In the course TMMT06, Mechanical engineering – project course, a mandatory project is carried out in a group, while a bachelor’s degree work is, in unison, carried out in pairs. The project, which in this case is shared by three pairs, aim to investigate the dimensioning of hydrofoils and their performance. This document is part of one of those bachelor’s degree works, and is focused on a static structural analysis of hydrofoil support struts under the influence of lift and drag forces.
The goal with this work is to create a construction foundation to be used in the shared project. The aim of the work is to find guidelines for the selection of strut and hydrofoil assembly angles, as well as strut dimensions, depending on the size and shape of the hydrofoil.
With the use of generally established equations for lift and drag forces, and data for hydrofoils the forces and stresses crucial to the dimensioning of the strut cross-section and assembly angles are analyzed.
Two streamlined struts, thickness 15 mm and width 100 mm, with the hydrofoil section design Eppler 818 mounted, are studied in a static structural case without dimensioning for unexpected events, such as collisions. Within the span of velocities that are probable for a boat it was found that there never occurs any danger of buckling. Within the same span, plastic deformation does not occur either, although the stress is high enough that the safety margin is relatively small.
The danger of buckling does not increase as the assembly angles are changed from a perpendicular position, and buckling ceases to be a factor for angles of attack lower than −4°. The stresses and the risk for plastic deformation, however, can increase as the assembly angles deviate from perpendicularity. By making the struts bigger, the construction will be able to use more demanding assembly angles, but to have anything but perpendicular struts is to be avoided. The analysis also shows that the boat should not be allowed to pitch more than 6° at high speeds to avoid large stresses in the struts.
The largest dimensioning stress in a strut acts at the connection point with the hull or with the hydrofoil. In order to minimize the total thickness of the strut, and thus lowering the drag, it can be made thicker at the connection points while keeping it thin otherwise. The analysis also shows that an increase in strut cross-section area will give large positive outcomes in terms of safety margins against buckling and deformation.
Eppler 818 was the only hydrofoil studied during the analysis, but the results provide good guidelines for construction of support struts in regards of strength.