Efficient solar powered (electric) kayak

Discussion in 'Boat Design' started by ElectricKayak, Mar 20, 2015.

  1. portacruise
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    portacruise Senior Member

    The key to running batteries in parallel is to only do so when they are delivering power. If they are not in parallel when at rest there should not be any issues. A momentary DPDT center off switch may be the easiest way to have 12v-24v with out being in parallel at rest. NiMH and Nicads in particular cannot be charged in parallel because charging stops when the strongest battery peaks leaving others uncharged, so charging in series is an easy fix. But Li types are routinely permanently connected in parallel to form higher capacities- up to 4 in parallel is Ok from what I have read.

    Hope this helps.

    PC

     
  2. alan craig
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    alan craig Senior Member

    I'd be very interested in seeing the results you get with the Turnigy SK3 motor. Will you use direct drive? If so what size propeller?

    In a competition for craft powered by cordless tools I used drills mounted on pivots driving propellers through long stiff (not bendy) shafts. I did not manage to finalise the mechanism for raising the drill/shaft/prop assembly and as a consequence had issues with the props leaving the water. I've just realised after re reading this thread that this was probably due to the props trying to align themselves with the flow as they would on a flexy shaft.

    Also I did not realise the possibility of raising the props to semi-submerged as a way of running temporarily in shallow water.
     
  3. ElectricKayak
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    ElectricKayak Junior Member

    Miscellaneous comments below:

    Multiple batteries with switches: It's a neat idea although I doubt I'd ever use it given the availability of ultra low cost PWM controllers. The controller I referenced above quite literally cost only $13 including free shipping from china...it works from 10-50 volts with power from 0-3000 watts (60A).

    Porta:
    I have two drill extension shafts. One 48" and the other 40". The ends of the drill extension are weak so need to be cut off making the longest reliable shaft length around 48". The 48" has a nicer bend but the 40" works fine too. I prefer shorter shafts as they are easier to handle and allow a higher top speed. The minimum limit is determined by reliability (infinite-life bend radius) and slowest operational speed. The maximum length is limited by desired top speed and ease of handling. I'm not sure how many kilometers I can go on a shaft before failure...unfortunately one has to exceed the limit a few times to know.

    The 11 kph limit was due to shaft buckling. Shortening the shaft or going to a larger diameter solved the problem. My goal for top speed was 10 kph on the sea kayaks and 7 kph on the inflatable. I can exceed both of these so am not pursuing higher speed options at the moment (the issue becomes one of battery capacity). I suspect significantly higher speeds would require a supported shaft for reliability (the compressive load otherwise becomes a significant portion of the shaft stress).

    The MinnKota motors are probably better than most people think. I've attached my efficiency plot for the C30 motor (their cheapest). As you can see it would be listed in a catalog as a 24v motor at around 80%. And this is fully waterproofed, it would be a few ticks higher efficiency without the seals. It is also fairly light for a 400W fully waterproofed motor at 7lbs. Of course there are better ones, especially when run as direct drive at 5 volts (where it's efficiency is barely over 50%). A lot of motors I looked at were also poor when run as direct drive at 5 volts.

    As an aside, when one does the math efficiency improvements don't dramatically affect top end speed. However they do strongly affect range. But even there a small drop in speed will achieve the same results. So if one has some latitude on speed then efficiency improvements becomes less critical.

    I raise/lower the motor by tilting it. Seems to work well.

    I haven't found a foolproof automatic solution to weeds. However I worry about weeds less and less it seems. I often don't bother shedding them until power levels or vibration skyrocket (!). Spinning the prop in the air sheds most weeds. However stringy weeds usually require raising the prop towards vertical at which point they tend to slide off. And if all else fails, I reach the prop directly and pull them off....this ended up being a design constraint for motor location.

    I'm quite interested in your quick release shaft connection. I wanted to do similar but wasn't smart enough to figure out a good solution. So currently I use a shaft coupler with grub screws. If you have pictures of your quick release setup or can describe specifics that would be great!

