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Discussion in 'Boat Design' started by Guest625101138, Jul 14, 2008.

  1. Guest625101138

    Guest625101138 Previous Member

    Beppe
    The power estimate in the previous post was based on one of my props.

    I did some back calculation from your published matabolic data using 26% biomechanical efficiency. Based on this your measured power is much higher than I calculate. It would be interesting to have the actual crank power.

    I have attached my comparison. It suggests you have measured power of more like 350W to do 12kph. This indicates either poor biomechanical efficiency and/or poor drive efficiency.

    Your data on the rowing scull at 4m/s is precisely what I predict for the hull using my standard figures of 22% biomechanical efficiency for rowing and 64% for rowing efficiency. This gives me confidence in my numbers.

    You could improve the biomechanical efficiency using cleats as I notice they are not fitted to your boat. This is well documented for recumbent cycling position.

    If you have a symmetrical foil section for the prop it will be less than optimal.

    Rick W
     

    Attached Files:

  2. beppe
    Joined: Jul 2008
    Posts: 51
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    Location: Udine, Italy

    beppe Junior Member

    more data and questions...

    Rick
    your calculated results are very very interesting, I believe we can get somewhere from here, and in particular we can understand in a better way better if building a long distance PPBs faster then a traditional high performance HPB is really possible, against the frustrating evidence of Michael (and others, including myself) in races. And if it's possible, of course, I believe the Open Waterbike will get there first...

    Have you got the whole paper we published in the Journal of Sports Sciences?

    The complete reference is:

    Zamparo, P; Carignani, G.; Plaino, L; Sgalmuzzo, B. and Capelli, C.; "Energy balance of locomotion with pedal-driven watercraft", Journal of Sports Sciences, Volume 26, Issue 1 January 2008 , pages 75 - 81

    I am one of the authors, but I believe it's against the rules of this forum to upload the whole paper since now that it's published it's copyrighted material.
    I have a zillion questions about your results but I can give you before that a few more data for refining the analysis:

    overall efficiency for the cat is 0.27 in this case;
    propelling efficiency is 0.57@2 m/s and increses slowly with speed (as usual) to 0.62@3 m/s

    The disappointing figures of measured propelling efficiency can have different explainations due to the conditions of the drive unit during the testing procedure, I'll tell the story later; I believe we can treat the drive system as a black box for now using these efficiency figures. This figures can explain why your simulation gives better results than the measured data, and a correction of your calcululation using the experimental efficiency should show an even better alignment.
    Also, the displacement of the craft during the tests was of course slightly different form the design displacement, due to different body mass of each subject (we tested seven of them, five males, two females).
    Effective displacement was 97 to 114 l. This also could have lead to worse results of course.

    The first of my zillion questions:
    Have you estimated the drag of the boatat different speeds?
    How much of the drag is due to hull drag and how much to appendages and hulls interference?
    Is the comparison with a monohull based on same displacement and same length?

    Thank you for your analysis Rick, I'm really impressed...
    Beppe
     
  3. Guest625101138

    Guest625101138 Previous Member

    Beppe

    This statement suggests to me that it is the "Open" waterbike providing you are not required to provide any intellectual property.

    I appreciate the information supplied on the hull and the time taken by you to understand that you need to know the sources of losses before you can methodically address them - the fact that you are impressed by it also indicates you did not comprehend what I was pointing out earlier.

    For the other questions I feel it is going to be a one way street with me providing you intellectual property freely and you claiming copyright as soon as there may be something I have interest in. Certainly not what I regard as an OPEN community.

    You collect all the information from others on the pretext of being OPEN and then clam up when it comes to contributing your own data. It certainly puts your motives into question!

    Rick W.
     
  4. beppe
    Joined: Jul 2008
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    Location: Udine, Italy

    beppe Junior Member

    Open Waterbike resources

    Rick
    you don't understand.
    The problem is not my intellectual property, but the rules of this forum (have you read them?) and the copyright of the publisher of the Journal.
    The paper is published, it's no secret at all.

    Please have a look at this page
    http://www.openwaterbike.com/our-boat/fastest-boat-in-the-world
    and download the file, it has always been there.
    All the other more detailed data we have recorded during the tests are also available to you and all the other colleagues, just ask.

    I humbly believe I understand what you pointed out earlier, what I said is just that a more aggregate concept of efficiency (the propelling efficiency, that is the data I gave you before) could be interesting for comparing purposes, because we have the data in literature, while we have not those of more detailed efficiencies that are meaningless for other kinds of watercraft and aquatic locomotion.

