Aerodynamics - thundercat racing inflatable

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

  1. Squidly-Diddly
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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.http://virtualskies.arc.nasa.gov/images/Spoilers.gif

    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. https://en.wikipedia.org/wiki/Roof_flap 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? http://www.speedhunters.com/wp-content/uploads/2010/10/5_V4A4_78.jpg Connect to an aerodynamic flap on the front that angles down when the Wheelie Fin feels pressure.
     
  2. Roflhat
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    Roflhat Thundercat - 70.10mph

    a diffuser is a part underneath a car, like in F1 - https://en.wikipedia.org/wiki/Diffuser_(automotive)

    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.
    [​IMG]
     
  4. Yellowjacket
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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.
     
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  5. Roflhat
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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
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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.
     

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  7. Montesafred
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    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
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    alex thomas New Member

    Hi could you email me at Lexprecisioneng@gmail.com
     
  9. Roflhat
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    Roflhat Thundercat - 70.10mph

    Finally getting somwhere with the CFD. Took a long time to get a solid model in Rhino which I could use. Cd, drag coefficient is around 0.438, now to try and reduce it.

    [​IMG]
    [​IMG]
     
  10. Roflhat
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    Roflhat Thundercat - 70.10mph

    I'm a litle worried about putting a cover over the boat, as both the copilot and pilot need to move around to balance the boat, there are no seats, you just sit on the floor and slide up and down to keep the boat level. The potential for getting caught under the cover if the boat goes upside down seems a little terrifying!

    I could probably extend the nose of the boat back if there's a benefit to that? Been modelling an extension of the engine cowling, struggling to get accurate drag coefficient numbers out though, just want to know if it's helping or not

    Would have thought it would help to round the front of the cowling and maybe extend the tail back? Can get moulding foam and make up carbon fibre pieces
    These photos are as close as I could get to identical sims
    [​IMG]
    [​IMG]
     
  11. CocoonCruisers
    Joined: Dec 2015
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    CocoonCruisers Junior Member

    I'd think you'll get more drag reduction from a fairing behind the engine.
    Something like the tail of a thick strut airfoil: Airfoil database search http://airfoiltools.com/search/index?MAirfoilSearchForm%5BtextSearch%5D=strut&MAirfoilSearchForm%5BmaxThickness%5D=&MAirfoilSearchForm%5BminThickness%5D=&MAirfoilSearchForm%5BmaxCamber%5D=&MAirfoilSearchForm%5BminCamber%5D=&MAirfoilSearchForm%5Bgrp%5D=&MAirfoilSearchForm%5Bsort%5D=1&yt0=Search

    A 3D visualisation with streamlines would probably be helpful.
    Does your CFD somehow take the ground effect into account ?
    (If it is simulated like an airplane high in the sky, your results will be wrong. And setting a 'zero' level doesn't necessarily mean your sim assumes a floor.)
     
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  12. Roflhat
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    Roflhat Thundercat - 70.10mph

    Thanks, so I'll probably buy some moulding foam and make up a tail section for the rear of the cowling like you say. Do you think it needs to come to a point like in those foils or can a "Kamm tail" be as good?

    I've set the ground as a "moving wall" so it's got the same velocity as the air flow, 70mph. But the boat is sitting clear out of the water in the simulations
     
  13. Jimboat
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    Jimboat Senior Member

    Looks like a fun study.
     
  14. CocoonCruisers
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    CocoonCruisers Junior Member

    Yes, and your outboard already has one when you look from above. But unknowns remain: which width for such speeds / where do you reach a point of diminishing returns ? Which length/curvature of the sides ?

    Huh that's fast, and don't exclude the leading edge fairing just because i suspected that the other one may be more effective :) To me it sounds a little masochistic to skip straight to the solution. Modelling sure is funnier than simulating but maybe you could start with thin plastic or ply sheets and a few bolts and ziplocks. It would allow to experiment with the form, including the transition between the top and the sides, and to figure out to which extend you should prolongate the fairing below the level of the motor mount . Stick telltales on it to see if you get an organised flow: from 1:04 in this video . You'll probably find that it depends a lot on your seating position.

    Sounds good but if your bottom moving wall is the lower edge of your picture, i'd think that the boat is twice as high as in normal flight. That would make an enormous difference for the air cushion below and probably to the wake behind as well. (You may have to cut off the outboard below the cav plate.)

    Good luck and stay (more or less) safe !
     
    Last edited: May 29, 2020

  15. Anders
    Joined: Jun 2020
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    Anders New Member

    Hi Roflhat,
    It is evidently a complicated topic you have started here, and many knowledgeable people have made good comments. I sail, but do not race Thundercats myself, and I am very interested in how you progress. My comments below focus on issues that seem not to have been mentioned before.

    It seems you're doing very well speed wise. If your photo shows the boat at its average draft at the speed it would appear you are almost fully supported by the aerodynamic lift.

    In respect of the aerodynamic drag you could consider removing the top of the nose cone and fibre glass it level with the deck plates.
    Fitting handles enabling both crew to assume racing bike style aerodynamic positions might also contribute to reduce drag.

    In respect of hydrodynamic drag the engine leg would seem to be the major cause of this. Shape and area would be the two things to look at. Removing the skeg would reduce the wetted surface, but obviously have other less desirable consequences. Maybe you already did that?

    In any case I suppose that at a certain total whetted area and speed the increase in hydro-drag will offset any gain from reduced aerodynamic drag, just like hydrofoils have a max speed depending on their area.

    I believe the guys working on the Sailrocket in the end had to modify the leading edges of the foils and struts from conventional rounded shapes to squared flat edges. This was against conventional theory. It might be relevant for outboard too and be worth looking into.

    You probably also know of Sharrow Marine's new propeller - that might be an option to get more speed.
     
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