Going straight a thruster's rail fins form a snowplow. The
central arrow shows the direction of water flow. The small thick
arrows near the fins show the directions and sizes of the forces
generated at each fin. The thruster rail fins each are toed in 3
degrees. Each rail fin transfers a force to the board toward the
midline and to the rear. The force towards the rear of the board
is drag. Surf Trux rail fins orient towards the water flow when
you go straight. They minimize drag and generate no centering
On rail the situation is more complex. The inside rail fin, shown on the left of the "On rail" diagram, transfers forces to the surfboards to the left, and towards the nose. The rear fin transfers forces to the left and to the rear. The outside rail fin is in shallow water, so its forces are diminished. It generates forces to the rear and to the left. The forces to the left are hold. The forces to the rear are drag. The inside rail fin has negative drag in this context. That is the thrust in thruster.
Surf Trux on rail has identical inside rail fins and rear fins to a thruster. The rear fin is fixed, and the inside rail fin rotates to hit the rotation limit. The outside rail fin toes out, so that it generates hold and negative drag. You can turn tighter turns, or carry and generate more speed through the same line, on Surf Trux than on a thruster as a result of the outside rail fin toe-ing out. Force arrows are for explanatory purpose and are not necessarily scaled perfectly.