Pedal (assist) boat prop pitch

Discussion in 'Boat Design' started by jakeeeef, Nov 7, 2021.

  1. SolGato
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    SolGato Senior Member

    This video and other reports seem to suggest that’s all in -boat, motor, battery, gear, and passenger.


     
  2. jakeeeef
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    jakeeeef Senior Member

    Thanks,
    I can find up to 6kw of power with my plans but what suffers at that level is the range and the sensation of pedal assist. Quite how much, I need to work out very carefully.
    That Honwave in the clip doesn't look like its planing to the point where it can be backed off and remain on plane. So it's only able to plane at all at a very expensive part of the power/ speed, range curve. One of the big challenges of low power boat design, and one not well met by a traditional RIB or SIB design.

    I worry about the efficiency of Torqeedo's hub motor.

    I know though that it's 6 of one, half a dozen of the other, when it comes to hub motor, versus a motor aboard the boat and bevel or Flexi drives. You either suffer the hydrodynamic drag of a hub motor's additional diameter or the mechanical drag of a mechanism.

    For them though it's a no brainer as thier design wins hands down on cooling.
     
  3. jakeeeef
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    jakeeeef Senior Member

    Ignore what I said about the $100 hobbyist version of solid works. It's called 3DExperience, has universally TERRIBLE reviews, saying it is buggy to the point of being unusable, with zero support.

    There are free packages that are said to be better.

    I need to take a look at solid edge, and the free tier of autocad 360 you suggest.
     
  4. SolGato
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    SolGato Senior Member

    Actually if a controller is designed properly, it provides the best efficiency in the higher RPM range.

    Is it using more power yes, but watts in versus watts out it should be more efficient at cruising speeds.

    Typically with a PWM controller this is around 75% throttle.

    The challenge you face is achieving the low RPM you desire while being able to swing a very large propellor.

    The only reason that Torqeedo motor can reliably make use of such a large aggressively pitched prop is because it uses a gear reduction and is being used to push a lightweight vessel.

    I think you would need a pretty big and powerful motor to be able to handle the loading associated with turning such a big prop to get a craft to the speed you are looking to achieve, and a controller designed to handle those loads. You’ll also need to make use of a gear reduction.

    The E-foil crowd has already solved a lot of the challenges you face with regard to propulsion -small compact motor, battery and control system, enough power to get on plane, good cruising speeds once on plane, minimal drag, etc..

    Their formula for success is built around very power small motors spinning at very high RPM coupled to a gear drive to turn a small propellor.

    Maybe instead of designing only around low RPM turning a big prop, you might think about a small motor and prop at high RPM to get the vessel going fast enough to plane, and then from there a pedal system spinning a big prop might be used to maintain momentum once the craft is on plane. This of course is based around incorporating some foiling technology into the craft. The idea being that a non E foil board requires a lot of effort to get going, and then once it is up out of the water and planing, only requires occasional pumps to keep it going like a bike.

    A hybrid if you will.

    I know it’s not the “pedal assist” experience you are looking to achieve.

    Anyway, just something to think about.
     
  5. alan craig
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    alan craig Senior Member

    That Torqeedo in the video above sounds worse than my home made brushless outboard. Reviews suggest that the E-Propulsion outboards are quieter, they are direct drive.
     
  6. SolGato
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    SolGato Senior Member

    Yes it’s one of the things I dislike the most about them, although phones and camera typically accentuate the frequencies.

    I can only imagine what the marine life thinks!

    My brushless motors are so quiet I often sneak up on Honu, fish, birds, even other boaters, kayakers, paddlboarders, etc..

    Torqeedo’s line of motors have been considered so noisy by some, that they introduced a new direct drive 1103 motor advertising the reduced db levels over their 1003 gear driven counterpart. They have outfitted the motor with a smaller less aggressive prop pitch (10.2” diameter x 6.2”) versus the gear driven 1003 equivalent (11.5” diameter x 8.1” pitch) due to it being direct drive, and yet it is said to still be more powerful and efficient.

    In fact I believe they may have phased out the old gear driven line of small motors all together now.

