A combination of hull and propeller cleanings can deliver nearly 7% fuel savings on an Aframax tanker
Underwater hull and propeller cleaning are established practices to help boost ship performance. Periodic cleaning can reduce fouling build-up and prevent increased ship resistance and fuel consumption in the period between dry dockings. Coupled with fuel consumption drivers, concerns over invasive species from biofouling as well as the need to maintain high levels of efficiency have made underwater cleaning a more attractive proposition to ship owners and operators.
Cleaning processes and standards have evolved informally, but there is now a need for formal standards to ensure cleaning is performed to a set of universal specifications and cleaning products are disposed of in an environmentally sustainable manner.
Further, ship operators often need to decide if a full-fledged hull cleaning operation would is required, whether or a propeller cleaning would suffice. A case study by Propulsion Dynamics Inc evaluated the fuel cost benefit from such cleaning and indicates an optimum cleaning strategy. A hull and propeller cleaning programme executed on an Aframax tanker can deliver nearly US$2M in fuel savings in the five-year period between dry dockings, the study shows.
Though the surface area of a propeller is rather small when compared to the hull, the effect of a rough or unconditioned propeller on fuel consumption can be relatively large. Propeller roughness is caused by cavitation damage to the metal, calcium deposits, mechanical damage and marine fouling, both soft and hard.
Aside from marine fouling, propellers develop a tenacious, hard, rough layer of calcareous chalk, produced as a by-product of the cathodic protection system, says Burkard Watermann, founder of LimnoMar in Hamburg, who studies fouling and coatings. He explains that ships usually have sacrificial zinc or impressed current anodes generating electrons that flow to areas of paint damage on the hull and the propeller and prevent corrosion. This causes the areas of bare metal to become cathodic and in so doing reduces oxygen and water to hydroxyl ions that react with calcium, magnesium and carbon dioxide to form calcium and magnesium carbonates (chalk). The chalk deposits add protection to the surface but also cause significant roughening. "The amount, rate and type of deposit are dependent on cathodic current density and ambient seawater conditions,” says Mr Watermann, noting “Chalks generally form faster in tropical waters."
If a propeller becomes fairly rough, then restoring it to its original state (or close to it) would require grinding away the material itself. Periodic underwater cleaning of the propeller can remove biofouling, prevent the effects of cavitation damage from spiraling out of control, and remove calcium deposits.
The underwater diver’s tool is the hydraulic polisher, which is a lightweight flat angle unit. Their flat and light design allows the diver to reach niche areas at the root and palm of propeller blades. The rotation of the disk creates a suction effect which holds the pad to the surface and allows the diver to move the disk like a floor buffer. Diamond pads or other abrasives can be used to remove roughness, but should always be followed by the surface conditioning disks, says Mr Watermann.
He adds that divers must be cautioned not to change the surface profile or cause any damage to the suction back leading edges, which could initiate cavitation. "A still photo and video report noting cavitation or mechanical damage, as well as fouling before and after cleaning, should be provided by the divers. The diver supervisor should be knowledgeable enough to explain the conditions the diver observed," says MrWatermann.
New propellers are typically delivered with a surface roughness in accordance with ISO standard 484. Propellers are delivered to either a Class "S" or Class "I" standard, which indicates surface roughness between 1.5 and 6 microns.
Still, it may not be practical to use expensive, state-of-the-art instruments to recreate such measurement accuracies after every cleaning. Divers have evolved methods whereby the roughness of machined and/or conditioned surfaces can be assessed by visual and tactile (fingernail) comparison with a standard set of surfaces machined in the same way. There could be roughness comparison specimens where standard references are available to demonstrate the likeness of the finished propeller to a particular reference.
The Rubert scale is the informal worldwide standard for measuring and recording the roughness of a propeller blade. The Rubert Scale ‘A’ grade is 0.65 microns in surface roughness. This was the highest attainable level some decades ago, says Mr Watermann. "Today with better materials, specially designed equipment and multistage polishing techniques, divers have been able to achieve surface roughness as low as 0.2 microns," he adds. But it is hard to obtain this finish everywhere on the surface.
