Archimedes (Helical) Screw Pontoons

Discussion in 'Boat Design' started by Rurudyne, Aug 26, 2014.

  1. Rurudyne
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    Rurudyne Senior Member

    Does anyone know where to find information on the use of Archimedes screw equipped pontoons for boat propulsion?

    (Not necessarily even amphibious, just shallow draft, or as they used to say of some small paddlewheel craft: able to run on a "heavy dew".)

    While there are snippets on the performance of particular craft, such as the Riverine Utility Craft , I suspect these don't necessarily represent horrible slip for this type but rather having the screws fairly deeply submerged so that they were pushing a fair chunk of water out of the way on craft that didn't have long LWLs to begin with.
     
  2. waikikin
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    waikikin Senior Member

  3. Petros
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    Petros Senior Member

    it is not very efficient, and it is heavy and bulky, so you sacrifice a lot of power and space to get the "crawling" capability and shallow draft.
     
  4. Nick_Sinev
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    Nick_Sinev Junior Member

  5. Rurudyne
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    Rurudyne Senior Member

    Thanks for the links. I was hoping for information of a design nature.

    Petros, what I'm thinking about is less, um, unconventional. Rather than supporting the boat with the pontoons they would be smaller and used just for propulsion, a use similar to a paddlewheel. To borrow nomenclature, it could be termed "sidescreweys" (people say I'm screwey anyway ;)).

    (Note: spelling intentionally squirrelly)

    The pontoon: smaller, slimmer, lighter and, to borrow a phrase, just kissing the water. The auger itself proportionally larger since land and muck use wouldn't be a goal. I suppose you could class them in that form as a kind of surface piercing drive, just one operating at potentially much slower RPMs.

    Which of course brings up the design issue.

    In simple mechanical terms it wouldn't be impossible to built a screw like this with a pitch of 8 or 9 feet.

    It's just such a large pitch may not work at all unless the screw itself were really large too, having a length several times a complete transit: something not desired. There are many unknowns ... though drawing a pretty picture on the computer is of course deceptively easy.

    If slip were 25%, a number outta nowhere just for speculation, then 10kt could be had at 150 rpm with a 9' pitch ... though of course a narrower -- say 4:1 or slimmer -- light displacement boat would have to be 40' plus to even hope for that much speed (I just don't see these powering a planing hull ... I'm not that screwey).

    Edit: by "complete transit" I mean of a single blade. I just realized someone might think I wasn't allowing for multiple blades.
     
  6. Nick_Sinev
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    Nick_Sinev Junior Member

    What is the point?
    If we talk about the propultion in the water, the "Archimedian spiral" is less efficient than a "propeller". Historically, the first ship propellers were "Archimedian spirals" and then they were reduced in length to form the modern shape.
     
  7. Rurudyne
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    Rurudyne Senior Member

    It's also less efficient to run a prop in a tunnel. There can be windage involved with any paddlewheel sizable enough to be efficient for a livable speed. There are always trade offs. But we aren't talking a blue water cruiser but one where shallow draft is desirable.

    Texas' "rivers" can provide such a need.

    For benefits, "sidescreweys" could have similar maneuvering capabilities to a sidewheeler without the windage. With potentially low operating RPM they wouldn't pose the same risk to swimmers or animals that props would. I'm guessing low wake effects like a paddlewheel too. No worries about cutting sea grass. Moreover, if they worked, they could be employed in place of stabilizing amas for a narrower main hull (which only make up a small percentage of displacement).

    I'm not sold on the idea, not by a long shot ... I still like paddlewheels. But if a guy's a dreaming why not dream?
     
  8. gonzo
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    gonzo Senior Member

    They would pose probably a higher risk to swimmers and animals. Getting caught by a long screw is more likely to make you into mince meat that just getting hit by a blade. Also, if you go in shallow water it will get jammed with weeds and other debris. I think it will be almost impossible to clean without dismantling when all the stuff gets wedged in.
     
  9. Nick_Sinev
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    Nick_Sinev Junior Member

  10. DCockey
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    DCockey Senior Member

    My guess is the efficiency of a long screw will be much lower than a reasonably sized conventional propeller.
     
    Last edited: Aug 26, 2014
  11. Petros
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    Petros Senior Member

    you can consider the helix blade a really, really low aspect ratio propeller. really bad efficiency in a fluid. a conventional prop would be lighter, cheaper, smaller and more efficient.

    What is the advantage or feature you are looking for? Might be better to have mounted tracks if you want to run up on muck and sand bars, with a separate drive for a prop for water usage.
     
  12. fredrosse
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    fredrosse USACE Steam

    As stated previously, the screws would evolve into a modern propeller for reasonable efficiency. If you must have the cylindrical screw, then the following would be a good starting point:

    The blade area submerged in the water should be about double what is proper for a normal screw propeller (screw propeller sized at about twice the RPM of the screw).

    The pitch to diameter ratio should be around 1.0, more or less, so if you want 9 foot pitch, your cylinders should be about 8 feet diameter. Unless you are talking about a very large boat, you probably would want the screw to be much smaller diameter.

