Gyroscopic Stabiliser


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Video demonstration of gyroscopic stabiliser

Inquiry Information

The range of nine gyroscopic stabilisers supplied by Ship Dynamics are tabled in the brochure downloadable above titled "Ship Dynamics Unit Size Range" . These are generally suited to monohull or trimaran type vessels where installations require 1% to 4% of vessel displacement. The largest eight unit sizes supplied generally suit vessels with a roll stiffness (Defined as: Displacement [t] * Metacentric height (GMt)[m]) of greater than 100 t-m (see download brochure). Owners of vessels with a roll stiffness of less than 100 t-m will most likely find the larger units are not suitable for their application and will need to refer to the smallest unit (GYR059).

When corresponding with Ship Dynamics in regard to a vessel application please provide at least the following information.

1.	Hull length overall (LOA)	[m]
2.	Hull beam overall (BOA)	[m]
3.	Displacement (for range of loading conditions)	[t]
4.	Metacentric height (GMt) (for range of loading conditions)	[m]
5.	Hull roll radius of gyration (if known, for range of loading conditions)	[m]
6.	Natural roll period (if known, for range of loading conditions)	[s]
7.	Operation speed	[kn]
8.	Vessel general arrangement drawing or hull description (i.e. hard chined, round chined, planning or displacement hull)

Vessel type suitability

The gyroscopic roll stabiliser is ideally suited to military, luxury yacht and ferry applications where low forward speed roll motion is important. Patrol boats that spend a large proportion of their time at low loiter-speeds could ideally benefit where roll stabilising fins that protrude from the hull have reduced effectiveness. Luxury yachts resting at anchor or dock-side can benefit from gyro-stabilisers as there are no moving parts protruding from the hull. Crew transfer and pilot boats might well find the benefits of these units in their operations. Catamaran installations are practically less likely.

A gyroscopic stabiliser does not preclude the necessity for a hull to be designed for seakeeping performance and therefore it is recommended that those skilled in this knowledge be consulted in the early design stages to get this right.

Planning vessels

High-speed planning vessels can benefit from a gyroscopic roll-stabiliser when at rest or low-speed. At high-speed the hydrodynamics forces on the planning surfaces can become quite high in both magnitude and frequency. Therefore, it is recommended for the gyroscopic roll stabiliser be supplemented with active transom interceptors or tabs for effective roll control.

What is a gyroscopic stabiliser?

Gyroscopes work on the principal of conservation of angular momentum where a spinning mass cannot undergo a change to its angular position without simultaneously producing a moment in a plane that is orthogonal to the spin axis. This physical law can be effectively harnessed to provide a means of roll stabilisation for vessels regardless of their forward speed. Therefore, as a vessel oscillates in roll due to the wave action on the hull, a spinning rotor correctly mounted can use this physical law of conservation of angular momentum to create an opposing roll moment to the wave action on the hull. The effect of this process is that the roll motion will be eliminated where the opposing rolling moments are equal. Where the wave roll moment is greater than the opposing gyroscopic roll moment then the gyroscope will at least provide some reduction of the hull roll magnitude. For this to occur the axis of the spinning rotor must be mounted orthogonal to the vessel’s roll axis and furthermore, the rotor must also be permitted to rotate (precess) about a secondary axis that is both orthogonal to the rotor’s spin axis and the hull’s roll axis. This rotation is referred to as precession.

The ability of the spinning rotor (gyroscope) to oppose the wave rolling action is primarily related to its size and speed of spin (i.e. the magnitude of its angular momentum). Theoretically it would be possible for a gyroscope to eliminate ship roll if it had sufficient angular momentum but practical limitations such as space, weight, structural and bearing design means some compromise is necessary. Achieving this efficiently and within the boundaries of practical engineering application is our job and we have been able to do this effectively with this design.

The illustration below (figure 1) shows an arrangement of two rotor disks mounted within the hull with a vertical orientation (spin axis in the plane of the computer screen) that are precessing (precession axis is orthogonal to the plane of the computer screen) over the range of +/-90°. Each image shows a precession increment of 45°. In this case the rotors work together to provide roll stabilisation whilst minimizing the unwanted effects from the pitch and yaw rates of the vessel. This arrangement also reduces the hydraulic power consumption to a minimum.

