How much fuel does hull fouling cost? What the evidence says and what it doesn't

What the research says

Fouling increases the roughness of the hull surface, and roughness increases frictional resistance. Frictional resistance is a large part of a merchant ship's total resistance at service speed, so even small changes in roughness show up in fuel consumption.

The IMO GloFouling Partnerships' 2022 study on the impact of marine biofouling on the energy efficiency of ships is one of the most cited sources. It modelled the effect of different fouling conditions on several ship types and found that:

• Even a thin layer of slime can increase fuel use and greenhouse gas emissions significantly
• Heavier macrofouling (weed and barnacles) increases the penalty much further
• The effect varies with ship type, speed and how much of the hull is fouled

That is the honest summary: fouling matters a lot, starting earlier than many people assume.

What the research does not tell you

The study's figures are model results for specific ships under stated assumptions. They are not a prediction for your vessel. The real penalty on a given ship depends on:

• Hull form and size
• Operating speed: frictional resistance matters most at service speed
• Where the fouling is: flat bottom, vertical sides, waterline
• The type of fouling: slime, weed or calcareous growth
• The coating and its condition
• Propeller condition, which is a separate effect

So treat any single percentage, including any a contractor quotes, as an indication, not a promise. We do not promise a percentage saving from cleaning.

How to estimate it on your own ship

Your own data is better than any study. The method in outline:

1. Collect speed and consumption from noon reports or a performance monitoring system.
2. Filter for comparable conditions: similar draught, wind and sea state, and speed range.
3. Look at the trend since the last docking or clean. A rising consumption at a given speed is the fouling signal.
4. Compare before and after cleaning, using the same filtering.
5. Put a cost on it by multiplying the extra consumption by fuel price and the days the ship will trade before the next opportunity to clean.

Standards such as ISO 19030 describe more rigorous methods for measuring hull and propeller performance, and many fleets now use monitoring systems that do this automatically.

Where carbon intensity comes in

Extra fuel means extra CO2, which shows up in the ship's Carbon Intensity Indicator (CII) rating. For ships close to a rating boundary, hull and propeller condition can matter commercially beyond the fuel bill.

What to do with the number

If the estimated daily fuel penalty, multiplied by the trading days ahead, comfortably exceeds the cost of cleaning and polishing, the clean pays for itself. That calculation, done honestly, is a better reason to clean than any headline figure. Read the complete guide to ship hull cleaning and what underwater hull cleaning costs for the other side of the comparison.

If you would like a second opinion, send us your noon report data and we will help estimate the fouling cost.