RPV hull shape and propulsion

Discussion in 'Boat Design' started by DianneB, Mar 2, 2011.

  1. DianneB
    Joined: Jan 2010
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    DianneB Junior Member

    I am undertaking the design and construction of a small Remotely Piloted Vehicle for for wildlife research and could use some advice for where to start.

    The vessel is limited to no more than 4 feet in length, 2.5 feet in width, and will need to carry about 150 pounds in payload (primarily computer and video equipment) including batteries for equipment and propulsion.

    The field of operations is freshwater lake shorelines and marshes. The RPV is required to be self-righting and waterproof as it will be subject to the wake of passing power boats. Speed is not a requirement - slow is fine - but it needs to run quietly (so as to not disturb waterfowl) and to be able to run through marshes without binding up the prop(s) with weeds or snagging weeds in the rudder.

    The "deck" of the vessel should be within a foot of the waterline to maintain an inconspicuous profile but high enough to keep multiple antenna and a camera above water. I am thinking of a profile similar to a turtle. Whatever the profile, it should provide the best stability in the water to provide a stable camera image in small waves or ripples.

    Hull construction will be either welded steel or aluminum with a removable top to access internal equipment.

    I do not have enough experience in the marine field to know what shape of hull would best suit these requirement or what kind of propulsion/steering can operate in weeds and reeds.

    You suggestions would be very much appreciated!
     
  2. Submarine Tom

    Submarine Tom Previous Member

    Well that's quite the undertaking. Are you thinking radio controlled then? Can you be more specific about the mission? What exactly will the vehicle be doing. Hours of operation? Fresh water then? Budget?

    If it's private, PM me and we can "talk".

    -Tom
     
  3. Jeremy Harris
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    Jeremy Harris Senior Member

    This sounds like an extremely interesting design challenge!

    I'm far from being an expert in boat design, but I think that I'd look at a catamaran configuration, to get the stability you need. Propulsion could be something like water jets, but these are horribly inefficient at low speeds and, given that your energy storage capability might be limited by the need to probably use electric power, they might not be a sensible choice. I know it will sound daft, but I think this is an application where a pair of independently driven paddle wheels fitted between the two hulls might be a workable solution. These could be well shrouded to keep splash noise to a minimum, could be independently driven to give good manoeuvring capability and would be relatively free from weed entanglement problems (I think). Paddle wheels work well at low speeds and can be reasonably efficient if they have a fairly large diameter.

    Ideally a bit more beam would be useful, for better stability, but I think you might be able to get away with the dimensions you've given. The displacement will be relatively high, though, as the weight you need to carry is pretty high for such a small boat. This may restrict the shallow water capability somewhat and may rule out a catamaran layout. Would it be possible to reduce the payload weight a bit?

    Jeremy
     
  4. rasorinc
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    rasorinc Senior Member

    I think I would use a rubber Cat track on each side or even under the hull though spaced a bit. Would make steering much easier, would be quiet if placed under, almost no water movement when going slow. A small electric engine(s) to run each track independently. No ideas on self righting. My 2 bits. Got an idea on righting--on each side a balloon attached to it a small CO2 cartridge the driver could set off.
    I would build out of rigid closed cell styrofoam and glass with 1 light layer. a cubic foot of styrofoam floats 55 pounds.
    Sounds like fun. A 30"x48"x10" piece of styrofoam is 8CF x 55 = 440lbs. floation. Carve out the center to fit battery, camera, electric motor etc and lower the weight and self righting becomes mute. A piece of foam that size weighs about 14 pounds. Go buy a Cat Dozer toy with rubber tracks that moves and you have all you need to put under.
     
  5. DianneB
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    DianneB Junior Member

    The mission is to approach and watch waterfowl and other "swamp life" without disturbing them. Mission duration should be at least 4 hours.

    The plan is to used commercial (domestic) radio control for direction and speed control of the vessel. The payload will be audio/video gear communicating back to the control base via 802.11 (Wi-Fi). An on-board laptop will interface to the camera and also provide GPS location information to the base. The payload and battery bank which accounts for most of the weight.

    Budget is rather limited but not overly so.

    The project came to me because I have experience with radio control, model building, and computer networking and a shop that can fabricate a lot of things from "spare parts" and inexpensive components.
     
