Pedal dual Kort nozzle props on a kayak??

Discussion in 'Boat Design' started by Deadweasel, Apr 7, 2015.

  1. Deadweasel
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    Deadweasel Junior Member

    Hello all. Been reading through these forums which repeatedly come back as results on my myriad searches on the general topic of DIY pedal powered kayaks, and now I've been motivated to join and engage the community.

    I've read and watched countless articles, posts and videos regarding the Hobie Mirage, ProPel and other DIY prop drives, but I couldn't help thinking everybody was missing some critical aspects that (I think) are almost deal breakers for my intended use.

    For example, the Mirage "tops out" at a certain point, and you simply can't go any faster even if the boat's hull speed hasn't been reached (and the non-circular pedal method is a major factor in that, too). From what I can tell, after a certain point, the wake turbluence of the leading fin will just corrupt the trailing fin's ability to generate effective thrust. ProPel and others like it have a semi-permanent draft requirement that complicates shallow water navigation with the tedium of dismounting the drive to pull it inboard, not to mention issues with weeds and submerged hazard strikes.

    So my research turned to ducted props, which I quickly learned are called Kort nozzles (I'm still new to marine terminology, but I'm getting better quickly at this rate!). As I'm looking at the sparse results I've been able to dig up, I can't help but wonder why it seems so few are using the ducted design that would seem to be a major improvement to an open prop. Also, I've read many cautions regarding weed/line tangles on props, but almost no mentions about cutter bar mods on the prop shafts that could help reduce entanglements.

    Then, finally, I discovered something that blew my mind and made the itch to build a pedal powered craft much more difficult to dismiss.
    Peddle-Powered Kort-Nozzled Watercraft (http://blohr.com/jetyak.html)

    It was something that made me literally jump up, stab at my phone's screen and cry out "THIS!". It was the eureka moment when all of the ideas and concepts and questions just balled up together, organized a fire team, and declared war on my wallet.

    So having found this, (and having drawn complete blanks on these forums trying to find any mention of it), I'd like to ask the thoughts of the community at-large. Is there any reason to believe that this concept couldn't work as it would seem to from the initial overview? What are the potential downsides, and might there be methods to overcome or at least partially address them?
    Initially, I'm thinking there could be issues with weight and a greater power requirement at the pedals (which could be abated somewhat by making it a tandem pedal system, possibly?). On the pros side, I'm thinking better maneuverability, no cumbersome reconfigurations for shallow navigation, and decreased opportunity for damage/function degradation due to strikes and tangles.
    If this gets folks as stoked as it did me, what about construction materials? Would this be best served by wood core or hard foam? Maybe something else I'm not yet aware of? I keep thinking about foam for the built-in flotation benefit, as well as more flexibility in building a smooth, flowing form, but I'm still in the idea stage so I'm content to consider anything.

    I'm excited about this concept, but the last thing I want to do is to start putting in the effort to design the hull and source components, only to discover the concept is too flawed to be useful (let alone fun).

    I'd love to hear others' input on the concept and its potential feasibility.
     

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  2. daiquiri
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    daiquiri Engineering and Design

    Ducted props have some merit, but I am somewhat dubious about their utility on a human-powered boat. Perhaps only as a protection for prop blades in shallow waters. Human powered boats require a maximum propulsive efficiency and minimum drag. Neither of those are brought by kort nozzles, apart at very low speeds. It is not just a case that tugs and other slow-speed towing and maneuvering vessels are the most common clients for that kind of props.

    Now, without further repeating of what has already been said here on that topic in the past, I would invite you to first check this discussion for more info about that topic: http://www.boatdesign.net/forums/bo...v-s-nonshrouded-opeller-24756.html#post235137
    and this one too: http://www.boatdesign.net/forums/education/propulsion-kort-nozzle-4525.html
    Then we can discuss eventual further doubts or questions. ;)

    Cheers
     
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  3. Deadweasel
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    Deadweasel Junior Member

    Thanks for your reply!
    I hadn't read the shrouded prop discussion yet, and now that I have, thanks very much for that link. I had previously run end to end on the Kort propulsion thread.