    I'm still investigating the sources of the residual vibration. In my tests I looked at blade imbalance and also considered asymmetry in blade shape/angle or center hole concentricity but non of these were significant contributors. If the prop is not aligned vibration increases but this is the residual after all those issues have been sorted out. Plastic shaft might be an option although I haven't looked up any properties yet so who knows...lol. In any case, the problem is minor but my outboard approach was vibration free until high power when a slight pulse from the 2 bladed prop became evident. So I'm trying to not lose such niceties! :)

    Alan: Yes, depending on speed, there is surprisingly strong "lifting" force as the prop tries to align itself. Also a properly aligned prop has noticeably less vibration under power. Remember that even when aligned your motor mount has to deal with shaft bending and compression forces. Depending on shaft and thrust requirements this can be substantial with an unsupported shaft. I'm curious what hull type and power/speed are you doing?

    I'll write up the Turnigy version. It is direct drive also. I'm currently using the same 10x6p propeller mainly so the shafts are interchangeable.
     

    Attached Files:

  4. portacruise
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    portacruise Senior Member

    Thanks for you feedback EK.
    Some of my thoughts/questions are within your message below:

    PC

     
  5. alan craig
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    alan craig Senior Member

    https://www.youtube.com/watch?v=FOfAA_rw4HY

    electrickayak - mine is the yellow boat Four Candles. In this clip you can see the problem with the props leaving the water, also the drills were shutting down driving 12" x 6" props, and had to be reset by momentarily releasing the triggers. If you search "cordless canoe challenge" a few more videos pop up. The winner was doing over 6kt, I was not too far behind. In other clips; Aero, a red skiff by Jeremy harris with his double UJ drive, and Fast, yellow with white outriggers by Dennis Adcock with just one drill and a bendy shaft. Both used home made folding propellers.
    I don't know how much power I was using but enough to shut down the drills as mentioned.
     
  6. portacruise
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    portacruise Senior Member

    Generally the longer, lighter and thinner the boat, the faster it will be with all things equal (hull speed). Moving the prop deeper into the water should solve the prop moving above the surface, you may have to attach a leg to lower it. Your short length shifts the weight around more easily - may cause control issues. The prop is too aggressive for the drill you are using, you can go to a 4 pitch and smaller diameter or go with a lower rpm drill, around 600 rpm at full power with more aggressive props. Trial and error process to get it tuned just right or copy some of your fastest competitors shaft drive setups to shorten the process.

    PC
     
  7. ElectricKayak
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    ElectricKayak Junior Member


    Haha, looks like fun. Although the various designs look a tad complicated if the winning speeds are around 6 knots. For reference, I can do over 6 knots with a single hand held Dewalt cordless drill (20V, brushless). I see you're using a kayak...mine requires about 200 watts to run 6 knots which is a little over half power on my drill.

    I'm not too fussed by the 12x6 prop as it only requires a couple Nm of torque to drive a decent kayak to 6 knots (at around 1300rpm). This should be well within any drill's torque capabilities although the 200-300 watts required to run at 6 knots might be a bit much for small drills.

    In the end there isn't a lot of magic. It takes power to drive a boat a certain speed and since your drills were shutting down they were at the power limit. Slowing the prop down will improve prop efficiency but the drill will probably lose efficiency just as fast or faster. My largest prop is a 13x11 and it only gains about 4% over yours and that's probably eaten up by the drill unless it can be mechanically geared down to 750 rpm. Running the drill at full throttle probably improves efficiency over part throttle but if full throttle is 2000 rpm then you'd need a 8x4 prop for the same speed and the drill will have to gain more than the 6% lost by the faster spinning prop. Regardless the gains/losses are small percentages and given the cube relationship between power and speed it isn't going to be significant. If you want to go faster, get bigger drills! :)

    Your setup isn't conducive to more power though. Getting a flex shaft drive and bringing it in close to the side of the boat would make the biggest difference. It would also allow more power to be applied without the boat becoming unstable. The tips of my props only run a couple inches under water even at 6 or 7 knots so they don't have to be very deep if everything is set up properly. Also if you support the prop end of the shaft so it takes the thrust load then the shaft can be very small in diameter.

    Anyway, there's my gratuitous comments for whatever they are worth. I especially liked your air turbine approach I saw in another video. That was very cool! :)
     
  8. ElectricKayak
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    ElectricKayak Junior Member

    >> PC: Water and especially salt spray and atmosphere can be hard on controller electronics.

