Ultra-budget electric kayak made from scraps, need all kinds of advice.

Discussion in 'Boat Design' started by Factoryninja, Oct 31, 2016.

  1. Factoryninja
    Joined: Oct 2016
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    Factoryninja Junior Member

    I, am a 100% boat noob and a 100% powerboat noob. Absolutely zero experience with any of it. My ignorance, is staggering, yet entertaining.

    That said, here's what's up, and why I'm here. I'd like to know where I'm doing this wrong and what I can do to improve this toy.

    The story: I'm an industrial repair guy, general technical hacker and career adventurer. I recently bought a house on a small pond about a mile long. I have never lived on water or owned a boat of any kind before. Naturally, first thing I did was buy a boat. All the boat I could afford is Wal-mart's cheapest... a plastic 8 foot one-man kayak, weighs about 35 lb. Spent about a year paddling happily around the pond with it.

    I recently came across some industrial scraps and couldn't resist finding out what would happen if I nailed a motor onto the kayak. :)

    This, is as crude as a powerboat can possibly get. Budget: 120$. (Harbor Freight 2 kilowatt inverter) the rest of this was built entirely out of whatever junk I had lying around.

    Please keep in mind I know absolutely nothing of the niceties of props, trim, anything about boats. So if any of this sounds stupid, 1, gotta learn sometime, and 2: The whole point was to see if I could build a workable powerboat from nothing, with nearly zero money spent.

    The motor: A Baldor industrial model. Weighs as much as the boat does- 35 lb. Runs on 90 VDC, pulls about 4.5A. Rated to 1/2 HP for continuous industrial duty. Seat of the pants estimate is that this is a ridiculously conservative rating and the motor can take 5-10x that. Turns out I was right.

    The drive: a KB industrial motor drive matched to the motor, can source up to 16 amps, runs on 120VAC. This is fed by a cheap Harbor Freight inverter (the only part of this I spent a dime on) rated at 2000 watts continuous, 4kw surge, in turn powered off an 80-lb marine deep-discharge battery I had lying around.

    The structure: Whipped up in a single weekend. A basic adjustable angled motor mount made of chunks of aluminum extrusion, crudely nailed to the back deck of the kayak with a bunch of U-bolts and a few bars of aluminum internally for strength.

    The prop: Best I could do was guess. Prop shaft is a 5 foot long 1/2" thick aluminum rod mated to the motor shaft with another chunk of extrusion bored out to the motor shaft size and secured with stainless steel hose clamps.

    The prop itself is again as crude as it can possibly be. Its two roughly carved pieces of aluminum plate, leading and trailing edges vaguely ground to something resembling edges, again nailed to some extrusion, adjustable blade angle currently set at about 45 degrees, prop's about 15 inches across.

    The battery and inverter live in the footwell, the trimpot for the motor drive bolted to the edge of the boat by my right hand.

    Took it out for its maiden voyage yesterday. Oh, my, god was that fun. At first the motor to shaft union folded. Fixed this and started going places.

    The result: Kayak does maybe a little over 10mph before the prop starts cavitating. At least I think it's cavitating. 5 minutes reading and I also learned of prop "ventilating"... it does a lot of that too, pulls in air from the surface. When it does, at first it feels like I hit the brakes. Can put more power to it, but now the power required to go any faster ramps up almost exponentially and very quickly the inverter alarms out.

    Looks like the motor is good for this... an hour of pushing north of 2 kilowatts through it and it barely got warm. It certainly hauled *** around the pond with great enthusiasm. The whole idea is to turn this kayak into something I can actually use for adventuring on real open water yet be so small and light I can stuff it in the back of the jeep and close the tailgate.

    The kayak also has a -very- hard pull to the right under power. There is no steering mechanism yet. I aimed the boat by dragging a paddle off the left side. Wastes a -lot- of power, but works. Will add a rudder or something later.

    So, for starters, looking for tips on the following areas:
    Prop angle... too steep? Not steep enough? Prop size: too big?

    Next changes I plan to make are doubling the blades to 4, and maybe making the prop a bit smaller, cut it down to maybe 12 inches or even 10, should allow it to spin faster, but don't know if that's going to screw up an already screwy ability to hook up with the water.

    Any advice is welcome. I'd like to see if I can make this efficient enough to get the kayak planing. I think the prop wants to be spun much, much faster... still, not bad for a first try.
    Here's the boat. It doesn't have a name yet.

    Attached Files:

  2. Heimfried
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    Heimfried Senior Member

    it is not only the driving power that makes a boat planing, her ability to plane depends also on her hull form.

