By Jamie Benoit
Gyrostabilizers have made an industry resurgence in the past 15 years, now becoming near standard equipment on vessels under 80 feet (24.4m), particularly as their price point, power requirements and overall size have become more manageable. In this regard, they have provided the option of roll reduction equipment in a market sector previously untouched. Active fin stabilizer systems, however, continue to dominate the larger boat space — well into the 328 feet (100m) and larger class.
For most new construction projects, fin stabilizers with the capability of roll reduction both underway and with zero speed of advance are fairly common. There are, however, a few new construction projects that deviate from this convention by offering both gyrostabilizer and fin stabilizers. This trend has also been seen in existing vessels – whereupon gyrostabilizers are added to the vessel to augment an older “underway fin stabilizer system”, as opposed to a fin stabilizer system retrofit. For these vessels, the operating designation is to use fin stabilizers whilst underway and the gyrostabilizer for anchor or dockside comfort.
Despite these industry practices, there are some unspoken recent launches within the megayacht category for which the builder opted to provide a gyrostabilizer(s) as the exclusive roll reduction device for roll reduction across the entire operating envelope. In such cases, criteria such as distinctive design, specific contract or performance measures are generally prioritized over vessel motions.
One public example includes the 207-foot (63m) M/Y Utopia IV, launched in 2018 by Rossinavi. Touted for being best in class for her speed, shallow draft, magnificent drivetrain, sleek lines and designer interior, Utopia IV has earned countless industry accolades.
Unfortunately, her overall open-water seakeeping under certain conditions raised serious doubt and often kept this thoroughbred in the barn. While initially led to believe that the gyrostabilizer — by itself — would perform satisfactorily for all encounters, critical limitations were quickly found with open-water seakeeping abilities. Thankfully, the determined and optimistic owners were not deterred and pursued the expensive process of adding fin stabilizers to restore predictable and reliable comfort to Utopia IV’s guests and crew. By retaining the gyrostabilizer primarily for zero speed use, the inherent benefits of both systems are now able to be maximized.
The theory and science of gyrostabilizers are proven and their usefulness in certain conditions and applications is not disputed. Gyrostabilizers require precession to produce a roll reducing moment.
An underway fin stabilizer, on the other hand, produces the roll reducing moment by a coupling of relative water velocity and fin angle of attack. The precession requirement of gyrostabilizers, however, becomes a critical limitation underway, for example when the precession angle has been exceeded or is too slow to be of use.
Further, the speed of the precession is influential in attaining the rated output of the machine. The direction and timing of precession motion must be further controlled and synchronized, all while constrained within less than 180 degrees of rotation.
As nearly all vessels are considered “weight sensitive,” there also comes practical balance between the portions of roll reduction energy attained by the rotor mass in the gyrostabilizer versus that contributed by its dynamic factors. In a gyrostabilizer, the rotor mass, rate of rotor rotation and rate of angular rotor precession are design variables and become performance-resulting factors — each with secondary implications.
As an example, for a given rated output, a gyrostabilizer can be designed with a larger rotating mass with rotating velocity and thereby utilize a lower precession rate. Alternatively, it can be designed with a smaller rotating mass with a higher rotation velocity and precession rate.
Nearly all stabilization devices are sized with consideration of the vessel’s natural roll period and sufficient bandwidth for variations in load, free surface effects, etc. In this respect and for vessels with short roll periods, the gyrostabilizer can respond well as its precession metrics are in better alignment with the vessel motion. As the roll period increases and as encounter frequencies decrease, however, the precession dynamics are challenged, if not in cases fully limited.
Operationally speaking, conditions include following seas, quartering seas, wind heel, long radius turning and list where the roll period is extended. In this regard, the underway vessel motions exceed the precession range whereupon roll reduction moment is intermittently or perhaps fully lost.
In summary, the industry’s expectations and passenger experience aboard a luxury motoryacht demand limited roll amplitudes, low roll rates, reliability and limited accelerations for the purpose of personal comfort, safety, health and damage mitigation. Where these objectives dictate vessel operations, a balance of equipment can be successfully deployed to best serve the entire operating envelope.
Lesson learned: At present, gyrostabilizers in megayachts have a role but not as the sole stabilizing mechanism.
Jamie Benoit is a licensed professional engineer and holds degrees in Naval Architecture and Marine Engineering. He currently serves as the engineering manager for Murray & Associates based in Fort Lauderdale. He has been practicing for 19 years, and has been involved in retrofit and new construction projects ranging from 26 feet to 585 feet. He continues to provide active support to Utopia IV and her owners. Comments are welcome below.