Slime lets go. Barnacles do not.

A hull fouls in a fixed sequence: a film in hours, slime in days, weed in weeks, barnacles in months. Each stage holds on about a thousand times harder than the last. That is the case for cleaning little and often, and for a robot that never leaves the ship.

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Back to Maritime Hulls
Fuel Consumption
Roughness → Fuel
Year One
Speed & Waiting
The Plot
Grooming
References

Fuel Consumption

Increase in fuel consumption from biofouling

Measured increase in fuel consumption from biofouling, plotted against equivalent sand-grain roughness Ks (µm). Ships are grouped by fouling condition: deteriorated coating or light slime, heavy slime, and small calcareous fouling or weed.

HYDRODYNAMICS

Small roughness increases lead to large fuel penalties

Equivalent sand-grain roughness Ks sets frictional drag. The first ~100 µm is the steep part of the fuel curve.

Validation against Schultz (2007)

Reproducing the published frigate table across a 333× range of roughness, RMS error ~2.0 percentage points:

ConditionKsSchultzModel

Shaft power increase at 15 kn, FFG-7 frigate, versus a hydraulically smooth hull. Chart curves use the Dream-class POW_KS/POW_SP kernel from THE PLOT (18 kn).

FOULING DEVELOPMENT

Once fouling takes a hold it goes quickly

A well-applied modern coating holds fouling off about a year after drydock. Once onset starts (~month 12), the fuel-relevant stages arrive inside four months.

It takes a year to get a hold. Once it does, the fuel penalty is built by week five.

This is not a coating-selection problem. Timings are from onset (~month 12), not from undocking.

7 days post-onset

First slime

Covers ~10% of the hull.

40 days post-onset

Light slime

Ks 100 µm · +14% additional fuel use.

125 days post-onset

Heavy slime

Ks 300 µm · +27% additional fuel use.

MECHANISM · ADHESION

Fouling adhesion increases exponentially

If you wait too long then cleaning the hull invariably damages the coating. Gentle cleaning is only possible if it is done frequently. Fast sailing can wash of the calcareous fouling but it does not protect against slime since those organisms live inside the viscous sublayer that does not move even at high speeds. The only effective defense against slime is frequent cleaning.

~19 µm

Viscous sublayer at 18 kn

0.06 kPa

Water drag at 18 kn

10–100 kPa

Diatom slime hold strength

THE PLOT

Hull roughness and fuel penalty over time

Roughness or whole-ship fuel penalty vs months after undocking. Economics for years two–three read from month 12 (onset).

The plot

Hull roughness and fuel penalty over time

Both coating families under the same duty cycle. Shaded bands are the IMO / Schultz condition categories.18 Drag the controls to change the operating assumptions — the model re-runs hour by hour.

Biofilm accumulates on biocide-free fouling-release surfaces far faster than on biocidal paint — by about two orders of magnitude in one seasonal study10 — but on this duty cycle the copper coating’s own leached layer closes the gap. Flat, well-flushed hull only. Niche areas — thruster tunnels, sea chests, fin recesses — see no flow at any ship speed and follow a different, much steeper curve; they are treated separately below.

Table view — modelled values at selected dates

INTERVENTION

Grooming intervals for years two and three after drydock

Mean roughness and fuel over months 12–24.

First-year averages — FR coating · 18 kn · 22% stationary · Q₁₀ 2.5 · 15,500 t/yr · $610/t

REFERENCES

Where the numbers come from

  1. Schultz, M.P. (2007) — Effects of coating roughness and biofouling on ship resistance and powering. Biofouling 23(5):331–341. usna.edu (PDF)
  2. Schultz, Walker, Steppe & Flack (2015) — Impact of diatomaceous biofilms on the frictional drag of fouling-release coatings. Biofouling 31(9–10). usna.edu (PDF)
  3. Finlay, Schultz, Cone, Callow & Callow (2013) — A novel biofilm channel for evaluating the adhesion of diatoms. Biofouling. usna.edu (PDF)
  4. Oliveira, D. & Granhag, L. (2016) — Matching forces applied in underwater hull cleaning with adhesion strength of marine organisms. Journal of Marine Science and Engineering 4(4):66. mdpi.com
  5. Tribou, M. & Swain, G. (2010) — The use of proactive in-water grooming to improve the performance of ship hull antifouling coatings. Biofouling 26(1):47–56. This one isn’t in the artifact’s own reference list (it cites the related Tribou & Swain 2015 and 2017 papers instead) — it’s the earlier paper that first defined grooming, and I’ve linked Florida Tech’s publication listing for it, since I couldn’t verify a live direct-journal DOI link: research.fit.edu
  6. GIA for Marine Biosafety / IMO GloFouling Partnerships — Impact of Ships’ Biofouling on Greenhouse Gas Emissions. Note: the artifact cites this report without a 2022 date attached; I haven’t independently confirmed “2022” as its publication year. imo.org (PDF)
  7. US Navy, Naval Ships’ Technical Manual, Chapter 081— Waterborne Underwater Hull Cleaning of Navy Ships (document number S9086-CQ-STM-010). No working direct PDF link found; the closest verified source is FAS’s NSTM chapter index, which lists it but doesn’t host the file: man.fas.org/dod-101/sys/ship/nstm
  8. International Maritime Organization — Biofouling Report (GHG). testbiofouling.imo.org (PDF)