    As far as I know, the hull form shown in the pic is a displacement hull with a lack of a broad flat surface at its rear and will probably never pane.

    But 10 mph is a lot of speed for such a simple short boat with an "experimental scrap" driving unit.
  3. messabout
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    messabout Senior Member

    How did you measure the 10 MPH that you said the boat would do? That seems excessive for an eight foot Tupperware kayak. Anything is possible but in this case unlikely.

    If you are having cavitation problems you may need to add a plate over the top of the prop, a part unsurprisingly called a cavitation plate.

    As Heimfried suggests, the prospect of having a kayak that planes is pretty remote. Sounds like you are a clever fellow so build a boat that will plane readily? You may need a lot more power to get it to plane in any case. There is the matter of power to weight ratio that is difficult to violate.
  4. Skyak
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    Skyak Senior Member

    At some point you are going to need to consider what the endpoint needs to be. As it is, you are progressing to a toxic heap poisoning your pond.

    The first thing to replace is the prop. What you have is not worthy to stir paint. The drive you are emulating is a "longtail" but you have not gotten to supporting the shaft or enabling steering. Longtails are primarily used in weedy shallow waters so the prop can be lifted clear.

    The next problem appears to be the hull. As previously stated it is designed to be efficient up to speeds of about 4mph. I suspected that you were already over the weight limit but I see it is rated at 260# (hard to believe since the 10ft I have is rated 250#). The power you need to go 4 mph is a tiny fraction of the rated power of the motor you have bolted on. You could buy a small efficient motor and controller and have a boat capable of several hour/mile range. There are several threads to guide you.

    If 4mph doesn't interest you then you are better off slapping together an appropriate hull from plywood. Before you go out on the water again you should check out how safe it is to be in the water inches away from your battery and electronics, because that is where you are headed.

    If you insist on keeping the hull and motor and going over 10mph you can 'fix' the squatting with a foil, or go full crazy and lift completely out of the water with two foils.
  5. Factoryninja
    Joined: Oct 2016
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    Factoryninja Junior Member

    Re: Stirring paint: Worked, didn't it? That prop now has about 3 miles on it. Maybe 4. It survived quite a few laps around the pond. The shaft is actually the worst part, I'm shocked that whole mess worked as well as it did. I gave it a 50/50 shot that flimsy aluminum was just going to shear off if I put any serious power to it, but it was the only thing I had at hand that could serve as an improvised prop shaft. Next time I might use a Swiffer handle. *jk
    I actually put some thought into trying to make the prop float when taken off the shaft so I could recover it if it just broke off. In the end I didn't bother. The pond is so shallow most places you could just see it and fish it off the bottom if it did break off anyway.
    I'm also surprised the disposable tupperware kayak actually even -has- a design spec. I never bothered to look.

    What's funny is you're judging this by the same set of rules I threw out the window from the get-go. I -know- a disposable kayak is not an ideal hull, and not designed to go faster than a human can paddle it. Question was, what happens if I made it do it anyway? Answer turned out to be, holy crap was that a fun way to spend a weekend.

    Anyway the whole point of the exercise was to see if it was possible to make a working powerboat -out of whatever junk I had lying around.- I kept looking over my shoulder at the rotating paint-stirrer behind me thinking, "I can't believe this pile of junk is actually working!" Spin the power knob and it actually pushed pretty hard, enough to set me back in my seat a little. Only time in my life since childhood that doing 10 mph or less was actually fun.

    It helps if you don't take this too seriously. This was essentially a "technical joke." Done more for fun and a good laugh than any serious goal. I know what motorboat props look like- nothing of their design parameters, but I know roughly what their shapes are, I've seen them before... and I had absolutely no means of attempting to create any such complex curved shape let alone one even vaguely optimized for the job. I just wanted to see if something that crudely fabricated could possibly work at all, and if so, how well.

    Also I think you may be overestimating the risk, here. As fast as it went (or didn't) there was no sign of instability let alone ending up on the bottom of the pond. As for buying a "small efficient motor and controller" again, whole idea was, see what I can pull off for nearly zero expenditure. The fun of trying to make it, instead of just going and buying it. And just from one afternoon puttering around in it, as awful as that prop and shaft are, it already -does- have several miles and several hours range. I know I must have used a good portion of the battery's capacity, but it was not yet showing any signs of running low on juice. I quit for the evening not because I ran out of power, but because I ran out of daylight.

    As far as the endpoint, it's already almost there, really. After putting up my initial post and giving myself a day to really think about it, I'm not going to bother trying to see if I can plane the thing or get any serious speed out of it.

