Blade Profile Design Considerations
Since all tidal current turbine blades are made from composites the designer will have to consider cavitation speeds at the blade tip. This parameter can be dealt with in a few ways. The designer can simply reduce the rotational speed of the generator and gearbox, thereby reducing the blade tip speed. Alternatively the designer could create a more durable blade. The tips which can reach these speeds could be laminated with a strip of high strength metal or more durable coatings.
In the wind turbine blade industry it is common place to have multiple blade profiles along the length of the blade. This group suggests that different profiles be assessed for the blade tip to improve performance at higher rotational speeds. For example, by using an Eppler hydrofoil profile a designer may find that the onset of cavitation occurs at a higher speed. Hydrofoils, rather than aerofoil profiles are designed specifically for the marine environment. The Eppler for example is considered an ogival section It is for this reason that the group has added the option to apply three blade profiles along the blade length. Please note that the Eppler profiles were not added.
In the maritime industry there is an extensive amount of research being performed to improve performance of hydrofoils and propellers by changing their profiles. If new profiles are found to be advantageous their shape coordinates can be used in the process described on this website. Some example profiles from “A study on the Performance of Hydrofoils with Thick Trailing Edges” by Richard kimball are shown in the following figure.
For more explanations of hydrodynamic terms this group recommends the “Dictionary of Ship Hydrodynamics” produced by the International Towing Tank Conference. The currently active link to this document follows.
http://ittc.sname.org/new%20recomendations/Dictionary/Alphabet.Dictionary.pdf
