Back to Ship Construction & Naval Architecture
Ship Construction & Naval Architecture

Methods of Reducing Rolling: Fin, Bilge & Tank Stabilisers

How active fins, bilge keels, and passive/active tank stabilisers fight synchronous roll and why tank water must be tuned to GM.

8 min read
Intermediate
Ship Construction & Naval Architecture
Key Principles at a Glance 6 points
  • Fin stabilisers work like aircraft wings: aerofoil-section fins at bilge level rotate in opposite directions to generate a lift force that creates a moment opposing the roll.
  • The main fin moves up to about 20° while its articulated tail flap moves a further 10°–30°, and the fins are driven by an electric motor and variable-delivery pump feeding oil to the fin tilting gear.
  • Typically two fins extend from the ship side near bilge level; turning a fin down produces an upward water force and turning it up produces a downward force. Most fins are retractable (sliding fin boxes or hinged inward) for shallow-water and berthing protection.
  • Bilge keels are passive roll-damping projections fitted at the bilge above the bottom shell and within the ship breadth; they must be deep enough to penetrate the boundary layer yet not so deep that rolling water force damages them.
  • Bilge keels of 250 mm to 400 mm depth extend about one half of the ship length amidships and are gradually tapered at the ends.
  • Free-surface tank stabilisers do not depend on the ship forward motion (suited to drill ships), but introduce a free surface that reduces stability; the tank water must be tuned to the ship GM and roll period or it can synchronise and worsen rolling.

1. Fin Stabilisers (Active Roll Damping)

Fin stabilisers are the most powerful active anti-rolling device. They work very much like an aircraft wing, providing lift — positive or negative — depending upon their aspect relative to the water flow.

Construction & Geometry:

  • Aerofoil cross-section: The fins are of aerofoil section and are provided with tail flaps which can be moved relative to the main fin. This movement is accomplished automatically as the main fin rotates.
  • Angles of movement: Main fins usually have a maximum movement of about 20° up or down, whilst the tail flap can move a further 10° to 30° relative to the main fin.
  • Location: Two fins extend from the ship side at about bilge level, one port and one starboard.
  • Opposite rotation: The fins are turned in opposite directions as the ship rolls.

How the Stabilising Moment is Generated:

The forward motion of the ship creates a force on each fin, and hence produces a moment opposing the roll:

  • When the fin is turned down, the water exerts an upward force.
  • When the fin is turned up, the water exerts a downward force.

Driving Machinery & Retraction:

  • The fins are turned by an electric motor driving a variable-delivery pump, delivering oil under pressure to the fin tilting gear.
  • The oil actuates rams coupled through a lever to the fin shaft.
  • Most fins are retractable, either sliding into fin boxes transversely or hinged into the ship. Hinged fins are used when there is a restriction on the width of ship which may be allocated, such as in a container ship.
Interactive 3D: Fin, Bilge Keel & Tank Stabilisers

Orbit the midship section, pick any part to isolate it, then switch modes to run the active fin stabilisers, the passive bilge keels and the three free-surface tank systems — passive, controlled passive and active controlled — and watch the water lag or be driven against the roll.

Loading interactive 3D roll-reduction model…
ACTIVE FIN STABILISER: AERODYNAMIC LIFT & TILTING MECHANICS AEROFOIL FIN SECTION & FLAP DEFLECTION Speed V Main Fin (20°) Flap (+20°) UPWARD LIFT (+L) Neutral Axis Dynamic Lift Formula: L = ½ · ρ · V² · A · CL Articulated flap boosts lift coefficient CL by ~60% without stall. MIDSHIP FIN COUPLE & ROLL COUNTER-MOMENT VESSEL HULL Wave Roll to Starboard → +LIFT (UP) -LIFT (DOWN) STABILISING MOMENT ← ACTIVE FIN STABILISER TECHNICAL SPECIFICATIONS • Kinematics: Main aerofoil tilts ±20° via electro-hydraulic variable-delivery rams; articulated trailing flap moves an additional ±10° to ±30°. • Speed Dependency: Lift force is strictly proportional to ship speed squared ($L propto V^2$); ineffective at zero speed / anchor. • Retraction Types: Sliding fin boxes (transverse retraction into hull) or hinged folding fins (inward fold for beam-restricted container ships). • Provides up to 85% to 90% roll reduction at design cruising speeds.
Figure 1: Active Fin Stabiliser System. Symmetrical aerofoil fins at bilge level rotate in opposite directions into the oncoming water stream. The submerged fin turned downward generates massive upward lift (+L) while the opposite fin generates downward force (-L), creating a powerful righting couple that counteracts wave roll.