  6. baeckmo
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    baeckmo Hydrodynamics

    The weight/length ratio will be quite high, making multihull configurations unsuitable. Think "self-righting lifeboat" of the type that is launched nose-down in free fall.
     
  7. DianneB
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    DianneB Junior Member

    Instinctively I am leaning toward a "flat fish" design and carrying the weight as deep in the water as possible with only a "conning tower" above water to minimize the effects of swell or ripples and provide a steady camera platform. All that needs to be above water is the camera, microphone, and antennas and since the weight is fixed, having only a very small positive buoyancy is sufficient and will also dampen the effects of water movement.

    Since the objective is to "sneak up on wildlife" and observe their behaviour without disturbing them, being quiet and unobtrusive is important. That's why I want to keep the above-water profile small and the propulsion quiet. If waterfowl and marine life think the RPV is another non-threatening critter, that's perfect!

    Time under propulsion isn't critical since most of the duration of a mission will be stationary but if we can get 1/2 mile to 1 mile out from the launch point and back it would be ideal.
     
  8. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ===============
    I think this is an excellent suggestion. Are there any restrictions on draft?
    I did a video piloted rc helicopter project back in the 80's and 90's. Back then it was hard to get a camera and video transmitter light enough and cheap enough-not today. Sounds like a fun project-good luck!

    PS- I assume that the frequency you mentioned is FCC approved for your application? My company was the first to apply to the FCC to transmit video from an RC heli-took almost a year to get their approval for 2.4ghz......
     
  9. DianneB
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    DianneB Junior Member

    There is no hard restriction on draft but I would like to keep it to 24" or less.

    The R/C equipment is approved for model (hobby) control and the Wi-Fi is all approved equipment.

    The camera is a good quality commercial grade PTZ (pan, tilt, zoom) - not HD but VGA resolution - and weights 1.5 pounds (mainly because it is already available at no cost).
     
  10. Submarine Tom

    Submarine Tom Previous Member

    Excellent.

    I'm for the displacement hull and, at under 200 pounds, you only need ~3 cubic feet, and at 4 X 2.5 = 10, then 4 inches of draft will do it. A little deadrise for each end, you could put some bulbs down on the outer edges for increased initial stability. Place as much weight inside on the bottom of the hull and seal up the rest. Keeping the props clear is going to be a challenge, I don't have any bright ideas there. Your power requirements don't sound high. Could you tether this mini-barge into position? How big a boat wake is it likely to encounter?

    -Tom
     
  11. Submarine Tom

    Submarine Tom Previous Member

    I think twin screw out the stern and forget rudders. Sharpen the prop blades to a knife edge and just cut through your weeds. You'll have to be really careful handling the vehicle so not to cut any one. Locking out the throttles would be a requirement as well when handling, just don't turn the radios on until it's in the water and clear.

    -Tom
     
  12. Submarine Tom

    Submarine Tom Previous Member

    Google: vancouver harbour survailence submersible (but try and spell everything correctly). This set-up approach would be elaborate but if you've got the time and inclination it would be in keeping with your posts thus far.

    -Tom
     
  13. anthony goodson
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    anthony goodson Senior Member

    Whilst planning this ,particularly if you are to be a mile away ,why not factor in a small inexpensive wide angle cctv camera to deter or at least identify those who would interfere with it ,for whatever reason.? The focal length of your main camera is unlikely to achieve this.
     
  14. DianneB
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    DianneB Junior Member

    Sharpening the edges of the props would be bordering on negligence (from a design perspective) and wouldn't stop weeds from winding on the prop shaft so I think we will just go with a fully screened prop.

    Since the Wi-Fi link requires "line of sight" (more or less) the RPV would always be in view of the operators (with field glasses) so intercepting troublemakers isn't an issue. The camera has a wide range zoom lens so there would be plenty of time to photograph the vandals and the images are collected on the mother ship so they are out of luck with getting away with anything. The audio link also provides two way communications so the RPV could have a voice as well as eyes - that could be fun with troublemakers! LOL!
     

  15. anthony goodson
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    anthony goodson Senior Member

    Glad you've already thought of it ,an unattended ,prepackaged ,assortment of electronics would certainly be a target here ,and not just to the curious.
     
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