Light Weight Diving Vehicle

Discussion in 'Boat Design' started by dtoshni, Jun 16, 2008.

  1. dtoshni
    Joined: May 2008
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    dtoshni Junior Member

    Hey there! :)

    I am working on a light weight diving vehicle, kind of like a vertical profiler, and I am having a lil bit of trouble regarding the design of the vehicle. Here are a few questions regarding the thing.

    1.) When the vehicle descends with its motor turned on, it kind of starts to rotate because of the motor. So the idea I had to counteract this was to use static fins to stop the motion. Now I was just thinking, can I use long fins, that is with small span(about an inch) but running along the body of the vehicle to counteract this rolling motion, keeping the area of the long fins the same as that of the fins I am using right now. Will this be helpful? Will the boundary layer separation affect this? (as I think that the small span fins may lie totally inside the boundary layer. Will this reduce their roll-resisting capacity?)

    2.) I need to calculate the propeller efficiency of the thruster which I'm using. Its a ducted propeller, a tecnadyne design. Can you tell me a way to calculate the efficiency of the ducted propeller? I seem to have lost my PNA too. If you can post images of the section it has regarding this, it'd be a lot of help. :)

    Thank you.

    Hoping to hear soon from you guys.

    Daniel T
     
  2. SamSam
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    SamSam Senior Member

    Maybe fins like on an arrow. It would seem they would have to be adjustable or something depending on how much power was applied.

    Counter rotating props would maybe work, at the expense of simplicity.

    Possibly a smaller propeller at higher speed would be an improvement.

    Maybe instead of the fins running alongside the body and being more or less passive and working only with forward motion of the contraption, there could be two adjustable rudders crossed like this, + , directly behind the ducted tecnadyne design propeller that would counteract the torque.
     
  3. dtoshni
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    dtoshni Junior Member

    Thank you Sam :)

    When you say two adjustable rudders behind the thruster, what exactly do you mean by that? I am having a little trouble understanding as to what will be the position of the rudders.

    And I also want to know? Will the fin design work? I know it is of passive nature as the fins will be static, but can this design(long fins of small span along the body) actually prevent rolling motion of the vehicle?
     
  4. Guest625101138

    Guest625101138 Previous Member

    You need to be able to counter the rotation created by the prop. A set of vanes behind the prop could straighten the flow. This is a simpler solution than counter rotating props.

    Fins could be used but long narrow ones will not be anywhere near as efficient as wide short ones. Something like the pectoral fins of a fish would do the job. These are not quite as effective as the vanes at the prop because they rely on movement to create the turning force.

    If you provide the dimensions of your prop, the thrust or power and required speed I can tell you the efficiency possible from a suitable prop.

    Rick W.
     
  5. dtoshni
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    dtoshni Junior Member

    Thanks a lot Rick. :)

    I really appreciate you looking into the problem.

    The thruster I'm using is a Tecnadyne 260 Model DC thruster..
    http://www.tecnadyne.com/Brochure/Model 260 Brochure.pdf
    The dimensions are given in this pdf.

    The velocity I'm getting(in the field) is 0.58m/s and the thrust is 0.532 Kg-f. Can you please calculate what the propeller efficiency is? Input is being provided at 24 Volts. And also, I think there might be some room for optimizing the efficiency by use of a better propeller. Ca you please help me out on that?

    Regards

    Daniel T
     
  6. Guest625101138

    Guest625101138 Previous Member

    Your performance figures do not align with the brochure. You are way down on the data provided.

    The spec data given indicates 5lbf thrust at 4kts. This equates to power out of 44W.

    You figures of 0.58m/s and .532kgf equates to 3W. A significant difference. What is the motor current? The magnetic prop coupler may be stuffed.

    I have taken a stab at the prop design using JavaFoil and arrived at the attached data using the dimension given and the performance at 4kts. The rpm I needed to get the right size blades looks low for a small diameter motor but it may be geared. Do you happen to know the motor rpm at 24V?

    At a speed of 0.6m/s the blades are stalled out so the efficiency drops way down to about 30% but this would still give a thrust of 2.5kgf.

    The specification data provided indicates very poor efficiency overall. For example the current required to get 5lbf at 4kts is 12A. This means an input of 288W to get 44W out. I get a prop input of 60W for this condition so there are a lot of other losses. You would normally expect around 75% efficiency from a good industrial type motor of fractional HP. So there seems to be a lot of power wasted somewhere in the system.

    The prop will work better one way than the other. All my data is for the most efficient direction.

    Rick W.
     

    Attached Files:

  7. SamSam
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    SamSam Senior Member

    [​IMG]

    What exactly are you building? The site you show says the machine is good for ROV's, remotely operated vehicles, or AUV's, autonomous underwater vehicles. Are you making something for people, a DPV, diver propulsion vehicle?

    What is a 'vertical profiler'? If you are making an unmanned machine, what will it be doing? Are you looking to eliminate rotation 100% or just slow it down?

    On a machine that travels horizontally, weight and bouncy might be used to counteract torque, that is a heavy bottom and a bouyant top. That wouldn't work vertically.

    Try searching under Google "Images" for image ideas and also sites doing whatever you are interested in. Here's an image search for "auvs" that brings up sites from colleges, aquariums, government, etc.

    http://images.google.com/images?cli...fficial&hl=en&q=auvs&btnG=Search Images&gbv=2

    On manned vehicles, I think a lot of control is done by the operators fins. Here is an image of what I suggested. It's hard to tell but I think there is a slight twist to the vanes behind the prop to counteract prop torque.

