How Gyrostabilizers Work
By Paul Steinmann
Product Manager – VEEM Gyro
By Paul Steinmann
Product Manager – VEEM Gyro
A marine gyrostabilizer is a device for reducing the rolling of boats and ships in waves.
The device comprises a flywheel mounted in a gimbal frame allowing two of the three possible rotational degrees of freedom. This gimbal frame is then rigidly mounted to the hull of the vessel**, with the flywheel gimbaled within the frame. Most often the device is located in the engine room of the vessel.
There are three inter-twined parts to the process of creating gyro- stabilizing torque. Note that each of these things is simultaneously occurring at the same instant in time, but it is helpful to consider each of them separately. Once the flywheel is spinning, the following process leads to the development of a stabilizing torque that opposes rolling motion:
The physics that causes these inter-twined actions is called gyro-dynamics. If the flywheel spins in the opposite direction, the induced precession motion will be in the opposite direction, but the stabilizing torque will be identical.
Because a gyro’s roll stabilizing torque is created by the rolling motion itself, there is absolutely no time delay, or lag, between the wave induced rolling motion and the stabilizing torque produced by a natural precession gyrostabilizer.
The result is an amazingly smooth application of the massive stabilizing torques produced. In practice, the experience of turning the gyro ON is fundamentally different from fin stabilizers. There is simply a calm, relaxing reduction of rolling motion.
This sensation has to be experienced to be understood. For too long, the yachting community has believed that the trade off for a reduction in rolling motion was an unpleasant jerkiness. This no longer is the case.
There is simply a calm, relaxing reduction in rolling motion.
So, what are the main differences between fins and gyros?
Gyros are Safe for Swimmers – Many Captains will not run zero speed fins when guests are swimming in the vicinity of the yacht for safety reasons. This understandable and responsible, however this compromise between the comfort of those onboard, and swimmers is not required with a gyro installation as there are no appendages in the water.
Reduced Drag, Higher Hull Efficiency – On most yacht installations, selecting a gyro over zero speed fins will result in Higher Speed, Increased Range, and Fuel Savings. The trade off is between the increase in mass between a fin installation and a gyro. The increased mass does represent a hull drag cost, however when compared to the drag of inefficient low aspect ratio zero speed fins (especially if they are actually working), a significant net reduction in drag is achieved.
No Risk of Grounding Damage – We have all experienced or heard stories of stabilizing fins being damaged by collision with floating debris, or grounding. This usually results in time consuming and expensive dry-docking for repair of replacement. This is simply not possible with a gyrostabilizer located inside the hull.
No Fouling with Nets or Cables – The risk of fouling fishing nets, long lines, buoy anchors, or cables is removed completely with a gyrostabilizer located inside the hull.
No Equipment Outside of the Engine Room – By locating the gyrostabilizer(s) in the engine room, noise levels are reduced as compared to fins (most important at night), and there is no requirement for technical personnel to enter the owners spaces for operational or maintenance tasks related to fins.
No Dry-Docking for Maintenance, Ever – Dry-docking is a hectic and crammed period, with many systems requiring attention. A VEEM Gyro can be fully maintained (including major over-haul) within the vessel. So take a few lines off the docking list by selecting a gyro over fins.
Simple Installation – No Need to Run Cables and Piping Through Frame Penetrations – Enough said. Having the gyro delivered as a fully self-contained item of equipment saves a vast amount of time, effort and money coordinating frame penetrations, cable runs and piping runs through the hull.
Gyros Alone Cannot Control Steady List, so Install with Transom Flaps or Interceptors – One characteristic of a gyrostabilizer is that it cannot sustain a stabilizing torque for an extended time (they are great at opposing motion, but when the motion stops, they stops creating torque). This means that steady list angles due to wind heel or induced during turning maneuvers, cannot be corrected by a gyro acting alone. Fortunately there is a highly efficient solution to this.
