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

    Ah. Well I suppose we could always look toward Carnival. They've been in need of human power on their vessels more often than others lately. Heh.
     
  2. Rurudyne
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    Rurudyne Senior Member

    I believe it has been said on one of those Ancient Impossible shows that she had a displacement on the order of 4000 tons. At 5-10 HP per ton, seen bandied about here and there, that's 10-20 times what you estimate.

    So 2000, ah, Oar Horse Power (OHP) isn't going to get her anywhere near a hypothetical "hull speed" of 28 knots displacement for a 400' length.

    It would probably be scary at 10 if you could get there, what with that big deck flexing.

    I think the estimated OHP should be lower because the oars would have been huge and difficult to handle, a good part of rower's work being just shifting them.
     
  3. daiquiri
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    daiquiri Engineering and Design

    My numbers are wild estimates, of course. But if they have claimed 5-10 HP/ton, then I'd say that my wild estimates are by far much better than theirs.

    Few numbers: 4,000 t x 5-10 HP/ton = 20,000-40,000 HP
    20,000-40,000 HP x 746 = 14,920,000-29,840,000 W

    They had 4000 oarsmen, so each oarsmen had to produce
    14,920,000 W / 4,000 men = 3,730 W
    29,840,000 W / 4,000 men = 7,460 W
    Now, the best competitive swimmers can produce approxim. 4,000 W only during jumps: https://online.medunigraz.at/mug_online/wbabs.getDocument?pThesisNr=17980&pAutorNr=63942&pOrgNR=1 - see the page 34, graph JP (Jump Power). For any longer performance their power output drops to 700 W for 2 min runs (TP - Transitional Power), and 220 W for 2 hrs runs (EP - Endurance Power).
    These number are compatible with 430 W produced during the world-record hour-run on a bicycle: http://mapawatt.com/2009/07/19/bicycle-power-how-many-watts-can-you-produce
    I was not able to find any data about any person ever being able to produce 7,460 W for any significant time period.

    So, I would say that, even if we admit that those oarsmen were in top physical shape and well-fed, they would never be able to produce that amount of power for any time longer than several seconds.

    400 W over an hour and perhaps 200 W for day-long journeys (and with tams of oarsmen rowing in alternating shifts) is a much more possible estimate, IMO. Especially after I have find out this interesting fact: http://www.leeds.ac.uk/news/article/581/fitness_has_fallen_since_the_days_of_ancient_greece :)

    Cheers
     
  4. Rurudyne
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    Rurudyne Senior Member

    Sorry if I wasn't clear but the 5-10 HP per ton is when powering a modern boat. A reference to be able to reach displacement "hull speed" for comparison.

    One thing to keep in mind is that power to a given speed comes more dear, in a non linear fashion, the faster you go, at least once you've overcome skin friction (as you know). So 1/10, or even 1/20th modern power, can still move.
     
  5. Gib Etheridge
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    Gib Etheridge Junior Member

    Getting back on track, you might find that the increased draft you get with a leg extending below the boat is not really a problem if the unit can tilt up. Also, if it's designed properly you can remove any weeds and it will tilt if you hit anything serious like a rock or the beach.

    Here's a link to photos I have in Photobucket of a 21 foot by 4 foot hull I've designed and am building specifically for a SeaCycle unit. These units are well designed, durable and efficient.

    Something like this may be just the ticket for the two of you, although it's only single station, but you can take turns. Two units would be no advantage at all. I estimate hull speed at cadence 60 to be about 4 MPH, with sprint capability of perhaps 6 MPH, maybe quite a bit faster single.

    http://s1050.photobucket.com/user/GibEtheridge/library/Pedal Dory?sort=2&page=1

    Note: There are 2 pages of photos, the pedal drive unit is on page 2. The unit is installed in a prototype trunk.
     
  6. PAR
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    PAR Yacht Designer/Builder

    With human powered craft, drag reduction is paramount, so the introduction of a Kort wouldn't be the wisest choice.
     
  7. Rurudyne
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    Rurudyne Senior Member

    G.E. ... really nice. :)

    One tame deer too.
     
  8. daiquiri
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    daiquiri Engineering and Design

    G.E. very neatly done, both the boat and the drive. I guess it uses a chain transmission?
     
  9. daiquiri
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    daiquiri Engineering and Design

    I have noticed this part only now, sorry.

    The speeds at which Kort nozzles are useful are in the bottom-low range, which means nearly-static thrust or just somewhat faster than that.
    In fact, ducted fans (that's what Kort nozzles are) are designed for thrust and are almost never analysed in terms of efficiency. They are usually characterized through their thrust-coefficient curve and through the figure of merit (FM=Ideal hover power / Actual hover power), the latter one being borrowed from the helicopter theory. When ducted fans are analysed in forward motion, and hence in terms of efficiency, then they usually show a peak efficiency at a very low advance ratio, like J=0.3-0.4 . For example, page 6 of this paper: http://4hv.org/e107_files/public/1386100085_2431_FT0_1353043533_2431_ft145838_duct_pdf.pdf or the page 5 of this one: http://www.marinepropulsors.com/pro...tigation of Ducted Aerodynamic Propulsors.pdf

    Hence, when you think of Kort nozzles, you are should imagine tugs towing an oil tanker or a hovering quadricopter drone, rather than high-speed boats or airplanes.

    Now you just have to decide what will your HPV be designed for:
    1) If, for some reason, you need a high static thrust and don't care about the top speed - then a Kort nozzle is a valid option.
    2) If your propeller diameter is limited, resulting in a high disc loading at the design speed, then a Kort nozzle might be a valid option.
    3) If you want to cruise at highest speed possible with lowest effort possible (minimum drag), then forget about a Kort nozzle and use a well-designed prop.

    Hope it helps.

    Cheers
     
  10. Gib Etheridge
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    Gib Etheridge Junior Member

    Yes, it's twisted chain. You can read more about it here;

    http://www.sea-cycle.com/accessories

    And thank you for the compliment.
     
  11. Gib Etheridge
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    Gib Etheridge Junior Member

    Thank you Ru.

    He's been hanging around for about 9 years now, used to come in the house when he was little and still will if I tempt him with a carrot, or his favorite, salted roasted peanuts out of my hand.
     
  12. gonzo
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    gonzo Senior Member

  13. Rurudyne
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    Rurudyne Senior Member

  14. Deadweasel
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    Deadweasel Junior Member

    That's very slick GE, definitely appealing and something I want to try out eventually.

    For comparison, my current 18'x6' concept looks like this

    General overview of the concept
    [​IMG]

    Close-up of the drive and prop retract mechanicals
    [​IMG]

    Overview of deck spars and pilot seating
    [​IMG]

    Based on the community feedback, I abandoned the Kort nozzle idea, and am now trying to optimize the hull shape (which is modified from a completely different functioning design) and drive system.
    It doesn't need to be fast like a racer, but I'd definitely want something better than those crappy paddle wheel rental boats. Something comparable to those Native Watercraft (in terms of speed) we could take out for fishing excursions.

    Am I in overkill territory with two props, or am I most likely looking at something that will wear me out quickly and unnecessarily?
     

  15. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

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