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Optimal design, water-wing - single oar propulsion

  1. Sep 11, 2013 #1
    Optimal design, "water-wing" - single oar propulsion

    Good evening, and hello to you all.

    As you can see I'm new here, so please go easy on me.

    A while ago I got to learn about the single oar propulsion system used in southeast Asia.
    The way I see it this is a principle with loads of potential, far superior to any alternative man-powered propulsion systems. Conventional rowing is of course faster, but it's restricted to small boats and short distances.

    For those who are not familiar with the principle it works like a wing. The oar moves from side to side astern of the ship, generating lift to propel it. The person operating it only has to overcome the drag of the water.

    I want to make an oar like this, but before I start designing it I need to narrow down the uncertainties, understand some basics - and hopefully getting some hints & help.

    First and foremost I want to focus on the "wing", both because it's the by far most important part and because it's the most physic-related problem.

    My suggestions and questions so far:

    The oar will enter the water with about 45 degrees. The steeper the angle, the less thrust will be wasted. However, if the angle is too steep, operating the oar will be very hard. To bend the shaft will be necessary, both to counter this effect and to easily twist it at the end of each turn. (This bend will be discussed later in the process).

    The profile of the oar will be like the wing of a plane, but it has to be symmetrical. The reason for this is that both ends will be the leading end every second stroke. This one of my most important questions: Does anyone have a good idea as to what a symmetrical waterwing should look like? I am thinking about a mirrored basic wing profile, but especially the ratio between thickness and width is puzzling me. Also angle of attack, length and speed are related issues here.

    Then there is the question about varying the profile over the length. Especially if the wing is long, the speed of the end will be much higher than the speed at the waterlevel. To me it seems like a must to take this fact into account.

    Finally, is there any easy way to calculate the lift and drag of a wing trough water? Any free software available?

    A familyfriend is in a line of work that enables him to construct this wing for me if I actually design something useful, in this case it will have to be drawn in ProE or something.

    I hope I can get some help here, this is getting much more technical than I had in mind..
     
  2. jcsd
  3. Sep 11, 2013 #2
    In english-speaking countries we call it sculling (although sculling also has another meaning). Blade sections are usually fairly simple because (i) there is little to be gained from an airfoil because unlike air, water is not compressible and (ii) controllability is as important as efficiency. Bending the shaft significantly can introduce unhelpful stresses at the pivot, in fact if there is a bend in the shaft it is usually designed to make the blade less vertical which makes it easier to control. One major factor to consider if trying to optimise design is that the speed of the blade (and the pressure of water) increases with depth.
     
    Last edited: Sep 11, 2013
  4. Sep 11, 2013 #3
    I appreciate that you took the time to answer, but I believe you are wrong. Both the Japanese and Chinese have sculled for centuries (at least), and with the technique and design that have developed over this time "sculling" (happy?) makes it possible for an average woman to propel sampans weighing tonnes over prolonged periods of time (hours).

    The bend, as well as the airfoil-profile, are both important enough to make the oar practically useless without.

    EDIT: I missread the text - I'm so sorry for the harsh response.

    I believe you are talking about "Bahamian" sculling, which is as you describe. The principle behind the Japanese Ro and the Chinese Yuloh are far more sophisticated and developed. If what you say about water being a useless matter for generating lift via pressure-difference I'm truly mislead.
    Can someone confirm this?
     
    Last edited: Sep 11, 2013
  5. Sep 11, 2013 #4
    No it was me - sorry I edited it after posting. If you google "sculling oar design" you will see (once you have eliminated the majority of posts which refer to sculling in the sense of rowing with two oars) that a fair amount of thought has been given to western designs for sculling oars too.

    Sculling is generally preferable to rowing in two situations: (i) where forward visibility is important (ii) where the vessel has a large amount of drag and so can only be propelled at slow speed by a human - conventional rowing is less efficient in this situation because the oars "slip" through the water losing a lot of energy to turbulence whereas slipping through the water is exactly how a sculling oar transfers its power. Because of the relative affluence of Western nations we tend to overcome (ii) by fitting an engine which is why sculling is less common.
     
  6. Sep 11, 2013 #5
    You are right, there have been a fair amount of tries by especially many sailing enthusiasts. But most of them have been fast "hobby"-projects with little or no scientific approach to it.

    The reason why I started thinking about this was actually an article in some magazine. I think the person who told me about it have an electronic copy, I will try to find it if interested. There the author of the article talked about the fact that western replication attempts mostly have flunked, mainly because they have been based on the bahamian and not the Asian sculling-method.

    Both the bend and the inverted wing-profile were main points in the design, for instance, the bend is responsible for countering the momentum of the lift trying to turn the oar (because of profile and angle of attack), making it a huge advantage in the Asian concept but a drawback on the bahamian.