    Agreed. All my electronics are in waterproof cases.


    >> PC: Again, because of unpredictable shaft failure, a quick connect backup spare shaft/prop setup is probably advised

    Currently our backup is to paddle. We are used to it. With the sea kayaks it is fine. But the inflatable wouldn't be fun if we had to paddle 20kms against a strong headwind! So I agree it would be good to have a quick connect spare. I had thought about a quick connect like you are using but discarded it because I couldn't figure out an easy way to connect a round 3/16" shaft to it. I'll have to revisit the issue.


    >> PC: Yes, but it's nice to have a total motor head assembly including reduction train weighing under 2# and running perhaps 50 watts to drive at close to your target speed- depending on hull efficiency.

    Agreed. The biggest improvement of the Turnigy solution is the greatly reduced weight. The majority of the weight is now battery. I've been able to reduce the battery size to a 10AH AGM battery weighing 8.5 lbs but would like to get it lighter for ease of handling off the water. On the water the weight isn't noticeable. I don't want to go below roughly 120 Wh capacity because of the way we use the boat.


    >> PC: How do you lock the angle of tilt and then readjust and lock for shallows/ surface piercing mode?

    I'm very lo-tech. I have a string with carabiners on the end and simply clip it to the suitable deck rigging location that gives me the desired angle. If the water is just a little shallow and not for long I simply lean the boat.


    >> PC: To clean weeds by hand, do you have to remove or spin around the motor head?

    The motor was originally mounted in a forward position so I could reach the prop easily. However in rough sea conditions the prop would hit the side of the kayak. So I've gone to a mount location behind me (this puts the prop near the back where the kayak has significantly tapered in and safe from prop strikes). I can reach back and tilt the motor vertically or keep going and spin it all the way forward...when the prop is rotated forward I can now reach it by leaning forward slightly.

    Thanks for the comments.
     
  9. portacruise
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    portacruise Senior Member

    Thanks, EK. Here's another QR chuck which might work more easily with the MK shaft and a coupler because it has a thicker shaft: http://www.rockler.com/rockler-insty-drive-quick-change-chuck
    Good quality and holds bits securely for the insty-bit.

    ADDITIONAL NOTES: The 3/16" version of GreenLee extension shaft referenced above comes with a REMOVABLE hex male which can TRANSFERRED to YOUR existing shaft. OR you can use brass/ss tubing from a hobby shop to build up the 3/16" to whatever diameter needed to match the diameter of a hex driver extension bit with tubular exterior. Shaft collars with set screws into drilled down holes and epoxy has held up well for me. PS: I found out the hard way what the difference is between waterproof and submersible with my electronics.

    Alan, sorry I mis read your post. Thought you were the slow guy in the first video because of the issues you mentioned. Your boat is quite fast despite the issues you wrote about!

    OE

     
    Last edited: Aug 2, 2015
  10. ElectricKayak
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    ElectricKayak Junior Member

    OK, here is the write up for version 4 of my current electric drive system. This is a direct drive version based on the Turnigy Aerodrive SK3-6374-149kv R/C motor.

    Since the R/C world is poorly documented I've included comments to help demystify the more arcane bits.

    Parts list:
    I bought all my parts from HobbyKing and the key pieces were:

    1) Turnigy Aerodrive SK3-6374-149kv brushless motor ($80). At less than 2 lbs it is quite a strong motor with a maximum current rating of 70A and peak output power of 2250 watts. Seems to be a good motor. The 8mm shaft size is probably the minimum suitable for this application.

    2) HobbyKing 120A Boat ESC with 4A UBEC ($43). An ESC is the motor speed controller. Same idea as the PWM controllers used for brushed motors. There are a gadzillion to choose from. I chose the 120A version for a conservative power rating and the boat version for waterproofness and reverse (aircraft versions don't have reverse, car versions do but car ESC are programmed differently due to wheels on the ground versus a propeller in air/water). So far it is working well although gets a little hotter than I expected. There may be versions that run cool but this one works fine so I'm not inclined to experiment.

    I bought a version with a Battery Eliminator Circuit (BEC) because I needed to power the Servo Tester.

