small jet-like electric drive

Discussion in 'Jet Drives' started by DrDistorzion, Aug 17, 2009.

  1. DrDistorzion
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    DrDistorzion New Member

    Hi guys! (my first post on this forum :)

    I'm planning to build a small toy-like foam boat for my little daughter. I'ts going to be electrically powered and as I need it to be very safe, I don't want any exposed prop so I'm thinking of building something that resembles common jet-drives with steerable exhaustduct but for low power ofcaurce.

    I'm new to this forum so I don't know if anyone has pulled of something similar so I'm really looking forward to hear if you guys have any ideas for this project. If you know of any good site with fundamental calculations for this kind of drives I would love to know, as I haven't built anything similar before. (I work with electric motors though, so that's what I know).

    To give you a hint of the kind of powersource I intend to use is 24VDC 0,5-1Nm nominal (ca 300W) and between 2000-3000rpm, so I need to know how to design an suitable impeller for this. Ofcaurce I have a lot of different motors to choose from if you think I should go with other specs.

    The kind of speed I'm looking for is no faster than the fastest an avrage adult can swim as this is a toy indended for children at an age from 3-4 years old.

    The boat would look like a waverunner and be about 1,2-1,5 meters long.
  2. marshmat
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    marshmat Senior Member

    Hi Dr.D, welcome aboard :)

    300 watts is not much power; you're in the range of human-powered vessels (ie, pedal boats) here. At the speeds you might be able to achieve with such a small motor, a jet pump of the type used on PWCs will be extremely inefficient.

    If safety is a big concern (as it ought to be), given your requirements, I'm not sure a waverunner-like style is ideal. It is very easy to fall off such a craft, and man-overboard is certainly one of the biggest causes of boating injuries. On the other hand, being cut by a propeller is an extremely rare occurrence, pretty much exclusive to manatees, Stock Outboard racers, and that small contingent of ****** who don't think to shut the engine off when swimming. The main reason PWCs have jet drives is- you guessed it- because it's so easy to fall off of them. Take away the falling overboard bit, and the jet's safety advantage is irrelevant.

    You need weight carrying capacity for the batteries, motor and controller, as well as your daughter. And you probably don't want her to outgrow it too quickly.

    Why not look at a longer, simpler boat, something like a canoe with outriggers? Rick Willoughby (member on here, see ) has a website with a few such boats, mostly pedal-powered, that might be inspiring. Add some Hobie-like trampoline netting and you'd have a pretty hard time falling overboard by accident.

    Finally, although you may disagree, I maintain that 3-4 years old is too young to take command of a powered vessel, no matter how small the engine. A hand-crank sidewheeler, or a rowing design, or a pedal-powered boat, is probably a better bet for such a young kid. A simple dinghy to play with will be more than enough to get her excited about the water for the moment, and if (as most do) she falls in love with it, a more complex electric/pedal boat could be a wonderful father-daughter project for a couple more years down the road.
  3. DrDistorzion
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    DrDistorzion New Member

    Thanks for the reply. :)

    I understand your intervensions. Kid's in that age are so very different, my friend's little boy of 4yo is already driving radio controlled cars very patiently and with very good control, so he would have no problem controlling the kind of boat I'm thinking of. At the same time, another child in the same age that I know can't manage that at all so It's just wait and see how my daughter handles things like that, but ofcaurce I haveto design it so that it can be used when she's older if it takes longer before she can start using it. I'm going to put my radio controll on it at first. :)

    I got this idea when we visited an adventure bathing facility where they have small toy-foam-boats that my daughter loved sitting on and beeing driven around. :) So atleast I know she likes boats. :)

    But cards on the table, I must admit that I'm probably more excited about this idea than my daughter will be. :p

    of cource a real waverunner-style is easy to fall of, so i'm thinking of making it lower and wider, but as children in that age often are used to sitting on toy cars holding on to steering wheel or bicycle steering I thought that they should have easier relating to this concept and no matter how it is designed, parents must ofcource attend constantly when it's used and i shallow waters.

    Other than that it's allso mostly a construction factor because I don't want to build a conventional hull.

    Thanks for the link! I really liked the yellow 3-hull design!
  4. DrDistorzion
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    DrDistorzion New Member

    I forgott to add why I thought of jet-drive, I had one more reason.

    What rpm's do most jet-drives operate at?

    I thought that 2000prm must be very much too high for a conventional prop design for this project and to avoid having to build it with a reduction gearbox I thought of making a pump-like design. And as you said, I'm in the power-range of human powered vessels so that's exactly where I want to be.

    One more reason for this drive was to achive a more fun stearing with the more direct control of thrust vectoring. But ofcource I understnad that a conventional rudder behind a prop will work just fine.
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Of course I concur with Matt that 3-4 years is too young. You'll never forgive yourself if something bad happens.
    But if you daughter has that age now, I would say start drawing designing, building, experimenting. By the time you are ready she will have the proper age for your boat.
  6. jim lee
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    jim lee Senior Member

    A lot of bath toys use a centrifugal pump as a simple jet drive.

    How about just enlarging this idea and use an off the shelf electric bilge pump?


    Just a thought..

    -jim lee
  7. marshmat
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    marshmat Senior Member

    Good point Jim. A Rule 8000 puts out a three-inch-thick jet that could probably serve as a bow thruster on some boats....

    Dr.D, a PWC style hull and pump just won't work very well on low power (ever seen one try to get through a marina's no-wake zone?). And a jet's vectored thrust steering only works at higher power settings- release the throttle, and they just go straight, regardless of where you point the handlebars. But if you think longer, narrower and two- or three-hulled, you can probably come up with something more stable, easier to swim or watch wildlife from, that will do well on either your 300 watt motor or a couple of pedals on a bike crank.

