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Building an electronic foil surf board

Discussion in 'General Electronics Discussion' started by billy bob surfer guy, Jul 11, 2017.

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  1. billy bob surfer guy

    billy bob surfer guy

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    Jul 11, 2017
    Hello everyone!
    I am making an electronic surf board using a motor and a hydrofoil. I will be attaching the motor to the hydrofoil with a propeller. I am trying to figure out what type of battery would be best for what I am trying to achieve. I would like the surf board to be able to move relatively fast. Does anyone have any suggestions? I appreciate all feedback as I am relatively new to electronic design. Thanks!

    Here is the link to the motor I hope to use: https://www.aliexpress.com/item/KZ5...id=90759d0b-9980-4105-bc2d-f2881e43e4a0&tpp=1
     
  2. kellys_eye

    kellys_eye

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    I'd check the thrust requirement first - seems awful low for what you intend to do.

    Then it's a case of compromise! Motor power versus run time versus weight versus recharge time etc etc.

    It's almost impossible to decide for you - though modern high power-density batteries (LiPo for example) seem like a probable solution.
     
  3. Audioguru

    Audioguru

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    The Chinese motor uses brushes that will soon wear out or burn out. Modern RC motors are brushless that last forever. I agree that the thrust is too low for it to carry you fast.
     
  4. kellys_eye

    kellys_eye

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    If the OP's suggestion to use a hydrofoil is true then that motor certainly won't provide the speed required to get lift.
    I saw a recent video of a hydrofoil-type board and they were travelling at at least 20kph! Mind you, it looks oh so impressive when it happens! If the vertical blade from foil to board is thin enough it looks like you're travelling OVER the water - there was hardly any wake at all!
     
  5. billy bob surfer guy

    billy bob surfer guy

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    Jul 11, 2017
  6. kellys_eye

    kellys_eye

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    Adding to my previous comment:

    That motor is rated at 150watts. If you want to run it for 1 hour you need 150 Watt-hours of battery storage (plus some extra for system 'losses' and redundancies) so for a 1 hour run period you need (say) 200 watt-hours of energy.

    If you use a 24V motor this equates to (200/24) = 8.33amps.

    Therefore a battery that has a voltage of 24V and a capacity of (rounded up) 10 amp-hours will easily do what you want.

    If it was a 12V battery you'd need a 20 amp-hour battery (if you half the voltage you have to double the current to get the same power).

    Next you decide the battery chemistry based on your specific needs - do you want 'instant' recharging (expensive) or swappable (cheap but slower to recharge)? Additionally the more modern the battery chemistry the more complicated is the charging and management of them.

    Taking a compromise, simple Nickel Cadmium (as you'd find in older power tools) can provide the voltage and current requirements quite easily but you'd be looking at a minimum of 2-4 hours to recharge them - or, carry two/three packs and swap them out. One will recharge before you've discharged the other two!

    Or, fork out lots of cash for the latest technology like LiPo (lithium polymer) that recharges in minutes and has high capacity in small packages - at a cost!

    This isn't something we can just 'tell you to do' - you need to make YOUR choices on how you want to use the finished article.
     
  7. billy bob surfer guy

    billy bob surfer guy

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    Jul 11, 2017
    Thank you for explaining. I appreciate your feedback.
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    The motor-driven hydrofoil attached to the bottom of a surfboard and powered by a battery in the surfboard is a fantastic idea that should be very successful both as a commercial product (if you can spare $12,000) and as a hobbyist project. Think jet skis that you can stand up on, but electric powered.

    I predict motorized hydrofoil surfboards will take California and Hawaii by storm, but they may also be popular on Gulf of Mexico beaches. Someone here in Florida should investigate obtaining distributor rights from Liftfoils, the original developer who is currently taking orders for delivery this fall and assembling boards in Puerto Rico (for now). It appears that pre-orders are threatening to exceed production capabilities, so these things may be in a situation similar to the Tesla automobile a few years ago. Definitely a hobbyist opportunity waiting here.

    @billy bob surfer guy is going to need some expert design help to successfully build this one. The Chinese ducted fan motors, available in both brushless and brushed versions, appear to be up to the task, but I would not use a brushed motor for this application because, as @Minder has pointed out, they are not reliable. The brushless versions with Hall sensors are the way to go, although the motor controller design is more complicated. Consider using this one with a 48V LiPO battery. You could also use a lower voltage (24V, 12 V) battery pack with a boost converter to obtain 48V.

    You really DO need a Bluetooth wireless, water-proof, hand-held remote control for the motor with a "dead man switch" motor turn-off capability when (not if) the rider falls off the board, or the remote is accidentally dropped in the water. This is a completely separate design consideration. Get some expert help because you probably will NOT be able to adapt an off-the-shelf commercially available product.

    The battery and its charger need to be considered in depth. The prototype that Liftfoils is currently using is 2 kWhrs and requires about two and a half hours to re-charge from a 1200 watt charger. A "spare" battery (recommended for use at the beach where power is scarce or unavailable) reportedly costs $3000, so budget for two of them. Also budget for a gasoline or natural gas powered generator with 2 kw capability so you can recharge anywhere.