    3) Turnigy Servo Tester ($6). ESC's are intended to be used in an aircraft/boat so they expect to be talking to a radio receiver and monitoring the transmitters throttle stick position for speed control. This little device emulates those functions and has a little potentiometer to control the motor speed. It needs power which can be wired in separately from a battery or supplied by the ESC if the ESC has a Battery Eliminator Circuit (BEC) which mine does.

    4) HobbyKing Boat ESC programming card ($7). ESC's are highly programmable and while it's possible to program via the throttle stick it is a mind numbing procedure. Programming cards are cheap and make it dead easy. Buy the one that works with your ESC.

    Not essential, but I also bought a spare shaft and bearings. The bearings were good Japanese bearings but very light duty. For long term reliability the motor needs to be protected from any significant loads.

    Electrical Connections:
    Given all those parts, connections were simple. The ESC controller connects to the battery and the motor. The order of the three leads to the motor doesn't matter. Changing any two leads will reverse the motor direction. One needs to solder suitable connectors on all the wires. The ESC controller also connects to the Servo Tester via a standard connector.

    The motor will run with the standard settings programmed in the ESC. But depending on battery voltage one may have to change the low voltage cutout to prevent the motor from going into limp mode. Also ESC's control motor timing electronically and most default to fairly aggressive timing advance for maximum power at the expense of efficiency/heat so I changed mine to neutral timing. The motor has plenty of power anyway for this application. The other settings are personal preference.

    Enclosure:
    Since brushless R/C motors are noisy I enclosed mine in a 3"x6" PVC white drain pipe with standard end caps (one rubber and one pvc). This dropped the noise to quiet (but not silent) levels. Unfortunately this also blocks all air flow and creates a heat problem. At a sustained 30-100 watts it seems OK (peak measured temp 60 C or less). I haven't measured long sustained runs at high power so cooling is still a work in progress. For now, I remove the end cap for higher power runs and accept the increase in noise. The enclosure also waterproofs the motor against waves in rough weather.

    Mounting:
    Most of the work in this version arises because the Turnigy motor is not suited for the loads arising from a direct physical connection to the propeller flex shaft. I machined a boat mount that included two miniature stainless bearings to support the inboard end of the flex shaft and take the loads. The weight of the motor is suspended on the end of the shaft. A flexible torque arm transfers the motor torque to the boat.

    The flexible torque arm is achieved via the rubber end cap on the PCV enclosure. Basically the rubber end cap is attached to the hard mounted boat mount and the motor torque arm connects to the rubber end cap. So the bearings take all the loads except the torque which is coupled to the boat through the flexible rubber end cap.

    Otherwise mounting locations and methods are the same as the MK version. So the motor is on a pivot that allows it to rotate vertically as needed to set the desired angle or for clearing weeds.

    Performance:
    Weight: The motor package weighs about 3 lbs including the motor enclosure, boat mount and shaft. The MK was almost 8 lbs. The perceptual difference is significant.

    Power: The MK is about 350+ watts, the Turnigy around 2250+ watts. Clearly evident in top speed and acceleration. I haven't pushed the Turnigy anywhere close to full power since I'm not sure my shaft coupler can handle it. Once I've made some spares I'll likely get bolder...

    Efficiency: I now cruise around 30 watts at 5kph versus about 45 watts with the MK (from memory the original stock MK was upwards of 150 watts). The original 35AH battery, which was quite insufficient for the stock MK, is now vast overkill so I've replaced it with a 10AH battery.

    Solar Panels: One 50 watt panel laid flat on the deck will keep the battery full charged at cruise speed so range becomes indefinite. I don't bother using any panel controller.

    Noise:
    The Turnigy is noisy without an enclosure. A simple baffle helps but a sealed enclosure makes a big difference and is needed to reduce the noise to "quiet" levels. In comparison the MK was dead quiet.

    Reliability:
    Maybe the question mark. I have about 100 hrs on the shaft version of the MK and more prior to that without any problems. The Turnigy has about 20 hrs on it wtih no problems. It appears well made but there are stories about loose screws, magnets etc so time will tell.

    Remaining issues:
    Cooling while achieving sound abatement. It seems OK up to 100 watts but sustained levels above that will need a better arrangement. Currently I remove the lid of the enclosure for higher power which works fine except noise levels increase.

