Aerodynamics - thundercat racing inflatable

Discussion in 'Hydrodynamics and Aerodynamics' started by Roflhat, Oct 13, 2015.

  1. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    WTF is a 'diffuser'? Is that what they call a 'spoiler' in Scotland?

    IIRC those are on the rear of kid's cars to (supposedly) prevent the REAR from losing traction due to aerodynamic lift at higher speeds, because cars are shaped like that to hold people and still be streamlined.

    Notice they aren't at the very rear on airplane wings, but about in the middle.

    I'm pretty sure that straight part at the bow between the tubes is already doing a great job creating lift destroying turbulence, the problem is the big flat center underneath traps air between the tubes, and does so even more when it tips up.

    Well, I don't think you guys every go into flat spins, or if you do you got other probs. But to me the obvious thing would be to vent the center. It doesn't do any actual "wave breaking" at speed, right?

    I could see how a solid bottom and 1/2 meter of rise at bow and transom would make the boat able to act like a barge and carry huge loads in shallow water etc but not needed for two guys wearing helmets in a race.

    How about Wheelie FINS? Connect to an aerodynamic flap on the front that angles down when the Wheelie Fin feels pressure.
  2. Roflhat
    Joined: Sep 2015
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    Roflhat Thundercat - 70.10mph

    a diffuser is a part underneath a car, like in F1 -

    If I cut any holes or vents in the deck waves will definitely go through, so unfortunately that just isn't a viable option.

    The problem is the entire lifting surface is ahead of the centre of gravity, so as the boat starts to lift there is no moment counteracting that. Adding a large rear wing with 0 angle of attack whilst travelling at normal speed and angle would then act as a counter moment, which could stop the boat flipping over backwards.

    I have considered wheelie fins, like wheelie bars on the back of a drag car, which I could deploy from the transom to prevent the boat flipping. Then when I'm using the boat in rough weather or surf I could either tilt them upwards out of the way or remove them completely. May be a possible solution...

    Your last link isn't working Squidly-Diddly ?
  3. Roflhat
    Joined: Sep 2015
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    Roflhat Thundercat - 70.10mph

    Bit of an update. I was down at Coniston last November at records week, managed 70.10mph average over two runs, max speed of 74.9mph.
    Super happy with the boat, it's very stable even at that speed. Going again this year, aiming for 80mph. Always open to advice for more speed, still looking at aerodynamics. Been trying photogrammetry to get a 3D file of my boat with myself and crew in it but it's not working well with the black hull.
  4. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    At the speeds you're reaching aero drag is starting to be the biggest thing you can do to increase the speed. As someone noted getting the driver, co-driver and motor all in one line behind a fairing would be a big improvement. This would also reduce the height of the drag resultant. Remember that your thrust line is down at the surface, and the drag is almost 2 feet up.. This results in a positive pitching moment and reducing the drag up high will help keep the nose down. Also you could think about a treatment to the leading edge that would spoil lift if the nose rises. If you look at some aircraft (like the Beechcraft Bonanza) there's a small triangular spoiler on the leading edge near the blade root. Also google ice buildup on wing leading edges. Such shape have normal lift at low angles of attack, but stall very easily at even at moderate angles of attack. On the Bonanza the purpose of the strips is to make the wing root stall while the wing tips are still unstalled and the alerions are still working. This provides the pilot control even when the wing is losing lift. You might want to do something similar. That is, you might put a strip in the middle so that as the boat starts to pitch up it stalls near the center of the hull and this dumps lift. Probably not good to have the strips outboard because if one side stalls and the other doesn't then you'd have asymmetric lift and that would cause a barrel roll.
  5. Roflhat
    Joined: Sep 2015
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    Roflhat Thundercat - 70.10mph

    Sadly it's not really possible to get the co driver and driver in line, the engine is tiller controlled but also we need to sit side by side to keep as much weight aft as possible. In the fastest run the copilot was tucked right in, so his head was below the level of the tubes in that photo. I was thinking of adding an aero section to the helmets, a bit like the olympic cycling style. I'm going to remove the visor from my own helmet too.
    Also getting a new boat, very similar but a couple of small changes. I've asked for no straps etc. in front of the nose of the boat, which should clean up the airflow going over the bow of the boat. But still thinking of adding a fairing like you say, but I'd really like to get a 3D file of the boat with crew so I can test different designs before going to the effort of producing one.
    Also thinking of adding a section behind the engine cowling, to join up the air more smoothly, it's just a flat section at the moment.

    Thanks for the info on the stall strips, I'm going to have to do some reading up on them, never heard of them before
  6. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    Hi Seaumus, I presume. Buoyancy plays no role whatsoever (responding late to a post), it's all water and air lift. Airplanes don't blow over because they try to place the center of pressure at the center of mass. That can't always be done so they use tail flaps to stabilize the plane. Blowing over is always a question of where is the cp relative to the cm. With a given boat you can only move weight forward to try to get an adequate balance. We do this all the time racing tunnels, where the goal is to drag as little sponson as possible underwater but at an angle of attack where the bow doesn't start lifting suddenly in wind gusts or waves. A three degree attack angle (between tunnel bottom and water horizon) is about maximum that will work. So the boat must have enough air and water lift to get the sponsons relatively dry without using much trim on the motor. For adequate water lift on the sponsons lift strakes are necessary.

    Attached Files:

  7. Montesafred
    Joined: Feb 2019
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    Location: Perth, Western Australia

    Montesafred New Member

    Roflhat, I have been pondering this problem for a while and recently installed a permatrim aluminium plate "hydrofoil" on the anti-cav plate on my thundercat. I tried a Doelfin branded foil on another boat a few years ago and although holeshot improved,top speed was clipped. What I figured is that the key is to jack the motor sufficiently to keep the plate just above the water surface at speed,mine is around 40 - 50mm, but not too high to allow the trailing edge to touch and correct the pitching motion prior to backward flip. It worked excellently and allowed me to do full revs with only 1 man in the boat on a lightly modified tohatsu m50d and 15p Yamaha bunny ear surf prop. I also went out solo in the open ocean in 2m swell for a play and the seakeepung is much better too. I think this combo will also provide much faster holeshot with a bigger prop and eliminate the Cavitation. I am pondering whether I also get a foil for my bigger 70c Yamaha firebreather?

  8. alex thomas
    Joined: Jul 2019
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    alex thomas New Member

    Hi could you email me at
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