Power Required

Discussion in 'Props' started by Asleep Helmsman, Jan 5, 2010.

  1. Asleep Helmsman
    Joined: Jul 2008
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    Asleep Helmsman Senior Member

    Can anyone tell me how many watts it would take to power an underwater hull, approximately 1.25 meters long, shaped like an airfoil revolved around an axis, with a length to beam ratio of one to five.

    The prop will be 25 to 30 millimeters in diameter, minus the hub, and shaped similar to an airplane prop with each of 2 or 3 blades having a length to width ratio of approximately 4 or 5 to one.

    I need one calculation for 4 knots and one for 8 knots or better yet, a power requirement graph showing the speed from 0 to 8 knots would be most helpful.

    Thank anyone in advance for this “above the call of duty” information.

    Joe
    PS. If there is any cheep or free software on the web that can do this for me; that would be as if Santa Clause made a special trip to my house.

    PPS. the prop will most likely be ducted using the Rice design, over the Kurt, which is supposed to increase thrust by 35 - 40% at these low speeds.
     
  2. Guest625101138

    Guest625101138 Previous Member

    An important question is - how deep?

    If deep so there are now waves you could pessimistically expect a drag of 50N for an elliptically shaped hull at 8 kts. If it is an optimised laminar flow hull it could be about 15N but this requires very steady operation and surgically smooth surface.

    Now I am assuming you have left a decimal point off the prop size so you can actually use a 300mm prop. In this case you need 230W to do 8kts. It does not need a nozzle.

    If you actually meant a 30mm prop then it is doubtful you can make blades strong enough to take the load (you are talking about blades 15mm long (about 1/2") and 4mm wide (say 1/8"). If you do manage to make a prop from some super strong material you will need about 500W to get 8 knots if it is in a nozzle and about 550W if it is open.

    Rick W
     
  3. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Thank you,
    Yes, I did leave off a zero.

    Would power consumption increase at the square of the speed? So then at 4 kts it would use 78 watts and at 2kts it would use 18?
     
  4. Guest625101138

    Guest625101138 Previous Member

    The power is roughly the cube of speed. Flow regimes change a little but from 4 to 8kts you could expect about 8 times more power. So say about 30W at 4kts if in deep water. Will be much higher if at surface.

    Rick W
     
  5. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Rick,

    I imagine that by the time I am finished with this project you and I will become well acquainted.

    Thank you Sir,

    Joe
     
  6. Guest625101138

    Guest625101138 Previous Member

    Hope you are not filling it with explosives!

    It is possible to optimise shapes for different speed with submersible hulls to get so-called laminar flow hull. There is significant benefit to lower drag if it can be achieved but I have never set a hull deep enough to get away from wave drag and cannot comment on practicality. I know it will work for hulls that use buoyancy as the driving force but do not know if it is sustainable when the hull is motorised.

    Attached paper shows the shapes and explains how they were derived. The low Re# hull has been extensively tested and you will find reference to the Carmichael Laminar Flow Hull.

    To my knowledge there is no freely available software that determines flow over a body of revolution but Javafoil does reliable flow modelling for a 2D section. The drag data is not directly transferable but you can use it to determine the points of flow transition for the 2D and this will not be much different for 3D. The laminar flow hull relies on carrying a favourable pressure gradient as far aft as you can to sustain the laminar flow. As Re# goes up the nose gets more bluff to achieve this and transition occurs closer to the nose.

    Rick W
     

    Attached Files:

  7. Asleep Helmsman
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    Asleep Helmsman Senior Member

    You’re absolutely correct about moving the maximum beam aft. I grew up on yachts and have spent the majority of my life watching water flow past hulls.
    The idea is to use a parabola shape to continuously reduce the forward angle of attack as the slip stream moves aft.
    The trick is to keep the aft section from separating the flow from the hull which increases the size of the vortices and creates drag, or in other words subtracts energy from the hull.
    The deeper the dive, the faster you can move without cavitations.
    A good design to study is the sail plane. They have really pushed the envelope for laminar flow.
    As you said earlier at speeds of 8kts or less, this is not an absolute design function. Other considerations like maneuverability become equally or more important.
    The main reason for having as high a ratio of laminar to turbulent flow is energy conservation. Submarines’ major considerations are energy conservation, maneuverability and stealth, the balance is solved in large subs by using nuclear power, unfortunately I don’t have too much access to that.
    This project will utilize the latest in battery, motor, and hull, technologies to achieve the highest possible range, and still perform its mission.
    Thank you once again, I will certainly keep you informed.
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Asleep Helmsman,

    You didn't answer Rick's question about whether or not you're carrying explosives.

