There is another factor in that the fins are arranged one behind the other so that as the fins cross at the bottom of their travel, additional power is generated via a squeezing effect as the rear fin picks up and works upon water that is already being propelled backwards by the front fin coming down in the other direction.
I have a suspicion that there may be some merit in having the rear fin angled "flatter" than the front fin and using a short stroke around the centre of the full travel so that the rear fin is working almost continuously on the front fin's slipstream water.
You're absolutely right. "Contra-rotating" blades are not new. Typically they get about 5 to 15% efficiency gain. Hobie fins are, in fact, highly modified (bi-directional) prop blades.
The Mirage Drive takes advantage of this concept when the fins rapidly cross each other. There is a small potential gain by tightening the clew outhaul on the rear fin (thus flattening the leech), but is is offset by a loss in performance in normal pedaling and an imbalance in the fins that begins to cause the boat to rock. So in reality there is no effective gain (yes I tried it).
My first time out on a Hobie boat with the Mirage drive I could feel that the amount of resistance from the pedals as the greatest in the second and third quarters of the stroke. I can keep up a much greater rate of speed over a period of hours with no more exertion on my part by doing a half stroke with the pedals.
When I am out on the water with other people with the same Hobie model they get frustrated as they see me going faster and not working as hard as they feel the are in trying to keep up with me. I believe it is because they are wasting time and energy with the last portion of the stroke.
A couple of things are happening here. 1) Due to increasing bio-mechanical efficiency, legs generate greater power as they straighten. 2) In bicycling, pedals generate their greatest leverage when perpendicular to the axis of the leg (close to the 3 o'clock position). Similarly (but to a far lesser extent), the Mirage Drive crank arms deliver their best mechanical advantage when nearly straight up, so the effect sort of depends on the length of your legs; they deliver more thrust in the #4 and 5 positions than the 6 and 7 positions. IMO, between the two, factor #1 is the more important when it comes to thrust and fatigue, but the net result is a combination of the two. There is also an unrelated but very important influence of Drive friction, commonly present, but easily minimized by proper tuning and lube.
I cannot see how the last 25% of the stroke when the fins are nearly flat against the hull is going to be as effective as the portion of the stroke when the fins are nearly 180 degrees from the bottom of the hull. There are probably multiple factors in play in terms of the hydrodynamic forces around the fins and under the hull of the boat.
You're right. There is a small negative effect called interference drag as the fins get next to the hull, but there is a much bigger gain called "ground effect" where the fins become more efficient. Very similar to an airplane floating over the runway just before landing or a pelican gliding very close to the water. Dr. Ben tells us rapid fin crossing is the more significant of the two.
I can also see there there could less efficiency with the rear drive unit operating in the wake turbulence of the front drive in a Hobie tandem boat.
Normally both fins have clear water except in the narrow zone where they cross (see above).
It would be valuable to put multiple pressure sensors on each side of the fins and measure the forces across the fin during each point in the pedal stroke. This would provide some of the empirical data needed to understand their output profile.
The best thrust zone in a fin changes as the boat accelerates, depending at any moment on hull speed, pedaling speed and fin adjustment. In addition as you already suggested, fins continually accelerate and decelerate within each stroke. It may be impossible to identify an optimal output profile that would apply in all these conditions. I think there is as much art as science that goes in to the design of these blades! The fins have improved a lot over time and I suspect it has mostly been by trial and error, and intuition. The neat thing about these fins is, unlike boat props, they don't have to be changed out for different boats and speeds -- they have the plasticity to adapt within a wide range of conditions.
Adding a paddle wheel device to measure speed and distance would be a lot more accurate than relying on a GPS to provide this information. It would also automatically compensate for current for the data gathered which GPS derived data cannot.
Great idea! The older Drives have a pitot tube speedometer insert that also accomplishes this -- has to be calibrated for boat and load though so it's not quite as good.
There is also the aspect of the Hobie being a displacement hull and so its speed is limited and applying more power past a certain point is wasted. In terms of human power expended per meter of distance there is going to be a point of rapidly diminishing returns.
Yes, but as a "semi-displacement" hull, conventional hull speed formulas don't strictly apply with kayaks. When and how the effect takes place depends on the length and width of each model.
No doubt, Dr. Ben will be able to answer these excellent questions and observations much better when he is able to address these matters himself. Great topics!!