    Best
    Beppe
     
  5. Guest625101138

    Guest625101138 Previous Member

    Beppe
    I did not realise the paper was on the web site. I would have looked at it earlier. I had only seen the graph - not the link to the paper.

    I will read through it to digest what you have actually measured.

    Rick
     
  6. Guest625101138

    Guest625101138 Previous Member

    Beppe
    Based on the SRM power measurement your propulsion system has very large unaccounted losses. There is an unaccounted gap of around 65W.

    My drag calculations and your total drag measured data are almost identical. At 10.1kph the power is apportioned as follows:
    Rudder 4.8W
    Drive leg drag 13.2W
    Windage 4.7W
    Wave making 26.9W
    Viscous hull drag 74.7W
    Prop losses 21.6W (this is for my V11J prop - sub-optimal for your boat)
    Mechanical losses 4.5W (this is with a precision right angle drive)

    Total calculated power is 150.4W.

    This compares with your regression curve of 214.9W.

    Hence there is something like 65W gone missing. More if the prop was optimum.

    There seems to be high no load losses on the drive unit of about 20W. You should be able to check this using the SRM crank without any load on the prop or even with the prop removed from the shaft. We eventually got Greg's unit below 2W and mine runs around 3W. His with new seals was costing between 6 to 7W at no load.

    It is hard to imagine your prop being so poor as to lose an extra 50W but I would need more detail on it to assess. From the photos it looks to be 420mm diameter with 50mm blade chord. I cannot guage the blade thickness. There is a Canadian firm who milled Greg's prop from my CAD file and is prepared to make other one-offs for CAD550. I hand fabricate my props but the ones Greg has had milled are more precise and give almost 1% higher efficiency. I have attached a photo of one of the props Greg had made.

    I expect if the chain was dragging enough to rob 50W or more it would be showing metal shavings in the oil by now.

    So you need to be looking for 65W that you are just throwing away. That is not to say you should be happy with the rest as there are many opportunities to go faster for much less effort.

    In calm conditions V11J will cruise at 10kph with 92W. Basically just rolling the legs around.

    Rick W.
     

    Attached Files:

  7. beppe
    Joined: Jul 2008
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    Location: Udine, Italy

    beppe Junior Member

    I take this as an acknowledgment that my intentions are not after all as vile and evil as you said before... Your words are a bit heavy sometimes, Rick; try not to scare off all the contributors of this thread you started ;-)
    Beppe
     
  8. Guest625101138

    Guest625101138 Previous Member

    Late Model Engine Repower

    Took my first steps to repower the new boat today.

    Conditions were not ideal as the wind picked up but Jeff Nielsen came down and had his first test ride. I will now do some alterations to better suit his preferred pedalling geometry. He is taller than I am and his recumbent racing position is more straight legged with lower bottom bracket. I will make the position of the crank adjustable.

    The testing gave me the chance to take videos off the boat and I have posted a clip of the session here:
    http://www.youtube.com/watch?v=8BiGp94RLX0

    The attached chart shows the GPS trace. Even with the wind up around the 20kph mark he was able to hold over 11kph without a lot of effort so he is a good engine.

    Rick W
     

    Attached Files:

  9. beppe
    Joined: Jul 2008
    Posts: 51
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    Location: Udine, Italy

    beppe Junior Member

    Open Waterbike ladies' day

    Nice testing day yesterday for Open Waterbike 'prototype 0'. Not really the Open Waterbike, just the starting point of the project...
    Nice weather at lake Cavazzo, our usual testing site. It is a small lake in North
    eastern Italy, not far from the Austrian border. There is also a monster in the lake, but it didn't show up. A bit windy at first, then flat water. Actually, we were not testing performances, just user-friendlyness, the perception of safety by common people and the reliability of the new drive unit.
    Several people tried the boat, most of them ladies and children. The registered speed (GPS) range was from 5 to over 7 knots , as expected. We discovered that ducks are not afraid of the waterbike, and the ladies just loved that.
    Maneuverability was also tested for duck-chasing with good results...
    We recorded a few videos, but most of them they are too cumbersome for uploading; we'll put them after editing on utube. Just a picture and a video for now, Open Waterbike with lady nere:
    http://www.openwaterbike.com/our-boat/our-boat-safe-fast-easy
    Beppe
     

    Attached Files:

  10. markdrela
    Joined: Jun 2004
    Posts: 307
    Likes: 28, Points: 28, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    Here's a potential system to consider for your open waterbike project.