    Battery at 8.5 lbs is almost 3 times the weight of the motor assembly. Time to start looking at options in this area!

    Conclusion:
    It's the best version yet...provided the reliability holds up.

    EDIT: Added some pictures that might help visualize the setup
    1) Motor mount in pieces
    2) Motor, ESC and Servo Tester with motor mount (3/16 drill bit showing where shaft would go)
    3) Complete Motor assembly with shaft and propeller
    4) Permanent kayak mounts (re-using existing eyelet holes...no mods allowed to kayak)
    5) In operational position (vertical stub is 6 inches and I clip my rope to it to set various positions)
     

    Attached Files:

  11. alan craig
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    alan craig Senior Member

    30watts? I would have expected the motor to lose that much power without even driving a prop. Anyway it's a fantastic set up; an order of magnitude away from my brushless outboard which consumes 135w no load, and 600w or more on a small rowing boat. My two 105Ah batteries weigh a total of 54Kg!
    I'm building a 14ft skiff at the moment which will use the outboard; I will eventually make a folding prop for this to solve the weed problem, And I have modified the outboard to run in both directions so that I can experiment with easily obtained model propellers.
    I can see myself in the future making or modifying a long thin boat possibly using two drive systems like yours.
    Congratulations, looks like you have achieved your goal - and not enough frustration or mistakes!
    Thanks for posting the results.
     
  12. ElectricKayak
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    ElectricKayak Junior Member

    With no load prop spinning in air, at 5 km/h equivalent cruise speed it consumes 4 watts and at 11 km/h (6 knots) it's around 10 watts. One needs to pay attention to both the motor and the drive train losses to get it that low. But worthwhile since it really helps on the battery size if the thing isn't killing the battery doing nothing!

    I suspect you'd find this setup would drive your rowboat or 14 foot skiff pretty well. I'm not sure what power levels the motor can comfortably run but I'd guess at least 500 watts and maybe even to 1000w.

    Unless the folding prop is 100% effective with weeds, one still has to deal with the exceptions. Might be easier to just design something where removing weeds is easy. That's the route I went and it is working for us. Although that air turbine approach is probably the best solution for weeds! LOL

    Thanks for your comments along the way, and good luck with your next project. :)

    EDIT: Added various comments and tried to fix the sentences that even I didn't understand...
     
  13. portacruise
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    portacruise Senior Member

    Thanks so much for posting your write up and pictures. Hope you can keep us updated on your progress as you further refine your design. I really like the potential for short power bursts well beyond your cruising speed, great for emergency situations.

    Did you make your own flexible torque arm, and do you have pictures, as I can't quite visualize the arrangement? I take it there is no quick release arrangement, but can't tell as the motor mount/housing covers that section.

    I've heard that the turbulence behind the prop is a more significant loss than what you get in front. If so, a longer tapered rear fairing may give a very slight improvement in efficiency. Maybe some experts can chime in on the matter.

    Kudos,

    PC



     
  14. ElectricKayak
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    ElectricKayak Junior Member

    Thanks a lot for your help PC. I'm finding the extra speed burst particularly useful to traverse narrow spots in a river where the current temporarily quickens. Previously those would stop me.

    I'm using the higher speeds more than I expected...so I need to address the stress concentration on my shaft or it will fail sooner than later. I think you mentioned you sheathed your shaft. Did you epoxy with fibreglass? You always have good practical solutions so I'd appreciate any input here. It would be nice to find a fibreglass "sock" I could cover with expoy and then pull over the shaft and coupler and tighten like a chinese finger trap but I haven't found a local source for something like that yet.

    I don't have a picture of my torque arm arrangement but it is quite simple. The motor comes with a torque arm designed to bolt directly to the frame. Instead I bolt it to my rubber end cap and then bolt the rubber end cap to my mount. So the torque is transferred through the rubber end cap.

    In addition to increasing efficiency, reducing the turbulence around the prop might also help reduce vibration. It may also increase the "clamping" effect of the propeller so the shaft could take slightly higher loads before buckling. So it's on my list but I expect the improvement to be quite small so it is a low priority.
     

  15. portacruise
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    portacruise Senior Member

    Thanks, EK. Comments below.

    PC

     
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