In order to optimise trim (to maximise fuel efficiency, and speed) and to manage list angles, it is recommended that the gyro be installed with either transom flaps or interceptors. By doing so, you get all of the comfort and low drag benefits of the gyro (including no ugly appendages prone to damage and fouling), as well as steady state trim and list control. Both trim flaps and interceptors are extremely efficient at controlling steady state running trim and list. Both solutions also maintain clean hull lines free of appendages and their costs.
VEEM can provide integrated control of the gyro(s) and an auto trim/list system. This system provides both auto trim and auto trim management. By allowing the gyro(s) to do the dynamic stabilization work, the transom flaps or interceptors power supply can be compact and efficient.
When two pairs of fins are installed on a vessel, they can be used to correct trim and list. However this detracts from the force available to roll control and is relatively inefficient from a drag perspective.
In fact, just as important is the way in which precession motion is controlled. The major considerations that define the performance of a gyro stabilizer are: flywheel angular momentum, the precession range allowed, the maximum precession rate allowed, and the ability of the gyro stabilizer to maintain full precession range when vessel rolling rates are low. All of these considerations are handled differently by the various vendors of gyro stabilizers. Understanding exactly how each unit works will allow the most informed selection of the best gyro system for your application. A good place to start is to find out exactly how much stabilizing torque is generated across a range of rolling periods. This will unearth many of the considerations discussed above. It is also very important to understand what the operational envelope of the gyro stabilizer is. Will the unit continue to operate in rough conditions when you need it most? In what conditions (if any) will the unit shut down or de-rate to protect itself?
This is theoretically possible, but not a practical reality. The resulting uncomfortable harmonics introduced into the rolling motions of the yacht would create a significantly less comfortable experience for those on-board. Given that there is a finite range of precession available before the stabilization torque starts to increase rolling motion, if you accelerate precession motion through some of that range then you need to decelerate this motion somewhere else in the cycle. These accelerations can be felt by guests as ‘wobbles’ in the rolling motion that are hard for a human to predict and therefore make walking and general balance more difficult. So while it theoretically possible to do this, it is not a practical solution. If it sounds too good to be true…then it probably is.
In fact in very small waves where the gyro is not overpowered, a vertical axis gyro could work without any control system whatsoever. A horizontal axis gyro would also work a little, but the very high resistance to precession of the slew ring bearings used on these systems would significantly limit the stabilizing torque generated. Stabilization torque is not caused by the precession axis braking torque, it is caused by the precession oscillation rate combining with the angular momentum of the flywheel to generate torque in the roll axis. The precession braking is only applied to manage the precession motion to within a nominated precession oscillation range and in most cases also to limit the rate of precession oscillation so that the gyro torque created is effectively capped allowing the supporting structure to be designed to withstand a defined maximum level of load.
Because a gyrostabilizer produces a pure torque, it can theoretically be located anywhere on the vessel. The stabilizing torque will always neatly oppose the rolling torque whether on or off vessel centre-line, or whether forward or aft.
To avoid high vertical accelerations that might shorten the life of the bearings, VEEM recommends that the unit(s) are located aft of mid-ships. However when required it is possible to locate them up to 70% of LWL forward of the transom.
So long as the overall mass distribution of the vessel is maintained, there is absolutely no performance disadvantage to locating the gyro(s) off center- line.
If the gyro(s) are located more than 2m above the waterline, please discuss this with VEEM. The flexible rubber isolation mounts may need to be transversely supported to prevent over-load.
In most cases, the convenience of electrical power supply and suitably strong supporting structure will result in the gyro being located within the engine room. This has the added advantage of enclosing the gyro within a noise lagged space. Where the gyro(s) are located outside of the engine room, noise isolation considerations should be addressed.
Gyrostabilizers can be conveniently located as far from the owner’s spaces as practical. This helps to eliminate annoying night-time noise, and to ensure that service technicians do not need access to the owners spaces.
