Dry Dock Hull Coating Selection & CII Efficiency Guide

A hull coating is specified in an afternoon and lived with for five years. That was always true, but it used to be a corrosion and schedule decision. It is now a carbon intensity decision, because the required CII tightens every single year of the docking cycle you are about to begin — and from 2027 it tightens faster than it has so far.

That is the part that changes how a technical superintendent should think about the specification. The coating has to be good enough not for the trade the ship is in today, but for a rating threshold that keeps moving underneath it until the next docking. This guide works through the systems available, what the fuel savings honestly are once measured rather than modelled, how surface preparation governs everything, where the anti-fouling regulation now sits, and what to write into the dock specification so you get what you paid for.

Why dry dock hull coating is now a CII decision

The carbon intensity regime applies to cargo, cruise and passenger ships above 5,000 gross tonnes trading internationally, under MARPOL Annex VI. Each year the ship calculates an attained operational CII — annual carbon dioxide mass over transport work — and is rated A to E against a required CII derived from a 2019 reference line and an annual reduction factor.

Those reduction factors are the reason the coating decision has changed. They are not a flat requirement held constant across a docking cycle.

Through 2026 the factor rose two percentage points a year. From 2027 the annual increment becomes 2.625 points, adopted via MEPC.400(83) in April 2025. A hull coated in 2026 and next docked in 2031 has to carry the ship across that step change on the performance it had on day one, minus whatever it has lost since.

The consequence of falling short is not a fine. A ship rated E in a single year, or D for three consecutive years, has to develop a plan of corrective actions inside the Ship Energy Efficiency Management Plan Part III, approved by the flag Administration or a recognised organisation, describing how it will reach C or better. MARPOL Annex VI imposes no direct financial penalty; the cost arrives through chartering and financing, which is precisely where a technical department has the least control and the most to explain. The wider emissions picture these sit inside is covered in our guide to the MARPOL annexes.

Hull roughness and the fuel penalty a coating has to beat

Before comparing products it is worth being concrete about the quantity being bought. Coating performance is a roughness story, and roughness has a scale a superintendent can hold in their head.

A new hull sits at roughly 120 microns average hull roughness. Fouling and deterioration take an eight-year-old hull into the 300 to 400 micron range. Readings are taken with a hull roughness analyser over around 100 locations including bow, stern, midships and the boot-top, each traverse sampling ten readings across 50 millimetres.

The soft side of the problem is larger than most people expect. A slime layer half a millimetre thick covering up to half the hull has been put at a 25 to 30 per cent increase in greenhouse gas emissions. Slime is not the dramatic barnacle growth that appears in presentations; it is the invisible film that a ship carries for months without anybody on board noticing, and it is the single biggest reason a coating that looked excellent out of dock stops delivering by year three.

Hull coating systems compared: SPC, foul-release and hybrid

Four families cover almost every specification written today. The last column is the one worth reading twice, because every system fails in a characteristic way and the failure is usually a mismatch with the trade rather than a bad product.

Underneath all four sits the anticorrosive scheme, which is not a coating choice so much as the thing that determines whether any of them survive. A premium fouling control topcoat over a tired, under-prepared anticorrosive is money spent on the wrong layer.

Foul-release hull coating savings: claimed, modelled and measured

This is where specifications go wrong, and it is worth separating three numbers that routinely get quoted as though they were one.

Manufacturer claims run to around 10 per cent. Modelled savings by vessel type range from 3.5 per cent for supply vessels through 4.8 per cent ro-ro and passenger, 5.1 per cent gas tanker, 5.9 per cent bulk carrier, 6.5 per cent container and 7.2 per cent Aframax to 8.6 per cent VLCC. Operator data from VLCCs in service showed improvement out of dock of up to around 5 per cent, and average fuel savings of around 3 per cent across the full docking cycle.

Three per cent over a cycle is a real and worthwhile number. It is not ten. Build the business case on the measured figure and the premium system still earns its place on the right trade; build it on the brochure figure and the gap will be found by somebody in finance two years from now, at which point the technical argument for good coatings gets harder for everyone.

Matching the hull coating to your trade profile

There is no best coating, only a best fit, and the fit is decided by four honest answers about how the ship is actually operated rather than how she was chartered to operate.

Cost belongs in the same conversation rather than a separate one. Silicone systems have been put at triple or more the cost of a traditional self-polishing system, against a service life at least equivalent to SPC at up to five years, with some products offered at ten or more. The arithmetic only works if the operating profile lets the coating deliver, which is why the four answers above have to be real.

Surface preparation decides what the hull coating can deliver

Every coating comparison assumes correct application, and in practice that assumption is where most of the difference between promised and delivered performance is lost. Release systems in particular are usually specified over full abrasive blasting to Sa 2.5, rather than the spot blasting a conventional system will tolerate — and that means an extended dry dock period, which is a cost and a schedule consequence that belongs in the comparison from the start.

The anchor pattern left by blasting is what the anticorrosive keys into; the smoothness of the finished topcoat is what the water sees. Both are set during the dock period and neither can be improved afterwards.

Anti-fouling regulation that shapes the hull coating specification

Two regulatory items sit directly on the coating specification, and both are now firmly in the present tense rather than on a horizon.

Both belong in the biofouling management plan and record book rather than in somebody’s memory of the last docking, and the broader inspection regime around hull condition — coatings, cathodic protection, in-water survey — is set out in our guide to hull maintenance and inspection.

Writing the hull coating specification for dry dock

The specification is the only moment at which a superintendent has leverage. Afterwards there is a painted ship and an invoice.

Keeping that package attached to the vessel rather than to a project folder is what makes the next docking a decision rather than a reconstruction, and it belongs alongside the rest of the maintenance record in the planned maintenance system.

Measuring whether the hull coating worked

The weakness in most coating programmes is that nobody ever establishes whether the last one delivered, which means every docking argues the same case from first principles. Three measurements close that loop.

The point of all three is the same: to make the coating decision at the following docking an evidence-based one. A fleet that can show the measured deviation curve for two systems on comparable ships has an argument no proposal can match.

Dry dock hull coating: frequently asked questions