    [​IMG]

    Most modern ones have the vanes in front of the prop, as in this one.

    [​IMG]
     
  8. dtoshni
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    dtoshni Junior Member

    @ Rick:
    The data IS way off whats given in the brochure Rick. :(
    I'm operating the propeller at a Control Voltage of 4 Volts(with an Input of 24 Volts). The Thrust versus Control Voltage graph I get for input at 24 volts(control voltage 0-5 V) fits the function
    0.00094*(x^3)+0.067*(x^2)-0.18*x+0.12
    where x is the control voltage.

    With this configuration when I operate it in the field, its terminal velocity is 0.58 m/s.
    The motor current is 3 A and I am sorry but I don not know the RPM of the thruster.

    @ Sam
    The Vehicle goes down vertically to collect samples. It is an unmanned vehicle.
     
  9. dtoshni
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    dtoshni Junior Member

    @Rick

    Can you please also explain all the parameters used in Javaprop to me. As in, why did you enter the velocity = 2 m/s, is this what you are designing the propeller for? How did you get the thrust 22 N ? The parameters which are calculated too(eg. v/omega*R and beta etc). I am sorry for all this trouble.

    Thank you.
     
  10. Guest625101138

    Guest625101138 Previous Member

    Daniel
    I took this data from the brochure for the 24V motor so I could get an operating point for the prop. One particular chart shows a series of the thrust curves versus current for different speeds. I selected the 4kt (2m/s) curve and the point that gave 5lbf (22N) thrust. This happens to require 12A.

    Having an idea of the motor rpm allows me to verify that I am close with the blade shape.

    The unit will not have the same thrust at rest for a given voltage as it does at some speed unless by pure chance. So thrust data taken at rest is meaningless in respect to the thrust required to do the 0.6m/s.

    I will redo the numbers for the same prop at 4V (lets say 400rpm) to see what conditions look like to do 0.6m/s. The reduced voltage explains the very low power output you have.

    It would certainly help if you gave an indication of the design objective. Are you particularly seeking long battery life for example? Does it really matter if the unit rotates while descending? I have seen real clever water gliders that pump ballast to rise and fall while gliding long distances. This system requires tiny amounts of power to cover long distances.

    Rick W.
     
  11. dtoshni
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    dtoshni Junior Member

    Yes Rick..The primary objective of the profiler is elongating the battery life. To maximize endurance.

    I have also heard of gliders that operate solely on ballast mechanism to adjust their depth, but that is a rather slow process(yes, its power consumption is very little but still, we need to get to the height a little more quickly). That is why we thought of using a thruster for the motion.

    And I'm worried about the rotation because I think it uses up some of the energy of the vehicle, and hence the linear motion(downward) has to suffer because of that. Is it not so?

    Daniel T.
     
  12. Guest625101138

    Guest625101138 Previous Member

    Daniel
    I misread this originally. You are supplying 80% of speed signal so presumably motor is running at 80% of speed.

    Taking your equation it means that at 5V control you are getting 1.0125 of some units????? of thrust.

    One of the curves in the specification provides data for static thrust. At full 24V the spec indicates it produces 8lbf of thrust. So unless one of your force units equals 8lbf the unit is not performing to spec.

    The other thing that does not quite gel is the 3A at 80% of max speed and only 0.6m/s. You would expect much higher current if the motor is doing 80% speed and unit is only moving at 0.6m/s. Again something looks wrong relative to the data sheet.

    Rick W.
     
  13. Guest625101138

    Guest625101138 Previous Member

    WE crossed posts - I will answer this one.

    The rotation does not cost anything if you have rpm to spare. The vehicle will rotate so robs a few rpm from the prop but very little power unless it is spinning fast. Having the vehicle spin will help it hold course if it does not have perfect weight distribution or symmetric body. The spin performs the same function as spin on a projectile from a rifled barrel.

    Fitting vanes on the body of the vehicle that have very slight pitch angle could be set up to counter the spin if it really is spinning fast.

    Rick W.
     
  14. dtoshni
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    dtoshni Junior Member

    ok. I see. Well, my vehicle is spinning pretty fast(124 deg/s).

    Also, while doing fin design, the approach I'm using is:

    1. I calculate the torque due to the thruster with the formula :

    Torque = Thrust*pitch/2*pi
    2. Then I am calculating the force at the fins due to this torque.

    3. Then I equate the drag on fins to the force calculated above and I calculate the area of the fins for the tolerable velocity.

    I am thinking of using a NACA 0012 fin for this. What would be the drag coefficient for this when its rotating, as in moving perpendicular to the water. Will it be almost equal to the flat plate drag coefficient(1.28 (wikipedia))?
     

  15. Guest625101138

    Guest625101138 Previous Member

    Daniel
    I am travelling right now so have limited time.

    The thrust stuff I need to think about whast you are doing.


    The anti-rotation fins will have a lift to drag coefficient of say 20 if they have an aspect of 4. That is 4 times wider than their length. So they will still rotate unless you slat them slightly to counter the spin. It is the lift that counters the spin. So they do not cost you a lot of set up well.

    I can do exact numbers but I would need data on the size of the vehicle.

    Rick W
     
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