Aerodynamics - thundercat racing inflatable

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

  1. Roflhat
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    Roflhat Thundercat - 70.10mph

    Hi,

    I'm keen to hear peoples thoughts on the aerodynamics of thundercat inflatables. I have one of these which I'm trying to go faster with. Currently at around 55 knots but I should be above 60 soon. It seems to me at this speed the aerodynamics would have a large impact on the top speed of the boat. Only around 10cm of the hull is in contact with the water. Would a diffuser placed behind the transom be beneficial? They become very unstable at these sort of speeds, the smallest gust of wind will blow them over backwards.
    Any ideas of how to improve the stability or performance? I was also thinking about extending the back of the top cowling on the outboard, to give it more of a teardrop shape like an aerofoil.

    Here's a couple photos, this one is to give an idea of the hull shape:
    saiba.co.za/images/profile5.jpg

    Here's mine on the trailer:
    [​IMG]

    And going at speed:
    [​IMG]

    Cheers
     
  2. messabout
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    messabout Senior Member

    The tween deck generates lift when it has a positive incidence angle. The pictures both show a positive angle. Think of that part of the boat as a wing. The lift generated at 55 is almost double the lift potential at 40. Blow overs in a boat of that sort are not unusual.

    Consider that if you personally survive a blow over, your engine will not survive. The connecting rods will be destroyed and it is likely that the crankshaft and pistons will become history.

    Slow down, live longer. Yeah, I know, that going fast is fun.
     
  3. Roflhat
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    Roflhat Thundercat - 70.10mph

    Thanks,
    I always wear a helmet now as I've recently gone from a modified 50hp to a 70hp, which I'm also going to modify.
    Yeh I can see how the deck acts as a wing, and the faster I go the less trim (angle of attack) I require to generate the same amount of lift. What I want to do is reduce aerodynamic drag.

    The engines usually survive flips, the cowlings often get smashed to pieces but the moving parts are usually ok. I don't plan on slowing down any time soon!
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You know you won't be happy till you flip one of these things, and I think anyone who advises you how to go faster, will only be hastening it.
     
  5. markdrela
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    markdrela Senior Member

    The real problem is not the lift. After all, airplanes generate lots of lift, but they don't "blow over".

    The real problem is aerodynamic pitch instability, which is when an increase in angle of attack produces a nose-up pitching moment which increases the angle further. On a boat at low speeds this is easily overcome by the opposing pitching moment from shift in buoyancy. i.e. as the boat pitches up, the buoyancy shifts aft to push the nose back down. But as you speed up, the unstable aero moments increase as V^2, while the stabilizing buoyancy moments stay more or less fixed. So an aerodynamically unstable boat WILL blow over at some threshold speed. No amount of pilot skill will prevent it.

    Sailrocket I found this the hard way. It was aerodynamically pitch-unstable, and sure enough it did an airborne backflip the first time it hit 45 knots or whatever. Sailrocket II was redesigned to be aerodynamically pitch stable, and showed no hints of blowing over at 67 knots.

    The main ways to obtain aero pitch stability, or at least weaken the instability are:

    1) Remove horizontal area up front. In your case I would cut away any deck that's in front of the cockpit.

    2) Add horizontal area in the back, as a T-tail or Pi-tail unit like on many hydroplanes. The vertical tail area will also improve aero yaw stability.

    3) Push the center of gravity as far forward as possible. Put ballast on the pontoon tips if possible.

    Sailrocket II did all three: The main horizontal strut was moved from front to back, and heavy pilot was moved from back to front.
     
  6. jimburden
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    jimburden Junior Member

    Make it a front engine front drive monohull

    The problem is weight distribution and high center of gravity compared to the outer shell surface area. The weight of boats should be ahead of the center of the hull with some kind of hydrofoil or planning step forward to support this weight. If the engine was placed horizontally as low as possible in the bow of one of the side hull shape of this boat, assuming it was not and inflatable and it was connected to a bow surface piercing prop in a bow step and the driver was sitting just behind the engine and fuel mass, inside a fully enclosed minimal frontal area and coefficient of drag canopy then this boat might do over 120 MPH with the same engine. You would have to gear up the prop or go with a larger diameter to reduce the prop wash losses to higher pitch to diameter rotation. The rudder could remain prominatly in the stern. A heavy low angle skeg could protect the prop from some floating object impacts. The bonus is This new boat at one fourth your boats frontal area and a tenth or less it's aerodynamic drag would take waves and winds like a fast deep ocean racer and not squirm about, bounce as much or flip over. The same self righting qualities that keep sailboats from turning over keeps power boats from turning over. Jim Burden, Lincoln Nebraska
     
    Last edited: Oct 14, 2015
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    How about you draw this 120mph machine for us Jim !
     
  8. Roflhat
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    Roflhat Thundercat - 70.10mph

    Hi Mark,

    thanks for the reply, I had a quick read of your background and work, very impressive. We had a look at sailrocket in our High performance sailing yachts class, I thought it was a fantastic piece of engineering.

    Interesting thought on adding a tail. Would the "T" part of the tail be completely flat at level trim? As in not creating lift? I'm trying to visualise where the tail would go, as far back as possible?

    I can certainly move some weight further forward, the fuel tank etc.
    I've noticed with the new engine and higher speeds I can go faster with another person onboard to act as ballast, the boat is much more stable.
     
  9. Roflhat
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    Roflhat Thundercat - 70.10mph

    Jim - Sounds like a very radical redesign of the entire hull! I'm not quite sure how it would look but I'd like to see.

    I should mention that this boat needs to perform in big waves, surf etc. as well as flat water. They're usually raced in the surf. They also turn completely flat as if on rails and can pull over 3G through the corners. I'd like to retain these properties if possible, rather than modify the boat so much that it can only be used in a straight line on flat water.
     
