[hpv-boats] Fin-drive redux

[Capn Jimbo]

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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