    The closer your design gets to hanging conventional propellers off the side of the boat, the better it will be in terms of propulsive efficiency.

    Note that sidewheelers have killed many people under their wheels, having lower RPM does not necessarily mean that a swimmer would not be damaged by a sidewheel, even though it is turning relatively low RPM.

    Also note that tracked paddles will work reasonably well, without the windage penalty that comes with conventional paddlewheel boxes. There are many machines underway on deep water using tracked paddles, they are conventional snowmobiles. Don't stop tho, they sink like a rock if you slow down too much.
     
    Last edited: Aug 26, 2014
  13. portacruise
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    portacruise Senior Member

    I like this concept for shallow draft myself, the Tuhhsnelda:

    http://www.humanpoweredboats.com/Photos/UniversityDisplacementHPBs/TUHHSnelda_2.jpg

    It should be more efficient than any screw hull type all things being equal. Possibly could be built to match prop craft efficiencies, because it is using hull water friction to an advantage rather than fighting water friction drag of a conventional hull...
    Design advantages of not needing the narrow conventional hull shape and potential for amphibious?

    PC

     
  14. portacruise
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    portacruise Senior Member


  15. Rurudyne
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    Rurudyne Senior Member

    Good responses. :)

    Without addressing the technical issues, thank you Fred for testable suggestions, I would like to rather look at safety issues for this type of drive.

    First off remember that I'm not talking about a system that supports the ship after a major fashion, nor am I speaking about a terribly fast craft -- though the speed of a boat is a matter for the hull design and adequate propulsion that can push hard enough will get it going ... I seem to recall a planing craft powered by a souped up high speed paddlewheel being talked about on this very forum, for example.

    So if the main hull is relatively light but still displacement, and maybe long but not "narrow" after the fashion of large stabilized monohull designs, that is the performance range I'm speaking to.

    Installed as a pair these can either rotate towards the hull from below or above. If from below, and they are close enough to the hull, there would be a real danger of pulling some critter up between the screw and the hull probably resulting in serious injury. If from above then there is a danger if someone should somehow fall on it being dragged down between it and the hull.

    Clearly, then, there may be a minimum distance desirable. Not necessarily big enough to swim through but certainly large enough so body parts don't get caught to then pull the rest in.

    In terms of safety a simple and sufficient platform above can keep all but the most industrious idiots from just falling in, so rotation top down would be preferable for normal use since the effect under water would be to move anything in the water getting into an argument with the drive away from the hull.

    Naturally, reverse would then be tricky ... something to be careful about, because at such times the things would push victims towards the hull.

    Of course the farther these are out the less this is an issue, but the trade off is weight plus a platform that may let idiots congregate farther from your centerline.

    Now, about the danger of slice and dice relative to a prop (getting bludgeoned paddlewheel style isn't a particular concern).

    The examples people can point to are things like the Fordson for land use (some engineering students thought to take their badly balanced Fordson clone out on the water ... sad) or amphibious. I'm actually not talking about either but a non-amphibious boat. The majority of the diameter would be blades, not pontoon. So the danger those blades represent could be very real ... if one were to start with Fred's advise at point blank value (and he generally seems to give good advice) then they could easily be three to four feet deep before you get to the pontoon.

    However, the primary danger from a prop comes from not just the speed of the thing but the many new cutting edges presenting themselves at your person every second. These are all naturally sharp.

    But with a helical screw is there a need for an edge that is neither the leading edge nor the trailing edge to be sharp? With these their function, such as it may be, is to just shepherd the water along in a controlled fashion. This would be a source of parasitic losses. So whatever length of that sort was absolutely prudent (and I'm wondering if this section wouldn't be a trick to not need so great an outer diameter, though by no means small: the speculative "pretty picture" I drew today FOR FUN would have 12-18" deep blades in addition to a 12" diameter pontoon hub) would not represent a significant cutting danger unless the water were really shallow and you just got run over by the barely floating boat ... which of course would be serious on its own.

    I'm suspecting that there are trade offs for amphibians that make them less ideal than either pure terrestrial or aquatic designs and that the form of the leading edge, as well as blade depth, would be one of these. The Fordson, with its shallow blades, looks like getting run over by it would be just about like getting run over by a tractor, while a Riverine Utility Craft with its deeper blades on top of those huge pontoons looks like it's be like getting run over by a tank ... with swords on the treads. Either would mainly be "getting run over" no matter how it slices you.

    I'm thinking that in a purely aquatic design the leading edges would be the main cutting danger ... is it prop like, standing proud, or does it grow deeper until at some point the blade has reached its greatest depth? The former presents a smaller area of greater danger, the latter a relatively larger area of, well, growing danger.

    These are the safety issues as I've so far pondered them so far.




    Frankly, making the "blades" swim by articulated oscillation rather than rotate as a screw would throw the whole techno-geek cool meter off the scale as well as be safe in a big way ... but who has money to pull that off?
     
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