I can't say for sure as we are still testing and they are collecting the data and interpretating it, not me. But I personally feel from experience the drag comes from the fluke being too bouyant. My tail with drainage seems to drag more than even bigger flukes ( but less silicone due to shape/ mono fin) without drainage. The more we test the more I feel it isn't one trait ( eg drainage, fluke, monofin) but rather specific combinations. However these are not official study findings. Just totally my opinion.
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