^Ship Propeller (Note the Size) – Image Courtesy of the United States Department of Transportation at https://en.wikipedia.org/wiki/File:Ship-propeller.jpg
Why is Propeller Maintenance so Important?
This article is based largely on the whitepaper Ship Propeller Maintenance Optimum Solutions by Hydex, a company known its quick supply of turnkey solutions for underwater maintenance and repair of ships.
On account of its chloride content, seawater is among the most destructive of environments. It unleashes a host of raiders that erode and corrode the hull as well as the propeller. As many as 30% of marine equipment and ships fail due to marine corrosion.
Seawater contains about 3.5% of sodium chloride i.e. common salt. This salt is the source of the caustic chloride ions. But this water also plays host to ships that transport over 90% of the internationally traded merchandise. The fuel consumption of the global fleet is colossal.
Any measure that cuts down fuel consumption not only slashes expenses but also limits toxic emissions. This is precisely why the study of propeller maintenance is so very important.
Just to put things in perspective, a 10,000+ TEU containership gobbles up between 175 and 375 tons of fuel a day when sailing at a normal speed (20 to 25 knots).
Even a 1% cut in fuel consumption substantially slashes fuel costs and emissions. Appropriate propeller maintenance limits propeller roughness caused by corrosion, fouling, and erosion. This can slash fuel use by 5% to 15%. Now that’s a mammoth saving.
But that is not all. The better you maintain propellers, the lesser material you have to remove from them during repairs. This restricts the pollution of seawater by propeller materials. Propeller maintenance is therefore a win-win measure.
Propeller Thrust: The Force that Drives Global Commerce
Propellers drive most ships. And, as mentioned earlier, ships drive global trade. In a way, propellers are the force that drives global trade and commerce. Certainly a force to reckon with!
Its blades act like aerofoils generating thrust by creating a pressure differential in water. This difference propels the ship forwards or backwards depending on the direction of rotation and the pitch of the propeller.
Such working is based on the famed Bernoulli Principle as applied to fluid dynamics. It states, any increase in the velocity of a fluid will be accompanied by a decrease in its pressure or potential energy. In turn, Bernoulli’s Principle is based on the Principle of Conservation of Energy.
For forward motion, the propeller creates low pressure (or high velocity) on its rear and high pressure on its front. Thrust acts from the higher side of pressure to the lower side i.e. from the rear of the ship to its front. The reverse is true for backward motion.
Anything that affects the shape or the surface finish of the propeller ends up disturbing its ability to generate the required flow and thence the required thrust at a given level of fuel consumption.
Although the surface area of the propeller is slight as compared to that of the hull, the propeller exerts a disproportionately large influence on the fuel consumption of the ship.
In other words, per unit area of propeller roughness affects fuel use more than does per unit area of hull roughness. But because hull roughness is more important in the absolute sense (vis-à-vis relative sense), it has traditionally garnered greater attention.
You can rapidly and inexpensively repair propellers. The repair of hulls however is longer, costlier, and lengthier. Propeller maintenance therefore offers larger and quicker returns on investment than hull maintenance. Please note, hull maintenance is also important.
Causes of Propeller Roughness
Factors that cause and aggravate propeller roughness complement each other. When you eliminate one, you end up automatically minimizing the others. These factors are:
- Manufacturing Defects
- Cavitation Erosion
- Calcerous (Chalk) Deposit
- Faulty Polishing / Cleaning
- Impingement Attack
- Mechanical Damage from Impact
Manufacturing Defect: is largely related to the material used for making propellers. Of course, the deficiencies of the manufacturing processes also roughen a propeller surface.
These days, engineers use nickel-aluminum bronze to make more than 80% of propellers. Designers also prescribe manganese-aluminum bronze and manganese bronze (high-tensile brass) for a small number of propellers while stainless steel finds application as ice-class propellers.
However, technicians do not choose a material solely on the basis of its intrinsic smoothness or its ability to resist roughness. Manganese bronze propellers for example are rougher than nickel-aluminum bronze.
Engineers also consider other material properties such as its ability to withstand cavitation and corrosion as well as its strength, weight, and compatibility with repair and casting.
Corrosion: is both, chemical and electrochemical. Most propellers are unpainted and uncoated. Corrosion starts the moment you bring the propeller in contact with water.
As mentioned, saltwater is very corrosive. Its chemical action causes erosion of propeller material. Furthermore, the propeller becomes the cathode of the hull-propeller electrolytic cell and stimulates the corrosion of the hull.
Corrosion is the destructive attack of environment on a material. Factors such as the temperature and speed of ambient water and air, humidity, pollutants, and dissolved oxygen determine the rate of marine corrosion.
