Are guide blades better than a bulbous bow?

Discussion in 'Boat Design' started by ivor Bittle, Mar 20, 2008.

  1. ivor Bittle
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    ivor Bittle Junior Member

    When I was exploring the physics of the bow wave and of the bulbous bow I came across the possibility that active guide blades fitted to a conventional bow might just be better than a passive bulb. I fitted blades to a test boat and there is no doubt that the blades gave me a measure of control over the bow wave and the wake. A significant part of the energy used to propel the boat is dissipated in these waves and the ability to control them may make it possible to reduce the resistance attributable to wavemaking.

    I wrote this up on my website hoping that someone would comment but no one has. The article is too large for an attachment but it is on my site at www.ivorbittle.co.uk

    Perhaps this is all old hat and abandoned long ago but hopefully someone can have something useful to say.

    Ivor Bittle
     
  2. Petros
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    Petros Senior Member

    It seems to me that you will have induced drag from the blades (unlike the bulb) plus the increased skin area. It is possible the induced plus skin drag is smaller than the reduction of bow wave drag. It seems unlikely to me, but I do not know that much about the mechanics of bow waves.

    You can reduce the size of the bow wave with blades, as you have found. Did you attempt to make any drag measurements? That is what is important, not just wave size.
     
  3. kach22i
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    kach22i Architect

    Nice site, is this part of your thesis or something?

    I want to see someone do something like this for hovercraft and hovercraft skirts.

    Nice documentation, very nice.

    I've bookmarked it for future reading.
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The pictures of wave patterns are lovely.

    On one of the pages you write:
    "It has taken me a very long time to find a mechanism that gives all the
    common features and fits in with the laws of physics."

    The equations for the waves made by objects travelling on or near
    the surface of water have been known for almost 100 years, so I'm
    not clear why it took you a long time to find them.

    There are some images that I made at:
    http://www.cyberiad.net/wakeimages.htm

    Good luck!
    Leo.
     
  5. tinhorn
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    tinhorn Senior Member

    A most impressive website, ivor! A non-engineer (I'm of the "Well, that worked/bombed" school), I'm trying to figure out the mechanics of bow bulbs, too.

    I especially appreciate your paddle pages. I'm trying to figure those out as well. I've compiled dozens of old engineering articles and dozens of old US paddle-wheel patents. Some of the patents were quite bizarre.
     
  6. rwatson
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    rwatson Senior Member

    On the subject of reduced wake, you might like to check out the "Allsphere" design, if you havnt come across it before, in an article at http://www.alsphere.at/download/The_Yacht_Report.pdf
    This design is discussed on other threads on this site as I recall.
    It mentions many of the observations you make on your web site about the water flow around and under the hull.
     
  7. Frosty

    Frosty Previous Member

    Ive looked at you web site. Your pics are great the way you got the reflection in the waves, because it is the photgraphs of the wake that we are judging are these constant.

    Could it not be that the increase in speed or the reduction of wake is merely that the bow is lifted and altered the trim?
     
  8. Guest625101138

    Guest625101138 Previous Member

    Ivor
    This is not necessarily so. A hull optimised for a given speed will lose very little energy to waves. So you are concerned about something that does not need to be of concern unless you intend to design and operate a boat in a particular regime that is not near the optimum for the speed.

    Next time you get a chance have a close look at the waves generated by a rowing scull - if you can see the waves. With these hulls less than 10% of the drag is attributable to wave drag from all sources.

    I used to worry about bow waves until I understood that you do not need to make very big ones. I have attached a video of one of my boats doing 7 knots over calm water. The bow wave is visible but it is not very big. There is far more energy going into the turbulent boundary layer, which is well developed within the frame of the video. If you look down the port side in the last frame you will see it is about 1.5 to 2" wide in the bottom of the frame. That is roughly 3m from the bow. It takes approximately 0.7m from the bow to be noticeable.

    Viscous drag is hard to reduce whereas reducing wave drag is not too difficult.

