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Thread: 20k rpm and bigger prop the way to go?

  1. #1
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    Default 20k rpm and bigger prop the way to go?

    Hello, this is my first thread start here at OSE, the reason is that I wanted get you thoughts on something that has been on my mind for a while now.

    I'm a part of a group of fun loving amateurs the meet up on a yearly basis to "compete" with homemade hulls, this is not at all that serious but of course winning is the game.

    The rules are:
    Max 1m hull length (drivetrain and rudder not inlcuded).
    Home made (DIY hull), no limitations on material or hull design (mono, cat, outrigger etc.).
    Electrical propulsion (although the number of motors to be used is not specified).
    Max 4S. (the number of batteries to be used is not specified).

    My contribution for the last couple of years has been a 750mm long home made aluminum rigger (race photos below), glued 0,5mm thickness, SSS 2970-2250KV, Flier 120A ESC, drivetrain from a dragon hobby sword (stiff axis), this boat is fairly light and surprisingly seaworthy since it has quite high sponsons and main hull, the prop's used has varied a lot and this bring me to my question.

    Swapping the 37mm three blade dragon hobby stock prop for a significantly larger 51mm Grauphner two blade made a lot of difference to the better, the boat sprints better, is less nervous, and achieves higher top speed, but it is noticeable that I do not get the same top revs out of the engine, I understand this puts a lot of strain on the setup (ESC, engine, batteries) but it held up fairly well the one day I used it ( ~4 runs) and the temperatures did not seem excessive.

    So now I'm thinking that I should take this (less revs more prop) further, and do it a bit more correctly (perhaps) when designing a new outrigger 1m long in aluminum.

    I'm currently looking at some motors from SSS, both 4092 size category, one is 1350KV (4D) the other is 1800KV (3D), they would provide roughly 19548 and 26064 rpm unloaded respectively on 4S, both of these are significantly slower (in terms of revs) but at the same time vastly more powerful in terms of torque compared to my current motor (2970-2250KV) which would give 32500 rpms unloaded.

    My thinking is to go for the extreme and try the 4092-1350KV (19548 rpm unloaded) hoping that it will not drop as much in revs once loaded, and pair it with a ~60mm high pitch 2 blade prop and get the speed that way, it makes sense to me go this route and I really want to try it out (please note, I'm an amateur), of course I need to step up in terms of ESC and drivetrain, perhaps go for a 180A and a 1/4" system.

    Please let me know what you think of my reasoning, I want to try the lower rev bigger prop route, if you have any better suggestions (most likely :) ) please let me know, but I would like to reside in the ~20k rpm range.

    All the best / Anders Martinsson

    RC2019winner.jpg
    Last edited by martinsson; 08-06-2019 at 01:05 PM. Reason: added picture

  2. #2
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    For an outrigger that size (<30”) really big props could be a bad idea due to excess prop walk. Comparing 28mm motors to 40mm motors is a mistake, you could just as easily use a 40mm motor with a 2200 Kv for more torque. I have gone very fast with relatively low rpm setups, and it can work with larger hulls. But IMO a 60+ mm prop is just too big for a 75 cm hull. For your rigger I suggest the following:

    36x75 2400 Kv quality motor
    150 amp ESC
    x648 or 1916/17* prop

    This should give you a substantial speed increase without the problems of a huge prop. (There is a noticeable difference just getting rid if the cheap plastic props.)

    You can of course try the large prop, but you may not like the results. Or you may.


    .
    ERROR 403 - This is not the page you are looking for


  3. #3
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    Thanks for answering Fluid and I value your opinions they do seem to make a lot of sense for my current rigger, but just to clarify, the new rigger hull will be 1m (~40") excluding the add-ons (rudder etc.). that's a bit of added weight but also more overall stability.

    The competition are all running aquastar/hobbyking gear which seems to be very nice stuff, not to mention cheaper than the SSS/Flier combo I use, both of my main competitors use the 3974-2200KV Aquastar motor and hobbyking 120A ESC's, one in a mono the other in a cat, both 1m, we all run on 4S yet I beat them mainly due to better cornering capabilities and the fact that I can go full throttle without going airborne despite having the lightest both on the course :) but i simply cannot keep up with them on the straights nor on the acceleration.

    I'm considering a 1/4 system mainly due to the availability of bigger props, but perhaps a 3/16 will suffice?

