Propeller particulars

January 25, 2017

Propellers are the driving force for our beloved boats, but how much do you actually know about them? There are a few important types and variations in design that are fit for different purposes. Incorporating the correct propeller type along with maintaining a clean surface, facilitating reduced drag by using an effective foul release coating can significantly improve boat performance and efficiency.  

What are propellers and how do they operate?

Propellers are fan like structures which use power generated from a main engine to convert rotational motion into thrust. When concerning boats, the thrust generated from propellers imparts momentum in the surrounding mass of the water which propels the ships forward by imparting a force upon it. The resulting pressure difference is produced between the forward and rear surfaces of the air-foil shaped blade and water is thus accelerated behind the blade. The energy for rotational motion is transmitted through an engine route consisting of the main engine crank shaft, the intermediate shaft then the stern tubes along with their bearings and finally to the propeller itself.

Types of propellers?

  • Controllable pitch propellers: (CPP’s) can be manually operated to change the angle of attack for the propeller blades. They offer enhanced manoeuvrability, especially when a ship is moving in confined waters. Additionally, these propellers allow for greater fuel efficiency to be maintained when travelling at different speeds. A controllable pitch propeller prevents the over-torquing of the cruise propeller that might be caused by fixed pitch propeller. These advantages mean that they are often used in warships where the boat may need to move in both high and low speeds (cruising speed is half the maximum speed).
  • Fixed pitch propellers: Conversely, fixed pitch propellers have blades that are permanently attached to the hub in a fixed position. Fixed pitch propellers don’t incorporate any mechanical and hydraulic connection as in Controllable pitch propellers and therefore they are more robust and reliable. Although the manoeuvrability is not as good with fixed pitch propellers, installation, manufacturing and operational costs are lower than for controlled pitch propellers.
  • Self-pitching propellers: In this case, the propeller blades swivel freely and automatically to set the optimum angle. Out of all the types of propellers, this type is most likely to face mechanical errors which means that they are more commonly used on smaller, low powered vessels.
  • Highly skewed propellers: The design of highly skewed propellers is made purely for the purpose of creating a smoother cruising experience and has negligible effect on boat performance. The skewed structure of the blades reduces vibrations that originate from the action of the propeller, this means they are mainly used on cruise ships. They can be either fixed pitch or controlled pitch propellers.
  • Blade number: Three and four blade propellers are those most commonly employed. The lower number of blades on propellers means a lower manufacturing cost and also greater speed and acceleration. However, more blades correspond to increased strength and durability along with better handling in rough speeds and low speeds.

Construction and materials of propellers?

A marine propeller is constructed by sections of helicoidal surfaces acting together to rotate through water with a screw effect. The two main methods of constructing propellers involves either forging in one piece or welding the rotational blades to the central hub or boss.  Between the two methods, forged blades are known to be the more expensive option but also have greater strength and are more reliable.

Propellers spent the vast majority of their time operational, submerged in sea water which has high corrosive nature. Therefore, the main consideration in the construction of propellers is that they are made from corrosive resistant materials. Alloys of stainless steel and aluminium are the most common material used although various alloys of bronze, nickel and aluminium are becoming more popular as premium options due to their greater strength and lightweight.

Factors to consider when selecting propellers?

Propellers have to be designed in a way to reduce noise/ vibrations and hence cavitation to the lowest possible level while also maximizing speed and propeller efficiency. Considering these factors, prioritising your choice of propeller will depend on the desired attributes of your boat whether it be a smooth cruise with minimum noise disruption, optimum speeds and manoeuvrability or strength and durability to cope with testing sea conditions. Importantly, all propellers of different designs and purposes can reach optimum propeller performance by having a clean, slippery foul free release surface which reduces friction, drag resistance and cavitation which directly improve speed and efficiency.

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