    Rick W.
     

    Attached Files:

  9. rwatson
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    rwatson Senior Member

    What Ricks film doesnt show is that all three hulls on the trimaran are very fine conical sections - like two witches hats joined at the base (without brims).
    This did reduce the bow wave, but in my opinion, the hull design was far from optimal, and would have suffered a lot of viscous drag because where the two cones joined, was a sudden angle - it wasnt a smooth curved transition.
    I think that particular design was just moving the flow problem away from the bow, and placing it midships, where the hull wave became much more pronounced. Though you couldnt see it when pedalling the boat, it wasnt much less than a kyak of the same length.
    Further research on the "Allshpere" design I quoted earlier on this thread, provides some really usefullinformation on water flow around a hull at particular speeds, and if the film is still on "youtube", you will be amazed to see this huge boat producing no wake at fifteen to twenty knots.
     
  10. Guest625101138

    Guest625101138 Previous Member

    The bow taper is 125mm in 2400mm so the actual transition to the centre portion is not what you would call severe (2.8 degrees). You do not need very well trained water molecules to negotiate this when they are moving quite slow relative to the hull forward motion because they are in the boundary layer. This simple developable shape costs about 3% in drag at design speed compared with the fully optimised shape.

    The attached shows a side view of the same boat. Not much wake to see.

    I have made the fully optimised hull but I do not have a good picture of the wake in calm water. You get some idea from this clip taken by a friend (it is the black boat):
    http://www.youtube.com/watch?v=ckWqIgmVM4Y

    Rick W.
     

    Attached Files:

  11. Guest625101138

    Guest625101138 Previous Member

    I should add that the bow on the yellow boat is only 40mm higher than the keel so the angle that the water negotiates at the transition along the keel line is less than 1 degree. Again not a big ask for untrained water molecules to negotiate when they are already spinning about in a turbulent boundary layer. The attached picture of the unpainted hull may give a better indication of the "severity" of the transition.

    As far as hull drag goes the concept of "hull speed" is more important to understand than hull fairing for displacement mode. In other words, getting the length, beam and distribution of buoyancy right will have more influence than a perfectly polished, perfectly faired hull. That said aluminium is fast to work, almost perfect finish and will be perfectly faired within the constraints of developable shape.

    The shape of the outriggers are irrelevant in calm conditions because they are not intended to contribute to displacement. They have dead-flat rocker so if they are slightly immersed there is no need for the water to worry about going around any abrupt transition.

    Rick W.

    Rick W.
     

    Attached Files:

  12. ivor Bittle
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    ivor Bittle Junior Member

    I am astonished at the flurry of replies. I will look at them carefully and try to respond properly.

    It may help if I say that I am 80 years old and my evening recreation after my active day finishes at about 3 pm is to sit with my computer, Prandtl's "Fluid Dynamics" and the contents of my brain and just try to understand things that interest me. Sometimes the result might be of interest to others and I add it to my website. Sometimes it is exhilarating.

    I think that it might be nice if a student in a suitably equipped university tried out these blades.

    Ivor Bittle
     
  13. ivor Bittle
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    ivor Bittle Junior Member

    PETROS
    I am grateful to have your reply. I am afraid that my resources do not extend to measuring drag. I took the matter as far as I could and working out of doors 40 miles from home is not easy.

    The trouble with this reasistance problem is that whilst we can all see the various elements that go to cause the drag we cannot quantify them with sufficient accuracy to be certain. With ships a small percentage of a lot of fuel is still a lot of fuel.

    kach22i
    Thank you for your kind note. I am well past the age for writing theses. However there is a bonus, I have no one to influence how I think so I can just enjoy myself in my own way. It is very liberating.

    Leo Lazaukas
    My pictures are really a comment on the success of digital photography. Nevertheless they did not come easily. My helpers on the pondside enjoyed helping and they looked at wakes for the first time.

