As the Arctic opens up to shipping, Van Voorden Castings has been developing its knowledge base in both the ice-class design of propellers and the materials of which they need to be made. The ice-cold waters near the polar regions make propeller design a challenge. There has to be a trade-off between the highest possible strength, the highest possible propeller efficiency, and the lowest possible cost of manufacturing.
Because of the growing need to be able to operate in tough conditions, Van Voorden has seen a rise in demand for stainless steel propellers in the last couple of years. Production has switched from making just a small number of stainless steel propellers five years ago, to a point where more than half of all cast propellers were made of stainless steel in 2015.
Traditionally propellers are made of various bronze alloys, which are used for their good corrosion resistance, ease of manufacture and reasonable material properties. However, Van Voorden believes these material properties are not adequate for vessels sailing through ice. Either they will result in a propeller design that is crude and thick and that deviates from its optimum shape, or the propeller will simply not be strong enough for the harsh conditions.
The solution Van Voorden has chosen instead of the bronze alloys is high grade stainless steel. Although it is more expensive to machine, the cost of the material is cheaper. Depending on which kind of stainless steel is chosen, its strength can be more than twice as great as the bronze alloys, as can its resistance to wear and tear. Van Voorden contends that this will permit a more efficient propeller design and deliver lower maintenance costs, reducing the total cost of ownership.
Starting with ice-classes equivalent to the Finnish-Swedish ice-class 1A, as the ice-class increases the need for a material stronger than bronze becomes greater. When optimisation is needed and even the smallest efficiency gains are worthwhile, high quality stainless steel can make the difference – although the extra fuel efficiency can be as little as 0.5 per cent, the longer maintenance intervals can provide a good return on investment.
To illustrate the effect of material choice on the propeller blade thickness, Van Voorden points to a typical propeller with a diameter of about 3,500mm without ice-class. The thickness difference between a Ni-Al-Bronze (nickel, aluminium, bronze) propeller and a high martensitic stainless steel grade can be more than 12.5 per cent. This is a considerable reduction in thickness, resulting in a propeller that is less heavy. Furthermore, the propeller is able to operate more efficiently because the profiles are more slender. And the propeller designer has more engineering freedom to modify the blade profile in order to decrease cavitation characteristics.
A slimmer blade combined with less bulk on the tip of the propeller will reduce pressure pulses, so that the stainless steel propeller will also increase comfort on board, Van Voorden believes.