A Dirty Ship is an Expensive Ship to Operate

Hull Roughness Survey

Image: International Paint

A half decade ago, hull performance sounded like some esoteric concept, but the ban on tributyl tin (TBT) coatings and increasing bunker prices has led to the emergence of companies specializing in hull and propeller performance monitoring.

Nowadays, there is widespread data from many academic and industry sources on the ship performance effects of:

  1. Basic ‘roughness’ (old paint systems attached to hull, even if smoothed over);
  2. Hull fouling (starts with slime, then algae, then grass, barnacles); and
  3. Propeller friction

Evidence gathered from full scale tests on over 300 ocean-going vessels from one company has shed light on these aspects of hull performance in relation to the clean smooth hull (physical trials model).

By calculating speed through water (true speed through water cannot be measured by the log or DGPS), and full correction for wind, waves, swell, sea current, draft/trim, and fuel oil quality, what we find is that the basic roughness made up of old hull coating systems can cause a significant performance loss on all ship types, but especially containerships with their long vertical sides and relatively small flat bottoms.

Hull performance graph, showing decrease in hull resistance after a full hull cleaning, then ship was docked and fully blasted after 3 years.

Hull performance graph, showing decrease in hull resistance after a full hull cleaning, then ship was docked and fully blasted after 3 years. The hull coating is performing well according to the post docking development in resistance.

Ships entering their second or third 5-year docking can significantly improve fuel efficiency by white-metal blasting (SA2.5) of the hull (most suppliers of Antifouling and Foul Release hull coatings now suggest white metal blasting for smoothest hull performance). Mediocre coating application or adverse weather conditions in drydock and/or slow steaming and long idle times exacerbate hull fouling. Also, the effects of slime and onset of fouling appear as a continuous increase of the ship’s resistance over a time function, the moment the ship hits the water.

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