Two hulls, the same sea, the same five degrees of roll. The wide, stiff hull swings through a shorter period — and roll acceleration, which is what the inner ear actually measures, scales with the square of roll frequency. This is the trade-off no spec sheet shows.
Both hulls shown at the same 5° roll amplitude. Angular acceleration scales with the square of roll frequency, so a 3-second period delivers approximately 2.8 times the acceleration of a 5-second period at identical amplitude. Illustrative comparison, not a measured trial.
Volume I made the case for building stability in rather than bolting it on. This volume assumes that case is accepted and deals with what follows: the physics a hull form cannot escape, the economics that justify the line item, and the engineering questions that separate a system that performs from one that merely fits.
01Volume I — The Stability Standard — covers the IG series specification, the 11-parameter forecast process and the OEM integration timeline. This volume does not repeat them.
Owners buy the beamier model for stability, then spend the season wondering why their guests keep reaching for the handrails. The boat is not wrong. The word is. “Stability” means two different things to a naval architect and to a passenger, and the two are close to opposites on a small hull.
The assumption that connects them — that a hull which is harder to push over must be more comfortable to ride in — is the error. It sounds obviously true and is straightforwardly wrong.
Natural roll period is set by two quantities: roll moment of inertia, and metacentric height GM. Widening a hull raises form stability, which raises GM, which strengthens the righting moment. The boat snaps back harder. The period shortens.
Total roll angle for a given heeling moment may indeed fall. But the motion becomes quicker and sharper — a continuous sequence of small abrupt corrections rather than a single smooth swing through the wave.
“It's like the boat is always second-guessing itself.”
— Owner of a beamier replacement hull, describing the difference from the narrower boat it replaced.
This is where the engineering disconnect lives. On a specification sheet, a 5° roll at a 3-second period and a 5° roll at a 5-second period are the same entry. In the cockpit they are not remotely the same experience.
Roll acceleration scales roughly with the square of roll frequency. At identical amplitude, the shorter period delivers substantially more angular acceleration — and acceleration, not angle, is what the vestibular system detects and what provokes motion sickness.
| Natural roll period | Roll amplitude | Relative roll acceleration | Perceived motion |
|---|---|---|---|
| 5.0 s · narrow, tender hull | 5° | 1.0× | Slow, predictable swing |
| 4.0 s | 5° | 1.6× | Noticeably busier |
| 3.0 s · wide, stiff hull | 5° | 2.8× | Sharp, twitchy, fatiguing |
Ratios follow from acceleration scaling with the square of roll frequency at constant amplitude. Real hulls vary with damping and excitation spectrum; the direction of the effect does not.
There is a second-order effect that cuts against the intuitive story. Coastal wave periods concentrate between 3 and 8 seconds. A narrow hull at 4–5 seconds sits inside that band and is more exposed to wave-driven resonance. A wide, stiff hull at 2.5–3.5 seconds has partly stepped below it.
The wide hull has not escaped the motion problem. It has changed the mechanism — from resonance exposure to a permanent acceleration penalty that applies whether or not the sea is in sync with the hull.
None of which makes beam a mistake. Beam buys deck space, payload, initial stability and often a drier ride. It simply does not buy ride comfort, and it cannot be made to. A naval architect can move a hull's period a second either way through beam, ballast and weight distribution — but on a boat already fixed in length and displacement, and with comfort competing against speed, efficiency, payload and cost, there is little room to manoeuvre. The band does not move. What remains is the dynamic response, and changing that means adding damping the hull does not generate on its own.
The answers may not change the beam. They will usually change what gets specified alongside it.
Put a 26-foot boat and a 60-foot boat in the same moderate sea and the smaller one rolls far more violently. That is not a matter of perception or of build quality. It is the direct consequence of scaling a hull down, and it operates through two independent mechanisms at once.
Easier to set rolling, and slower to settle once rolling. The effects compound rather than cancel: a hull that resonates readily and damps poorly experiences both at the same time.
It is worth being specific, because “damping” is often treated as a single property when it is three mechanisms working together — and all three weaken with size.
Research on small-craft roll dynamics indicates this weak damping also becomes less predictable precisely as wave encounter frequency approaches the natural period — the exact condition where predictability matters most, because that is when amplitude is already building fastest.
The physics above has been understood for a long time. What changed is not the understanding but the hardware: gyro torque scales with stored angular momentum, so a unit matched to a given righting moment has historically demanded proportional mass, power and volume. Shrinking that without surrendering torque is the whole engineering story of the last decade.
A 6.27-metre unmanned surface vessel of about 4.5 tonnes, fitted with a 115 kg unit providing 1000 N·m·s of angular momentum. Natural roll period: 2.07 seconds — about as demanding as small-craft stabilisation gets, sitting at the peak of the resonance curve for short-period chop.
Full-scale trial result rather than a laboratory simulation. A 6-metre hull under 5 tonnes with a 2-second natural period — the least favourable combination in the small-craft range — still achieving over 85% reduction.
Run a boat at 15 knots beam-on to a 1.5-metre swell at 6 seconds. Then stop and let it drift. The difference in motion is not subtle: what was manageable underway can become marginal at rest, with no change in the sea at all.
