Compressed air powered boat

  1. I saw on myth busters where they took compressed air tanks an attached them to a boat and then opened the valve and then the boat started to move. They did 2 trials, one where the exhaust nozzle was pointed above the water and one where it was pointed in the water.
    Then one that was pointed above the water went faster. Is this because the air can leave at a higher speed so there is more momentum transfer?
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,261
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    Yeah, it's about conservation of momentum - the point of the test was to demonstrate that rockets don't have to "push against" anything to move. (A popular misconception used to be that the Apollo missions could not succeed because the rocket would have no air to push against in space.)

    The compressed-air rocket would work even better in a vacuum - in air, the speed of the exhaust depends on the difference in pressure between the inside and outside of the cylinder.
  4. interesting, so would a jet boat work better if the water jet was shot above the water into the air.
  5. No, because the whole idea is to do with pressure, and water can't be pressurized, as it is a liquid.
  6. Simon Bridge

    Simon Bridge 15,261
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    Water can be pressurized - it just cannot be compressed.
    You'll have heard how the water pressure increases with depth in oceans right?

    A jet boat collects water from below water-level (well doh!) so it would require energy to lift it above water-level to shoot it into the air.
    So there is a balance between the back-pressure from the surrounding water and the loss due to lifting it a few feet.
    In fact, the balance means best thrust comes from ejecting the water horizontally just above the waterline, and this is how many modern jet-boats are built.
    It's not normally obvious because of the wake, but I used to ride one every day :)
    Last edited: Jan 5, 2012
  7. I see. Thanks for correcting me.
  8. ok so is it better to shoot the jet just above the water level.
    Ok this might sound crazy, but what if we had a water tank in the boat, so it wouldn't have to lift it against the gravitational field. And then we have a helicopter flying above draining water into the tank so it doesn't have the added weight. Obviously it takes fuel to run the helicopter but I just want to see if the boat will work better this way, and the boat ejects the water jet above the surface of the water, will this work better than if the jet pushed against the water, or am I crazy.

    Tow the boat with the helicopter and forget about it.

    Unless you're paying. Then I propose we build and test this.
  10. Wouldn't back pressure at the nozzle also be bad?

    Are you discussing the inlet or the outlet?
  11. Simon Bridge

    Simon Bridge 15,261
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    I was talking about the jet outlet - yes, bearing in mind I am not a jet-boat engineer, the back-pressure at the immersed nozzle would be the same for a water jet as the air jet in the original example.

    The back-pressure depends on depth - but in a boat we are close to the surface anyway.

    Water is also lots heavier than air, so it takes more energy to lift it.
    As mentioned, the jet-boat lifts water to just above water level - it also has a very shallow draft so that is not far (and it can hydroplane). Lifting any higher would use more energy for no gain in terms of reducing the back pressure.

    You could also mount a water tank on the deck of the boat and pressurize it - say, boil the water, drive a plunger into it, pump compressed air into it, whatever.
    Then you have a water-rocket and the physics/engineering simplifies quite a lot.

    One could use a helecopter to counter the added weight and so do do a fair comparison ... but you could also just add sandbags to the other boat ;)

    The Mythbuster's compressed air boat would also have worked better if they had some water in the cylinder ... ever built a water rocket? If you haven't, you should! Add too much water and you cannot build up the pressure enough before it pops, and too little and you don't have enough reaction mass. Nice investigation into Newton's Third Law.

    You could repeat the experiments by powering a model boat with a water-rocket. You can try three options: tossing the water in the air, horizontal, and with the nozzle submerged.
  12. It is slightly more complex than attaching a water rocket to a model boat due to the need to keep the output nozzle horizontal. The boat will need to be trimmed (attitude adjusted) as the speed changes and the water is used. Otherwise the experiment is no good.

    With waterjets on full power you might not need to lift the water from the intake if you plane or trim the bow up. Having it designed to do this means performance at low speed will be reduced, but this sort of boat only has (judging by the owners) two speeds: berthed and full ahead.

    edit: I should add most waterjets have steerable nozzles.
    Last edited: Jan 6, 2012
  13. Simon Bridge

    Simon Bridge 15,261
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    yeah yeah yeah ... good thing the Wright bros hadn't met you eh?
    These are all solvable engineering problems - details of the experimental design.
    I didn't want to go into details because (a) that would be boring and (b) working this out is part of the fun of doing experiments.
    Technically true but not needed - the reaction mass is ejected from the rocket in less than a second - you just have to watch the center of mass ... the effects you are talking about are too small to notice against the overall statistical uncertainties. I know this because I have done the experiment... have you? (I used a curved length of stiff plastic pipe to redirect the thrust.)