    Finally, you need someone with expertise in laying up and building fiberglass surfboards. It's not difficult, but it does require planning and the use of noxious chemicals. Lots of surfers took to this as a hobby and a way to earn a little money, so you may want to see who is doing this sort of work in your area. Getting that motor mounted properly on the bottom of the supporting "keel" along with the hydrofoil lifting wing is of paramount importance. If possible you should build a parametric 3D model using appropriate software and analyze the stresses on all the parts involved, possibly using aluminum or stainless steel struts covered in fiberglass in appropriate places.

    Sounds like a fun project if you have a few thousand bux to throw at it. Don't skimp on the electronics. Learn how to waterproof everything electrical. I would suggest using elastomer-sealed ("O"-rings) access covers, drawn down tight with quarter-turn Dzus fasteners. Upload some pictures of your progress.
     
    davenn likes this.
  9. kellys_eye

    kellys_eye

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    A 2kWhr battery :eek:

    Must be one heck of a motor! Puts the 150W motor to shame!!!
     
  10. davenn

    davenn Moderator

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    they already are a commercial product as indicated :)

    the interesting thing is, I assume the current manufacturers have patented their product
    so others would have to be careful not to run into legal difficulties trying to commercially manufacture them
     
    Last edited: Jul 12, 2017
  11. hevans1944

    hevans1944 Hop - AC8NS

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    It is rather large, but appropriate for this application. You need a LOT of power to plow through the water at 25 mph, even with a low-drag wing and a low-drag mast to lift the surfboard out of the water. These sealed, underwater motors with integral ducted fan appear to have been originally designed and built for remote-controlled submarine applications, either commercial or hobbyist. That they are now available in a one kilowatt brushless three-phase design with Hall sensor feedback of rotor position says a lot about their possible application.

    Drug smuggling comes to mind, but this would require very sophisticated navigation electronics and very high endurance at depth to avoid detection. Communication is always a problem with submarines, and reliable solutions are both expensive and susceptible to interception. OTOH, using large electric motors to power hydrofoil-lifted surfboards appears to be a sustainable business model worldwide.
     
  12. davenn

    davenn Moderator

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    .... and the weight of the rider and air resistance he poses :)
     
  13. Audioguru

    Audioguru

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    The fairly low max RPM of 3000 and graph of thrust vs speed (no Chinese translation, is 1 the speed of sound or the speed of a snail?) shows that the thruster is not made for speed. It is made for slowly pushing a submarine or slowly mooring a large boat.
     
  14. hevans1944

    hevans1944 Hop - AC8NS

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    I cannot intelligently argue this point because the physics is beyond my capabilities. However, the one kilowatt three-phase brushless motor I suggested the OP try out is rated for 25 kgf thrust at 4500 rpm, more or less linearly decaying to 15 kgf thrust at 1 m/s, according the thrust graph provided on the web page. It would be fairly easy to guess how long a constant 25 kgf would take to accelerate a 250 Lb payload (surfboard plus rider plus battery) to 25 mph, neglecting friction and wind resistance, but the thrust is NOT constant, decreasing with increasing speed.

    How do you calculate this decrease in force? What happens when the flow of water through the ducted fan exceeds a speed that represents the maximum thrust possible from the motor? Can this even happen? It is possible that the working models of these powered hydrofoils use a much higher rpm motor to maintain thrust at higher speeds. To duplicate the prototype working model will require some REAL engineering design to avoid a LOT of expensive trial-and-error construction. What is encouraging is seeing video of the powered hydrofoil in action. Having a working "proof of concept" beats all the pie-in-the-sky imagineering and provides a place to start from.
     
  15. Audioguru

    Audioguru

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    I have an RC airplane with a ducted fan brushless motor that flies very fast. I think its motor spins at 30000 RPM or more. Of course the airplane is very lightweight and does not carry a heavy passenger.
     
  16. davenn

    davenn Moderator

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    and it isn't propelling through water
     
  17. billy bob surfer guy

    billy bob surfer guy

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    Jul 11, 2017
  18. kellys_eye

    kellys_eye

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    That motor is some 4" x 2" in size...... I think you've mistaken the 500KV rating for something other than what it is.

    The 500KV figure indicates that the motor will rotate 500rpm for every volt applied (as good as).

    Some of the figures given for their operation are also misleading to those unfamiliar with the technology.

    You need to find the 'maximum continuous amperage' for the motor and its 'maximum usable voltage' to get a comparison between 'power' availability.

    https://www.tfl-hobby.de/Motore--Ha...FL-56-mm/56104/BL-Motor-56104-790KV---9D.html

    the link shows data for a motor similar to the one you are considering. It states that it's a 14000W motor! Not true. If you use it's 'maximum' amps and maximum volts you find the motor power is 2.8 x 63 = 176 (or approximately 180W, pretty standard for the size).

    Those brushless motors use pulsed power to generate rotational movement and the maximum pulse amplitude that motor can take is 220A - as a brief pulse - whose average value works out at 2.8A. The advertisers are bending the description somewhat by using the 'peak' pulse values to give a wattage rating.

    180W is 1/4 of a horsepower...... no where near enough to do the surf thing. Hevans1944 quoted a typical board using a 2kW motor - some 2-1/2 horsepower in real world numbers.
     
  19. Audioguru

    Audioguru

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    These little motors are designed for toy boats, not large hydrofoil surf boards.
     
  20. Audioguru

    Audioguru

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    Sep 24, 2016
    The hydrofoil lifts the board out of the water decreasing drag which increases speed. But the motor's fan must spin fast enough in the water to do it.
     
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