Anti-Roll Performance

- The worst roll motions tend to occur at low and zero speeds as boats roll less and more slowly than at higher speeds due to the increase in hull damping.

- At rest and at low speeds the uncontrolled roll will be dominated by resonant or natural roll and it is this motion that causes discomfort and motion sickness.

- Properly sized, Seakeeper gyros can eliminate 60-90% of the resonant boat roll whether the vessel is slowly moving or at anchor.

- For zero speed stabilization, an external fin system utilizes hydraulic power to rotate large "panels" through the water at high rates of speed to attempt to hold the hull in place and counteract the wave action and resulting boat motion.

- External zero-speed fins are considerably larger than standard underway fins as they must actively move large volumes of water to stabilize a vessel the hydraulic power required to move the fins is much greater than the electrical power to spin the gyro. These oversized external appendages result in significantly more drag and must be compensated by increasing engine horsepower or acceptance of a slower top speed. In addition, external fins negatively affect fuel efficiency due to the additional drag.

- To be most effective, external fins should be located close to mid-ship on a given vessel. Unfortunately, with modern yacht designs, this forces builders to locate the shafts and hydraulic actuators near the main stateroom which creates the potential for noise during fin operation. In contrast, gyros can be mounted virtually anywhere there is access to the hull frames or stringer system. For planing boats, gyros should be located aft of mid-ships while on displacement boats anywhere is acceptable; and there is no requirement to be on the hull centerline.

- The maintenance requirement of any item that is constantly submerged is, by definition, going to be far greater than any item installed inside a dry machinery space. In addition, the requirement of through-hull shafts, seals and fittings that can be damaged if an impact or grounding occurs adds another level of risk that must be considered. Gyros are always located safely inside the hull where there is no risk of damage.

- Fins can correct small steady/static heels or listing due to beam winds, off center weight or long period rolls running downwind due their inherent ability to apply a steady lift force under the hull while the vessel is in motion. In contrast, gyros cannot supply constant forces and as a result cannot correct a heel or list angle. This is not generally an issue as the vast majority of boats have a need for roll damping at zero and low speeds where heel and list angles are not an issue.

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