is a DIY jet drive feasible?

Discussion in 'Jet Drives' started by black_sails, May 13, 2016.

  1. black_sails
    Joined: May 2016
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    black_sails Junior Member

    Pretty self explanatory... just like people who have done various homebrew prop drives of their own design and such, i'm wondering if something similar has been done with a jet drive before. (it's not really a complicated machine, just really like a specially shaped prop inside a specially shaped hydrodynamic tunnel with close tolerances and engineering concerns for the relevant loading)
     
    dsigned likes this.
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    As a one-off, properly done, it is obviously not going to be a practical proposition. Unless there is some peculiar requirement that existing product does not meet. Which seems unlikely.
     
  3. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I guess that would depend on your definition of feasible.

    You could put a prop inside a pipe, build a steering nozzle on the back, include a thrust bearing and your DIY jet would move a boat. If this is your definition of feasible, then certainly you could DIY a jet.

    If your definition of "feasible" is to build an efficient DIY jet, efficient in this case being input horsepower to effective thrust, then the answer is "not likely"

    Your comment that its not a really complicated machine is quite far off the mark.

    The shape of the inlet is critical to reduce turbulence and ensure as far as practical that the water flow reaches the entire inlet area of the impeller. The shape of the impeller vanes is critical for horsepower input and load and the pressure that you are trying to build in the downside bowl to increase velocity to optimize thrust. A guy who jots drawings down on a table napkin will not produce this profile, computors are used now.
    Then there is the set of stators, for increase thrust recovery, another computer aided design project. Cavitation calculations and limits are critical for jets and the vane design.

    Then the easier stuff, thrust bearings, lubrication, nozzle sizing, steering, vibration, etc all put building an efficient jet for a DIYer quite far out of reach.
     
  4. gonzo
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    gonzo Senior Member

    Every jet drive started as a prototype. However, considering your previous posts, I don't think you have the expertise and equipment to do it. To start with, jets are run by impellers not propellers. There is a lot of advanced engineering involved in the design. Not just on the shape and size, but also the materials.
     
  5. black_sails
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    black_sails Junior Member

    Thats understandable, so basically what you are saying is "they are pretty hydrodynamically complicated if you want it to perform well and efficiently, the manufacture shapes are fairly complicated/you'd need a large CNC mill to make all the parts, and it might have to be out of high strength materials as well - and by the end of the day with all that even with free CNC time you're still reengineering a wheel better optimized by others".
     
  6. Emerson White
    Joined: Aug 2012
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    Emerson White Junior Member

    A DIY jet drive would be like a steam powered outboard. You could probably do it, and it would make an interesting conversation piece, but you shouldn't expect anything that performs within an order of magnitude of a professionally designed and manufactured drive.
     
  7. Ben Landgren
    Joined: Jun 2017
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    Ben Landgren Junior Member

    Hi, Few things to take into consideration: First of all DIY jet is feasible if you have some knowledge, funds and the impeller size is not very large. The major risk, which is not present in DIY open propeller, is that if your waterjet duct is inside the boat hull, a break down of a component can lead into a fatal leak. By saying this, a safer approach could be something similar to Alamarin-Jet 230, where the jet is mainly outside the hull. The maneuvering mechanism of waterjet, while giving nice thrust vectoring, is very complex compared to open propeller + rudder.

    About the geometry: impeller, stator, nozzle, deflector, intake are the components that require some knowledge in hydrodynamics. Sand-casting would be the way to go when manufacturing these components (maybe up to 700 hp?). I know one case of a successful DIY waterjet but if you really are up for the challenge, maybe try to acquire a used jet from somewhere and do some reverse engineering first.
     
  8. Ben Landgren
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    Ben Landgren Junior Member

    To add one more thing: if you successfully manage to build a jet drive that actually works at some stage, without proper hydrodynamic design, cavitation will most likely occur, which will destroy your components pretty fast. The open propeller cavitation does damage usually only to the propeller, but in waterjet case cavitation can typically occur in the inlet, impeller and stator.
     
  9. Eldias
    Joined: Jul 2017
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    Eldias New Member

    I've been watching a thread on RCGroups (a Radio Control forum) of a 3d printable jetd rive project. Is this like what you're thinking OP?


    And to Ben, any thoughts on the design they've been working on? I want to stick it inside of a Stand Up Paddleboard eventually and I'd rather not have it self-deconstruct while I'm riding it!
     
  10. Ben Landgren
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    Ben Landgren Junior Member

    Hi Eldias,
    I got an assignment from a client (waterjet manufacturer) in the end of last year to scale down one of their jets to 1:4, design the shaft line and bearing assembly, print the parts and conduct bollard pull tests. I just finished these tests in Newcastle University towing tank few weeks back. Impeller, stator and duct were printed of ABS and the shaft was naturally stainless steel. Even though 100% fill was used in printing the duct leaked surprisingly much so all parts were eventually coated with epoxy. 30 min Zpoxy was found to work well and give nice smooth surface. Tests were run from 400 to 5400 rpm in bollard pull condition and maximum thrust of 50N was achieved. Despite the epoxy coating, the impeller blades had most probably deflected under the loading, so I would suggest to use another material than ABS for impeller or add some reinforcing fibers around the blades. High blade loading is a thing for bollard pull though and if craft is in forward speed the blade loading is typically smaller.

    Hope this helps.
     
  11. Eldias
    Joined: Jul 2017
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    Eldias New Member

    I appreciate the input Ben! I'd suspected I'd need to glass reinforce the parts and was considering having the major pieces printed by Shapeways in nylon with a brass impeller. Im concerned about cavitation damage so I'll wait for Sundogz to run through his testing before I have anything made.
     
  12. Lepke
    Joined: Sep 2015
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    Lepke Junior Member

    I ran a couple different twin jet setups in the military. Prop was bronze and housing was cast aluminum. When new, the tips of the blades were about .002" clearance from the housing. In dirty water, like most river water, the housings wore quickly. As the clearance got bigger, the top speed went down. Top speed was originally about 42 knots but went down to 35 knots after several months of continuous use.
    I don't see why you couldn't make one, but I'd copy an existing design. The jets I ran had about a 12" x 18" opening in the bottom with like a flat bar grill to stop big things from getting into the jet. The intake runner needs to be smooth and transition to the prop in a gentle curve. The nozzle & reverse gate should be easy but the gate in neutral needs to send thrust forward and aft so there's no thrust causing boat movement (no transmission).
     

  13. dsigned
    Joined: Aug 2017
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    dsigned O.R.C. Hunter

    Short version is that yes, it's feasible. Long version, you'll never know unless you try...Bear in mind that this forum has a lot of retired types who have nothing better to do than to hang around a forum trying to make themselves feel smart, which they do by telling you every possible reason your idea is dumb and can't work.

    To be fair, fluid dynamics is crazy complex, and cavitation is a *****. On the other hand, there's tons of information publicly available on both, and with a little bit of luck, you can probably avoid having to deal with either of them.

    I would also ask the question of what your end goal is in making the jet drive. If you're hoping to learn about jet drives, there's no better way. If you want to convert some boat to jet drive, just bear in mind that building it yourself is going to be more time, energy, frustration, etc. Again, if you're looking for a project, and the fact that you're asking at all seems to indicate you do, then it sounds like fun.
     
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