So in summary, the gyro can be located:
• Up to 70% LWL forward of the transom
• Off centre-line
• Up to 2m above waterline
In fact, a pre-1900 gyro stabilizer invention claimed to work without precession. This was eventually debunked and the invention discredited. The stabilizing torque is created by the combination of the flywheel’s angular momentum and the precession oscillation rate. If the flywheel does not precess, no stabilization torque is generated. This is how a gyro stabilizer can be turned OFF without stopping the flywheel from spinning. The precession oscillation axis is simply locked.
In fact a spinning flywheel does not have any inherent stability, or tendency to remain at its current orientation. As we have discussed above, a flywheel does have very specific gyro-dynamics that cause it to bend and applied torque through 90 degrees as a rate of precession, or to bend a rotational motion through 90 degrees as a torque. However there are many specific flywheel applications where the flywheel does provide a stabilizing influence. These include the spinning top children’s toy, the front wheel of motorcycle or bicycle, and happily, a marine gyro stabilizer. However each of these applications applies gyro-dynamics in a unique way, and is not related to any inherent stability of a flywheel, but the way that it bends torque through 90 degrees.
The key technical features that differentiate between modern marine-gyrostabilizer products are as follows:
Although theoretically both of these approaches produce effective stabilizing torque, there are a couple of noteworthy differences.
The main problem with a horizontal spinning axis is that it does not allow the use of natural precession. The resistance of the slewing-ring type bearings used is excessive, and requires that the precession motion be driven to overcome this resistance.
Another limitation of a horizontal spin axis is that it is not convenient to provide the precession motion with an equilibrium point at zero precession angle. For vertical-spin-axis gyros, it is possible to arrange the precession bearing shafts so that the CG of the cage assembly holding the flywheel is lower than the shaft-line. This ensures that the precession angle always tends towards vertical. This feature allows the advantages of natural precession (see over) to be utilised.
VEEM Gyros all feature a vertical spinning axis.
Gyrostabilizer precession motion is a naturally occurring response. Utilising this naturally-occurring-precession-motion means that the stabilizing torque of the gyro is always perfectly synchronised with the vessel roll, regardless of how quick, slow or random the rolling motion may appear. This eliminates and inefficiencies caused by slow sensors, electrical or hydraulic systems and ensures a perfectly timely response in all conditions.
For vessels with longer roll periods, the lower roll rate in small waves may result in less torque created to provide full precession range. This can create a band of rolling motion in which the gyro either does not respond, or responds less vigorously than it could. Onboard, this may be seen as a lack of responsiveness in small waves. Driving the precession motion, can virtually eliminate this dead-band. This option may be advantageous for some mega- yachts, or larger commercial vessels. The downside is the additional power requirement, additional space, and additional cost of the motive power unit. The driven precession option requires either PTO hydraulic pumps or a separate power pack.
I recommend considering driven precession only where the benefits clearly outweigh the disadvantages.
VEEM Gyros are supplied with either natural or driven precession.
Most modern gyros feature active precession control. This is a key technology advancement that gives modern gyros their high efficiency across a wide range of conditions. VEEM Gyros have a highly sophisticated control system which is a point of differentiation to other market offerings.
In order to ensure optimal performance across a wide range of wave conditions without the need for user adjustments, the control system should also be adaptive. Adaptive control systems automatically search for optimal control settings without needing to be tuned by an operator. When executed well, this means the system is both simple to use, and also continually optimised.
VEEM Gyros all feature active, adaptive control systems.
A key, but often over-looked feature of gyrostabilizers is the robustness of the base frame and the precession-motion-control system. As larger waves cause larger rolling rates, the torque induced in the precession axis continues to grow. In order to control the increased precession rates, the mechanism for controlling the precession motion must be able to over- come these ever increasing torques. When the torque induced in the precession axis exceeds the capacity of the precession control mechanism, the gyro must either shut down to protect itself from damage or progressively de-rate to achieve the same. An under-sized precession control mechanism will result in premature shut-down as wave conditions build.
VEEM Gyros are designed and built to ensure that they continue to provide roll stabilization as seas become severe.