School me on jets. Looking for something that may not exist

Discussion in 'Jet Drives' started by curtis73, Oct 3, 2021.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    You appear to be in some fantasy where an inboard engine and jet drive with controls is vastly lighter than an outboard of equivalent power, that simply won't be the case, and at least the outboard has the virtue of being demountable, so manually moving the thing will be easier. 6hp should be enough, I think the main problem would be getting the right prop
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

  3. curtis73
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    curtis73 Junior Member

    I'm hoping for constructive ideas and objective input. I have - in my possession - a 14 hp 150cc motor that weighs 30.2 lbs. Small pumps that would work with it won't weigh much. There is no fantasy about it. The entire drive with engine, pump, and even the cut hull immediately around it out of a 600cc Jet Ski weighs 133 lbs and makes 55hp at the crank or about 35 out of the nozzle. A typical 35hp 2-stroke outboard weighs 190-200. I'm not sure what the fantasy part is. I have a 3.5hp and a 6hp outboard. I'm not going to use either. Period.

    I asked about sourcing a jet to match an engine for water that eats lower units for lunch... and you keep suggesting the exact thing I have said 5 or 6 times that I refuse to use.

    I
    Will
    Not
    Use
    A
    Propeller
    For
    This
    Project.

    Is that quite clear now? I feel like I'm asking what economy car I should buy and you're recommending a Gerbil for my next pet. Thank you for the input, but unless you have specific information related to my question, please stop telling me I'm an idiot for wanting what I want. Maybe I'll do this jet and hate it. I might even say "I should have listened to Mr Efficiency." Until that day, how about we get to the meat of the questions. Maybe the answer is "can't do it, no one makes a pump that size," but at least we'll stop talking about outboards.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    If you already know the right solution, why have you not already executed it ? Whatever power you choose, collision with these rocks is going to occur, and that hull in its present form will be destroyed, being lightweight GRP. I'll leave you with it, that idea I drew, with skegs that protect the boat as well as the prop is my best suggestion. if anything gets destroyed it won't be the main hull as it now exists, and you will still be afloat
     
  5. comfisherman
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    comfisherman Senior Member

    Sat down at a computer so I can type more, hopefully it allows me to close with my end comments with0ut being a complete big ol meanie head.

    Your asking for a product that doesn't exist especially in gas form. That leaves you with extreme diy, and that requires you to be extremely skilled or willing to accept poor performance.

    Here is a youtube video of the latter, guessing by the abysmal state that the entire jet unit was home built. If your into good solid machining and quality builds avert your eyes and don't click the link.
    Homemade Jet Unit Build, Part 4, 6.5hp 212cc Test - YouTube

    So your best bet is to find a 440 jet unit and adapt it, looks like you can get pretty small like down to 13 twist impellars for it. Now will that be enough for a 14 hp motor..... I'd guess probably not, but I'm not a mathamagician and pay people money to figure out the kw rating for my impellars on my big boats.

    My buddies all have mini jet boats, so I've ridden in them as small as the watts 3m with a 701 Yamaha up to a 12 footer with s supercharged monster out of a modern Yamaha jet ski. At 230 your not a small person to push around the water, I'm 6'4 and have ridden standup skis ranging in weight from 230-250, it takes some oomph to move that even with my 71 hp ski with a proper matched impellar. Add weight and resistance of a wider platform plus a sub optimum intake keel design for a mono jet and your not going to be super stoaked with performance.

    Electric surfboards are your closest option in power range, if it were me Id be looking in that range and maybe twins.
    Jet Water Thruster 120mm Boat Rc Model Parts Aluminum Metal - AliExpress

    Seems like a lot of work to build essentially a mokai kayak, cursory look around has the second hand around 3500. Doubtful all in you'll save much for a better option.
     
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  6. curtis73
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    curtis73 Junior Member

    You do realize you are a rather frustrating bloke right? :) I haven't executed it BECAUSE I CAME HERE TO ASK HOW TO SIZE A JET FOR A MOTOR. You suggested a Gerbil and when I said "I don't want a Gerbil" then you said "well you must already have all the answers, so why isn't it done yet?" I know the solution is jet. I don't know how to engineer it to work with the power I'm targeting.

    Collisions will occur, but you have to admit that a rock 3" under the water will completely miss the hull and jet, whereas a lower unit that is 10" below water will be destroyed. The point (as I said earlier) is to move cautiously in water that I know is treacherous, and plane in water that I know is open. Give the fact that there could be a rock anywhere, having a lower unit in the water means that I'll never be able to comfortably plane with a lower unit. At all. Ever. So what I'm saying is, if I have a 6hp propeller outboard, I'll only ever be using 2hp of it and constantly whacking things, whereas with a jet, I can at least plane where I know it's safe. I'm basically reducing my overall draft by 10" in water that is sometimes not much deeper than that.

    I appreciate the drawing and the idea of skegs/tunnel, but I don't see that as a direction I am wanting to or willing to go with this. Far more fabrication than just cutting a hole.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Just cutting a hole ? I think rather more complicated than that. Frankly if you want a jet boat, get or build one that will survive hitting rocks, that one won't.
     
  8. curtis73
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    curtis73 Junior Member

    Meanie head I can respect. Straight dope.

    The thought had crossed my mind, and I've been successful at similarly difficult engineering feats (also done some spectacular fails). One thing I'm sure you all don't know about me is that I have no problem going super deep into a subject. Before I built the from-scratch suspension on the $2000 race car, I sat down for some nice reading... of an 863-page suspension engineering textbook, and spent weeks building 1/8 scale prototypes to test camber curves, bump steer, and ackerman. Not saying I'm ready to dive into building my own pump, but the thought has tickled my frontal lobe.

    I did watch it. Threw up in my mouth a little. Still, nice to see what not to do.

    I actually used to have a Mokai. It was nice for what it was, but I'm getting too old and stiff to be crawling in and out of one.

    Thank you for the straight answers and the suggestions. I will look into them.
     
  9. ondarvr
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    ondarvr Senior Member

    What river(s) do you plan to use this on?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Mr. E - unless you have something to contribute to this thread and the subject matter at hand, I would suggest you do as noted, butt out.
    You're becoming a troll with such pointless replies...
     
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  11. curtis73
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    curtis73 Junior Member

    Susquehanna primarily. Central Pennsylvania. It will also likely get used in the lower Conodoguinet, possibly Juniata, and Conestoga rivers.

    These are photos of the river a little below zero stage, but you get an idea of what the rocks are like in the shallows. Further down toward the breakwater, you get pockets of 6-10 feet and pretty muddy bottom. Lots of pontoons and some props down there. So the short version is, if my travels take me to where it looks like the photos below, that is where I putter through slowly so I don't (as Mr Efficiency puts it) rip a hole in the boat. I don't plan on zipping around them like cones on an autocross course. You putter through these spots. Then when I get through sections like this to the 6' deep areas, THAT'S when I open it up and plane.

    [​IMG]

    This photo is a little further north of where I usually boat, but it's a good example of the conditions. There is just zero way a prop outboard is the right equipment for this water

    [​IMG]
     
  12. comfisherman
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    comfisherman Senior Member

  13. curtis73
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    curtis73 Junior Member


  14. comfisherman
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    comfisherman Senior Member

    It's a vertical oriented jet and engine power package similar to the mercury sport jet. Would be a much better option than a repurposed jetski that's linear oriented.
     
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