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

    It looks cool. How did it work out (the pedal boat)?
     
  2. Guest625101138

    Guest625101138 Previous Member

  3. Frosty

    Frosty Previous Member

     
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    That torpedo-hull boat is an interesting and unusual configuration, indeed. At first sight it looks like it would operate at some heel angle, maybe around 5°... Is it so?
    How does that torpedo-hull version compare to your actual trimaran version of the boat, in terms of total resistance or max speed attained? Was the propulsion system of the same flexible-shaft type?
     
  5. Joakim
    Joined: Apr 2004
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    Location: Finland

    Joakim Senior Member

    In order to minimize drag you need to make the wetted surface hydraulically smooth. Making it smoother than that does not decrease drag any further. This maximum roughness depends mainly on speed and at 8 kn it is about 20 um, which is equal to surface made out of 20 um grains = equal to P800-P1000 paper. A well painted antifouling is likely to be about 50 um.

    When you are sanding the surface will become smoother than the paper. Thus about P400 is OK for sanding it and it is used for towing tank models and P600 for Americas Cup boats, which are a bit faster.

    Here is a table of paper grain sizes: http://www.cs.rochester.edu/u/roche/rec.wood.misc/grit.sizes.html
     
  6. Guest625101138

    Guest625101138 Previous Member

    A circular section gives maximum volume for wetted surface but there is no point having more volume than you need.

    Attached is a drawing of Albacore. It has slightly wider beam than depth in the body of the hull. I modeled my submerged buoyancy boat on this but with circular sections. They just happened to be easier to cut with a hot wire than ellipse and I was after maximum volume with no internal dimensional constraints.

    One possibility for propulsion of a craft like this is to use buoyancy. You can achieve very high efficiency with this concept particularly if you are operating in deep water and design speed is low.

    Rick W
     

    Attached Files:

  7. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I thought there might be some kind of transverse flow causing cavitations or something like that.
    So if there isn’t that problem, then I guess it would follow that the less round the better as long as you are reducing one of the dimensions.
     
  8. Asleep Helmsman
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    Asleep Helmsman Senior Member

    The main mission is side scan sonar in shallow water. From a software development point of view, the more constants, the easier to program. So maintaining depth would be one less set of variables.
    Maneuverability is actually the main objective. This entire discussion was to establish the range for a given speed.
    I’m glad we all did this, and I’m thankful to everyone’s contribution. I looked at the JavaFoil site and discovered that my original concept (done completely from a half a century of looking at hulls), was within 5% of the same as many of his foil designs.
    So I’m betting it will be in the accepted range of efficiency, and at the high range of maneuverability. Which of course are, in some ways, in direct conflict with each other.
     
  9. Asleep Helmsman
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    Asleep Helmsman Senior Member

    The concept is not secret.
    both programmed and autonomous.
     
  10. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I am not, do you have a link?
     
  11. Asleep Helmsman
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    Asleep Helmsman Senior Member

  12. Asleep Helmsman
    Joined: Jul 2008
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    Location: Republic of Texas

    Asleep Helmsman Senior Member

    There must be an acute point of no return when a prop diameter reaches a certain size.

    Does efficiency relate to slippage?
     
  13. Guest625101138

    Guest625101138 Previous Member

    There are practical limits to size but the fundamental, in terms of efficiency, is to act on the largest area of water and accelerate it a tiny amount. That prop in the image has a velocity ratio of about 1.03 at design condition. So effectively 3% slip. The rest of the power loss is related to moving the blades through the water. As the diameter goes up the blades rotate slower.

    In practical terms there are limits. If you you use a very large slow turning prop then the craft will tend to spin rather than the prop. You will need some way to counter the prop torque. Some submarine craft use counter rotating props but it is more efficient to use gravity through location of the CofG or control surfaces.

    If you use JavaProp to design a prop you need to check that the resulting blades will handle the loads without undue flexing. Your loads are quite small though.

    Rick W
     
  14. Submarine Tom

    Submarine Tom Previous Member

    Asleep Helmsman,

    I don't want to hi-jack your thread but I am interested in a couple of "off topic" questions when, and if,

    you feel like commenting on them. What collision avoidance (if any) system will you employ?

    How, or will, you protect your prop(s).

    If you go with a large diameter prop(s), how will you gear your motor(s) to achieve the low RPM.

    Have you considered a dimpled surface finish to maximize a turbulant flow concept?

    What type/style of control surfaces are you considering and where will you locate them?

    How important is manoeuvrability to your mission requirements?

    Are you planning on neutral buoyancy or slightly positive?

    What colour will you paint the fish? No, just kidding...

    Okay, I give up, what movie is your signature sign-off from?

    -Tom
     

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

    Most of these questions will be answered soon.

    So in reverse order. My signiture is my own.

    Once when I was 14, I stood watch on a 65 Wheeler. The whole crew was sick that night from 20 hours of nearly unendurable rough seas, and some bad water in Panama.
    I went aft to relieve my poor stomach and 'we ran over what must have been a 500 year old mahogany tree.
    Ever since then I don't generally take my eyes off the water, for more than a minute of so at a time.
    I’ll tell this tale in another thread, sorry but time and fast living have contributed to all of the photos being lost.
    I will try to use Google Earth as much as possible to illustrate the voyage.
     
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