[hpv-boats] Fin-drive redux

[Bob Stuart]

Thanks for the data on the Hobie drive.  I wouldn't have bothered  
thinking about fin drives if I had not seen a lot of room for  
improvement in all previous systems.

Starting from the fin, the usual scheme has been a rigid leading  
edge, with a flexible wing.  This gives a shape like an airplane wing  
flying upside down.  The first level of improvement is a rigid,  
symmetrical foil, as on an aerobatic aircraft, with a pivot that lets  
it automatically flip over to the best angle of attack for each  
stroke.  This pivot can have a second degree of freedom under manual  
control that lets it be set for reverse thrust.  For greater   
efficiency, we can use two wing spars, fore and aft, so that a  
flexible fin assumes nearly the ideal camber for each direction.

Next, the fins should be placed so that there is no yaw imposed on  
the boat.  The Hobie is fine in this respect, whereas Harry Bryan's  
boat was getting thrust from all three fins as it wiggled through the  
water.  Two fins off the stern with opposite motions are one  
solution, but I prefer having a compact arrangement, with the fins  
nearer the pedals.  Three options are: a single fin, with the side   
force reacted against the mass and shapes of a multihull, two opposed  
fins mounted like bilge keels or leeboards, or fins on the tips of  
sweeps, like oars on their way to being parked on the gunwales.
The single fin can be mechanically simple, with force in each  
direction coming from one pedal.  The bilge-keel mounting emulates  
the pectoral fins of whales and penguins, and avoids disturbing the  
surface of the water.  The leeboard style avoids a through-hull  
fitting and gives a longer sweep, but makes waves of it's own.  It  
might achieve a constructive interference with the hull, giving a bit  
of jet propulsion and suction at the hull end of the stroke.  Only  
the sweep mounting gives a really decent angle of attack, applying  
most of its force in the proper direction.  My propeller tips could  
move over 15' per pedal stroke (half rotation of cranks.)  However,  
sweeps need compliance and surface-followers to keep the fin working  
properly on most boats and sea conditions, and rigid parts to  
minimize hysteresis  losses.  They probably need an extra link to  
maintain a parallelogram to keep the fin angle accurate.  They might  
also want a compact-operation mode for use within a marina where oars  
are a clearance problem.

The single-fin type requires a separate rudder, but the others are  
able to handle steering duties.  The leeboard style would allow  
course corrections, or errors, depending on the listing of a single  
hull.  The double fin types, with reversing switches, would allow the  
boat to turn with no forward motion.  Various degrees of adjustment  
could be applied for course keeping, saving the drag of a rudder.   
Ideally, steering could be done by either varying the force on each  
leg, or, preferably, by touching the reversing switch or trim  
adjustment with the sides of the heels.

In shallow water, the fins could drag on the ground and still work,  
and when surfing onto a beach, they would dig in and resist the  
retreating wave.  They could also just swing back to shed weed if any  
drag built up.

One great advantage of pedal cranks is that they conserve momentum.   
Linear drives often waste energy by requiring muscle power for  
deceleration, like shadow boxing.  I have not tried it yet, but a  
system like the Hobie drive, but with the pivot at the top might work  
well, like a pendulum.  If the natural frequency works out to the  
right range, it could be a sweet deal, combining easily with the  
sweep or leeboard schemes for a neat, drop-in installation.  The mass  
of the whole system would affect the frequency.  It could probably  
benefit from springs for adjustment to different speeds or leg weights.