    Adding a picture from the start (stand still) just to keep the entertainment value up :)

    PS. the unpainted alu mono had some power issues this race, and the yellow cat did a slight head start.

    123go.jpg
    Last edited by martinsson; 08-06-2019 at 03:34 PM. Reason: added photo

  4. #4
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    I have done some further research and seen that lower Kv is not equivalent to more torque straight off, rather is more torque for a given current, and that this gain in torque decreases up to a certain level of rpm, so it seems to indicate that lower Kv means more torque below a certain level of rpm.

    Where the break point is is not clear to me at present since it seem to depend on the internal resistance of the entire distribution chain from the battery to the motor put in relation to the internal resistance of the motor is self, and this is not even including what propeller I will be using, which will impact the rpm, so it will be (at least for me) a bit of guesswork.

    I want to run on 4S with lowers revs and more torque, making it possible to use a bigger prop (60+mm) with decent pitch.
    Why I want to try this is more difficult to describe, it just makes sense to me, I understand that this may not be the fastest option, but I'm fine with that.

    Having thought a bit more about it since the previous posts, is to go for something like this :

    TFL SSS 4092-1800Kv (in theory 26064 rpm unloaded on 4S)
    3/16" drivetrain
    60-65mm prop with decent pitch (65mm being sort of the limit on 3/16)
    Flier 250A ESC (just to be safe)
    The batteries will need to be >4000mAh with a C of >50 (capable of delivering ~200A)

    The hull will be similar in design to my current 750mm (see picture below) but larger (1m in total hull length).
    A bit more wight, partly from the new components (motor, esc, bigger batteries etc.) but also from the hull it self

    What do you think? any help is greatly appreciated.

    martinsson 2019 rigger.jpg

  5. #5
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    60 to 65mm prop is too big. A 50mm prop with the sss 4092 1800kv. Is a better option for 4s.

    Having an ESC that can handle over 200 amps, and maybe a 6500 mah 50c or greater lipo would be a good idea.

  6. #6
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    Thank you Speed3, I'm very grateful for you answer, even if it completely crushed my dream :)

    Here I was thinking that surely a monster of a motor such as the 4092-1800KV was powerful enough (4300W) to turn a 65mm prop at just above 20.000 rpm (under load) with 4S given that the ESC and battery could cope, sticking to a 3/16" drivetrain, how wrong I was, by 10-15mm no less :)

    Damn... my low rev big prop dream just went south, so what kind of gargantuan motor would allow me to achieve my goal? or can my dream still flicker back to life if I go down in Kv to say 1650, or 1500? (more torque/ampere)? please? did I mention that it is going on a light and easy to propel 1m rigger

  7. #7
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    All I have heard here is prop diameter. Are we assuming every prop is a 1.4 pitch ratio? Also blade area and rake come into play as does 2 or 3 blades. Every hull has a certain diameter and blade area needed for maximum efficiency. Buy props in incremental changes. On my P rigger 38X84 2050kv I run X545 X546 1815/3 1817/3 1816/2 1817/2 and 1915/2. All work but on the data logger amps and rpm plus lap times will decide what will be a 2 lap or 6 lap setup. A larger motor will run more prop and take more juice. Going to a 40mm X 84 or 92mn motor will add speed and create handling issues. Somewhere there is a perfect breakpoint, weight to HP and handling. No magic one prop fits all you need to test.
    Mic

    Mic Halbrehder
    IMPBA 8656
    NAMBA 1414

  8. #8
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    I hear you HTVboats, thank you for providing a bit of insight into how it is done properly, and for the example, please note that I'm merely a fun loving amateur by comparison although I've been dabbing around with RC since the mid 80's, but back then it was very different as far as electric propulsion is concerned, as I'm sure you know.

    The "racing" I'm involved in is nowhere near the level of seriousness and cutting edge performance as one might find experience from here, and for sure I will not get even close that performance, I wouldn't even see it using a telescope, we are but a group of semi-old RC enthusiasts that meet up on a yearly basis to have fun, it is more of a social thing really, but when the countdown starts we all get rather focused :)

    I do plan to test several props, and as you say the diameter is but one of many factors to consider, I will try find a good starting point and go up from there, I have never used as powerful motors as the SSS 4092-1800Kv before so it is difficult to know what to expect, I currently use a 2970-2250Kv from SSS, and I understand there is a substantial difference in power, and torque.

