Shielding Propellers Against the Forces of Roughness

^Shipping is the Most Potent Agent of Globalization and International Trade
Image Courtesy of Atiketta Sangasaeng at shutterstock.com

The Big Picture

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.

Propellers drive most of the ships around the world. And ships transport over 90% of the globally traded cargo. Seen from this angle, propellers are mightily important for international commerce and the global economy.

At a normal speed of between 20 and 25 knots, a 10,000+ TEU containership guzzles about 175 to 375 tons of fuel per day. Not a meager statistic by any standard.

Therefore, anything and everything that slashes this fuel use commands immense interest from all stakeholders. And because we live in a globalized world with reciprocal linkages between almost all of us, we too are stakeholders.

Adequate maintenance keeps propellers smooth and cuts fuel consumption by a whopping 5% to 15%. Considering that even 1% fall in fuel use represents hundreds of thousands of tons of fuel, the 5-15% range represents gigantic savings.

 Approach to Propeller Maintenance

Prevention is the definitive word in propeller maintenance. The better you maintain propellers, the greater value they offer – economically, environmentally, and logistically.

Cost, time, fuel penalty, operational complexity, and eco-friendliness establish the broad framework for us to set up best propeller maintenance practices.

Frequent Propeller Maintenance is Very Important – Economically, Environmentally, and Logistically  Image Courtesy of PHOTO FUN at shutterstock.com

Frequent Propeller Maintenance is Very Important – Economically, Environmentally, and Logistically
Image Courtesy of PHOTO FUN at shutterstock.com

If you allow a propeller to get substantially rough before undertaking maintenance:

  • the propeller will be more badly damaged and will get rough more frequently
  • you will have to remove more material from the propeller during each maintenance operation, something that intensifies heavy metal pollution of ocean water
  • the ship will consume more fuel
  • you will need personnel with greater skill levels (commanding larger fees) who will have to use harder abrasives or brushes to maintain the propeller
  • each maintenance operation will take more time

In short, if you delay the maintenance of your propeller, you will end up unnecessarily pushing up the cost, complexity, and duration of maintenance. Plus, the useful life of propeller will fall.

This also makes maintenance less environment friendly as more propeller material flows into the sea. Now, the amount of material removed from a single propeller is slight. But if you add the metal removed from the propellers of all ships across the world, the quantity becomes colossal.

Ships are Dry Docked Once Every 2.5 to 4 Years for Thorough Maintenance Image Courtesy of Mariusz Niedzwiedzki at shutterstock.com

Ships are Dry Docked Once Every 2.5 to 4 Years for Thorough Maintenance
Image Courtesy of Mariusz Niedzwiedzki at shutterstock.com

Most propellers are made from heavy metal alloys such as nickel-aluminum bronze, manganese bronze, and manganese-aluminum bronze. A lax maintenance approach therefore becomes a source of heavy metal pollution.

Propeller Maintenance Practices

Before we get into the details of propeller maintenance, we must take note of the techniques used to measure propeller roughness. When the propeller is outside water, technicians use stylus-based equipment for precise roughness measurement.

Divers use the Rubert Comparator for measuring the roughness of propellers inside water. The scale runs from A to F in increasing order of roughness. The diver inspects the propeller visually and by touch. He then compares it with samples to establish the level of roughness.

Most owners and operators of ships polish their propellers when they dry dock the ship. The normal interval between successive dry dockings is anywhere between 2.5 and 4 years. This is not frequent enough.

Aware of the perils simmering inside rough propellers, some owner-operators undertake this exercise once every year. The more vigilant ones do this once every six months. Even this is not enough.

Most Propellers are Unpainted & Uncoated Image Courtesy of Eastimages at shutterstock.com

Most Propellers are Unpainted & Uncoated
Image Courtesy of Eastimages at shutterstock.com

The Naval Ship’s Technical Manual of the U.S. Navy lays down the following guidelines regarding the interval for cleaning propellers and hulls:

  • Order of precedence for partial or total cleaning is:
  1. propeller
  2. forward one-third of the hull
  3. after two-thirds of the hull
  • Except in dire exigencies, do not clean a freshly painted hull for the next 12-18 months. In any case, maintain a 6 month interval between successive hull cleanings

This does not apply to unpainted surfaces such as propellers and masker emitter belts

  • Do not postpone in-water hull cleaning even if the ship is scheduled for dry docking within the next 1-2 months

This is because cleaner hulls quickly recuperate the expenses (on cleaning) by saving on fuel costs. Furthermore, cleaner hulls will lower the time and cost of the scheduled dry docking

These recommendations are in perfect sync with the ‘little and often’ frequency and style of propeller polishing suggested by John Carlton in Marine Propellers and Propulsion.

Best practices are those that enable simple and quick propeller maintenance at the least possible cost and with minimum removal of propeller material.

Power Washing Ships @ Dry Docking  Image Courtesy of Glen Jones at shutterstock.com

Power Washing Ships @ Dry Docking
Image Courtesy of Glen Jones at shutterstock.com

Getting into the specifics, the question that now remains to be answered is: how often is neither too frequent nor too infrequent? There is no one-size-fits-all answer to this query.

It all depends on how frequently and how long the ship is in use. And the quality of waters through which it sails. It is generally a good practice to polish in-water propellers once every month or two months. This allows excellent cleaning even with softer abrasives and brushes.

If you stretch the interval between polishing too far, you will have to use stronger brushes and abrasives. These remove more propeller material and, as mentioned, pollute the ocean water.

Minor propeller repairs involve cleaning and buffing the surface. Major repairs involve polishing and grinding that removes more propeller material. Propeller Buffing is a novel technique for polishing propellers.

Buffing removes less material as compared to polishing. It also leaves behind a smoother propeller surface and takes less time than polishing. Some of the better propeller maintenance companies polish a propeller within four hours. Similarly, cleaning is faster than polishing.

In order to minimize the amount of material removed during buffing / polishing, the U.S. Navy:

  • prohibits the use of water jets with over 10,000 psi pressure while cleaning propellers
  • recommends the deployment of only the most skilled workmen while cleaning propellers with wire brushes
  • forbids the use of wire brushes to clean sensitive regions such as critical areas, outer 3-inch periphery of the propeller blades, and areas of high curvature
Nomenclature of a Ship Propeller  Image Courtesy of BoH at https://en.wikipedia.org/wiki/File:Sketch_propeller.svg

Nomenclature of a Ship Propeller
Image Courtesy of BoH at https://en.wikipedia.org/wiki/File:Sketch_propeller.svg

Trailing edges and the 3-inch periphery of propeller blades are the most sensitive areas. The U.S. Navy manual allows their cleaning only with abrasive hand pads, abrasive disks, hand scrappers, water jet guns, and brushes made from polypropylene, nylon, and polyester.

You can clean non-critical areas with all the aforementioned instruments plus wire brushes, non-abrasive nylon brushes, high-pressure water jet guns, and nylon brushes studded with silicon carbide.

Finally

Summing up, propeller maintenance is not an exact science. Although rise in fuel use for separate voyages on the same route is a reasonable indicator of the state of your propeller and hull, propeller maintenance is more of an art where you have to feel your ship. And listen to it.

For more such valuable insights on the improvement of your ship’s performance, visit our blog. And for the very best in marine fabrication services, marine pipe fitting, and large scale custom metal fabrication, contact Kemplon Engineering.