    -Tom
     
  9. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I thought it was tongue in cheek?
    No actually, the exact opposite.
     
  10. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Rick,
    I'm reading that paper on hull design.
    Very nice.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Now you have my interest. What is the exact opposite of explosion?

    1. A black hole - that would be nasty.
    2. An implosion - yep, dive deep enough and that will happen.
    3. Figuratively - a bouquet of roses or white flag.

    Javafoil is quite nice to play around with if you are looking at flow transitions. Thick 16-series foils with zero camber are a good starting point. You could tabulate the results for the laminar flow 3D shape and place the data into Javafoil. I am not certain how far the similarity goes between 2D and 3D but when I played around with the shapes in that paper they resulted in the lowest drag coefficient I could get in 2D as well in the respective flow regimes.

    There are some things to be aware of. With bodies of revolution the drag coefficient is often given as a volumetric basis rather than surface area or frontal area. Hence you need to be careful what the coefficient is referring to.

    I have looked at all sorts of shapes from fish to submarines. The best submarine I could find was experimental Albacore. From memory it had a volumetric coefficient of 0.02. That is half of what I used for your proposed hull but about twice what a laminar flow hull gets. The drag would be very low if you can get into laminar flow.

    Rick
     
  12. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I'll be fifty this month. This is the most excited I have been about anything for quite sometime.

    Once around the age of forty I built some robotics for the movie industry.
    Now maybe I will do something equally as much fun.

    Ultimately this machine will seek out and identify underwater anomalies.

    I write software for the US government, I design robots, I grew up the son of a professional yacht captain, and finally add that to the software thing and I think you can see where this is headed.

    So, the opposite of carrying explosives.

    Here are some picture of the robotic stuff we built back in the 90s

    http://www.flickr.com/photos/30885456@N02/sets/72157608418121223/show/

    I also built the CNC machine that made most of that. Or rather I converted a 54" knee mill to CNC

    We’re gonna have some fun boys.
     
  13. apex1

    apex1 Guest

    Who knows?

    And the opposite would be implosives then?
     
  14. Submarine Tom

    Submarine Tom Previous Member

    LOL, interesting though.

    Yes, implosion was my first thought for the opposite of explosion.

    So, programmed trajectory or autonomous?

    Happy Birthday, I'm right behind you...

    -Tom
     

  15. jehardiman
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    jehardiman Senior Member

    So basicly you want to compete with a REMUS vehicle. That's a tough row to hoe...

    http://www.whoi.edu then click ships & technology/Autonomous Underwater Vehicles (AUVs)

    Now a lot of what Rick says is good, but it is not absolutely correct either. There are a lot of pitfalls designing a hull do accomplish your envisioned mission and a small error in power comsumption will be huge in the field. You will spend 90% of your time and money trying to suqeeze the last 10% out of power comsumption so just give it more battery volume to start with and and take the drag hit. FWIW, it is not advisable to design for laminar flow at your size and speed unless you never plan to put it into real water. AUVs look like AUVs for a reason and do not use laminar hulls; ignore that at your own risk.

    Also, never use a 2d program to estimate 3d body of revolution drag, you will be off considerably more than if you used Hoerner's (Fluid Dynamic Drag)curve fit. Additionally, appendage interference drag will be 30-50% of the total drag at your size and speeds so make sure you front load it into the analysis.

    Finaly, as Rick and Submarime Tom alluded to, this touches on the Wassenaar Arrangement (http://www.wassenaar.org) and US technology export law , so heads up.
     
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