Bob Stuart


On 26-Feb-11, at 12:39 PM, Capn Jimbo wrote:

> Thought it well to post the results of the definitive HPB study of  
> Hobie breakdown prone flipper system.  However successful the  
> marketing concept the results were dismal.  I quote:
>
> "At 3.5 mph the Mirage drive was only 22% efficient (heart rate  
> 107). The drive doesn't really get efficient until it reaches 5.3  
> mph at 46% (heart rate 132). Now when you realize that many prop  
> driven HPB's approach efficiencies are closer to 70-80% efficient,  
> it's no wonder that in fair, head to flipper competitions the Hobie  
> sucks hind teat."
>
> Rick has designed some systems on flexible drives that are easily  
> raised, or reversed in cases of heavy weed.  Keep in mind that  
> flipper drives are not immune to the same issues.  I am a lifelong  
> kayaker and I can assure you that weed is rarely an issue, unless  
> you are in extremely shallow water, when neither system is  
> practical.  Try "flipping" in 1 foot of water.  In these cases a  
> flex drive prop system can be created that is easily and  
> temporarily lifted out of the water.  Compare to the Hobie flipper  
> system. which is a trial to remove and replace.
>
> To me anyway, and even under the best of conditions, props are so  
> much more efficient, simpler and far less subject to breakdown.   
> Chasing the flipper concept is like trying to replicate sharkskin  
> or whale fin tubercles. Nature simply does these better.
>
> But have fun... but if Hobie couldn't do it...
>
>
> ----- Original Message ----- From: "Larry H. Smith" <lhsmith at wvi.com>
> To: "Human Powered Vehicles -- Boats" <hpv-boats at bikelist.org>
> Sent: Saturday, February 26, 2011 12:26 PM
> Subject: [hpv-boats] Fin-drive redux
>
>
>> Hi Bob,
>>
>> I'm confident that  Rick is right on top of propellor-stuff, but  
>> I'd like to hear more about your fin-drive ideas.
>>
>> Max efficiency is less of an issue when "messing about " is the  
>> goal, and freedom from weed-wrapped props with the convenience of  
>> a kick-up drive, combined with the fish-like grace of a flexible  
>> fin appeals to me.
>>
>> I wouldn't mind resorting to a paddle for the occasional reverse  
>> requirement.
>>
>> I liked the relative simplicity of Harry Bryan's rig, as opposed  
>> to the bulkier drive that was later developed and discussed for  
>> awhile on this list. Seems I remember some sort of transparent  
>> flex fin on Harry's early experiment. I like the esthetics. ;-)
>>
>> Harry ultimately used a forward fin(skeg) to counter yaw  
>> effects, ...I was wondering if you had any other ideas on yaw  
>> control.
>>
>> I was wondering whether a split fin with stacked halves in opposed  
>> motion would be feasible, or would turbulence between the flow  
>> between the fin-halves interfere too much to be useful? (Picture a  
>> swim-finned diver swimming on his side)
>>
>> I would value any comments you might make (Others' opinions  
>> welcome too.)
>>
>> Larry (Bob, I'm not sure if we have communicated via e-mail since  
>> your move(s?). [ My e-address is lhsmith at wvi.com ]
>>
>>> I used to make pedal-propeller drive units.  They were quite  
>>> successful, but perfection as a product became quite expensive.   
>>> One peculiarity of human power is the fluctuating torque, so that  
>>> most of the time, the propeller is operating far from its most  
>>> efficient rate. The fin-based drive on the Hobie Mirage has to  
>>> reverse direction at the end of each stroke, but this probably  
>>> involves less friction than keeping a propeller moving at a  
>>> constant speed.  If I were building another boat now, I would use  
>>> some kind of fin drive, and try to design the basic unit so that  
>>> it could be built and maintained wherever wooden boats are made,  
>>> without any purchased hardware.  I have sketched out several  
>>> possibilities of varying complexity, size, and efficiency.  I  
>>> expect that they would be as fast as propeller drives, and better  
>>> around shallows and weeds.
>>>
>>> Bob Stuart
>>>
>>
>>
>> --
>> This message comes to you via the hpv-boats at bikelist.org mailing  
>> list, sponsored by http://www,HuPI.org/
>> Visit http://bikelist.org/mailman/listinfo/hpv-boats to manage  
>> your subscription.
>
> --
> This message comes to you via the hpv-boats at bikelist.org mailing  
> list, sponsored by http://www,HuPI.org/
> Visit http://bikelist.org/mailman/listinfo/hpv-boats to manage your  
> subscription.