2. Bilge Keels (Passive Roll Damping)

When ships were first built of iron instead of wood, a bar keel was fitted, one of its advantages being that it acted as an anti-rolling device. With the fitting of the flat plate keel the anti-rolling properties were lost. An alternative method was supplied in the form of bilge keels, which are now used in the majority of ships.

Arrangement:

  • These projections are arranged at the bilge to lie above the line of the bottom shell and within the breadth of the ship, thus being partially protected against damage.
  • They extend for about one half of the length of the ship amidships and are tapered gradually at the ends.

Depth — The Two Governing Factors:

The depth of the bilge keels depends to some extent on the size of the ship, but there are two main factors to be considered:

  1. Boundary layer penetration: The web must be deep enough to penetrate the boundary layer of water travelling with the ship. If it is too shallow, it sits in still, entrained water and develops little damping force.
  2. Damage risk when rolling: If the web is too deep, the force of water when rolling may cause damage to the keel or its connection to the shell.

Bilge keels 250 mm to 400 mm in depth are fitted to oceangoing ships.

BILGE KEEL HYDRODYNAMICS & BOUNDARY LAYER PENETRATION Side Shell Bottom Shell Plating Entrained Boundary Layer (δ) Depth: 250 – 400 mm Tip Vortex Shedding (Damps Roll Kinetic Energy) Ship Breadth Limit Keel Baseline PROTECTIVE GEOMETRIC LIMITS • Must NOT extend below keel baseline (Prevents damage during drydock resting) • Must NOT project beyond maximum beam (Prevents contact during quay berthing) • Sacrificial weld protects shell if torn! BILGE KEEL DEPTH CRITERIA & SURVEY RULES • Criterion 1 (Penetrate Boundary Layer): Must project beyond the entrained laminar/turbulent boundary layer into undisturbed water to generate roll drag. • Criterion 2 (Prevent Shell Tearing): Depth cannot exceed 400 mm; excessive depth creates enormous hydrodynamic bending that rips shell welds. • Fitted over ~50% of ship length amidships; tapered at forward and aft ends to avoid generating excessive frictional hull drag.
Figure 2: Bilge Keel Hydrodynamic Action. The 250–400 mm deep web projects beyond the hull boundary layer to shed vortices, creating passive hydrodynamic drag against rolling. The keel remains strictly inboard of the vessel breadth and above the baseline, mounted on a sacrificial flat bar.

3. Passive & Controlled Passive Tank Stabilizers

Three basic systems of roll-damping use free-surface tanks:

  1. Passive Tanks
  2. Controlled Passive Tanks
  3. Active Controlled Tanks
The Free Surface Trade-Off

These systems do not depend upon the forward movement of the ship and are therefore suitable for vessels such as drill ships. However, in introducing a free surface to the ship there is a reduction in stability (loss of GM) which must be considered when loading the ship.

(a) Passive Tanks:

  • Two wing tanks are connected by a duct having a system of baffles.
  • The tanks are partly filled with water.
  • When the ship rolls, the water moves across the system in the direction of the roll. As the ship reaches its maximum angle and commences to return, the water, slowed by the baffles, continues to move in the same direction.
  • Thus a moment is created, reducing the momentum of the ship and hence the angle of the subsequent roll.
  • Tuning is critical: The depth of water in the tanks is critical and, for any given ship, depends upon the metacentric height. The tank must be tuned for any loaded condition by adjusting the level, otherwise the movement of the water may synchronise with the roll of the ship and create dangerous rolling conditions.
  • Alternatively the cross-sectional area of the duct may be adjusted by means of a gate valve.

(b) Controlled Passive Tanks:

