Airboat with Horizontal Fan

Discussion in 'Projects & Proposals' started by tom kane, Nov 2, 2010.

  1. gonzo
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    gonzo Senior Member

    Basically you are describing a very inefficient hovercraft. I see no advantages or improvements.
     
  2. Yellowjacket
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    Yellowjacket Senior Member

    Sorry, but mounting the fan horizontally is not efficient and holds no benefits over a conventional fan. In fact it has substantially higher losses when compared to a normally mounted fan. Hovercraft using horizontal fans and then dumping some of the fan output aft to create thrust have been found to be terribly inefficient. The reason is that you are accelerating the air, dumping it into a plenum and taking a penatly of slowing the air down to low speed and then accelerating it back out the back of the craft.

    You need to think of it in terms of propulsive efficiency. The less you turn the air the more efficient you propulsion system is going to be. You are taking air that is moving across the top of the fan, turning it 90 degrees and accelerating it downward, and then turning it aft to creat thrust. Basically you are doing two 90 degree turns with the airflow and that will result in much higher losses than if you use a horizontal shaft and a prop to simply accelerate the air aft.

    Moreover, the fan blades are moving at different air speeds, which hurts the efficiency of the system. On one side of the hull the blade is moving faster because the blade speed is added to the hull speed, and on the other side the blade is moving downwind and sees a lower airspeed. This is a well known and understood aspect of aircraft using lift fans and in helicopters.

    Models are fine, but one aspect of models is that it is relatively easy to provide much more power than can be provided in a full size system.
     
  3. tom kane
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    tom kane Senior Member

    My understanding of how a fan works. The blades displace air, (in this case down) and the 14 lbs + of atmosphere above rush in to replace the displaced air.no suction is involved. It is the same as sucking on a straw.You displace air by sucking and atmospheric pressure pushes fluid up the straw. As for efficiency I have yet to see an efficient hovercraft, cushion craft. I would be happy with a 2.5 liter diesel motor pushing my 2 ton BOAT around in shallow water and perhaps a small outboard in deep water.
    The craft in the last image I would be happy with the same motor draging that craft around,under strict control.I don`t want to apply for a pilots license so it is ground effect vehicles for me.
     
  4. Yellowjacket
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    Yellowjacket Senior Member

    You need to change your reference system, try looking at it this way.

    The thrust gained by any fan or jet is a direct function of the change in velocity of the air multiplied by the mass flow, or simply put the momentum change across the fan or jet engine.

    Since the boat is moving forward, the air around the boat is moving relative to the boat, and therefore has momentum. If you do not caputure this momentum, you have to accelerate the air from zero speed to the speed of the exit air, and that takes more energy than it does to accelerate the air a small mount across a conventional fan that is facing the airflow.

    If you are talking about very low speeds, it doesn't matter that much, but a fan drive system is very inefficient at low speeds. This is why you don't see low speed air boats, unless it is a very specialized application where you are operating over a mud flat. Otherwise, a propeller in the water is far more efficient than a propeller operating in the air. Do a search on propulsive efficiency of propellers and you will see that at very low speeds propellers operating in air are simply not efficient.
     
  5. tom kane
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    tom kane Senior Member

    The most important change to making this model (appear) more efficient would be to use a light weight more powerful gas engine to eliminate the massive battery weight. Which I used 30 years ago and it went well and fast. I don`t think Yellow jacket and myself are saying anything much different but just in a different way. Yes I am wanting a specialised shallow water propulsion. I would use a water propeller (outboard) in deep water. I know that a air propeller pushing a boat is not very efficient but the only way to push a boat across shallows (provided the wind is not to strong). I am sure that accelerating stationary air is more efficient. We don`t see low speed airboats because most people want to go fast. I understand that propellers lose efficiency quickly running faster than 2000 RPM. Some people my like to ride a John Deere air boat. www.bleedinggreen.com
     
  6. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    Changing the fan on the model takes seconds and the very obvious result is stability especially turning,even though the battery in the bottom of the hull would be the equivilent to 2 or three v/8 engines. Turning shap moving fast would roll the boat over, come to think about that I do not recall seeing real airboats copy-cat jet skis. They seem to always go streight ahead. There was an obvious increase in fan noise and no real gain in speed,if you can call a fast walking pace speed.
    The real air boat shown with wheels struggles to do anything, probably because they wanted to make it look conventional.
     