Chemical Corrosion is rusting i.e. combination with oxygen. In case of mild steel, the most commonly used shipbuilding material, the oxide layer is unstable and does not protect the underlying material from further attack.
Paints and coats protect from chemical corrosion because they lower the exposure of the metals to oxygen and water, the two elements needed for rusting. Most propellers are however uncoated and unpainted.
Electrochemical Corrosion is the result of an electrical connection between two separate materials through an electrolyte. Seawater acts as the electrolyte and the less noble material becomes the anode (positive terminal) and corrodes.
A common way to deal with electrochemical corrosion is sacrificial anodes. You maintain a more anodic material in the vicinity of the propeller. That material becomes the anode and corrodes. Likewise, you can make the propeller the cathode for the same result.
Calcerous Deposits: are a by-product of the cathodic protection system according to Dr. Geoffrey Swain, Professor of Oceanography and Ocean Engineering at the Florida Institute of Technology.
Sacrificial anodes generate electrons that flow over to those areas of the propeller and hull where paint has come off. Although the electron flow prevents electrochemical corrosion of such paint-less areas, it promotes calcerous deposits.
This is because the electrons cut down the supply of oxygen and water to the paint-less areas. The hydroxyl ions in the area therefore react with calcium, magnesium, and carbon dioxide to produce chalk i.e. calcium and magnesium carbonates.
Now, chalk offers some protection to the propeller. But this shield comes at a cost – it roughens the propeller surface. Tropical waters allow for faster chalk formation as do still waters. Furthermore, blade areas near the hub are more prone to calcerous deposits.
Chalk cannot form on moving propellers because it takes time for the carbonates to precipitate and take root. And because the propeller tips move at faster linear velocities, they easily lose the chalk that forms over them when the propeller rotates again.
Cavitation: results from the bursting of the vapor bubbles or vapor cavities near the propeller surface. Any turbulent flow creates low pressure areas inside the fluid.
Flow around the propeller is turbulent and the low pressure causes water to vaporize. Remember, boiling point comes down with falling surrounding pressure. This creates the said vapor bubbles.
These bubbles move with the flow. But because they are inherently unstable, they collapse. If they collapse on or near the propeller surface, surrounding water (at higher pressure) gushes in and creates a very-high-pressure wave that removes material.
Often, it is near-impossible to tell between damage caused by cavitation and that by chemical or electrochemical corrosion. Proper propeller design goes a long way in minimizing turbulence and the consequent cavitation.
Fouling: is the growth of colonies of organisms such as algae, tubeworms, encrusting bryozoa, barnacles, and mollusks. Their growth disturbs the propeller profile that is unable to generate the required quantity of thrust.
Ships moving at over 1 knot do not allow these microorganisms to take root on their hulls or propellers. But at some time or the other, ships will be in still water. High salinity and temperature aggravate fouling.
Anti-fouling paints are effective but environmental regulations are phasing them out. Florida University researchers have developed Gator Sharkote, a non-polluting, anti-fouling paint. It is modeled on shark skins that are fouling-free despite sharks living underwater.
Faulty Polishing or Cleaning: create a scenario wherein the treatment becomes worse than the disease. Flimsy grinding, polishing, grit-blasting, and surface treatment make the propeller undesirably rough. Improper hull painting that spatters paint on the propeller does the same.
Available statistics usually indicate the rise in fuel consumption that results from a rough propeller and hull, not propeller alone. Some authoritative sources however have made this fine distinction:
- Propeller fouling can escalate fuel use by 5-6% and hull fouling by 15% (according to Christian Schack of FORCE Technology in his Green Ship of the Future Seminar in March 2010 at the Asia Pacific Maritime in Singapore)
- Fouled propellers hike fuel use by 6-14% while biofilms on hulls do the same by 8-12% (according to Advances in Marine Antifouling Coatings and Technologies by T. Munk and D. Kane)
Impingement Attack: is the result of the impact of abrasive particles in ocean water on the propeller. The tips of the propeller are most exposed to such attack for they move at the highest (linear) speed.
All parts of a rotating body move at the same angular speed (measured in radians / degrees per second). However, the linear speed of a point on a rotating body is directly proportional to its distance from the axis of rotation.
Mechanical Damage: results from the collision of the propeller with solid objects. The location, speed, and contour of the propeller make it particularly vulnerable to destructive impact.
Although some degree of propeller roughness is inevitable given the conditions we expose it to, dealing with the issue only requires frequent and meticulous maintenance. We look into the preventive-curative approach in the next article of this series.
Interested to know more ways to improve your ship’s performance? Visit our blog. And for top class marine fabrication services, marine pipe fitting, and large scale custom metal fabrication, contact Kemplon Engineering.