How fouling develops
Fouling is a continuous, compounding tax on every voyage. It develops quietly, it is invisible from the deck, and by the time the master notices the ship "feels slow" the vessel has been burning several percent extra fuel for months.
It is also the most controllable influence on fuel consumption available to a ship manager — more controllable than speed, which the charterer decides, and far cheaper than any retrofit.
| Stage | Timescale | Effect |
|---|---|---|
| Conditioning film | Hours | Negligible |
| Bacterial slime | Days to weeks | Measurable added resistance |
| Algae / soft fouling | Weeks | Significant added resistance |
| Barnacles / hard fouling | Months | Severe; often requires mechanical removal |
Three factors accelerate it: warm water, idle time, and low speed. A ship waiting three weeks at anchor off a tropical port in summer can acquire more fouling than in the previous six months of steaming.
That interacts directly with routing. Cape of Good Hope routings and extended waiting time — features of the market since 2023 — have increased fouling exposure for fleets that previously traded through cooler waters with shorter port stays.
Measuring it properly
The metric is speed loss (or added power) versus a reference condition, corrected for weather, draft and trim.
The curve is indicative, and it steepens with idle time in warm water. What matters is that a fleet measures its own curve per vessel, because coating type, trade and operating profile change it substantially.
Requirements for a defensible measurement:
- A reference speed–power curve from sea trials
- Continuous or high-frequency logging of speed, shaft power, draft, trim
- Weather correction using an agreed data source
- Filtering to comparable conditions before comparison
- Trending over months, not comparison between two voyages
The economics
Consider a vessel burning 25 tonnes per day at sea, 250 sea days per year. A sustained 5% consumption penalty is roughly 312 tonnes of fuel a year — before counting the EU ETS allowances that fuel now requires and the FuelEU intensity consequence.
Against that, propeller polishing and in-water hull cleaning are modest costs. The arithmetic almost always favours cleaning earlier than operators actually do it.
| Intervention | Typical trigger | Effect |
|---|---|---|
| Propeller polishing | Every 6–12 months, or on performance signal | Often 1–3% consumption |
| In-water hull cleaning | Speed loss above ~5% | Restores much of the loss |
| Full blast and recoat | At docking | Resets the cycle |
Coating strategy
Coating choice at docking determines the shape of the fouling curve for the next cycle.
| Coating type | Characteristics |
|---|---|
| Self-polishing copolymer (SPC) | Controlled release; performance depends on activity level |
| Fouling-release (silicone/fluoropolymer) | Low friction; best on high-activity, higher-speed vessels; vulnerable to mechanical damage |
| Hard / conventional antifouling | Robust, tolerant of cleaning; higher friction |
| Hull air lubrication (separate technology) | Reduces friction directly; capital intensive |
Match coating to the actual trading profile, including expected idle time. A fouling-release coating on a vessel that spends long periods at anchor in warm water will underperform its brochure.
Biofouling regulation
Beyond fuel, biofouling is an invasive species vector. IMO guidelines on biofouling management recommend a Biofouling Management Plan and Record Book, and several jurisdictions — notably New Zealand and Australia, with California operating its own requirements — impose enforceable biofouling standards on arriving vessels.
Practical consequence: in-water cleaning is not permitted everywhere, and where it is, capture of removed material may be required. Plan cleaning around jurisdictions that allow it, and keep the Biofouling Record Book current.
Building a hull performance programme
- Establish the reference curve for each vessel
- Log high-frequency performance data continuously
- Report speed loss monthly, per vessel, with the trend
- Set a cleaning trigger — commonly around 5% speed loss — and act on it
- Inspect the propeller and hull by diver or ROV on a schedule
- Record every cleaning and coating event against the vessel
- Feed results back into the next coating specification
speed-loss curve and savings figures are indicative; measure per vessel. Biofouling requirements vary by jurisdiction — confirm before cleaning. Reviewed by the Zeaclub Editorial Team, 24 August 2026.
Frequently asked questions
How much fuel does hull fouling waste?
Published estimates vary widely, but a sustained speed loss of 5–10% between cleanings is common on vessels without an active programme, translating directly into equivalent extra fuel.
How often should a propeller be polished?
Commonly every six to twelve months, or when performance data indicates. The propeller is a small surface with a disproportionate effect on efficiency.
Is in-water cleaning allowed everywhere?
No. Several jurisdictions restrict or prohibit it, or require capture of removed material. Check local rules before arranging cleaning.
Does hull condition affect CII?
Yes, directly. Lower required power for the same speed reduces CO₂ per capacity-mile, improving the attained CII and reducing EU ETS cost at the same time.