  • The principle of action is the same as for the passive system, but the transverse movement of the water is controlled by valves operated by a control system similar to that used in the fin stabiliser.
  • The valves may be used to restrict the flow of water in a U-tube system, or the flow of air in a fully-enclosed system.
  • The mass of water required in the system is about 2% to 2.5% of the displacement of the ship.
  • With the valves closed the system is put out of action — a useful fail-safe feature.
U-TUBE PASSIVE / CONTROLLED PASSIVE FLUME ANTI-ROLL TANK Midship Cargo Hold / Machinery Space Flow Damping Baffles Air Throttle Valve HEAD WEIGHT (+W) 90° PHASE LAG DYNAMICS • Ship reaches max roll to Starboard. • Baffles delay water surge by ¼ period. • Water piles up on HIGH Port side! → Restoring moment dampens roll return. OPERATIONAL PRINCIPLES & STABILITY TRADE-OFF OF ANTI-ROLL TANKS • Zero-Speed Capability: Unlike fins, anti-roll tanks do not depend on ship forward speed — essential for stationary offshore drillships & seismic vessels. • Critical Resonance Tuning: Natural water oscillating period $T_{tank}$ must equal ship natural roll period $T_{roll}$. Mis-tuning can cause dangerous roll amplification. • Free Surface Effect (FSE): Tank fluid creates virtual loss of GM ($Delta GM = - rac{ ho_{w} i}{Delta}$); system mass accounts for 2.0% to 2.5% of total displacement.
Figure 3: Free-Surface U-Tube Anti-Roll Tank. Two wing tanks are linked by a baffled transverse flume duct. As the vessel rolls to starboard, internal baffles delay fluid transfer by ~90° (one-quarter roll period), causing elevated water mass on the high port side that generates a powerful counter-roll restoring moment.
Flume stabilizer tank arrangement for passive roll reduction
Photo: Flume stabilizer — wing tanks connected for free-surface roll damping.
Flume duct connecting port and starboard roll reduction tanks
Photo: Flume duct and tanks — transverse connection between wing tanks.
Flume duct baffles restricting water flow to tune roll period
Photo: Flume duct baffles — flow restriction tuning the tank water period.

4. Active Controlled Tanks & Roll Period Tuning

In the active controlled tank system the water is positively driven across the ship in opposition to the roll. Because the direction of roll, and hence the required direction of the water, changes rapidly, a different arrangement is needed:

  • It is necessary to use a uni-directional impeller in conjunction with a series of valves.
  • The impeller runs continually and the direction of the water is controlled by valves which are activated by the control system.

Design & Phase Relationship:

Careful design of the tank in terms of its shape, water capacity, and vertical positioning in the ship allows control to be exercised with respect to rolling. With correct design of tank:

  • The water oscillating period will equal the roll period of the ship.
  • Its motion will lag behind that of the ship by one quarter of the roll period.
  • It will lag behind the wave by half of the roll period.

Water in the tank thus opposes the wave action producing the roll. Water movement between the tanks is regulated to some extent by the air valves. With the valves closed, the system is put out of action. Under this arrangement — known as the controlled passive system — the mass of water is about 2% to 2.5% of the ship's displacement.

Why Tuning Matters — The Synchronous Roll Danger

If the tank water period does not match the ship's natural roll period, the water can move in phase with the ship and amplify the roll instead of damping it. This is why passive tanks must be tuned (by adjusting water level or duct gate valve) for every loaded condition, since the roll period changes with GM as the ship is loaded.

ACTIVE CONTROLLED TANKS & COMPARATIVE ROLL REDUCTION vs. SPEED ACTIVE UNI-DIRECTIONAL IMPELLER PUMP Port Tank Stbd Tank PUMP High-Speed Hydraulic Reversing Valves Active Water Shift Against Wave Roll Impeller runs constantly in one direction. Fast-acting valves switch flow direction in < 0.5 sec. % ROLL REDUCTION vs. SHIP FORWARD SPEED 0% 30% 60% 90% % Roll Red. 0 kts 6 kts 12 kts 18 kts 24 kts Active Fins (90%) Tanks (Steady ~65% at 0-24 kts) Bilge Keels (~30%) SELECTION SUMMARY FOR ROLL REDUCTION SYSTEMS • Active Fins: Unmatched performance (up to 90% reduction) at cruising speeds (15–24 kts); zero effectiveness when stationary or anchored. • Tank Stabilisers: Consistent 60%–70% roll damping at zero speed; optimum for drillships, cable layers, and stationary offshore units. • Bilge Keels: Universal, 100% reliable passive damping (25%–35% reduction) with zero machinery moving parts, zero power, and zero failure points. • Modern cruise vessels combine active fins (at sea) with passive tanks (in port/anchorage) and permanent bilge keels for total roll defense.
Figure 4: Active Controlled Anti-Roll Tanks & Speed Performance Comparison. Active tanks use a continuously running uni-directional impeller with rapid 4-way reversing valves to pump water against roll. The graph shows that while active fins dominate at high speeds (up to 90%), tank stabilisers provide steady 65% damping at zero speed where drillships operate.