    Which is how the smaller boats are designed :)
    Note - the bigger the tilt at full speed, the higher you have to lift the water at low speed. If you google for jet boat engines you'll see the typical perpendicular separation between intake and outlet. The really fast ones have the intake tilted slightly aft at rest so it's pointing in the right direction while planing.

    I found a pic showing the jets in operation:

    This is the boat I used to ride all the time:
    ... in motion The Jet Raider nozzles are slight...ed to [i]surf[/i] the waves into the harbour.
    Last edited: Jan 6, 2012
  14. I've never seen a waterjet yacht underway on the open sea that wasn't planing. I guess ferries might not as it upsets the old ladies. Never really look at the ferries.

    With waterjets you have to trim. I concede that with a rocket it might not be necessary. I was thinking about jets and copying your text (that's my excuse and I'm sticking to it).

    Stuff on boats breaks all the time. You just get used to it. Regarding plant matter, you can fit a grill over the inlet and all newer jets should be able to reverse the pump. If all else fails there will be an inspection hatch in the inboard part of the unit. The best solution is not to use terrible ports but I guess ferries don't have a choice in that regard.

    BTW am considering your photo and she should be able to plane at her length (as you know, it planes when the bow wave length exceeds the hull length. That might not be technically correct as I sail better than I do maths, but it's the concept of it)

    edit: she does seem slightly bow up in the video but isn't planing

    edit: boats that length can plane on screws. is she speed limited or did they go for jets due to requiring shallow draft? that might fit will with your experiences of plant crap in the intakes. You get a lot of stuff floating about in shallow water.
    Last edited: Jan 7, 2012
  15. Simon Bridge

    Simon Bridge 15,261
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    The Jet Raider was out of action far more often than the screw ferries that did the same route and suffered a lot from engine overheating ... more, I don't know. I only rode the thing.

    iirc: What usually clogged the intakes was plastic bags.

    You are right, it didn't plane any more than a regular boat - but more than the cats on the same route. She could go faster than in the video ... and very manoeverable. I've not seen her plane more than 15-20 degrees (from memory - if I squint: the inside had airline-type seats), but I have been a passenger when she leaves the water at the top of the crests.

    I guess there's a reason ferries tend not to be jetboats. I don't know what the rationale was to run her in Auckland Harbour but when new was the fastest boat doing the Auckland-Waiheke Island run.

    Most recently there was a fire.

    iirc there are slow jet ferries doing the whale-watching and dolphin-watching runs too - deep water but screws damage the animals.

    The smaller boats on the rivers around NZ seem to do just fine, and, of course, with NZs shallow, shoaley, rivers, they are the perfect craft (they were invented for these rivers so that's not really all that surprising.)
    Shotover River tour footage ... people have died on that tour BTW.

    But I think we are digressing very much here.
    The fact that you get better thrust by lifting the jet out of the water tends to puzzle people not familiar with Newton's Third Law. Even students who can apply the correct math tend to have the wrong feel for real life situations ... you'll know this effect from the other way around ;) ... which is why we like to give students experiments for these things.

    Boat stuff makes for great open-ended project work because there are so many dependent variables to control and lots of balancing and trade-offs... and everyone thinks they understand boats ;)
    Last edited: Jan 7, 2012
  16. > everyone knows about boats

    Maybe in NZ which (I am told) has a sailing tradition. Lifeguards are overworked here. :-)

    On to Physics:

    I can't math so please feel free to correct.

    I understand the mechanics of this device as Newton 2:

    Thrust (F) = mass of water moved / time * (jet exit speed - boat speed).

    This is effectively change in velocity of water in the tunnel over time. Which being change in velocity over time is an acceleration. Therefore F = ma (ultimately; the original equation is probably(?) more useful).

    Being underwater should affect you by resistance to accelerating the exit jet due to the difficulty of compressing(?) water. Plus there's probably something to do with cavitation and sucking in your own exhaust.

    But, what I don't understand is nozzle output cross sectional area.

    Because of (jet speed - boat speed) you want a smaller nozzle giving higher jet speed, which would allow for a higher top speed. However, I (incorrectly?) see the mass of water moved should be inversely proportional to the change in nozzle cross section, for the same power input, which would imply no net change in momentum. This sounds odd to me for two reasons. (a) KE having the square of velocity and (b) I know larger diameter jets with lower exhaust velocity are used on boats requiring torque or manoeuverability at low speed.
    Last edited: Jan 7, 2012

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