    I'm going to make some improvements on the paint stirrer, not to go much faster, really just to make it run better at the speed it already goes, smoother. The two-blade paint stirrer had a pretty hard wobble to it that averaged out some the faster it went. Might get another 2 mph out of it at a guess, get it so it isn't wasting so much energy thrashing bubbles when it's up to speed. That's about it. If I can get it so it runs smooth at the speeds it already does, I'll consider it a completed successful joke project and use it to amuse myself when bored with just paddling.

    I'd thought of trying to do some foils, but if I succeeded and they actually worked and started to get the hull out of the water entirely, then I -would- be looking at a serious possibility of ending up in the water and I don't want to bother going there. This was meant as a lighthearted humorous take on boat design not a serious vessel, or I wouldn't have started with a disposable tupperware kayak in the first place. The battery's weight did a pretty good job of keeping it stable, I think. It never acted as if it were anywhere near about to tip over or turn over, in fact I was surprised that aside from feeling twice as heavy, it handled about the same as it did when it was just a kayak. It was just a kayak that was being pushed much faster than a human could paddle it.

    Re: Messabout: Speed: The reason I said "Maybe a little over 10 mph" is that was a simple rough guess as to how fast it went. Keyword was "maybe", as in seat of the pants estimate. Mostly based on the observation that as fast as it went before bad prop effects kicked in and kept it from going any faster, it was pushing a pretty solid wave ahead of it, moving 2 to 3 times faster than I could possibly paddle it, and an observer chasing it along the shoreline would have had to be at a dead run and maybe a full-on flat-out sprint to keep up with it. Figure, 4 mph is a fast walk, 6 mph is a firm jog, 8+ is running.

    Anyway aside from trying a few slightly better props just to see how they affect the equation this whole thing has already exhausted most of its easily available possibilities, and most likely will never be taken off this one little pond. Thanks for the feedback, guys. I might pop back in here sometime and report on whether a slightly less badly-made prop works any better, other than that, got what I came for.
  6. rwatson
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    rwatson Senior Member

    As you have found out by now FJN , the shafts need very little support at low speeds.

    The prop and shaft becomes self aligning as power is applied, as owners of pedal baots have found.

    The shape of the prop will be essential, but that hull will never plane, as has been mentioned bfore.

    For that power, a long skinny non-planing hull will performa better.
  7. Factoryninja
    Joined: Oct 2016
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    Factoryninja Junior Member

    Well, I just scrounged some lovely 3/4 inch aluminum rod stock and some steel couplings to bolt the lengths together to make a nice rigid and straight shaft of any reasonable length. I'm currently trying to figure out how to make an adapter from a 5/8" motor shaft up to a 3/4" coupler bore, without a lathe, on a small tabletop mill, and keep it concentric. When I have one that doesn't wobble much if at all, I'll try nailing some more paddle-like 4 to 8 blade prop shapes on it and go for a prop radius under 10 inches so I can really spin the crap out of it. I knew the big paint stirrer was probably too hard of a torque load vs water coupling, and yeah... motor was pulling kilowatts to do a couple hundred RPM tops, maybe 3 to 5 rotations per second. Be interesting to see if a much smaller faster prop with more blades on a much better shaft improves this much. For a kayak it was seriously hauling a$$, I'd just like it to do it without feeling like it's gonna fall apart any second.
  8. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Get a plastic canoe... Size does matter in this case. The longer the better. I have an 18 foot one...with 5hp, it flies with very little effort. It is of course much harder to carry around.
  9. Irie
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    Irie Junior Member

    If you have a tractor supply store you can get a set of nova jaws to fit your shafts. Definitely get a new prop, aluminum longtail props are very inexpensive, as are pin drive props.
  10. PAR
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    PAR Yacht Designer/Builder

    If you can get the shaft angle at or below 7 degrees you'll see a marked improvement in propulsive efficiency and also why (well at least one reason) the long tail's have huge, long shafts.

    The hard pull you're experiencing is prop walk and typical of anything spinning without a counter rotational balancing element. Cant the shaft off to the side a little off the centerline, say a couple of degrees to start and this will help mitigate this a bit. A small vertical trim tab will help too.

    A fairly small plate above the prop, mounted at least 10% of it's diameter away from the tip, will help with ventilation.

