[hpv-boats] Fin-drive redux

[Larry H. Smith]

Terrific overview, Bob, ... thanks for taking the time!

Plenty of food for thought.

As a side observation (OT), your reference to "sea conditions" brought 
to mind a similar issue with one of my FrontRower rigs, where the 
changing freeboard with lighter/heavier loadings, and the varying 
surface height with chop, dictated differering "settings" of the 
mechanism, to insure proper oar-blade immersion. ( I built an 
adjustment feature which helps with the freeboard issue, ...somewhat 
less helpful with chop, but helps a bit.)

Your idea re. multihull yaw resistance interests me. Considering the 
added advantage of inherent stability(no outrigger drag at any speed or 
degree of list), and the overall desirability of a single fin's 
simplicity, do you believe that the lateral resistance of two hulls 
would effectively reduce the overall drag, yaw plus hulls, enough to 
equal-or-better that of a monohull with yaw-countering and stabilizing 
appendages? I envision hulls each optimized for its half of the total 
displacement at expected speeds, and an overall beam dictated by 
Godzilla(etc.) for minimum intra-hull wave interference.) My weight 
hovers around 200 pounds these days, so I generally think in terms of 
about 325-350 for all-up weight of a "pleasure" craft with lunch, 
camera, etc.)

I took pleasure in watching the relatively slow-turning 18" prop as Jon 
Knapp's Sea Saber moved away from the dock, which of course the rider 
could not see. There seemed to be a somewhat magical amount of forward 
motion from the prop activity.  I had a similar impression while 
watching the (8 ft.?) air prop on the (Steve Ball's?)Dragonfly 
hovercraft at (14-15mph? IHPVA Championships - Visalia Ca.) speed. I 
imagine similar pleasure from watching the fin working between the cat 
hulls, in this case perhaps visible to the rider, in a configuration 
which placed the fin closer beneath the pedals, minimizing linkages, 
etc.

Maybe a "Feathercraft" type kayak  drop-rudder(s) to assist turns, but 
raised for straight runs, minimizing appendage drag?

Jimbo,
Could you link-to or reference the source from which you took the quote 
re. Hobie efficiency? (i.e., the "definitive study")
Thanks.



On Feb 26, 2011, at 7:58 PM, Bob Stuart wrote:

> 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."