Session: 06-04-02 Marine Engineering and Technology II

Paper Number: 80646

80646 - Composite Propellers Showing Passive Automatic Bend Twist Characteristics by Combining Several Design Concepts 

Composite marine propeller blades are a promising alternative to traditional metal blades. While traditional metal blades usually are solid, isotropic and are often assumed to be rigid, composite propeller blades can be built to incorporate intrinsic bend twist characteristics that allow automatic passive adjustment of the pitch angle in the propeller blades. The pitch adjustment is advantageous when a propeller operates in periodic variable loading conditions. Common causes for periodic load variations are when the propeller operates in the ship’s wake or when the propeller is on a thruster used to change the heading of a vessel. This work focused on an operational scenario where a propeller on a thruster was turned 8°, causing a doubleing in total load between the minimum and maximum periodic loading. The advantage of bend twist coupling is lower fuel consumption, less vibration and less noise.

Bend twist coupling can be obtained by making the blade with specially designed non-symmetric laminates utilizing the anisotropic characteristics of composites. This study used anisotropic characteristics of asymmetric composite laminates and explored other concepts: hollow blades, local stiffening, modifying the blade-root attachment, and adding internal support structures. Combining all these concepts to achieve a high bend twist coupling in one blade construction gives better results than using only special laminates with anisotropic properties.

Initial studies explored the standard concept of special laminates with anisotropic properties and local stiffening patches.  Numerical finite element analysis was used to identify promising solutions. A full scale propeller blade was physically built using one promising solution. Some of the production challenges when constructing a physical prototype are described here. The blade was experimentally tested under simple static loads in the laboratory to verify that the prototype blade and the numeric model showed similar elastic response. Measured deformations and strains agreed well with the numerical predictions. 

Based on this experience a virtual design exploration of all the considered concepts was done leading to another more optimized blade design. The final blade achieved a twist of up to 3.3°, which is slightly beyond what is necessary to counter the load variation in the operational scenario of this study. Typically a blade with bend twist characteristics increases its twist from the root towards the tip, while idealy the twist should happen close to the root and stay constant in the rest of the blade. This blade achieved some of this characteristic. The twist per bend is a good, normalized measure of the bend twist capability of the design. The final design developed here achieved a twist per bend of 0.15°/mm, 300% more than reported for other composite propeller blades in the open literature). This static analysis shows that the potential for adaptive composite propeller blades exists and that they can be built with a predictable elastic response.

Presenting Author: Sondre Ø. Rokvam Norwegian University of Science and Technology - NTNU

Authors:

Sondre Ø. Rokvam Norwegian University of Science and Technology - NTNU
Andreas T. Echtermeyer Norwegian University of Science and Technology - NTNU