A simple hydromechanical model of the influence of pectoral fin geometry on thrust is developed. The model contains a shape factor (analogous to that employed by Weis-Fogh (1973) in his analysis of the influence of wing shape on normal hovering in insects and hummingbirds), which is calculated for various hypothetical fin shapes and the pectoral fins of Pterophyllum eimekei and Trichogaster tricopterus. The model predicts that fish which swim in the drag-based labriform mode are more likely to have triangular fins than square or rectangular ones, and this is generally the case.
The influence of pectoral fin shape on labriform locomotion is investigated further experimentally, using dead fish and a model Angelfish fitted with various model pectoral fin shapes (circular disc, square, rectangular or triangular fins of equivalent wetted surface area). Frictional and pressure drag determinations were made with the model fins attached to the end of a narrow spar (so that the flow around them would not influence that around the body) with the fins orientated either parallel (for frictional drag determinations) or “broadside on” (for pressure drag determinations) to the incident flow.
Values for the frictional drag of the various fin types were essentially the same for any given terminal velocity. The pressure drag of square and rectangular fins is about 14% less than that of triangular fins of the same area over a range of terminal velocities. When the fins are directly attached to the sides of the model fish and orientated “broadside on” to the flow the total drag force is higher (for any given value of the terminal velocity) than the values measured when the fins were placed, with the same orientation, on the ends of a spar. The difference is probably due to “interference drag” produced by the fins, due to the effect of the vorticity which they shed into the flow over the body.