    I take your point about looking at other peoples work of 100 years ago. I am an engineer and I have lived by the creed that I needed lots of relevant questions to which experience, physics and sheer curiosity would provide answers. In the end it is quicker than looking for something that might not be there. I must add that my early experience taught me that most of what I read was suspect and, as a result, I have become too independent really.

    I did enjoy your pictures of wave patterns. They are delightful. I will look at them much more carefully later. I was pleased to find that I could understand them without difficulty from the excercise of looking at other wave patterns over a long time and final exercise in thinking about them. You have to go for lots of walks to see the patterns produced by sticks and stones under the water in a flowing river at various speeds. In the same way I found that I could sort out seamanlike rigs on sailing vessels after I sorted out how sails work.

    tinhorn

    I did not set out to be impressive but thank you anyway. I just wallowed in the freedom to use as many words as I liked, to have no one to say no and just to be who I am. It is great fun.

    I am surprised at the interest in paddle propelled boats. I made my model as an exercise in engineering. It involved designing an engine, a hull and the paddle wheels from scratch and making them. It kept me out of mischief for two years. Just finding out about William Stroudley was a sheer bonus. We need more like him.

    rwatson

    I have visited the Allsphere site but I cannot take it all in at once. I will revisit it shortly. Thank you for the direction. As I got to my understanding of the flow round hulls in my own way it will be interesting to see the route taken by others.

    Frosty

    Thankyou for the comment on the pictures. I am not qite sure what you mean at the end of the first sentence.

    The wave pattern for all sorts of moving objects is both very similar and robust. It is very hard to alter it. the test boat weighed about 30 pounds and I could change the angle of the guide blades at will when it was running. I could see no changes in trim at all let alone of the magnitude required to alter the wave pattern as much as proved to be possible.
     
  14. ivor Bittle
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    ivor Bittle Junior Member

    Can guide blades replace the bulbous bow?

    Rick Willoughby,

    I am well acquainted with racing shells having rowed in my twenties but, whilst I used the picture of a toothpick to show both the bow wave and the eddying wake, (I am not sure that the boundary layer is turbulent.) I set out to find out why a large ship and a duck produced the same wake. I am sure that if one can design a hull solely to reduce the bow wave one can get the rsistance it causes down to a small fraction of the total. But most ships have a commercial function that determines its essential dimensions.

    When I go to Gravesend in England I can see the foaming bow wave on light tugs long before I can see the tug. They represent the very opposite boat being short, wide and heavy. They are practical boats for which the main resitance to motion when they are in transit is caused by the bow wave. The other boats lie in between and the current device for altering the drag due to wave making is the bulbous bow. This is a passive device and the ship has to be trimmed to keep the bulb working properly. The guides could be active.

    I liked your video.The bow is very fine and I concluded that, whereas the bow wave for a duck is in front of the duck, as the angle of the bow is reduced the primary bow wave forms first at the bow and then as it gets smaller still it forms back from the bow that then appears to cut through the wave. I think that I can see this on your final picture.

    When I was rowing I had plenty of time to look at the layer of water being dragged along by the hull and as I rowed in various seats in the eight the change in thickness was quite obvious. However my interest is in the bow wave.

    I note that rwatson has more information about your boat and I agree with him. Any sudden change in curvature is a mistake. Even the change in curvature between the two parabolae used to create the upper surface of aerofoils initiates saparation. American wartime naval vessels sported very fine bows and reverse curves. The transition to the to conventional curvature took place at perhaps 20% of the waterline length. It must have produced its own wave.

    When I retired I had time to procure and read all the papers from Froude, Reynolds, and Rayleigh just to find out what they actually said. It was sheer joy to read them and very instructive. What Froude would think of the way we use his work now I hate to think nor what Reynolds would make of the current use of "turbulent". Ah me such is life.
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm a mathematician so I prefer to walk around with my head in the clouds :cool:

    You might find the attached wake patterns made by islands poking through clouds interesting.

    Leo.
     
    Last edited: Mar 27, 2015
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