I tried the Skat-Trak Swirl 13/18 Impeller that weighed 2.5lbs; it gave very good bottom end acceleration. The Scat-Trak Swirl blades are longer, thicker and weigh a bit lot more than the Concord. I switched it to a 1.8lbs Solas Concord 13/18 cut back 3mm and lost some of the bottom end, but made a huge improvement in mid range acceleration. Going from the Swirl to the Concord was like installing a light-weight flywheel on the other end of the engine.