    There are quite a few aspects covered on the topic in both threads, but what caught my eye is that there seems to be a general agreement that Kort nozzles (shrouds) can at least be of some benefit to low speed craft, as opposed to speed boats or outboard motors in general.
    What doesn't seem to be clear is if there's a useful benefit in terms of a human powered craft. Most references to low speed craft were in context with speed boats or outboard engines, so I'm not sure if the term "low speed" is intended to include velocities attainable by human powered boats.

    That said, the design I referenced incorporated not just a ducted design but also five blade props, plus TWO of them to boot. I can understand that driving to two props would naturally require more energy to move, but the question for me is whether the ducted design combined with five blades would make any positive net difference in operating a boat powered by such a system.
     
  4. Rurudyne
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    Rurudyne Senior Member

    The (presumably patent) drawings you post are not full Kort nozzles.

    IIRC, Kort nozzles rely on the wing like cross section of the nozzle and it's interaction with the prop to improve power or efficiency at moderate speeds. Normally it seems that placement of the nozzle is clear in what little good water flow there is to be had under the boat. By having the top half of the duct as shown this effect is lost and I would question that the lower half, within the hull's influence as it is, would produce the desired results.

    This may be addressable by separating the props and nozzles from the back of the boat by some distance, not necessarily much, sorta after a fashion similar to a surface piercing prop, but of course fully submerged. The tunnels in the hull still facilitate getting water to the props even though they do not contain them. The central portion of the hull between the props could modestly extend rearwards too to support them, again, not unlike with some surface piercing props though less extreme (as those are high speed planing craft).

    With even a modest middle, ah, tail it may be possible to also have the nozzles be independently steerable rather than use rudders, though doing so will make the drive linkage more complex. I would refer you to Glen-L's DIY pod drive for ideas on that.

    EDIT: something to consider about the nozzle foil shape ... http://www.olds.com.au/marine/maximizing_propulsion_efficiency/
     
  5. philSweet
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    philSweet Senior Member

    Just a comment on the engineering. The sketches are just suggestive. They don't show what is actually needed to be effective. As a former competitive cyclist who weighed 145 pounds, I had a 1200 pound leg press, a 740 pound calf curl, and I could snap racing bike cranks if I wasn't careful on hill climbs. When you figure the actual engineering on those gear cases, you aren't going to like the price or weight very much. (think gears from a school bus tranny)

    For a one-off for the designer who knows the machine's limitation, not a huge problem, but for something like a rental at a resort or summer camp, you have a big problem.

    So in order to get the weight down, you have to get the sprocket diameters and chain speeds WAY up. And I can pedal at about the same torque as the diesel in my sailboat. It runs a solid 7/8" bronze shaft to a 14" prop. No, I'm not suggesting you need a 7/8" shaft in the kayak, but a fiberglass pole-bean stake isn't going to do the job either.

    Since you mentioned speed limitations, you have to realize that the torque that can be fed to a prop depends somewhat on boat speed. Using the prop size to limit the crank torque may not be very efficient, but might work for a rental of modest performance as long nothing jams the works. For a higher speed boat, the shaft torque of an efficient design can be quite large.

    I looked at a one man, two prop design a while back and figured I could just get by with repurposing a pair of the biggest deepsea fishing reels as gearboxes. Cost about $2000 each and would last a couple of days of endurance racing. (and they had an electric motor option too:))
     
    Last edited: Apr 10, 2015
  6. Deadweasel
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    Deadweasel Junior Member

    Thank you very much for your insight. I think I understand what you're getting at, and agree the layout is more in line with what I might have expected to see with a jet pump drive instead.
    Of course, it could also be a simple matter of dropping the props down from a hull a bit to clear the interference effect (at the cost of additional draft of course, but still nowhere near what the ProPel system requires).

    But then philSweet came along with...
    ...and I realized he too has a point. This concept would probably cost a lot more to realize than I'll be able to wrangle away from the little lady in any meaningful time. :D

    Nuts. Got all excited for nothing, it seems. Still, it gave me other directions to consider for this dream project, and again I thank you for the use of your more experienced eyes on the idea.