    One other nasty little problem with the older model is it’s track record for shearing prop pins. In my opinion it was undersized likely due to a limited prop shaft diameter, and with the torque and gear reduction, and probably the users enjoyment of “gunning it”, they would shear the pins quite easily according to a number of owners and the complaints I’ve read.
     
    Last edited: Nov 14, 2021
  7. Skyak
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    Skyak Senior Member

    PWM on conventional brushed DC motors have poor efficiency except at a specific rpm, toque and voltage. What Jakeee is talking about is responsive to human input -can't use that setup efficiently. He needs brushless DC. It still has a peak efficiency, but the controller is always working to maximize efficiency off peak. Again, I suggest watching Will Frasier's videos. Prop efficiency favors larger slower higher toque props. The mass of the water accelerated X acceleration = thrust. Putting the motor down by the prop minimizes the drive and simplifies cooling, but it limits torque gained from geometry. Gears can convert speed to toque at a cost of efficiency, money and complication.
     
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  8. portacruise
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    portacruise Senior Member

    Isn't any planing boat going to be wasteful In Design, compared to a model that is significantly utilizing solar power? What are some examples of planing boats that have very large slow-moving propellers? If a hydrofoil boat cannot be significantly sustained on HP, why is there expectations that a planing boat can be significantly sustained on HP alone? How is it possible to amplify the contribution of HP, by adding more weight and complexity associated with battery electric powered boats? I have lots of questions, just don't know what am I missing here?
     
  9. jakeeeef
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    jakeeeef Senior Member

    You have correctly spotted that with the parameters I have given, something has to give.

    And that is the only parameter I have not given. Range.

    But bear in mind ebike and e-scooter batteries are relatively good value, and increasingly becoming available in standard fitments/ configurations.

    It's partially why it's a leisure craft, not a more serious professional use boat. Never too far from a spare battery!

    With it being pedal assist it's still navigable when out of battery or when conserving battery range, albeit at a vastly reduced rate, and definitely not planing.

    It's a big change in ethos from where (internal combustion) boats are now. In terms of performance per £ or $, a big backwards step! But this is where we are.

    Because of this it clearly won't be for everyone, but the only real way to find out whether a boat of this 'ethos' is something that people want to and can live with is build at least a prototype and see if it's offering something genuinely new that people can't get elsewhere.

    After all, who, apart from a handful of manta owners know what a pedal assist electric boat actually feels like? They just aren't something you can go out and try or buy.

    How far can a trained athlete get in minimal assist, maximum range mode? How far will an overweight guy who doesn't want to pedal at all get?

    Will either or both of them gain pleasure from the experience? Will either of the numbers be deemed acceptable? They will all look poor for people used to thinking about electric land vehicles!

    How will the boat be optimised re the above scenarios? Will it end up a 4.5 knot displacement design, but one that anybody can do 4.5 knots with but the athlete can do it all day long.

    Or will it end up optimised to let a fat guy do 15 knots for 10 minutes?

    I'm aware planing is extremely marginal, and I will be working with a NA on that score. If it has no hope of planing performance, then I have big decisions to think about.

    One thing that in my view isn't open to question is that in ten years pedal assist electric boats will be 'a thing'. Maybe not a big thing, but a thing. I'm sure of it.
     
  10. kerosene
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    kerosene Senior Member

    for ref. KTM has had electric dirt bikes out for quite some time. I think the current one is 2nd or 3rd iteration of the freeride. 18kW motor with 3.9kWh battery. swappable battery but battery costs £2.800

    They have decent volumes so I don't think that is far of from realistic retail pricing of a Lithium kWh. I know that bare cells and bike batteries are much cheaper.
    But can ebike battery give 3-4kW constant duty?

    Screenshot_20211117_014358.jpg Screenshot_20211117_014309.jpg
     

  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    As previously noted - this is your first and only route.
    You need to establish this from the outset, otherwise it is just hot air and speculation.

    Just as Sir Clive Sinclair was ..?? :rolleyes:
     
    BlueBell and bajansailor like this.
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