Hull Coating Degradation

Coating degradation

Coating degradation

“A ship scheduled for such surface preparation [blasting down to bare steel] – whatever coating system is being used – would normally be 10-15 years old. The blasting will change the hull condition from rough and possibly fouled, to smooth and clean. We know that this surface preparation can improve fuel consumption by about 25-40 per cent, depending on prior condition.”

The statement, made by Bjørn Wallentin, Jotun Coating’s global sales director for hull performance solutions, appeared in an article in the June/July 2011 issue of Marine Propulsion.1

Mr. Wallentin’s statement represents general conventional wisdom on the subject in the shipping industry. It is well known and accepted: by the time a ship with a biocidal antifouling or with a fouling release hull coating system reaches 10 years or so since the last time it was fully blasted to bare steel, it will have increased fuel consumption by 25-40% compared to initial sea trials, even when it is not heavily fouled.

There seems to be very little scientific information which quantifies the exact proportion of fuel penalty which can be attributed to hull coating degradation as opposed to biofouling, but the evidence that there is a combined fuel penalty of this magnitude is very clear and well known to informed technical superintendents and those responsible for the fuel efficiency of ships around the world. A 10-year-old ship goes to drydock, the hull is grit blasted, a full new coating system is applied properly (any type) and the fuel consumption subsequently drops dramatically.

This increase in fuel penalty does not occur suddenly. It is a gradual process from when the ship is first launched, through the various drydockings in which the hull coating is patched, touched up, partially repaired and reapplied until, after 10 or 12 years the coating  has degraded so much that it has to be entirely blasted off and reapplied completely. Throughout those 10 years, the fuel efficiency has gradually become worse and worse. A great deal of money has been spent unnecessarily to maintain power and speed despite increased hull resistance. In days gone by, a ship’s engines were built with 40% surplus power. One reason for this was to compensate for what was thought to be “engine degradation” as the ship aged. But was it “engine degradation” or was it simply “hull coating degradation”? The evidence would indicate that the additional power was needed to maintain initial trial speeds as the hull friction increased over time. This White Paper aims to collect available information on the effects of hull coating degradation, invite reader participation in gathering additional experiential information, and highlight a system which does not undergo degradation over time but in fact becomes hydrodynamically smoother as the ship ages, operating as it does on entirely different principles than the coating systems in general use.

The rewards of successful application of such a system include a greatly reduced fuel bill for ship operators and a consequent reduction of CO2, NOx, SOx, black carbon and other environmentally unwanted emissions.

1 Bjørn Wallentin, Jotun Coatings, “The illusion of fuel savings,” Marine Propulsion June/July 2011.

Source: Hydrex whitepaper No. 09