    I've still got a lot of reading to do in order to ensure I'm not going to wind up with a barely floating Frankenbarge in the end, but I'm still having a lot of fun discovering the many different variations I never even knew existed until a few weeks ago!
     
  7. Rurudyne
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    Rurudyne Senior Member

    Have you considered an Autocanoe arrangement? It not only offers shallow draft but is amphibious.
     
  8. jehardiman
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    jehardiman Senior Member

    Kort Nozzles are for when you have an excess of HP but not enough speed. That is never the case for an HPV.
     
  9. Deadweasel
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    Deadweasel Junior Member

    While that's an interesting concept, I have no need for amphibious capability, plus that's an awful lot of extra hardware (and weight) that would be simply wasteful for my purposes.

    I'm still trying to get the Mrs to help refine what we're looking to do, but so far we're agreed about something similar to the size of the Blue Skies design (possibly with outriggers?), accommodating a crew of two and including dual pedal stations for additional power. Essentially we want something that could be equally fun to use for single or multi day trips.

    It doesn't need to be ridiculously fast, but it does of course need to be as stable and accommodating as we can make it without relying exclusively on sail or gas engine power (though I think I'd like to have sails as an option in open waters).

    With the objective shifting away from small solo crafts for each of us, we're also able to move away from shallow draft limitations as well, so I'm back to thinking about the dual prop aspect of the design in my original post, but with the props moved away from the hull.

    As may be evident by now, we're still in the dream phase of what I hope to begin as an actual build project as complete nautical amateurs, so please forgive any questions that may seem like extreme flights of fancy. I'm just starting to realize how far behind I am in the technical considerations!

    So on that, I'd like to ask: what's the largest human powered design in use out there? Are our "requirements" effectively beyond the abilities of pedal power, or is it more capable than the dearth of smaller designs I've been seeing would seem to indicate?
     
  10. daiquiri
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    daiquiri Engineering and Design

    Don't know about the present HPV designs :p, but the "tessarakonteres" of the ancient Greece were 128 meters long, 17.5 m wide, had 4000 oarsmen and carried 2850 marines:
    http://en.wikipedia.org/wiki/Tessarakonteres
    Some pics here: http://bellumartis.blogspot.it/2015/04/tessarakontera-el-gigante-helenico.html
    After 23 centuries, it is a still unbeaten world record in the size of human-powered vessels. Quite a feat, isn't it? :)
     
  11. Deadweasel
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    Deadweasel Junior Member

    Haha holy crap! I don't think I can fit that in my garage...

    That certainly is quite a big boat. Don't think I could afford to feed the crew needed to get it away from the dock either.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Well, we'll just have to build a 420' rowing shell and beat this record Slavi. I'm quite sure we can easily beat the speed attained by the tessarakonteres, but do we have a speed, let alone one that's verified? Assuming it can do displacement speed (I doubt it), her length places this about 30 MPH, so it's a tall order, but I'll bet you can get much more speed with modern design and construction, though that capacity would just be showing off.
     
  13. daiquiri
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    daiquiri Engineering and Design

    If their 4000 oarsmen were strong and fit, but not at the modern competitive levels, we can assume that they were able to maintain some 100-150 W over longer distances, with peaks of 500-600 W for short sprints. That's something like max. 3000 HP they could count on for the propulsion. How much is the efficiency of the slow-pace traditional oars? Let's assume 0.65 (but if you have a better estimate, please correct me). That gives approximately 2000 HP effective power for that ship.
    Now we just have to estimate the max speed of a 128x17.5 m ship (of unknown displacement) under that power - that's the speed to beat. :)
    Can't do it now from my smartphone, but it sounds like an interesting exercise. ;)
     
  14. Deadweasel
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    Deadweasel Junior Member

    It may be cheaper and faster to get a hold of a cruise liner and pressgang the passengers into service. Anybody know if the Costa Concordia is intended to go back into service? :)

    /too soon?
     

  15. NavalSArtichoke
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    NavalSArtichoke Senior Member

    The 5th of Never. The hulk was towed to Genoa and is currently being scrapped. :(
     
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