    The attached sketch shows an HPB drive system which I and two other guys built back in 1994, right after the Decavitator project. It worked very well. The thin shaft was quite stable, and did not exhibit any whipping tendency even at full sprint power.

    The nice features are:
    * only a simple and cheap 1:1 bevel gearbox is needed
    * no complex mechanism or seals under the water
    * no additional axial-loaded bearings besides the gearbox itself
    * very simple prop replacement system

    The "screwdriver" shaft-end system mimics a bike crank axle taper,
    and allows a much smaller flex drive shaft than what a shear-pinned joint would need.

    The 1/4" pitch chain and sprockets are easy to get.
    The only special item was the large 120 tooth chainring, but that was
    a fairly simple CNC job from aluminum plate.
     

    Attached Files:

  11. Guest625101138

    Guest625101138 Previous Member

    Mark
    You may be interested in this photo:
    http://boatdesign.net/forums/attachment.php?attachmentid=23439&d=1216205907

    That shaft operates without any support other than at the gearbox. A pushing prop is self stabilising so as soon as it starts to turn the prop just rises and aligns with flow.

    This video clip shows a completely unsupported prop using an 8mm aluminium shaft:
    http://www.boatdesign.net/forums/attachment.php?attachmentid=23455&d=1216266495
    I pushed the OC1 to 15kph with just that shaft. The aluminium section is 1.2m long.

    I normally use a tension strut to enable going in reverse. With this and the prop beside me I can reach down and pull it up to remove fouling. Also for beaching or obstacles the prop just bounces over.

    There is a balance between torsional rigidity and fatigue life. With good spring steel you can get a good compromise. I design for prop speed of 300rpm at design boat speed and 1/4" steel is just a bit thin. For sprinting I go up to 10mm spring steel and live with the extra weight to get a tolerable stress range. There is very little loss due to shaft torsional constant once you get to this thickness.

    This system does result in very low appendage drag for the prop.

    The other thing I have posted in earlier posts on this thread is my dipping rudders. These are only immersed when you want to turn or one just immersed enough hold a straight course.

    I can provide the design data if anyone wants to have a go at a reliable curved shaft system.

    Rick W.
     
  12. markdrela
    Joined: Jun 2004
    Posts: 307
    Likes: 28, Points: 28, Legacy Rep: 324
    Location: MIT Aero & Astro

    markdrela Senior Member

    Yes, but only if the shaft is thick enough (and hence draggy enough). With the minimal 3mm shaft there is a buckling mode at low speed and high power, since then the P-factor forces are not enough to restrain it. Also, a dangling prop like that is not attractive if the boat is to be usable in knee-deep water (e.g. beach launching).
     
  13. Guest625101138

    Guest625101138 Previous Member

    Current shafts are either 6 or 8mm spring steel and approximately 3.5ft immersed length. The tension strut is faired 20mm long by 1.2mm thick aluminium. The stainless steel bearings I use in my prop strut fit within a 14mm OD tube.

    I will need to do some calculations to see what is more efficient. What I have is very low drag and uses standard bicycle chain with 4:1 reduction so chainring is only 52T.


    Rick W
     
  14. Guest625101138

    Guest625101138 Previous Member

    I should add that the system I use has a side mounted shaft to avoid through hull penetrations.

    One common problem with the prop drive boats is prop fouling so mounting the prop beside me allows easy access to remove weeds.

    Also a prop mounted on the stern under the hull would be wiped out on the first log encountered. I have designed for things to just flex out of the way to avoid fatal damage.

    Rick W
     

  15. MLampi
    Joined: Aug 2008
    Posts: 74
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    Location: Bellevue, WA

    MLampi Junior Member


    It depends on the durability of the prop, too.

    I've encountered submerged rocks with an under the hull stern mounted prop. Yes, it makes a big CLUNK sound, and yes the propeller blade is no longer quite the same shape. Still, the propeller worked quite well and I was able to complete the journey some 5 miles later without much loss in speed.

    With a manganese bronze prop the blade curls over a bit, and can be easily repaired at a prop shop.

    With a ZnAl alloy or a plastic prop, I'm sure that bits and pieces of the prop would have been left behind.

    A free prop shaft such as yours would enable the prop to experience far less damage, regardless of the material, but you lose the ability to go in reverse via pedaling action. Whether that is significant depends on where and how you use your boat.
     
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