Returning to service after months at anchor: hull and propeller fouling on idle ships

Why idle ships foul so fast

Three things combine:

• No water flow. Self-polishing and foul-release coatings rely on the ship's movement to shed early growth. At anchor that mechanism stops.
• Warm, nutrient-rich water. Growth rates in the Gulf, the Arabian Sea and tropical waters are far higher than in temperate seas.
• Light near the surface. The waterline and boot top get the most light, so weed establishes there first.

The result is a hull that moves quickly from slime to macrofouling, with barnacles on the flat bottom and in niche areas, and often a propeller with calcareous growth on the blades.

What it costs when the ship sails

A heavily fouled hull and propeller cost speed, fuel and carbon intensity from the first day back in service. They can also bring commercial trouble:

• Performance claims under the charter if the ship cannot meet her warranted speed and consumption
• Biosecurity problems if she is bound for a country with arrival rules on biofouling
• Port refusal of in-water cleaning on arrival, leaving the ship to sail on fouled

That last point matters. Many ports will not allow heavy macrofouling to be cleaned in their waters without capture, or at all. Cleaning before departure, near where the ship has been waiting, is often the only practical option.

What to inspect before the ship trades again

A short underwater inspection before committing to cleaning tells you what you are dealing with:

1. Fouling type and extent on vertical sides, flat bottom, boot top and waterline
2. Niche areas: sea chests and gratings, thruster tunnels, bilge keels, rudder and stern frame
3. Propeller: calcareous growth, roughness and edge condition
4. Coating condition: whether the antifouling is intact or breaking down
5. Anodes: wastage after a long period stationary

Video from the inspection lets the superintendent decide on scope before the cleaning team mobilises, and gives a fouling rating to show any port or authority that asks.

Planning the clean

• Clean hull and propeller together in one mobilisation where possible. Polishing a propeller while leaving a fouled hull, or the reverse, recovers only part of the loss.
• Include niche areas. Sea chests, gratings and thruster tunnels foul heavily at anchor and are a biosecurity concern in their own right.
• Use a method suited to the coating. Heavy growth on a foul-release coating needs gentle, patient work, not hard brushes.
• Time it close to departure. Cleaning a week before sailing and then waiting another three weeks undoes much of the benefit.
• Keep the record. A dated report with photographs is the evidence for charterers, port authorities and biofouling record books.

Checking the rest of the underwater body

While divers are down, it is worth a look at rope guards and seals, rudder pintles and the condition of the stern tube seal area. Long stationary periods can leave fishing gear, rope or debris around the propeller and rudder, and finding it at anchor is far cheaper than finding it on sea trials.

Where Cleanship can help

Our UAE bases at Fujairah and Khor Fakkan are close to the anchorages where many idle ships wait, and we can inspect, clean and polish under one mobilisation, with a report for the vessel file. See the complete guide to ship hull cleaning for background, our underwater hull cleaning service, and hull cleaning at Fujairah.

If your ship has been waiting at anchor and is about to trade again, send us the vessel details, time at anchor, last docking date and where she is now. Submit vessel details for a hull condition assessment.