[hpv-boats] Fin-drive redux

[Bob Stuart]

Hi Larry,

I'd have to get serious about the math to work out the relative  
efficiency of hulls vs fins at resisting the side force of a single  
fin drive.  At least, having it in between the center of resistance  
and center of gravity lengthwise should keep yaw to a minimum,  
although there would be some sideways wiggling.  Hobie-cat style V  
bottoms would do much better than minimum-resistance round bottoms,  
too.  With round bottoms, a foil-shaped skeg would take much of the  
load, and add a bit of thrust itself. The skeg might be all the way  
forward, to balance the side force provided by the rudder.

The Sea Saber was my first good HPB ride, and it did feel like magic  
once I centered the rudder.  High pitch numbers give a magic  
appearance, too, but they do add a lot more invisible spin to the  
wake than is ideal.  OTOH, they waste less power at the low-torque  
parts of the pedal cycle.

My recall was that the Dragonflier had a 10' prop, but I'd have to  
check.

There are two general approaches to the whole efficiency debate.  One  
is the sailor's definition of "A Race"- two boats within sight on  
approximately the same heading.  The other is from my friend Gary  
Marshall, regarding our kayaks - "But Bob, once I'm on the water, I'm  
THERE!"  That didn't stop him from spending weeks inventing a new  
rudder style, though.  Boats are what sculptors do to avoid art dealers.

Bob

On 28-Feb-11, at 12:19 PM, Larry H. Smith wrote:

> 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."
>
>
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