In addition to a lack of formal industry standards, there are concerns over environmentally sustainable methods of disposing of cleaning products. In-water cleaning is only allowed in a few locations around the world and there is a tendency for coastal and port states to have rules which, at best, allow in-water cleaning under certain circumstances and at worst, prohibit it. There is a requirement for a standard that can be accepted by relevant stakeholders, which can enable and increase the quality and safety of in-water cleaning. As part of this effort, industry body BIMCO, together with shipowners, paint manufacturers, in-water cleaner companies and ports, is preparing a Standard on Hull and Propeller In-water Cleaning.
BIMCO representatives say that the standard will ensure that the cleaning and its outcome is in accordance with a set of specifications; the environmental impact of the process and coating damage is controlled; and the cleaning process is planned, safe and effective. The working group is drafting the standard and a draft text is expected to be finalised by the end of 2019.
The goal of hull and propeller cleaning is to reduce the "added resistance" that a ship acquires while in service. Added resistance is the roughness that the ship’s hull and propeller develop when in service, due to marine biofouling and other causes.
Added resistance is the addition to the "clean ship smoothness" measured during sea trials – speed-power tests at design speed and design draft. Added resistance is expressed as a percentage of the total clean resistance. It is a non-dimensional key performance indicator of the fuel efficiency of the vessel.
During the first five years of a ship’s life, this figure increases from 0% to 30%, assuming no in-water cleanings are performed and a reasonably good hull coating system was applied at the newbuild stage, according to Propulsion Dynamics vice president Daniel Kane. A 30% increase in resistance sounds high; however, any tanker operator knows his/her Afra will have lost about 1 knot in speed at in-docking, he adds.
Typically, the added resistance increases at the rate of one percentage point on an average every month; however, it is not a linear development over the course of the docking interval. As a range it varies from 0.5% and 1.5%. An increase of 20% would mean the ship is fouled and a gentle hull cleaning and propeller polishing may reduce this to approximately 10% - 15%. Long idling periods or slow steaming in warm waters could further increase the rate, he adds.
The Propulsion Dynamics case study of an Aframax tanker uses the CASPER database service, developed by the firm. The CASPER service is a ship performance mathematical model or a digital twin that calculates propeller thrust, true speed through water and added resistance. The model calculates the power required to keep a ship loaded to given drafts and running at a given speed and trim, taking wind, waves and the actual hull/propeller condition into account.
The CASPER service output is in two parameters: added resistance caused by roughness, biofouling and so on of the hull and the propeller; and the rate at which added resistance is developing. The first number shows the actual condition of the ship’s underwater surfaces. The second is an indicator of how the hull coating system is performing. The service allows for a daily check of speed, power, RPM and weather to ensure the data received is of high quality for the analysis.
The case study involves a 105,000 tonne deadweight Aframax tanker with a length of 223 m, breadth of 42 m, depth of 21 m, and a speed of 13 knots laden and 14.5 knots ballasted. Calculations were made for the interval between the second and third dry-docking.
For the fuel saving calculation, the following values were used: fuel oil price US$425/tonne; cost of propeller polishing US$3,000; cost of hull cleaning US$20,000. The tanker operates 250 days a year, 150 laden and 100 ballasted.
In the interval between the second and third dry-docking, three propeller cleanings will reduce the added resistance from building up to 44% to 34%. Without any cleaning, the ship would consume an average 52 tonnes a day in laden condition and 46 tonnes in ballast. Three propeller cleanings would reduce the daily consumption to 48 tonnes laden and 43 tonnes ballast. This would amount to a total fuel savings of nearly US$1.9M over five years, after accounting for cleaning charges.
Three propeller cleanings and one hull cleaning would reduce the mean added resistance to 31% from 44%. This would result in a laden fuel consumption of 47.3 tonnes per day and 42.3 tonnes ballast, providing a savings of US$2.2M over five years. If nine propeller cleanings and one hull cleaning were done, a saving of US$2.4M is typically achieved over five years.
Losses due to hull fouling are often much greater and hull cleaning can potentially offer more savings. But propeller cleanings are far less expensive and can be done in a shorter span of time, whereas hull cleaning carries the risk of divers damaging the coatings. The incremental benefit from each propeller cleaning tapers off.
Mr Kane advises gentle hull cleaning once every one and a half to two years and propeller polishing every six months. But condition-based cleanings are much more effective than calendar-based cleanings, he says. If fuel consumption increases by five tonnes a day, for instance, then it may be time for cleaning the propeller or hull. "In general, hull cleanings will not be required frequently since the coatings resist build-up. But idle conditions or slow steaming in warm waters can quicken hull fouling," he adds.