  9. #9
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    In response to your dream about turning a 60mm prop, the 60mm prop is more suitable for a 50 plus inches boat. I have seen a 60inch mono hull run really well with a 55 to 60mm prop.

    A 39 inch rigger should do well with a 47 to 50mm prop.

  10. #10
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    Thank you Speed3, very valuable info, I'm starting to have doubts if a 4092 1800Kv motor is really necessary in that case, maybe a smaller prop like 50mm can be just as easily handled by a smaller motor (although something inside me falls apart at the thought :) ).

    Is your recommendation to go with a max 50mm prop purely performance based, or am I looking at something catching fire in the case of using 4092 1800Kv, 250A ESC, and ~200A capacity lipos, if attempting to use a 60-65mm prop?

    Sorry about all the questions, your recommendations are clear, I only need to understand the reasoning behind them a bit better.

  11. #11
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    With a 4092 1800kv on 4s. From my experience I will tell you trying to spinning a 60 to 65mm prop the amp draw is a major problem you will have to face.

    I can see even the 50mm prop pulls huge amps with the 1800kv 4092. I have done enough dataloggings to have an idea.

    You might be looking at 150 to maybe 200 plus amps with the 50mm prop alone.
    I really want to tell you to go 45mm with the 4092 1800kv. But I realize you want to use a big prop so I said 50mm.

    At 150 to 200amps you need a real good battery. 10 gauge wire, at lease 6mm bullet connectors, certainly a high amp ESC.

    I warn against a 60 to 65mm prop because you might burn up equipment or have your boat stop due to a melted solder joint, which will cost you a race.

    I relation to going to a smaller motor than the 4092 for the 50mm prop. I suggest don't go smaller. The smaller motors cannot handle the level of current that a bigger motor can. Especially 150 plus amps.

    If the races are a drag race that will last less than 6 seconds. You can get away with many things.

    But if you are doing an oval race. The bigger motor and a reasonable prop is the safe option.

    If you are doing an oval race parallel 4s would be a good choice with 150 plus amps.

  12. #12
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    Great information! you just saved me a lot of pain and money, I can now more clearly see how you arrived at your recommendations, and that in it self is very educational.

    See this is why I like forums, I have been developing and publishing DIY loudspeakers for professional applications with great worldwide reception going on more than 10 years now, and it was all made possible by the exchange of knowledge trough forums like this, it lets you access and take part of actual experiences, and learn from very skilled people each one perhaps with their own expertise in the field, just imagine how you otherwise would have gone about obtaining this knowledge, I almost dare not think of it, I write this with the purpose to let you (all) to know that I really appreciate the help and guidance I get here.

    I have so many more questions, but some of it need to learn myself, empirically, making mistakes is just as much a part of this field as any other and it makes you grow in your interest, and believe me, I will make a lot of them :)

    Back to the topic - would dropping the Kv from 4092-1800Kv to say 1650Kv, 1500 or perhaps even 1350Kv make any difference with regards to the "prop size - current draw" issue? I ask since I have heard that lower Kv means more torque per ampere, but realize I will drop below of the recommended RPM range of 25.000-35.000, but if this allows me to up the prop size perhaps I can get something back in terms of speed that way?

  13. #13
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    You may need to run less than 1000kv if you want to use a 60 to 65mm prop on 4s.

    The rpm will be low, less than 11000 rpm. With that low rpm your boat will be much slower than a 1800kv 50mm prop and 4s.

  14. #14
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    Thanks, good to know how the Kv translates in this case, in some way I wish I had the means to test it, just to see what it would be like in reality, but it is a bit to much money just for that.

  15. #15
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    I will say that 1800 kv motor should turn a 50 to 52 mm prop. easy. On a rigger you can probably go with a 55mm prop. If it were me I would go with a abc 2015, 2115 prop on your set up which is around a 52mm prop.
    Last edited by MarkF; 11-11-2019 at 09:10 PM.

  16. #16
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    I wouldn't put a huge prop with low KV on a rigger, I prefer small props and higher KV for riggers. Fluid gave you a setup that will work well, as he implied riggers and hydros in general are sensitive to prop walk, this is because the prop is so far from the CoG and the running surfaces that the leverage it has on the hull is huge compared to the leverage the prop has on a mono or cat, and props that work fine on monos and cats can result in very poor handling on a rigger of a similar size and weight. A rigger also has less drag when on plane and therefore needs less thrust, allowing you to get the same acceleration from a smaller prop and gain speed by spinning it faster.
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

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