Pedal (assist) boat prop pitch

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

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    As you wish.
     
    Last edited: Nov 9, 2021
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  2. kerosene
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    kerosene Senior Member

    outboards power is at the prop shaft. E-motors frequently report at the motor or, even worse, the power consumption.

    Outboards are compromises in terms of speed range and prop but I doubt the power numbers are off. Of course it is more pleasant to operate e-motor at peak power than a 2-stroke.
     
  3. portacruise
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    portacruise Senior Member

    How to cut down the weight even more, haha:

     
  4. portacruise
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    portacruise Senior Member

    Lots of losses when using a pedal generator and involving batteries, see post 363.

    Pedal Powered Boats https://www.boatdesign.net/threads/pedal-powered-boats.23345/page-25#post-293555
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    150kg assembly, excluding the user ? Hull, batteries, ironmongery, the whole lot ?
     
  6. portacruise
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    portacruise Senior Member

    Just because the Manta riders are cranking real hard and doing work, doesn't mean they are necessarily contributing anything towards the propulsion is my opinion. That smallish looking propeller doesn't look compatible with human power unless they're using a huge step up in Gearing. That huge step up in Gearing might have a lot of efficiency losses...
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    Do you read what you write? A 4 hp outboard is around 55#, and 3kW. Fuel tank and inflatable puts you and your bateau at around the power I said, at the weight I said. And then you tell us that it's the shape I described. Perhaps if you use math more, and intuition, whims and hunches less, you will accomplish something mighty. I certainly hope so. I'm irked, but still in your corner.
     
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  8. jakeeeef
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    jakeeeef Senior Member

    But from what I've read elsewhere, 3 KW net is optimistic for a 4 hp outboard with its standard little bunny ears prop and its big draggy gearbox, bevel drive operating in thick final drive oil, turning an impeller etc.
    My drive leg will have less than half the frontal area, no bevel gears, no impeller.

    It's important though that I get this ironed out, because I can be reasonably confident that my boat would plane if I was to bolt a 4 hp outboard to the transom, based on the above inflatable dinghy analogy, so it's a reasonable starting point.

    At some point, I'll be employing a naval architect to optimize my design, but at this early stage for me it's about working out whether planing is broadly feasible, and in KW at the motor end roughly what it would cost.

    If it doesn't plane, it will go roughly the same speed as a well designed kayak, only at 4 times the cost, so definitely a commercial failure.

    I can see though it's not easy answers, and no answers at all when I can't provide details of the gearing and transmission system that it will use ( for calculating losses).

    However, it's a small boat, lying within my skill set when it comes to building a prototype, but as I say worth chatting to a NA under NDA before I start cutting carbon.

    This whole process is a massive learning experience for me, including and not limited to:
    Obtaining and learning Solid Works,
    Researching self patenting best practices and pitfalls and the patent process timelines,
    Possibly setting up a limited company,
    Studying crowd funding successes and failures.
    Researching available funding opportunities.

    The way I see it, if the boat succeeds or fails, planes or doesn't, sells or doesn't, flies or flops on crowd funding etc. I will have learnt such a lot of valuable stuff on the way, the exercise as a whole will have been a valuable one that has given me knowledge I can apply to future innovations. Perhaps sow seeds to turn a profit on a future project.

    I think it's the only attitude to have for the solo inventor. Otherwise the likelihood of disappointment is far too high.

    Because I will be doing all the above personal development work alongside prototyping, it's likely I'll be quite a way down the track before I can confirm real world performance. If performance was to be disappointing to the point of scrapping the project, then the skills learnt so far can be moved across and continued on another project so it's not wasted time.

    This is what I'm telling myself anyway!
     
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  9. jakeeeef
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    jakeeeef Senior Member

    ...And it's going to teach me a lot about composites, (already has done), and force me to finally set myself up with a much more professional ( mini) composites lab etc.

