Stabilisers reduce roll at anchor and under way. Surveying an existing system and retrofitting a new one start with the same question: how is the system's force carried into the hull?
Two main types
- Fin
- Moving fins below the waterline. Effective under way; some types also work at anchor. They need large hull openings and strong internal reinforcement.
- Gyroscopic
- A spinning mass inside the hull. No hull opening needed; weight and position, the supporting structure and electrical capacity decide.
Surveying an existing system
- Fin: shaft seals, the hydraulic or electric actuator inside the hull, leaks, bearing play and fin damage.
- Gyro: mounting to the structure, cooling circuit, running noise and vibration.
- Both: service records and operation on sea trial.
When retrofitting
- Structural calculation. Reinforcement for the loads going into the hull is calculated by a designer.
- Location. Hull form for fins; centre of gravity and access for a gyro.
- Power. Generator and battery capacity checked against the system's draw.
- Cooling and hydraulics. Seawater intake or hydraulic circuit.
- Sea trial. Roll reduction and vibration measured.
Types of stabiliser
Stabilisers on yachts fall into two main groups. Fin stabilisers project from the hull below the waterline and reduce roll with moving fins; some work only under way, some at anchor too. Gyroscopic stabilisers use a heavy spinning flywheel inside the boat, with nothing projecting from the hull. Each has its own power needs, maintenance and survey points.
Surveying a fin system
Ashore, the fin surfaces, the seal where the shaft leaves the hull and the anodes are examined. Inside, the hydraulic or electric actuator, leaks and connections are checked. The fin's internal support structure takes heavy loads; cracks or damp there are examined carefully. On the sea trial the system is engaged and its effect on roll observed.
Surveying a gyroscopic system
Noise from the unit's bearings, the cooling circuit, running hours and service records are examined. The unit is heavy and must be firmly attached to the structure; mounting points are checked for cracks. As the flywheel takes time to spin up, the sea trial allows for the system to come up to speed.
Retrofitting
Adding stabilisers to an existing boat is possible but a substantial job. Space and weight, power supply, reinforcement of the hull structure and the maker's installation requirements are weighed together. Gyroscopic systems are more often chosen for retrofit because no hole is cut in the hull, but their weight and position affect the boat's balance.
Maintenance
Both systems need servicing at the maker's intervals. A stabiliser with no service record is treated as an uncertain cost item at purchase.
Power
Stabilisers draw significant power, particularly gyros at anchor. The generator and electrical system must carry the load alongside air conditioning and other consumers.
Stabilisers in a purchase decision
A stabilised boat is noticeably more comfortable, especially with guests and on long passages, so stabilisers add value at sale. But if the system does not work or servicing is overdue, repair can be costly. Trying the system ashore and at sea on a pre-purchase survey, and reviewing the service records, shows whether that value is real. If the system cannot be run, the report says so plainly and service records are requested from the seller.
Findings are read together with the boat's electrical and hydraulic systems, since reliable operation depends on what feeds it.