  10. tspeer
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    tspeer Senior Member

    I agree, but there's more to it than the aerodynamics. At Boeing, we learned the hard way about unstable pitch-heave coupling on the DarkStar UAV, which turned out to have unstable ground dynamics on takeoff when the nose gear was off the ground and the main gear were in contact with the ground.

    The unstable pitch-heave coupling is driven by the contact with the water, which is predominately at the stern, as well as the aerodynamics. Think of it in terms of heave stiffness - how much additional vertical force is generated (or lost) with a change in immersion depth. The aerodynamic heave stiffness is very weak, as it's mainly driven by pitch attitude (angle of attack) and changes comparatively slowly with a change in altitude. But the planing surface of the hull has very high heave stiffness. It goes from supporting the boat to zero in just the change in depth of the V of the bottom.

    Consider the boat running in an equilibrium condition with the boat largely supported by the hydrodynamic lift and partly by the aerodynamic lift. If it gets a bow-up disturbance, this will increase the both the aerodynamic and planing lift, and the boat will accelerate upwards. As it moves up, the planing lift starts to drop off. The planing lift is located near the stern, so as the support is lost there, the boat wants to pitch up more. Which increases the aerodynamic lift and makes the boat rise faster. And pitch up more. The boat basically wants to pivot about the patch of hull planing on the water. This is the source of the unstable coupling between the aerodynamics and the hydrodynamics.

    There are several possible ways to solve this problem. One way to attack the pitching moment from the aerodynamics. This can be done by reducing the horizontal planform area, but that's hard to do because the boat already has about the minimum area necessary to support the pilot and engine. Another way would be to add aerodynamic area behind the transom. An efficient wing aft, especially if its span was substantially larger than the beam of the boat, might pick up lift faster than the bridge deck and hulls as the pitch angle changed, and reduce (or eliminate) the bow-up pitching moment.

    Another way to attack the problem is through the hydrodynamics. It sounds like an unnatural act, but if the boat were planing on the bow instead of the stern, it would have stable pitch-heave coupling instead of being unstable. This is the solution adopted by Sailrocket, when they moved the pilot from the stern to the bow and placed a planing hull forward. The aerodynamics lifted the stern completely out of the water and it planed on the forward surface. As it moved in heave, it pivoted about a point that was forward and pitched down, reducing the aerodynamic lift and bringing it back toward equilibrium. Having the engine at the stern makes this solution problematic because lifting the stern will bring the prop out of the water, too.

    So solving the unstable pitch-heave coupling has a lot of practical obstacles to overcome. But if you're hunting elephants, you have to go where the elephants are.
     
  11. Roflhat
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    Roflhat Thundercat - 70.10mph

    Thanks for the reply Tom, a lot of good reading there.

    I think I understand most of what you are saying. It looks like the best solution would be to increase the aerodynamic area behind the transom (behind where the boat is in contact with the water) so that as the bow rises the air hitting this part would push the bow down.

    Changing the hydrodynamic properties of the boat would be very challenging I think.

    Would changing the shape of the engine cowl help any? In terms of speed or stability. It has a relatively flat frontal area, which I think creates a moment (although fairly small) which causes the bow to rise. Would there be any benefit to elongating the rear of the cowling? I was looking at this image here and thinking the same could be applied to the cowling:
    [​IMG]
     
  12. whitepointer23

    whitepointer23 Previous Member

    See if you can find a used outboard racing hydroplane with no motor. I don't know if they race boats where you are but here used hulls come up for sale quite often and cheap because of their limited application.
     
  13. markdrela
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    markdrela Senior Member

    OK. But if the boat has aerodynamic pitch stability, then it will not pitch up uncontrollably after contact with the water is lost. That's what I was driving at. If it's aero-stable, then the chance of a blow-over is greatly reduced or even eliminated. Clearly, aero stability is desirable if not getting killed is high on the priority list.

    The planing step on a typical hydroplane seems to be relatively far forward on the pontoons, which indicates the CG is also well forward which is good for aero stability.

    The layout of the boat we're talking about here seems to have the planing surface and hence the CG quite far back. So getting true aero pitch stability may not be feasible, at least not without a huge rear horizontal tail mounted well behind the transom.
     
  14. markdrela
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    markdrela Senior Member

    The farther back the horizontal tail is mounted, the more effective it is.
    I'm picturing a "Pi tail", with two highly-swept vertical tails, one on each pontoon. The horizontal tail spans across the tops of the verticals, and extends out on each side if necessary. Much like what you see on many hydroplanes.

    You definitely don't want it to carry significant lift. But you probably want its angle to be dock-adjustable by a jackscrew of some sort. That way you can bias the boat's high-speed pitch orientation.

    BTW, the profile drag of an unloaded tail with a good airfoil will be miniscule compared to the drag of the rest of the boat. So adding tail drag is the last concern here. If anything the tail will reduce boat drag since it should reduce pitch oscillations at high speed.
     

  15. jimburden
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    jimburden Junior Member

    Imagine the smallest of the jet skis or like one of your boats pontoons. engine in bottom of the bow, followed by the prop in a slot, followed by the fuel tank, followed by the operator fully enclosed. The goal is to keep the weight low in the hull for self righting. The same ultra deep vees on the sponsons of your boat would slice through the higher waves instead of going air born, You would llean into ti=urns like a racing motor cycle. 3 G turns might be a little much accept on very flat water but possible like ice racing motor cycles. the bow should have small hydrofoils to limit plunge depths through waves since now most of the weight would be forwrd of the hulls side and top area.
     
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