A hull moving forward generates a flow field that interacts with its own rolling motion. The effect is modest — forward speed contributes less to roll damping than it does to pitch or heave — but it is real and it is constant. Appendages, propellers, rudder and shaft struts all add resistance to lateral motion.
Because that contribution is constant at speed, it goes unnoticed. The moment the vessel stops, it vanishes — leaving a hull still fully excited by waves but stripped of its most consistent source of roll resistance. Amplitude rises, the period lengthens, and the motion becomes harder to anticipate.
This is why anchorage is so consistently reported as the least comfortable phase of a passage. Not because conditions worsen, but because the boat has lost something it never advertised having.
Fin stabilisers generate lift from flow across a foil. Zero-speed variants exist and work, driving the fins actively through hydraulics rather than waiting for flow. It is a genuine engineering achievement, and the price is structural rather than electronic.
Owners who accept that trade-off get a workable system. But cutting multiple large openings below the waterline is a non-starter for most small-craft owners — and it remains a permanent underwater structural intervention even when everything performs exactly as specified.
A gyro's counter-torque comes from conserved angular momentum in a spinning mass. There is no flow to build up, and therefore no threshold speed below which output tapers off. It is either spinning and producing torque, or it is not. Nothing in between, and nothing that depends on how fast the hull is moving.
| At anchor / on station | Passive fins | Anti-roll tanks | Gyro |
|---|---|---|---|
| Effective at zero speed | No | Partially | Yes |
| Hull penetrations required | Yes · one per fin | No | None |
| Hydraulic circuit | Yes | No | No |
| Vulnerable to ice and debris | Yes · external foils | No | No external parts |
| Primary cost to the vessel | Structure + maintenance | Volume + free surface | Mass + electrical draw |
A day-boat owner who stops for lunch tolerates a few degrees without consequence. For any vessel that spends real time on station, the calculation changes: overnight anchorages are where fatigue accumulates, and a crew that rolls through the night starts the next day already depleted. Over several days the effect compounds.
The same applies to working boats holding station rather than making way — survey, dive support, buoy maintenance, fishing stops. Moving around the deck, handling equipment and keeping watch all get harder when the platform is unpredictable, and it is precisely during those static hours that forward-speed damping is not there to help.
These are engineering constraints, not barriers, and they are managed through placement and structural integration. The useful question for a specifier is rarely “can this be fitted?” It is “what is the alternative for anchorage comfort, and what does that alternative cost the hull?”
“You get used to it” is the standard reply to a motion complaint, and it is not wrong — people do adapt. But adaptation is a survival mechanism rather than a solution, and it carries a cost whether or not it is noticed day to day. Here is what that cost looks like when it is itemised.
Fatigue is a safety cost, not a comfort cost. The constant micro-correction of standing on a rolling deck means recreational owners arrive back at the dock tired and less inclined to plan the next trip — and commercial crews start the working day already depleted. A tired crew is a less observant crew, a slower-responding crew, and eventually a crew more likely to make the error that becomes an incident.
The equipment bill goes unattributed. Design loads and test protocols for most marine equipment emphasise vertical impact and steady-state vibration — not continuous lateral cyclic loading. Multiply that 100–140 N by tens of thousands of cycles across a season and the result is hardware whose fasteners, brackets and solder joints were never engineered for it.
Cracked mounts, loosened connections, intermittent electrical faults. These get blamed on bad luck or poor-quality gear. It is rarely either. It is cumulative fatigue in an environment the equipment was never specified for.
Figures are drawn from industry-reported ranges, broker experience and published ergonomics research rather than a single controlled study. They are an order-of-magnitude framework for building a business case, not guaranteed outcomes for a specific vessel.
Start with the counterintuitive part, because it disposes of the question most owners ask first. A gyro applies a pure couple to the boat, and a couple is the same about every point in the structure. The unit delivers identical roll torque whether it sits on the centreline or off it, in the bilge or up in a leaning post. Manufacturers say as much: above or below deck does not matter, provided the unit ties into primary structure and can be serviced.
What position does affect is everything around it. Fore-aft placement shifts trim and changes pitch inertia. A unit set well off the centreline introduces a standing heel that has to be trimmed out. Both are straightforward at the design stage and awkward afterwards.
In a finished boat, the spaces that work — near the longitudinal centre of gravity, close to a stringer run, within reach of a cooling loop and a service hatch — are rarely empty. They hold the fuel tank, the battery bank, the steering gear. So the gyro goes where it fits rather than where it belongs.
Reaction torque runs to thousands of Newton-metres, and it is fully reversing — swinging hard one way, then the other, thousands of times in a rough passage.
| Unit class | Typical vessel | Order of reaction torque | Load character |
|---|---|---|---|
| Small | 23–30 ft | ~2,600 N·m | Fully reversing, high cycle count |
| Mid-range | ~50 ft | 17,000+ N·m | Fully reversing, high cycle count |
On a GRP hull that is not a trivial load. A properly designed foundation spreads it across the grid of stringers and frames. A retrofit foundation, bonded in after the hull is finished, is always a patch — a localised reinforcement rather than an integrated load path. The bolts go through existing laminates, the backing plates are fitted in confined spaces, and the bonding is applied to surfaces that were never prepared for it. It works, usually. It is simply not as good.