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  7. mydauphin
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    mydauphin Senior Member

    A lot of the experimenting you are doing is pointless. The reason is air pressure, lift, etc... do not scale linearly. For example a 10 times or 100 times sized bumble bee would never fly or a shrunk bird would fly differently. By the time you scale the project 100 fold, all kinds of resistance numbers would go off the scale. The weight of the powerplant and boat is too heavy to be lift by the puny fan.

    OK so you don't believe me, try this. Get you little boat and tie a ballon with rear facing tube to exhaust the air. Watch it fly across pond. Make a boat just 4 feet in size and fine yourself a bigger ballon, it will barely move it. Same problem above.

    You need more horsepower as you get bigger when using air.
     
  8. tom kane
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    tom kane Senior Member

    There has been a lot of vehicles for years using ducted fans and props and there is lots of research documents and perhaps all of this has been a waste of time too. Efficiency is a measure of what you get out for what you put in. If you want to make a brick wall fly just put in lots of power. If you want a quiet safe cheap to run vehicle big power is not a necessary inclusion. You are not reading the thread properly we are not asking the suggested design to fly. There is lots of research going on all the time particularly regarding air propulsion for instance stealth craft air intakes,all have different theories and all actually work to some degree.
     
  9. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    I am sure you can give a better explanation than that gonzo I know that you are a WOT man.

    What I have re- confirmed re-visiting this experiment.
    There are very few if any airboats using horizontal fans.
    A horizontal fan would help make an airboat quieter,safer,more stable manouvering and allow a more user friendly design and reduce the possibility of torque turning the airboat over.I hate wind and rain.

    Efficiency is not to much of a concern to me in this case because the running cost of a small diesel would be small. I have always enjoyed drifting over mud and sandbars somtimes hoisting a small sail to save fuel going the long way.
     
  10. gonzo
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    gonzo Senior Member

    Now, a sailing hovercraft would be a strange animal.
     
  11. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    The airboat in the image is more toward my comfort zone,but it does not appear too efficient. I would still like a horizontal fan incorporated in the design. Airboats are rare in New Zealand probably because they appear to old fashioned and uncomfortable and noisy and operatig one would get you into no end of conflict with neighbours and Environment groups. Helocopters,jet boats,jet-skis, outboard.s do the job for most Kiwi.
     
  12. latestarter
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    latestarter Senior Member

    It just struck me that your original design is akin to an upside down jet boat.
    The air is drawn in from above and shot out the back whilst a jet boat takes in water from underneath and is shot out the back.
     
  13. tom kane
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    tom kane Senior Member

    Airboat_Experiment

    Well... I did try that concept to take a long line out.Built into an old surf board,it was safe and good. Here is another airboat design which defies convention.
     

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  14. mydauphin
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    mydauphin Senior Member

    Why is the intake of a jet engine in front of the plane and not the top? Airboats are all very inefficient, that is why they have 300 hp engines on a tiny boat. They only get somewhat more efficient because they get up on plane easily because of their flat bottom and light weight. But speed vs hp, they would lose to a propeller in the water boat. If you make air move 90 degrees it makes it even more inefficient. I saw a flat bottom hovercraft boat with a fan pointing down and back in a 45 degree angle providing lift and propulsion, but I think it was design more like hovercraft than an airboat.
     

  15. tom kane
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    tom kane Senior Member

    some Stealth air intakes are at the top and are controversial at the moment. Engines are buried in the hull. I would use a water propeller where the water was deep enough ( it would need to be at least 2 feet deep) A horizontal ducted air propeller would not change air direction twice,90 degree in 90 degree out.
    Trouble with airflow across the horizontal may start above 50 MPH. www.fas.org/spp/aircraft/part06.htm
     
    Last edited: Nov 9, 2010
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