Pedal Powered Boats

Because it doesn't matter if your air prop sucks (that's a pun) if you have minimum water drag. This is where the kinematic viscosity difference between the two fluids matters. It is not the prop, it is everything else hanging out in the specific working fluid.

 

They didn't! A Tailwind helped the decavitator more than the water propelled boats, and the official rules allowed for it. As I recall, in a later low wind competition the best of the water propelled boats beat out the decavitator, but the records were never updated, since a certain amount of wind speed was allowed in the original competition? I believe it's discussed somewhere in the archives of this particular list?

Ps. Reasonable efficiency estimate for a pedal drive https://www.boatdesign.net/threads/reasonable-efficiency-estimate-for-a-pedal-drive.66338/page-2#post-921042

See linked article towards the end of discussion, post number 24.

 

Many boats there had minimal resistance. I'm interested that the air propeller drive came close to the water propeller drive. They did not come close, they won. It was a surprise to me.

 

That thread is interesting . Thanks for the link and explanation. Despite that , I am surprised that with such small powers both drives are almost similar.
If I compare airboats and outboards , airboats are much more power thirsty.

 

<sigh> If you have something in the water more than the minimum drag hull, you can't have minimum drag. The very fact of having an in-water propeller means you don't have minimum hydrodynamic drag.

 

Ok. So show me any example where using similar power results in an advantage for air prop over water prop. ap wp .

Can you convince me , that using the same fuselage (edit hull), weight, and the same available power you will achieve the speeds from the video above?
Air propulsion setup can be a propeller or ducted fan, all perfectly matched. I think it is impossible to beat wp.

My conclusion about human powered boat is , they were using the wrong propellers and gearboxes ratios at that competitions .

 

Is that not also true for the air prop? Which would cause more drag, the 10" prop, shaft and bracket, in seawater, or the 60" propeller, cage and all that come with that, of an airboat? Given that the tips of the blades are the driving areas, the full diameter might be discounted as not contributing to drag, but when measuring power output, rather than forward resistance, full blade surface area, as well as sectional shape need to be factored back in. The surface area of an air propeller is exponentially larger than a water propeller.

-Will

 

My opinion, larger than one man typical Airboat hulls are not built for efficiency, with the lossy Grill covered propeller drivetrain, and other air blocking aspects. Airboat purpose is not efficiency/ speed but stability /utility/ large water surface contact area providing low draft in shallow water. Similar size and displacement outboard hulls can be generally designed to be more efficient hydrodynamically without compromising function.

The one man pedal racing boats were both hydrofoils meant to run a single time through a speed trap for perhaps a few seconds maximum, optimized in every way possible including the lowest possible propeller slippage once at their target speed reached Plus using trained athletes specifically matched to the particular gear trains.

From what I understand, everything begins with a Statement Of Requirements, SOR. If your requirements are for the fastest model RC boat drag racer, that is quite different than having an SOR to run in under 5 cm / 2" of water, where any submerged propeller might be useless. So my guess for the fastest RC drag racer compared to your link, would be one with Rocket motors, and the best hull design might also work in very shallow water. Another possibility is a very fast RC model car with balloon tires launched at high speed from the shoreline which continues at that high speed skimming on a very thin layer of surface water, and doesn't slow down enough that it sinks below that layer. So it appears that hulls don't have to be identical for fair comparison, but only that the best one for the particular application has to be used. This particular list has discussions of speed and efficiency considerations with just about every type of driving force for HPV; simple and unique versions: kayak paddles (Ofira, etc), water wheels, fish type fins, plus those already discussed. One of my favorites was the hydrocopter Drive patented by a Boeing engineer.

Those guys that did the HPB records had been around for many years and done extensive experimentation and research backed by MIT (for Drella) so I imagine they tried just about every possible combination of gear trains and propellers to arrive at their particular craft. Almost all were also trained athletes capable of putting out human power close to that of Olympic cyclists.

As the saying goes, "A man convinced against his will, is of the same mind still"..

Hope this helps.

 

Allright .
Flat water , no wind same power available . Let say 5 power and weight formulas ,from 2hp to 1000hp . Hydro efficient hulls e.g. some old hydro class boats or racing cats . Should be same in every level.
Start on the fly , 300m distance.
wp vs. ap.
How do you bet the results ?

 

True that the air prop surface has drag also, but its surface is not 800 times larger...which is the effective mass flow equivalence. So the air prop being 10 or 100 times larger is still a win.

 

Low power, air.
High power, water.
Just because of the thrust to drag moment balance makes high powered airscrew propulsion more problematic.

 

It was a rules exploit. They were allowed a small tail wind under the rules. So they were geared a bit lower in effect. They have been beat by a water-prop boat indoors head-to-head.

Gearing and inertia greatly favor the air prop when you look at the torque ripple of a skilled cyclist. Using cams to eliminate the torque ripple hasn't been looked at enough, and would significantly change all the prop designs, including Decavitator. If you go for tandem cyclists - trained cyclists - this can be almost a non problem. So the advantage with the air prop comes almost entirely from it's better suitability when connected to a single cyclist.

 

For the hydroplaning quad in post 2262, the water looks kind of shallow in some spots. I'm wondering if that planing would work in deep water as well as it seems to work in the film clip? I like the kind of "out of the box" approach to a craft of that type, because it takes the normally efficiency robbing detrimental hull water friction and uses it with the spinning rear wheels instead to stay on top in a planing mode. My guess is it probably matches if not surpasses the speed and efficiency of an air or water prop driven craft of the same medium range horsepower without needing a more unstable longish hydrodynamic shape! Rider is kind of high up, giving some instability because of the high C O G, but maybe the design or concept could be widened or tweaked in various ways (4WD?) to improve the stability part. Maybe could be altered and used as an amphibious cheap water surface torpedo drone, with RC controls instead of the weight of a driver, if it moves fast enough.. not a positive thought though.