    If you were actually doing 10 MPH, this is a S/L ratio of 2.55 (assuming a 12' LWL), which is about what you can expect of this hull form and will require a lot of output from your rig to maintain. Pushing past this point will be difficult given the available power and hull form. At these S/L ratios, the hull form wave making resistance rises exponentially, so the power needs do as well. Your wattage requirements will drop considerably if you back down below 2.0 S/L and possibly more than 50% lower, if you back down further than say about 1.3 S/L.

    Making your own prop isn't the best choice, even if it seems to be easy enough. Ideally, you'd want to "gauge" the prop's pitch and diameter, so she's pulling the recommended amperage at full speed, no more or less. When testing, run her up to max speed and see what's see's drawing. If she's under amp'd a touch, add some pitch and/or diameter and the reverse is true was well.

    I'd like to see a video or at least some photos of this underway at various speeds. At full speed, I'll bet she's squatting quite a bit and given the weight she's carrying, a huge quarter and stern wave train is well developed. You can play with things like cup and rake to squeeze a bit more from her, while still keeping the motor in it's load range, but these will be marginable improvements at best.

    This hull and drive is likely working on the old "inclined bottom" principles, so you're seeing much higher speeds than conventional wisdom would normally suggest. If you'd like to go faster, just cut off the last 2 feet of hull and glue a piece of plywood over the cut out portion of the hull, vertically, making a little transom. Though you'll lose some length and volume, you'll also have better cleavage for better flow exit at the stern, which will help a lot. If the hull is left the same, you'll run smack into (now currently are close to it) a "drag wall" that's unsurmountable, with current power available. Further, an instability will soon arise, if you do manage to push this puppy past say 3.0 S/L. I've personally enjoyed being towed by a powerboat, while in a canoe, to explore what happens when you push a displacement hull form well past it's limits. It's a fun ride, while it lasts, but it will roll over, you can bet on it, though on a warm summer's day in an empty canoe, no big deal. With your gear taking a swim, maybe not as much fun.
  11. Skyak
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    Skyak Senior Member

    PAR, I have the 10ft version for fun experimentation and this seems like a good 'teachable moment"


    I do wonder about your cut-off recommendation (I don't think he has buoyancy to spare he has 50 lbs of lead and acid between his legs). Is the idea more about flow separation (because that can be accomplished in other ways) or is it about freeing the hull to angle up and lift sooner (also other ways)? I have heard talk of an optimal transom/beam ratio per Fn but I have never seen it graphed.

    I was wondering how close I could get to the efficiency of a full width transom hull (same 10ft/30in) by adding a 30in wide foil below (anywhere) and upsets or a step anywhere on the hull aft of center to 'cleave'? Fn 1.3 to 4. My reasoning is that if the drop in drag comes from hull lift, I can get more than twice the lift/drag out of an immersed foil.
    Last edited: Nov 4, 2016
  12. PAR
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    PAR Yacht Designer/Builder

    A horizontal foil(s) can work, though you'll have to play with incidence angles for a while, until you find something that works well. I'd start around 2 degrees and move up from there. Have a look at the Bartender for an idea of where they might be placed.
  13. fredrosse
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    fredrosse USACE Steam

    Not possible?

    "an hour of pushing north of 2 kilowatts through it "

    I don't think an 80 pound marine deep cycle battery is capable of putting out 2kW for an hour, significantly less in real terms. What is your statement based on?
  14. portacruise
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    portacruise Senior Member

    Um, some mixed messages were posted here.

    FN: "The whole idea is to turn this kayak into something I can actually use for adventuring on real open water yet be so small and light I can stuff it in the back of the jeep and close the tailgate."

    Would seem to be excluded by:

    FN:"Anyway the whole point of the exercise was to see if it was possible to make a working powerboat -out of whatever junk I had lying around. Only time in my life since childhood that doing 10 mph or less was actually fun."

    So I think most of the postings have been directed at the first message.

    Congrats, on achieving your second goal. A search on the web will turn up similar things have been done with discarded lawn chairs, dock floats, and donated or second hand troll motors ($100 new) requiring no expenditure on inverters (120V ouch on water).


    PS: A cheap APC model airplane prop around $5, will work a lot better than a home made one, if it is within your budget rules.

  15. Factoryninja
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    Factoryninja Junior Member

    2 kw frequently, not necessarily continuously. It's not as if I put it at that output and then kept it there for exactly one hour, I was ramping it up, down, maneuvering, etc, which eases the duty cycle considerably.
    And the statement was based on the inverter's rating. It can sustain 2kw... anything more and it will begin to complain with an alarm. I spent much of that hour with the inverter pushed pretty close to its maximum and often -at- max but still constantly varying the load, as I was exploring the handling, behavior and maneuverability of the boat.
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