Some were used at the back of a twin slightly outside to capture the pressure coming off them and adding to the sweet spot and drive.
i have a Mac after PC world and need help in posting these pics.
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Intersting greg, i like "supercharged" twins, which have very similar sized fins just in front of the main fin, and closer to the rail. Theory being they "break up" the water so that the main fin is moving through turbulent water, reducing drag. A vane i think they're called.