Figure 1: Precession action of two rotors mounted in a hull to oppose the roll motion created by the passing waves.

History of gyroscopic roll-stabilisation

It is not our intention here to provide a detailed history of the gyroscopic stabiliser as there are many perhaps better qualified to provide a more apt historical description. However, it has been our observation that gyroscopic stabilisers were used effectively for many years particularly in the early part of the 19th century on ocean liners. They tended to loose favour because of their power consumption and a general preference that developed for the roll fin stabiliser, which required less internal volume and weight. These vessels spent most of their time at speed on transit voyages and thus the fin stabiliser was more suited to this application as roll stabilisation at low forward-speed was not necessary. With modern vessels having a requirement for low-speed roll stabilisation the gyroscope is reemerging as an option where alternative systems are limited.

Installation arrangements

Each gyroscopic stabiliser installation must be sized to suit the hull particulars whilst considering the balance that must be acheived between the weight and space requirements or limitations. Clients should be able to make an estimate of their requirement from the information supplied above in the PDF downloads. Otherwise, Ship Dynamics is pleased to do this for its clients. The Ship Dynamics gyroscopic roll stabiliser consists of two sets of rotors that are mounted and coupled together. These can be mounted in either the transverse or longitudinal directions in the hull with equal effect depending on the space requirements of the client (see figure 2).

Figure 2: Twin rotor units can be arranged to suit the installation.

A unique feature of the gyroscopic stabiliser is that the rotors can be mounted anywhere in the hull, even on a deck or bolted to the transom where they will continue to perform, but it is recommended that they be mounted aft of amidships to minimize the design loads.

Although there are many factors that influence the ultimate size and weight of a gyroscope installation, typical installations require 2% to 5% of vessel displacement to provide 3° to 5° equivalent heel force capacity (i.e. if the roll force amplitude produced by the gyroscopic unit could be maintained as a steady constant force this would be the induced heel angle on the vessel). For example, a typical 250 tonne patrol boat installation would require a gyroscopic unit of under 3.5% of vessel displacement with a volume of approximate length 4m, width 2.1m and height 2.1m. This is illustrated in figure 3 for a 56-metre patrol boat with a transverse orientation. As mentioned, a longitudinal orientation is also possible.

Figure 3: Typical unit suitable for a 56-metre patrol boat. (Note: Person figure shown is scaled only to the image of the gyroscopic stabiliser unit)

For most vessel sizes, there is a gyroscopic stabiliser size that can be selected suit (see figure 4).

Figure 4: Gyroscopic stabilisers sized for 40t to 2000t and larger vessels.

List control

A gyroscopic stabiliser is only suitable for reducing the oscillatory roll motion resulting from the wave action on the hull. Therefore, it cannot be used to control the static heel or hull list that may result from uneven cargo loading or a wind loading on the beam. If stabilisation of this type is required, a gyroscopic stabiliser must be supplemented by roll fins, wing tanks or a moving mass, depending on the speed at which list control is required. At low-speeds, fins are not an option and wing tanks or moving mass type configurations should be considered.

Effect on vessel speed

The question that is often asked is how the weight of a gyroscopic unit will affect the speed of the vessel considering it is 1% to 4% of vessel displacement. The answer to this is not necessarily straight forward but we can perhaps offer this comparison. If one is able to determine the speed reduction that is imposed on a vessel by an equivalent set of roll fins at operational speed, one is also able to deduce the equivalent amount of weight that can be added to a vessel before a comparable speed reduction occurs. It has been our observation that the loss in speed from a set of roll fins will usually exceed the weight requirement of a gyroscopic stabiliser. On this basis one can easily justify the added weight incurred from the installation.

Furthermore, it can be shown that there may even be a significant fuel saving because of the reduced vessel drag that can add up to tens of thousands of dollars/euros per year.

Supply process

Each gyroscopic stabiliser unit size is selected for each vessel application through an assessment of roll stabilisation performance, size, weight and arrangement. This process considers both the operational objectives and the constraints of installation.

Each gyroscopic stabiliser unit is manufactured and tested under the supervision of Ship Dynamics before being dispatched for installation.

For further information please contact Mr Tony Elms.