[hpv-boats] Fin-drive redux

[Larry H. Smith]

Hello Jimbo,

Boating in internet waters is fraught with the icebergs of 
misunderstandings, misperceptions, and misinterpretations. To attempt 
to dodge a few of these mis'es, let me state that my own interest in 
the fin drive discussion is almost completely curiosity-driven 
(sometimes a very inefficient drive-system itself ;-). I have no 
"dog-in-the-fight", particularly not the Mirage system, which has never 
appealed to me. Hobie or not, I don't accept that their drive is the 
be-all, end-all of fin systems. Of the systems I am aware of, only the 
Harry Bryan fin I mentioned caught my fancy at all, and I considered 
the yaw issue the greatest drawback, which was an entirely subjective 
objection on my part. It seemed to be a fly in the soup of what was 
otherwise the simple fish tail on-a-boat concept that intrigued me. 
Thus my question about a possibly yaw-balancing "split-fin drive"  ( no 
comments yet on this).

As regards the Bryan fin-system, specifically, I would submit that it 
is hardly more complex or subject to breakage than a prop system. (IF 
comparing them was the topic - see next to last sentence below)

As  tongue-in-cheek observation, I note that the Hobie effort , while 
solving the yaw issue, seems to me a fish trying to swim with its 
ventral fins as the main propulsors, rather than the tail. Other fish, 
smarter, I think, have chosen the tail as a better way to go.

I have watched Parker McReady on his Preposterous Pogofoil,  one of the 
earliest whale-tail creations and the horizontal fluke drive, at least 
in his hydrofoiling configuration,  was definitely a high-effort 
short-term proposition. Nothing relaxed about it.  Same goes for 
similar-but-simpler hydrofoil jumping-jack drive craft recently 
marketed (name escapes me at the moment.) Any horizontal fin "fluke" 
drive, introduces "vertical yaw"(pitching) which hurts efficiency also, 
of course.

Harry Bryan's simple displacement hull with a swish-tail fin flexing 
gracefully behind the boat much better suits my fantasy concept of a 
fin-craft for easy-does-it messing about. (Another marketing attempt, 
the Pacific Tail-fin, seems to have stalled, - the latest web-site 
update is dated 2006, I think. This is the "more bulky" unit I first 
mentioned, when compared with Harry's rig. Yaw is also evident in the 
site's videos.)

For elegant simplicity in a prop drive, Bob Stuart's drop-in Spin-Fin, 
which I have tried in his own kayak, very much impressed me, and if 
production/marketing considerations had been favorable enough that Bob 
had chosen to continue pursuing its development, I believe it would 
have become very popular. (More so than the larger Sea-Cycle units, 
IMO.)

(Relatively) recent progress in design and experimentation with 
flex-shaft propellor units, as exemplified by Rick's successes, with 
some collaboration by one or more others, I believe,  seem to me to 
represent the state of the prop-drive art at present, along with the 
interesting recent attempts at hp folding props. I also think the 
"kissing outrigger" stabilized monohull, again highly developed by 
Rick, Bob and others, has been adequately demonstrated to be the way to 
go for displacement speed.

In any case, I make no case for a fin's efficiency compared with a 
prop. Apples and oranges, I believe.  The trophy for ultimate speed, 
which I suppose is the highest measure of hp "efficiency" as we seem to 
be measuring things, seems still to be held by an air-prop driven 
hydrofoil (Decavitator) unless I am too far behind the times.

But the maximum-speed efficiency machines are not what I, at 70, am 
interested in spending my time on. Thus my intent in pursuing "fin 
discussion" is not to compare/compete with prop drives, but to explore 
fins as a separate subject.

I am slowing down, but still enjoy reading about and watching the 
experiments and progress achieved by all the human-power group, 
whatever their goals.  So I add my encouragements to all, whatever 
envelope they may be pushing. ;-)

Best wishes,

Larry






On Feb 26, 2011, at 10:39 AM, 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
>>>
>>
>>
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