    Whether you use electric, pedal or both, there's no need for a gearbox- at these power levels, a simple V belt on pulleys will work just fine.
  8. apex1

    apex1 Guest

    Mine played on micro bikes at the age of 4. I will never forgive my wife that she was unaware the risk!
    A sweet girl with a heavy scarf is not what we want. And scarfs can be at their innocent mind as well as on their legs. My daughter had it just for a year or two (at the leg), but I have it forever.
    Let her enjoy boating in a safe and reliable vessel, pump or pull, or sail or what so ever, but do not make your phantasy come true for your childs toy, please. It is not the same as a model train.
    In all honesty and decently, my two cents.

    the left one..................................

    Attached Files:

  9. aboyd
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    aboyd Junior Member

    I love the idea of you allowing your daughter to take the reins of a such a project. The only thing I would make sure of is having some sort of dead'man kill switch. Wether it's a tether to her wrist or life jacket hwich is the preferable one because the cord doesn't get in the way.Or like the riding lawn mower type pressure switch that as soon as her weight is of the seat engines dead. I take it that you are into radio controlled stuff so why not incorporate some sort of over ride to be able to shut it down or better yet over ride the controlls. Kudo's to you, my daughter's were rippin around on my 110 horsepower snowmobile at a very young age as well. Granted they were 2 years older but with the right understanding and guidence anything can be achieved. Especially with girls they seem to be a little more aprehensive to givin her all she's got untill they are totally comfortable with the situation. Just remember safety is a Key factor. Why not trying a pair of the bilge pumps on on each side of keel and controlling the vector of the out put nozzle. They are cheap water proof and available just about anywhere..
  10. Guest625101138

    Guest625101138 Previous Member

    The attached shows what a shrouded 100mm prop would achieve with 300W and 2m/s. The 93N force is quite useful.

    To get the 2m/s you would need maybe 6ft long cat hulls or 7-8ft stabilised monohull but this is a rough approximation. It will depend on the weight. If you want a full hull design you need to make a weight estimate.

    The prop is shrouded rather than a jet. You could vector the output but I am not sure if there is value in lifting the water to jet in air. The exit velocity is around 5m/s. It is more than a trickle but not a full blown jet. The higher the exit velocity the lower the efficiency.

    The pitch for 2000rpm is 125mm. You can probably find a 4X4 prop off the shelf that will do. Maybe start with a 5X5 and trim perimeter so the blades sit neatly in the shroud.

    At least a weekends work to put something like this together.

    This shows the sort of performance you could expect:
    The little ICE can produce more than 300W but then your load is much lighter.

    Using something like 100mm PVC drain pipe would make a reasonable shroud. Losses in a short length of pipe would not be much. A 45 degree bend from the bottom of the hull to the pipe would serve as an inlet.

    Rick W

    Attached Files:

  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "...To give you a hint of the kind of powersource I intend to use is 24VDC 0,5-1Nm nominal (ca 300W) and between 2000-3000rpm, so I need to know how to design an suitable impeller for this..."

    Put simply, a prop that will allow the engine to rev up to its full rated RPM at full throttle.

    But before you get there, you need to 'design' your little 'floater' first!...without knowing any more about what the boat is like, size, shape, weight etc, with all relates to the overall drag which in turn relates to the thrust from the prop to over come this drag (via the motor), your motor could be grossly over powered or grossly underpowered.

    Nice little project though.. :)

    Richard...., i think the term is, sweeeeeeeeet :)
  12. DrDistorzion
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    DrDistorzion New Member

    Ofcaurce safety allways comes first!! This toy will be used in shallow water with parents walking along ready to grab her arms, it allmost sounds like you think I will let her out on the open sea at 20 knots!

    You should have seen the smile on here face as whe was pushed around on the lilttle styrofoam boat that gave me this idea! my modification to that is just bringing some interactive controll for her to learn how to use, trust me, the project of making it electrically powered is not at all meant to make it faster.

    Just a thought, I think a boat like this when used under close guard from parents is safer than the avrage playing yard or playing with a tricycle.

    About safety, I intend to build the throttle handle so that if you release it, it goes back to zero power.

    jim lee; I am actually looking more to the design of avrage pumps than the waterjet impeller type of construction, as I understand that it's not at all meant to be used in this way.

    Rick; thanks fot the data! I know that I'm starting of in the wrong end, but the reason I gave a suggestion of powersource was to see if my intuition was anywhere in the right area, and it seams lika I wasn't that out in the blue after all. But I will wait until I have the hull-design before I choose the size of powersource.

    Anyone know how much power a small electric outboard engine usually sports? I guess they are called trolling engines right? (not that I would use one, just wondering out of pure curiousity).
  13. apex1

    apex1 Guest

  14. Guest625101138

    Guest625101138 Previous Member

    They come in quite a size range. Attached gives an idea of the smaller ones in the range. They are normally connected to 12V but will survive 24V if you do not exceed the current rating.

    The largest in the attached list will consume 45A x 12V = 540W. The motors achieve around 80% efficiency - maybe a little better. If you go up in voltage the prop will need to be changed or the boat very easily driven.

    Rick W

    Attached Files:

  15. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    You can buy electric boats here for kids to use so check the web....certainly a trolling motor would be simple and in my experience with a canoe 45 lb thrust gives about 3 mph .Minn Kota have sold the tooling to china so look for i-force motors and indistinguishable for Minn Kota ..if its not going on the plane why not conceal it in a tunnel under the boat ..or fit in from the transom and make her wear a switch lanyard so if she falls over it cuts the motor ..remember also wind can move the boat a long way and of course current ....
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