No one likes rolling. It can be bilious without a sail to steady you, but it’s potentially worse at anchor. That is especially true when the swell aligns with the vessel’s natural roll frequency, thus providing an amplifying effect. So just what options are there to reduce this rolling? Let us remove the commerciality here and speak to the various technologies available.
Passive roll mitigation. In this class, you have “Flopper Stoppers”, Bilge Keels, and Anti-roll Tanks. Flopper Stoppers are the arms that spread out like derricks on either side of the vessel. Attached to those arms are paravanes held in position under the water surface. As the hull tries to roll, the paravane on the side rising will meet resistance from the water. It results in a resisting moment and limits the rolling. About as cheap as it gets but won’t work well in large waves or at higher speeds. Bilge keels have a similar though lesser effect acting to resist rotational motion. Roll tanks do not suit small vessels with limited room aboard.
Flopper Stopper on a Trawler
Active Fin Stabilizer
Then on to fin stabilizers, typically electrically or hydraulically operated and act as it says on the box. They are fins, pointing the leading edge upwards, creates a downward acting force lifting the side and V/V. They have been around for a long time now, with early designs balanced, i.e., a similar area in front and behind the fin, similarly to a balanced rudder. Two drawbacks are a vulnerability to impact damage, coral, logs, quayside. They also need fluid motion to work, so they are ineffective at zero speed. A more modern version has an offset driveshaft, so it is more like a flap than a fin. These produce rolling forces on the hull at low speed (a bit like a kicking flipper), aptly named Zero Speed fins. Some larger stabilizers are of the folding type retracting in to pockets in the hull.
Then we have a newer type of stabilizer based on the Magnus effect. Essentially a cylinder spinning in a laminar flow will produce a force perpendicular to that flow. So we can replace a fin with a rotating cylinder to generate an upward or downward roll moment. At anchor, we can use the same device, but it must sweep back and forth as there is no laminar fluid flow unless anchored in a river or tidal stream. However, one advantage is that they are typically more effective at lower hull speeds, and they can be folded away to lie in line with the hull, thus minimizing potential damage. Again you can fit hydraulic or electrically actuated versions.
Sea Keeper Gyro Stabilizer
Lastly, the final active stabilizing system is the gyroscope. These systems rely on gyroscopic forces acting to counter a change in motion. They are high mass, high inertia machines that are mechanically linked to the hull, so as the hull rolls, they produce a force to counter that roll. A range of sizes are available, and they will work at all speeds from zero to 25 knots and above. They do, however, take up internal space and need relatively significant power to maintain operation.
Whatever type of stabilizer fitted, you should consider the effects of impact. Some fin stabilizers shear off, needing a replacement fin but not the actuator or damaging the hull. Some Magnus stabilizers work similarly, while gyroscopic stabilizers are internal to the hull.
What do we fit Vanguard? DMS Magnusmaster. Self-folding Zero Speed Magnus effect stabilizers. The stabilizers fit into the hull side via cofferdams sighted in the aft cabin. When folded, the stabilizer is shielded from impact by the hull. A NMEA 2000 data feed links to the Praxis MIMIC display for control and alarm functions. Operation is 230VAC.
Magnus Effect Stabilizer (Folds parallel to hull when not in use.)
We also fitted all bunks aligned Fore/Aft, so in the event, nothing is working, we can at least use leeboards for a night’s sleep.