC2X5 model (rpm: 4800; torque: 5.5 kNm; weight / mass: 265 kg, price around 17,500 euros) to the MC2X56 (rpm: 3500; torque: 56 kNm; weight/mass: 1400 kg; price …). The flywheel rotation system is on a horizontal axis instead of a vertical one. This is why bearings suffer less friction and maintain lower temperatures, thus not requiring water cooling.
Even the Japanese company brings its expertise to smaller vessels with the ARG 50T / 65T model (rpm: 4200; torque: 5.0 kNm; weight / mass: 234 kg, price around 15.500, 11.600 €) designed for boats from 7 to 10 tons. Among its peculiarities, the battery-based operation with direct current. The largest model of gyroscopic stabilizers of this company is the ARG 375 (rpm: 3250; torque: 37.5 kNm; weight / mass: 910 kg; price: na).
www.samos.it / www.antirollinggyro.com
Zipwake’s interceptors automatically correct speed by reducing roll and pitch. In addition to the comfort on board, they also improve the fuel efficiency of the boat. Suitable for boats from 20 feet , they can be connected to a single electronic control unit managed by a digital control panel with an integrated gyroscope that automatically sends the impulse to the interceptors to correct speed.
Price starts from 2,000 euros.
The most popular interceptors. They work actively and automatically to adjust the longitudinal trim (Automtic TRIM control) and also transverse (Automatic LIST control) on semi-planing or planing boats from 8 meters. They offer the coordinated turn function (Automatic Turn Control), with which they intervene on the heeling angle when turning to cancel lateral accelerations for the benefit of comfort. Basic prices from 3950 euros plus VAT.
The system uses the Magnus effect (read in the text). It combines lightness and immediate functionality (it is totally operational even when it is switched on) typical of a fin with a reduced increase in hydrodynamic friction (if it is switched off, it can also be installed inside the hull). It guarantees three possibilities of use – Zero Speed; Cruising, between 4 and 14 knots;
and Rake, at above 14 knots.
Among the stabilizers “for all”, the great recent growth has been that of gyroscopic (some look like a ball, others a nut, depending on the container that closes them.
The gyroscope, consisting of a high-rotating mass, when perturbed, i.e. when a torque is applied to tilt its axis of rotation, reacts to the phenomenon called “precession” by tilting instead on the perpendicular axis (try with a bicycle wheel). By mounting an appropriate gyroscope on board (for example with a vertical axis) it is possible to exploit the basic fact that the gyroscope has a certain capacity to react to the annoying roll generated by the wave motion by transferring part of the energy on a less annoying plane (e.g. on the pitch plane).
The gyroscope for stabilization purposes can be mounted at any point on the boat plan (bow/poop/left), but for trim purposes the transverse or longitudinal distance from the boat centre of gravity alters the original dynamic characteristics unless corrected by suitable devices (flaps/interceptors). Typically a stern mounted gyroscope worsens the trim (longitudinal trim) by increasing the forward resistance. The vertical position of the gyro on board alters the characteristics of static stability (read tilt angle) as well as dynamic (roll), usually it is mounted as low as possible (never on the fly!).
When the boat starts to roll, the mass does not move, it turns on itself, it reduces the roll thanks to the gyroscopic effect.
Installation on board a gyroscope always introduces greater resistance to the boat’s forward movement, not only because of the greater displacement (more weight on board, more draught), but because it is impractical to mount the gyroscope in a barycentric (longitudinal and transverse) position. As a result, the original trim of the hull is modified, tending to make it more difficult to glide and consume more fuel at the same speed. Contrary to what would be intuitively thought, instead, the stabilizing fins, thanks to their profile studied from the hydrodynamic point of view, introduce a minor increase in the resistance to forward movement (e.g. a good fin of 0.6mq resists at 20kn for 38kg V ALL). Keep in mind that in the boats of our interest the resistance given by the bow wave far exceeds the resistance introduced by wing shaped appendages (such as fins and rudders).
They have a greater ease of installation and more compact dimensions, so much so that they are interesting even for boats from eight meters in length and up.
Compared to fin systems, the gyroscopes are not easier to install, due to the complicated plant engineering (especially for those with forced cooling), the need for cranes for handling: the hull strengthening works necessary for the base for the gyroscope are comparable to those necessary for the fins. The gyroscope also provides important periodic maintenance, being a heavy mechanical system with high rotation (wear), which will affect the life cycle cost, which should be taken into account during the purchase phase. Do not forget that the gyroscope always having to work at full speed to be able to stabilize, affects the consumption of the boat.
The fin, on the other hand, has a consumption more limited to “zero speed” while in gait the consumption is totally derisory (as it is the speed of the boat to amplify the stabilizing effect given by the fin, which therefore rotates a few degrees). For medium and large boats (>15m) the fin system is much more compact and always lighter than the equivalent gyroscope.
Attention, however, the gyroscope must reach its normal rotation speed (Spool-up) to be efficient: this means that it takes about 30 minutes to switch it from off to operational (at this stage the consumption is higher than normal use). Just as it takes as much time, if not twice as long, to switch it off completely (Spool-down). The defect is the weight that is added (from a few hundred kg per ton) and the idea of having a mass of that size in perpetual movement on board.
The third possible option to stabilize your boat is the Magnus effect: a rotating cylinder, immersed in moving water, will tend to move from the side (down or up) where its rotation and the movement of the water have an opposite direction. If the boat is at anchor, the system works just as long as the cylinder is moving. Less expensive and heavier than gyroscopes, these systems always have immersed parts, but about a tenth of a fin providing the same thrust.
IT DEPENDS ON WEIGHT
The size of the stabilizer should be calculated according to the anti-roll torque it delivers, ie the force with which it tends to straighten the boat. Expressed in kiloNewton meters, this means that you need a 5 kNm stabilizer for a boat of about seven tons; a 6.5 kNm stabilizer if the tons become 10 kNm and up to 130 tons of displacement if the kNm produced are 120.
From a user interface point of view, the situation is fairly equal for the various manufacturers since they all have a digital control panel directly on the machine and, often, there is also an app you can download on your smartphone or connected device to manage and monitor the whole system.