    And about what you are saying is the premises for making sculling worthwhile: What about the chinese women propelling a large vessel for hours on the river? What about sailing enthusiast who could, if they had worked, sculled in harbor etc? There isn't many ways to power a big boat with your own body that I know off, so getting this to work might be worth a shot. If there is a chance.

    But I understand that I don't understand. I have just started to read up on this, and I'm sorry if i'm seeming very certain here. I have written my understanding like it was a known fact - I'm to tired to express my doubt clearly. But trust me, it's there :)
     
  7. Sep 11, 2013 #6
    If you are talking about a bend above the pivot then this is not there to make the blade more vertical, it is there to make the top of the oar more accessible for the rower.

    If you put a (downwards) bend below the pivot it will make the blade more vertical, but the forces on the blade twist the oar in the pivot and in the hands of the rower meaning that more effort has to go in controlling the blade.

    As for what you call profile (usually called section in foil design), yes this is important to reduce drag but at the speed a sculling oar travels its contribution to pressure differential is negligible. Hydrofoils certainly can generate lift through creating pressure differentials but at the speed of flow over a sculling oar the vast majority of the pressure differential is through angle of attack rather than fluid dynamic effects.
     
  8. Sep 11, 2013 #7
    In the UK this is probably the most common application. There is an article on it at least every year or so in most (cruising) sailing magazines and forums e.g. here and here. In the US I gather it is most popular for shooting water fowl where (i) the forward looking aspect and (ii) the ability to do it while lying down and hiding are the most important advantages.
     
  9. Sep 11, 2013 #8
    I should have picked up on one thing in your original post:

    You do realise that the only way to propel a boat forwards is to push water backwards? The amount of work done in overcoming the drag of a boat to move it forwards a given distance at a given speed is the same whether you are using conventional oars, sculling or a propeller. The reasons sculling is easier than rowing for a large vessel is because (i) there is a mechanical advantage similar to having a bicycle with a low first gear for going uphill compared to one with a fixed ratio (ii) less energy may be lost through turbulence and (iii) sculling applies a more consistent driving force than the stop/start force of conventional oars.
     
  10. Sep 11, 2013 #9
    I did not know this. It's getting late, but I will definitively check it up tomorrow. Thanks alot.


    I am starting to realize it, I did not =P

    My perception was that it was possible to create lift by moving a foil back and forth, thus creating a lift. With this lift countered with the loom(?, end of the shaft) attached to the "floor" of the boat I figured it was a pretty smart device.

    But with this lift-illusion of mine falling to pieces the whole project is as well.. I get what you are saying, and it all makes perfect sense. It just takes a little more time to sink in when you know you were wrong.

    But thanks for your time, if you or anyone else has any other input that would breathe life into my project (or just behead it) it would be much appreciated.

    Good night for now.
     
  11. Sep 12, 2013 #10
    Good morning, I gave up earlier than you!

    As you can see from the above and further research the design of the sculling oar has been refined in both East and West in a number of different applications, however if you are really keen on research and development and believe that there is something to be gained from looking at foil sections then there is one area I can suggest:

    Traditional sculling involves rotating the oar through an acute angle, probably about 30-60 degrees, at the end of each half-stroke so that the leading edge alternates from one edge of the oar to the other as you note above. This means that the blade must be designed straight and with a symmetrical section which may not be the most efficient shape. If instead the oar could be rotated 120-150 degrees in the opposite direction the leading edge on the next half-stroke would be the same as on the one just completed, however it is not practical for human wrists to rotate and power an oar in this way efficiently - if you try to do this you will find that you are only rotating the oar through about 90 degrees and end up pushing as much water sideways as backwards with the back of the boat waggling from side to side.

    However if you use a mechanical means of rotating the oar, and/or a flexible blade and/or other innovations it may be possible to maintain efficient forward thrust while keeping the leading edge constant. The other area where a mechanical linkage, or a carefully designed pivot could improve effeciency is by allowing the blade to be completely vertical avoiding the inefficiency of pushing water upwards (and the back of the boat down). Take a look at this video to see one solution that combines elements of both of these developments.

    Finally, for another method of manual propulsion that can be more efficient than rowing particularly for sustained periods look at this, although this is only efficient for craft with less drag (to efficiently propel a light craft quickly through water you want to push a small volume of water backwards quickly, which means a propeller, long conventional oars or a geared paddle; to efficiently propel a heavy craft slowly through water you want to push a large volume of water backwards slowly, which means a different design of propeller or a sculling oar).

    And finally finally, remember that whilst there are similarities between foil design in water and in air, there are also huge differences (due mainly to the much higher density and incompressibility of water) - to illustrate this look at the difference between propellers fitted to ships and to aeroplanes.
     
  12. Sep 12, 2013 #11
    You have been more help than I would ever imagine when I wrote the post. Thanks again.
     
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