What if you could improve your freediving performance by 5% with a gear modification? Yesterday, Chip Bissell and I tested the drag of the Freediver’s Recovery Vest (yes, we changed the name.) In order to mitigate the increased drag of the back-mounted inflation unit, I fashioned a turtle shell to cover the apparatus. See (http://www.oceanicss.com/Graphics/inflation/turtle.htm )
Chip and I each made 20 dives to 50 feet. We made 10 with the vest and 10 without and we followed the exact path each dive. Our idea was that the time it took to complete the circuit would be a good surrogate for drag—provided, however, that we kicked consistently on every dive. We were very careful here and we think that our results are accurate (spreadsheet results: http://www.oceanicss.com/Graphics/inflation/shell_test_results.htm )
We were astounded to find that when we used the vest we moved 7% faster through the water column than without it. We made the average circuit 3 seconds faster with the vest!
These results were unexpected but on reflection make some sense. Adding the humpback makes us more dolphin-like in shape. In addition, because most freedivers look forward and dive face-first (excluding record attempts where the diver descends head first), there must be significant hydrodynamic drag caused by the head. This is similar to the dynamics of an aircraft wing that stalls without a smooth laminar flow of air across its surface. When I watched Chip, I could see that the back of his head fared neatly into the hump, thus minimizing turbulent flow behind the head.
As scientists, we know the importance of independent confirmation of our results. Advisory Board member David Sipperly will work with the University of Rhode Island to resurrect a similar drag-testing protocol the university used to assess BC drag. The implications are profound. By a simple gear change, we might be able to increase our efficiency. Actually, when you look at bicycler helmets, they have a duck-tail fairing. We might find that the addition of a simple flap of rubber extending off the hood to fare into the back might enhance the hump. Maybe, this is all we need? For the vest, now that we know there is value to the hump shape, we will experiment with different shapes to come up with the optimal shape.
Terry Maas
Chip and I each made 20 dives to 50 feet. We made 10 with the vest and 10 without and we followed the exact path each dive. Our idea was that the time it took to complete the circuit would be a good surrogate for drag—provided, however, that we kicked consistently on every dive. We were very careful here and we think that our results are accurate (spreadsheet results: http://www.oceanicss.com/Graphics/inflation/shell_test_results.htm )
We were astounded to find that when we used the vest we moved 7% faster through the water column than without it. We made the average circuit 3 seconds faster with the vest!
These results were unexpected but on reflection make some sense. Adding the humpback makes us more dolphin-like in shape. In addition, because most freedivers look forward and dive face-first (excluding record attempts where the diver descends head first), there must be significant hydrodynamic drag caused by the head. This is similar to the dynamics of an aircraft wing that stalls without a smooth laminar flow of air across its surface. When I watched Chip, I could see that the back of his head fared neatly into the hump, thus minimizing turbulent flow behind the head.
As scientists, we know the importance of independent confirmation of our results. Advisory Board member David Sipperly will work with the University of Rhode Island to resurrect a similar drag-testing protocol the university used to assess BC drag. The implications are profound. By a simple gear change, we might be able to increase our efficiency. Actually, when you look at bicycler helmets, they have a duck-tail fairing. We might find that the addition of a simple flap of rubber extending off the hood to fare into the back might enhance the hump. Maybe, this is all we need? For the vest, now that we know there is value to the hump shape, we will experiment with different shapes to come up with the optimal shape.
Terry Maas
