Efficient Model hull design.

Discussion in 'Boat Design' started by awhapshott, Feb 7, 2013.

  1. awhapshott
    Joined: Apr 2012
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    awhapshott Junior Member

    I've been tasked with making a spring powered model to get as far as possible across an 8m long tank, static water.

    So far we have been thinking of long, thin catamaran with a paddle wheel powered by the spring.

    (Undecided on how to couple a spring to our paddlewheel, will need to be geared carefully, we intend to reach the other side of the tank!)

    Considering that I am working on a design under 1 meter LOA, does anyone think there would be any merit in a sort of wave piercing design, with 2 submersed hulls and an extremely thin structure leading through the surface to a platform where the paddle wheel, gearing and spring would be mounted?

    I thought of this design as it will cut down the drag caused by the surface tension of the water? Rather than to reduce the effect of waves, as, obviously there are none! Any design ideas/thoughts would be welcomed, all my experience is in skiff sailing dinghies/yachts...

    Hope you can help!

    Andrew
     
  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    A long skinny hull, driven by a propellor, is likely to give you the best result. A twisted rubber band will deliver plenty of power to traverse the 8 metre distance and at good speed. If you are required to use a spring then that is still a possibility for driving the prop.

    Check out model aircraft sites for information about rubber band power. Lightweight, simplicity, and economy are hard to beat in a boat design.
     
  3. tom28571
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    tom28571 Senior Member

    A governor on speed will be very useful in controlling thrust over the distance. Most devices waste a lot of energy in getting started. Think an air paddle like music boxes use. Could even be used to generate thrust. Catamaran is ideal for directional stability. Since maneuvering is not required, thrust can be delivered at the bow instead of the stern.
     
  4. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    I assume you didn't think of the actual wetted surface. You might want to.
     
  5. awhapshott
    Joined: Apr 2012
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    awhapshott Junior Member

    Thanks all, some very good opinions which are very handy..

    I had thought of wetted surface area increase, but my dad told me that wouldnt make a difference... Think I plan on something more simple though. I was thinking more about how this model is very small, and the surface tension will stick to the hull slightly.

    Unfortunately it must be a spring.. we'd thought about a music box kind of mechanism, I think we will head in this direction..

    Better get building!

    Thanks again!
     
  6. messabout
    Joined: Jan 2006
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    messabout Senior Member

    I will risk the ire of your dad. The drag of the boat is subject to skin friction. Wetted surface is, indeed, to be taken into consideration. The boat may not gain enough speed so that form drag is the dominant drag factor but that may be so. In any case the acceleration is a factor to be dealt with. If it gains sufficient speed such that Reynolds number of Froude number is signifigant, then form drag may be worth some attention.The immersed surface of your boat must have a superior finish. That means very carefully done finish work with no ripples of lumps of any kind. Do not wax the elegegantly finshed surface. Make the boat long and skinney, It will be a monohull not a multi hull.

    Make the boat as light as possible. Be vigilant about that because Sir Isaac has instructed us about the reality that F=ma. and you have limited force. DIminish the mass but hopefully not acceleration. I am confident that you can understand the relationship.

    The music box mechanism is the way to go. You may have to fiddle up a gear ratio that turns the prop faster than the governed music box movement would permit. Maybe you can fiddle with the air paddle to make it go faster. Your boat, presuming a standing start, will start slowly but accelerate to whatever its termianal velocity. With these parameters you will surely have a contest contender and with the references to Newton, Reynolds, and Froude you will have gained some favor with your physics teacher.

    Best of luck, but remember that you can make some of your luck by being more informed that your competitors.
     
  7. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    The drag of the boat is subject to skin friction. Wetted surface

    I like Toms idea of a fwd propeller , but does the clean water the prop encounters make up for the extra drag of pulling the boat thru prop accelerated water?

    With the prop at the stern the accelerated water can be fed to the prop as an added source of energy?
     
  8. tom28571
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    tom28571 Senior Member

    Having been in such a contest with one of my sons and won the event, albeit with a sailboat, I have some thoughts. Ours was the only boat that sailed a very straight course and it was a catamaran. I would make the hulls from foam and only enough beam to satisfy both needed displacement and immersed volume. Model shops sell a heat shrink plastic film that makes a superior finish at almost no weight. A forward propeller between the hulls will make directional stability much easier. Don't minimize the importance of steering straight.

    If the expected speed is high enough to make planing either possible or necessary, a monohull will probably be best.
     
  9. Submarine Tom

    Submarine Tom Previous Member

    Update?

    Andrew,

    How is the build going?
     
  10. awhapshott
    Joined: Apr 2012
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    awhapshott Junior Member

    Hi Tom,

    Sorry I went a little quiet, I have been Ill and had a few other things on..

    Thank you to everyone for posting with useful information and experience, very helpful.

    In the end we had to finish in a bit of a rush, as we mucked up our first hull build.
    We built 2 hulls (roughly identical, long and thin with lots of deadrise but only a little rocker) Carved them from polystyrene, laminated a thin layer of fibreglass over the top and a layer of vinyl over that to make it smooth (Last minute.. long story)

    We laser cut a paddlewheel from thin MDF (Again, not my choosen material, but didn't have long)

    Decided in the end we would be best off using the spring coil from inside a tape measure. A little agricultural, but after going to the £1 shop and buying 6 tape measures.. we eventually made something that worked... we finished it with literally minutes to go
    (no time for testing) The tape measure was directly coupled onto the paddlewheel shaft.. no gearing, no bearings.. straight onto the spring!

    Each tape measure was 7.5m long, so I figured with our paddlewheel being 3.5 times the radius, and assuming a loss of distance due to the blades not being 100% efficient at turning all that into forward motion, we should reach at least 8m of the tank..

    Dropped it straight in the tank and it had absolutely no trouble.. It reached about 28m on its best run (tank was too short and we had to turn it around)

    A short video of one of its first runs is here;
    http://www.youtube.com/watch?v=hlZb3VA_PFA

    All other groups used mousetraps and paddlewheels.. Most went round in circles, but even when aided to go straight the best one only reached a maximum distance of 5 meters, so we were fairly happy.

    Always room for improvement, but considering we only spent 6 hours or so on the build, not bad! (What can you do when your mate decides it would look great in red, spray paints it and dissolves it..)

    Thanks again,

    Andrew

    Ps. Forgot to add, all the duct tape was entirely necessary!!
     
  11. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    haha spray painting styrofoam - ruined a nice space ship in my youth like that.

    But congrats on your success - looks great on the video.
     
  12. messabout
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    messabout Senior Member

    And some of the contact cements will also render styrofoam into a gooey mess. (do not ask me how I know that)

    Congratulations for your success. You have done well.
     
  13. JRD
    Joined: May 2010
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    JRD Senior Member

    Great work Andrew, that looks very efficient and tracks well.

    PS. Back in the day when Cherubs were built of ply most of them required several rolls of duct tape by the end of a regatta :cool:
     
  14. awhapshott
    Joined: Apr 2012
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    awhapshott Junior Member

    JRD, they still require many rolls of duct tape, Carbon or not!

    Just started a very similar project with a 3V motor, but have to carry 1kg this time and have a motor and tiny propellor already supplied.. More polystyrene Catamaran carving I think!
     

  15. El_Guero

    El_Guero Previous Member

    Andrew,

    Was your foray into boating a one off? Or, do you plan more boating in the future?

    I notice your channel is full of trains, is your focus upon trains, or do you also focus upon boating?

    Wayne
     
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