A gyro reacts against its foundation. If the foundation flexes, two things happen, and the second is the more damaging of the pair.
None of this makes a retrofit a bad decision. A retrofit still works, and against no stabilisation at all it is still transformative. But it is not optimal, and on a small hull every percentage point of damping is one the boat needed.
Installers commonly report a retrofit costing in the order of a third more than the same unit fitted during production. That margin goes to the installer — not to the builder, and not back to the owner.
A gyro has no external appendages, which is its central structural advantage over fins in ice-prone water: brash ice, floes and pressure ridges can bend a foil, damage an actuator or open a shaft seal, and a gyro presents nothing to hit.
The exposure sits elsewhere — in the cooling circuit. Most units reject heat through a glycol loop into a seawater heat exchanger, and they need real flow to do it; even the smallest call for several litres a minute of raw water. An intake that ices up, a strainer that packs with slush, a through-hull sited where brash collects: any of these shuts the unit down, and none is visible from the cockpit.
Which is the whole argument in miniature. The boat does not care when the gyro is specified. The performance, reliability and cost of the installation depend entirely on how early the question was asked.
The smallest units from established manufacturers sit around 130–165 kg, built for the 23–30 foot range. The newest class — the ones actually sized for a 1–2 tonne hull — lands around 60 kg. On paper, lighter looks like the obvious choice for a small boat. It is not automatically.
| Established class · 130–165 kg | Light class · ~60 kg | |
|---|---|---|
| Share of a 1 t displacement | 16.5% before any payload | Under 6% |
| Torque headroom | More, relative to rated hull size | Sized to the hull, less margin |
| Service history | Longer · more field data, more technicians | Shorter · newer designs |
| Fits a sub-5 m hull at all | Generally no | Yes — that is the point |
A heavier unit typically carries more torque headroom relative to its rated hull size — built for a boat at the top of its bracket, not the bottom. That headroom matters when the hull is fully loaded, carrying gear, or working in sea states rougher than an average day trip. The light class earns its place differently: not by out-performing the heavier units, but by being the first ones that fit at all.
A gyro's stabilising moment comes from angular momentum — moment of inertia multiplied by spin rate — combined with how fast the gimbal can precess against it. Two units of similar mass can deliver different righting moments depending on how that mass is distributed.
A flywheel built as a thin, dense rim rather than a solid disc generates more angular momentum per kilogram, because moment of inertia scales with the square of the radius the mass sits at — not with mass alone. The number that predicts roll reduction is rated torque, and it does not track proportionally with mass between manufacturers.
Spin rate is the other lever. Running the flywheel faster raises momentum for a given mass, which is part of how light-class units close the gap with heavier, slower designs — but it also raises bearing loads and shortens service life unless the bearing design compensates. A unit that reaches its torque figure by spinning faster rather than through better mass distribution is not automatically the wrong choice. It is a different trade-off, and worth knowing you are making it.
Inside the sub-5-metre bracket the spec sheets converge on similar weight and similar footprint. What actually separates one unit from another mostly lives inside the housing, where a buyer cannot inspect it.
Purchase price is the number everyone compares. It is not the number that decides whether the unit fits the boat's electrical budget.
A 1–2 tonne day boat runs a modest battery bank sized around electronics, lights and perhaps a small trolling motor — not around a continuous 150–300 watt draw plus a spin-up spike on top. A gyro pulling 200 watts at cruise can turn a bank sized comfortably for a weekend into one that needs recharging every outing.
The upgrade that follows — a larger alternator, a second battery, a DC-DC charger — can rival the price difference between a light-class unit and a mid-weight one. The cheapest unit on paper is not always the cheapest installation.
Maintenance follows the same logic at a different scale. A glycol loop, a raw-water strainer, a bearing service interval: none expensive per event, but on a boat that is trailered, stored or laid up seasonally, the interval needs to match how the boat is actually used — not how a marina-berthed 40-footer is used. Stagnant cooling loops, harder start-stop cycles, and a duty cycle the stated interval may never have been tested against.
The question was never “does stabilisation work on a boat this size?” It is which version of it matches what this particular boat is actually asked to do.
A short list worth putting to any manufacturer, whichever unit is on the table. None of these appears on a standard spec sheet. None of them is difficult to ask for.
These are the questions that separate a unit chosen on weight and price from one chosen because it matches the boat and the job.
Hong Kong Decho Technology Development Company Limited is the European distributor of the IG series — gyro stabilisers developed for boats up to 5 tonnes or 10 metres. Chapter 6 ends with six questions to put to any manufacturer. They apply to us as much as to anyone else, and we would rather answer them early than have them surface at commissioning.
Performance figures in this volume are drawn from published small-craft research, full-scale trial reports and industry-reported ranges. All predictions are indicative; actual performance varies with hull geometry, loading condition and sea state.