Sponson Design

Discussion in 'Boat Design' started by aaronhl, May 7, 2014.

  1. aaronhl
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    aaronhl Senior Member

    Posting here because I didn't get any response from the rc boat community.

    Questioning performance when the sponson's top is parallel to the waterline at speed versus angling the sponson nose up or down at speed? Here is a picture of the boat to give you an idea how the sponsons are shaped.

    Since the angle or pivot will also change the planing angle:
    Angle the nose down- decreases the planing angle,
    Angle the nose up- increase the planing angle

    I have referenced on one sponson (different than pictured below) it has a 6 degree planing angle when you reference the top of the sponson parallel to the waterline. However, when it is mounted on the hull the planing angle decreases because the top of the sponsons angle downwards.

    Should the sponson be built to have a high planing angle and when mounted on the boat angle down? Or should it be made to have a low planing angle and when mounted on the boat the position doesn't change?

    Does this make sense? What are your thoughts?

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. aaronhl
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    aaronhl Senior Member

  3. gonzo
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    gonzo Senior Member

    I think that you need to see what the angle is when the boat is running.
     
  4. schakel
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    schakel environmental project Msc

    Man, that's spectaculair fast!
    The sound is like a aggressive fly!
    It corners like glued to the surface.
    Great design.
     
  5. daiquiri
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    daiquiri Engineering and Design

    I think that you will have to find the optimum position by trial and error.

    Savitsky theory says that the optimum planing trim (for max. Lift/Drag ratio) is somewhere between 3-5°, depending on deadrise angle and slenderness ratio. But I am not sure how applicable it is to narrow hulls like your sponsons. A higher trim will get you on plane first but at the expense of higher drag and lower speed.

    Now, thinking out loud... How about making a setup like the one in the attachment, which would allow you to adjust both the trim and the draft of the amas on the go? In that way, you could find your optimum by performing multiple runs for each combination of trim/draft positions? The spring has to be pretty rigid and well-compressed.

    Cheers
     

    Attached Files:

  6. aaronhl
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    aaronhl Senior Member

    How can I do that? If this is a calculus equation count me out hahaha, trial and error?
     
  7. aaronhl
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    aaronhl Senior Member

    Nice info there- I find the rc outriggers are 3-5 degrees planing angle. The one in the video above has a 4 degree planing angle and a 4 degree dihedral or deadrise.

    I am confused because another sponson I have has 6 degree planing angle if sponson top is parallel, but 3 degrees when mounted in the boat. The top isn't parallel at that point. Should the top be parallel?

    Very cool design you provided...that might be something to look into. Usually guys use shims under the sponson tubes inside the boat hull/tub to jack it up.

    Ultimatley I want to build a boat where no adjustment is necessary. If you find the right degree and angle then you should be go to go on the next build.
     
  8. aaronhl
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    aaronhl Senior Member

    IF everything else being equal "SHOULD" the sponson stops be flat.

    The tub or center part of the boat is built parallel on top and with a slight taper up towards the transom.
     
  9. rwatson
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    rwatson Senior Member

    You shouldn't waste time considering the angle of the sponson to the top of anything on the hulls - the problem is the angle of the sponsons to the *water* when running.

    This will depend on lots of things including main hull buoyancy.

    If you dont have the math, trial and error will be the only method to evaluate this. Try large painting white stripes that show up in a video to obtain measurements.
     
  10. aaronhl
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    aaronhl Senior Member

    I guess I could always run it at 4 degree planing angle as long as the strut/prop is in the right direction. The video shown above have the prop at 0 zero angle 1/4" 3/8" above waterline. If my full size diagram is correct that would be running sponson tops parallel. It does stay on the water really well.
     
  11. Tom.151
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    Tom.151 Best boat so far? Crowther Twiggy (32')

    Spectacular at speed! What is the scaled speed?

    Show us a picture or drawing of the whole boat in profile?

    Would like to see the relationship of the sponson planing surface(s) to the aft planing surface. If you can approximate the waterline (with the boat at full speed) that would be really great.

    As for the pitch up/down of the sponson top surface - I assume you are concerned about the aero lift and the consequences?

    Do you intend to add a fairing that covers the tubes between the hulls to provide downforce to help keep the boat pinned down at high speeds? If not for lift, at least for drag reduction?
     
  12. aaronhl
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    aaronhl Senior Member

    The boat is 41" long plus prop shaft so about 45" long total with 5 horsepower gasoline engine. It should be going 75-80 mph...so scale would be over 300? Not sure exactly hahaha

    Here is a side profile of the boat. You will notice the blue waterline. This is designed so that at speed the tops of the sponsons and middle section are parallel with the waterline. There are rear sponson skiis with 2 degree planing surface and front sponsons have a 4 degree planing surface.

    The bottom of the hull tub is 1" above the water surface at speed (measured at sponson planing surface) and then at the transom 1-1/8". The picture might not show that slight taper.

    So far in my research, I have found the sponson tops should be parallel or slightly angled downwards 2 degrees or so. Any more you will have a hull that won't plane very well with pressure on the tops of the sponsons. If you have them tilted up then the hull will blow over.

    For now I don't plan on adding the fairing that covers the tubes. But that might not be thrown out in the future...

    [​IMG]
     
  13. lohring
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    lohring Junior Member

    Mike Bontoft has designed several record holding model boats. His gas hydroplane still is the fastest RC gasoline fueled boat with a 109+ mph two way average set in 2004. His gas sport and 3.5 cc outboard hydro records also sill stand. Below are pictures of the boats with some of the many sponson designs we have tested. There's no substitute for days of testing various designs using a radar gun or GPS. We found 2 more mph at around 105 mph with a design we didn't expect to work.

    Lohring Miller
     

    Attached Files:

  14. aaronhl
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    aaronhl Senior Member

    Yea those are some unique designs. I like how you said what you didn't expect to work- actually worked well.

    Since I will be designing heat racing boats/no necessarily straightaway speeds- What do you think the most important design features are? I know that is a very broad question..
     

  15. lohring
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    lohring Junior Member

    Since water is 800 times as dense as air, the drag of an object in water will be 800 times as great as the same object in water. Minimizing contact with water any way you can is the main design consideration. However, stability is critical. Wings in ground effect aren't stable without serious help. The propeller needs to run at a stable immersion level. The boat needs stable directional control. Balancing all these factors with the least amount of wetted surface is tough.

    As an example, we added 2 mph at 108 mph by cutting 1/8" (3 mm) off the rudder. Previously we added 6 mph at 100 mph with a thinner, steel rudder blade. When we tried the same trick on the outboard hydro, the boat became directionally unstable. I keep thinking that an air rudder would have less drag. The best example of this was Joerg's 140 mph electric hydro pictured below. There the water rudder was immersed at low speed and partially lifted out at full speed. The air rudders held the boat straight. The down side is that for the same effectiveness, an air rudder needs 800 times the area of a water rudder.

    There is a lot of room for new ideas. Most are going to be small refinements on the basic outrigger hydroplane design. Andy Brown has been refining your Eagle for years. Models don't seem to need the lift of a wing in ground effect design like full size hydroplanes. Our experiments with air traps on the rigger center section lifted the boat but weren't an improvement. We actually added spoilers to the wing on the sport hydro (see the first picture above) to kill the lift. This added 25 mph. It's all an ongoing experiment. The only way to know what will work is to test your ideas. If 1 in 10 works out, you're doing very well.

    Lohring Miller
     

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