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Hobie Mirage Drive Kayaks
The Hobie Mirage Drive propels the boat easily and smoothly, and reaches hull speed with minimal effort. It also generates significant static thrust.
This is what the manufacturer say:
"Even we were surprised at the efficiency of the Mirage Drive. In a test to compare the efficiency of the Mirage Drive, we measured the heart rates of several kayakers at varying speeds in several paddled kayak models. In every case, the heart rate-or effort expended to maintain a particular speed-was three to ten percent less for pedalling versus paddling."
Translation? The Mirage Drive converts the effort of the human body into forward thrust more efficiently than a paddle! Allow us to explain. The Mirage Drive creates less turbulence in water. This becomes apparent when you compare the wake of a Hobie Mirage to the wake of a paddled kayak. With each stroke of the paddle, you'll see two vortices, or whirlpools, on the surface of the water.
These vortices are connected underwater, and there is considerable energy in these rotating masses of water. There are vortices in the wake of the Mirage Drive, but since the Mirage Drive acts on a much larger volume of water, they are much smaller and therefore contain less energy.
To create forward thrust on the water, a boat must move water backward. It can either move a little water quickly, or a lot of water slowly. The key to efficiency is to move a lot of water slowly with the least amount of turbulence. The volume of water that the Mirage Drive acts upon is approximately proportionate to the area that the fins sweep in one cycle, or about 226 square inches.
The volume of water that a paddle acts upon depends on the type of stroke. A basic stroke would act upon a volume of water proportionate to the area of the paddle, or about 90 square inches. This is just a fraction of the area "swept out" by the Mirage Drive, which explains the difference in efficiency.
Why not use a propeller? Human-powered propeller drives are typically smaller and therefore less efficient. We compared the performance of the Mirage Drive to a propeller drive, and found the Mirage Drive to be faster and more efficient. Studies on tuna and penguins show that oscillating foils such as the Mirage Drive are more efficient than propellers. Oscillating foils can make use of vortices that are naturally shed from anything going through the water to offset the vortices that would normally be generated by fins.
This equates to less turbulence in the water. The Mirage Drive fins "feather" into the flow when not pedalling and create very little drag; a propeller creates significant drag when it is not spinning. The back-and-forth motion of the pedals provides a long, smooth stroke. Pedals that go in circles on a boat have a much different feel than pedals on a bike.
On a boat, there are portions of a circular motion that are more difficult, so the cycle is not smooth. The back-and-forth motion allows the pedals to be positioned much lower in the cockpit.
The Mirage Drive allows any length of stroke desired, and performs well with both short and long strokes. The pedals easily adjust to accommodate different size peddlers.
The oscillating motion allows the use of a simple chain and cable system that is unaffected by sand and dirt, without the use of complicated seals.
The fins shed seaweed because they do not make a full rotation. The Mirage Drive fins fold up next to the hull for beaching and in shallow water by simply putting one foot forward.