    If in a year or two's time, I'm reasonably confident with Solid Works, have built useful contacts in funding bodies, and naval architecture, can pen a reasonable self patent, have moved my composites skills on a long way, maybe got started on 3D printing it won't really matter if this thing cost me £20 grand does 3 knots and takes 6 KW to do it!

    But I digress, and still do wonder what net power a 4hp outboard produces!
     
  10. bajansailor
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    bajansailor Marine Surveyor

    Jake, did you hear about the Walker Wingsail Company in the 1980's?
    The Walker Wingsail Updated? https://forums.ybw.com/index.php?threads/the-walker-wingsail-updated.456603/

    John Walker was of a similar (?) mindset to you - he had the gift of the gab, and sold his idea to investors (is crowd funding the modern version of this?) - and it was like a new religion, where he was the High Priest.
    Whenever he needed more money (which was frequently) he would put out an appeal to his fan club, and they would hurl $$'s at him with gay abandon.
    So it is possible if you adopt the right mindset :)
     
  11. jakeeeef
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    jakeeeef Senior Member

    As a former UK yachting journalist the main thing I know about John Walker and his wingsail was the legendary amount of money he (allegedly) took off Yachting World as a settlement after they wrote a bad review of it.

    It nearly finished the magazine I'm reliably told.

    I guess we're of a similar mindset in that he identified that you have to take your profit where you can in the innovation game. Probably earnt a lot more off IPC Media (or whoever owned YW at the time), than he ever did trying to make and sell the things!
     
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  12. jakeeeef
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    jakeeeef Senior Member

    You vastly overestimate my interpersonal relationship skills!
     
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  13. kerosene
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    kerosene Senior Member

    outboards power rating is measured at prop shaft. ie all those things you list have already been taken out. At flywheel the power is more than the rates number.

    I think this is true for sub 10hp motors just like it is true for the larger ones.

    As I mentioned earlier electric motor sales people and evangelists like to make these claims to justify low hp on electric motors.

    I am a fan of electric but hate the inaccurate in dishonest sales tactics.

    Many small OB props are not efficient as they have been designed to work for many uses. But that doesn't change the shaft power.
     
  14. DogCavalry
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    DogCavalry I aim to misbehave.

    "Many small OB props are not efficient as they have been designed to work for many uses. But that doesn't change the shaft power."

    Indeed not. And Crouch came up with his formula after a large amount of empirical testing in the 1920's. Drive trains were hardly efficient then. Props in particular are better now. Even bad ones are still better than 1920's props.
     

  15. jakeeeef
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    jakeeeef Senior Member

    It's funny you mention the 1920s.
    It chimes with I thought I've been having.

    That's about when power to weight ratio of internal combustion engines really sky rocketed.
    In my view. Since then, naval architects have been spoilt by the easy available power.
    Your boat doesn't quite plane? Stick a bigger motor on the back, more power for very little penalty. It's been the get out of jail card for too long and it's made naval architecture a lazy discipline in my view. Certainly on small boats. I talk to NAs often in my job, but I've given up trying to talk to them about getting maximum performance from very low powered boats, they have no interest in it at all and say shocking things that suggest they have not thought about it for a very long time.

    Naval architects are also taught almost nothing about how to make small boats go fast on very constrained power.
    It's never been any use to them in their career for designing a tug, supertanker, or even a sailing boat, so you can't blame the universities for trying to make their graduates employable!

    The only place I know of that seriously gets into pedal boats, low power small motorboats and seriously challenging things like getting a sculling boat up on foils is TUDelft. But that's the exception.

    But my view, as an outsider to the industry is that a lot of the answers re hull optimization and decarbonisation are to be found by limiting the power. A lot. Soon we'll have no choice.

    So, no surprise to me that the rules of thumb we are relying on relate to 1920 s technology. The 1920s was about the last time, professionally, anyone cared about how much net propulsive thrust a 4 hp engine actually does give!

    Probably also why the world speed record human powered boat (wind assisted!?)was designed by an aeronautics engineer.
     
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