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Thread: Help please. These motor Kv`s are really confusing me.

  1. #1
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    Default Help please. These motor Kv`s are really confusing me.

    Ok – Noob question alert !! Please be gentle with me

    I have a couple of FE monos that I am about to start building and am collecting stuff for. I would like (eventually) like to race them (at club level) and obviously want to start with something that is heading towards a race set up.

    I have searched high and low for info on set up and motors and things, put together loads of kv/rpm spreadsheets and interrogated FE people until their ears bled and am still none the wiser.

    Please help before I give up and set light to it all and dance around the flames with a pair of pants on my head.

    Mono 1 –
    620mm hull length
    3s

    Mono 2 –
    800mm hull length
    4s

    So lets start with a motor

    On 3s the kv x v thing comes out at
    38850 rpm with a 3500v motor
    At 80% efficiency that gives around 31000 rpm

    On 4s the kv x v thing comes out at
    38924 rpm with a 2630kv motor
    At 80% efficiency that gives around 31000 rpm


    All of these seem OK for RPM according to the web people and their calculations

    Chatting to the FE people it seems that they are all running 2500kv or less at 3s and 2200kv at 4s

    They all seem to have pretty fast boats that run the required 5 mins for a race

    What am I missing here? Why are my calcs so much different to theirs?

    Andy

  2. #2
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    Where abouts are you? the 2 letter location for the forum doesn't really work well for those outside USA.

    What hulls do you have?

    If you are still collecting gear for the Mono2 go with 6s not 4s, the lower amp draw makes things run cooler and more reliable, and you may be able to use a cheaper ESC. At this pointonly a few people with old setups are running 4s, as you're starting fresh there is no need to look at 4s unless you have a bunch of suitable 4s gear from another hobby.

    What the web people are running is much closer to what people are racing in my area, 3000-3900kv with 3400 being the most usual for Mono1, and 1500-1900kv with 1700 being the most usual for 6s. I have seen a few Mono2s on low kv as the bigger hulls will take a bigger prop, but I've never seen a Mono1 with as little as 2500kv, and I don't think it would work well.

    All the Naviga surface drive classes inc Mono1 and Mono2 are of 6 minutes duration with 10 second mill and in lap, so you need a run time of about 6.5 minutes.

    I am not sure why the disparity, it may cause offence and I am probably wrong, but with the 4s, low KVs and 5 minute races, all I can think of is that it is an isolated club with no outside competition, and is a dozen or so years behind the times. Are they using stepped hulls? If not that would explain the low KVs, unstepped boats needed a lot more prop to push them.
    Paul Upton-Taylor, Greased Weasel Racing.

  3. #3
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    Paul, Andy T is listed from the UK.

  4. #4
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    It's possible to run lower kV on 3s and be competitive.

    My revolt on 3s would be way faster than those mono 1's and I am using a 2080kv rated motor.

    I have a mono1 myself, an exceed shark, I had in a 5050kv 2860 tacon ( leopard 2860 5050kv equivalent) and the prop was an octura m435 3 blade on 2s. For the record the octura m435 3 blade could be one of the fastest 35mm props you can get your hands on.

    The speed my shark was doing my revolt probably would lap it a few times in a 6 minute race. To put it in perspective the revolt would be doing 3 laps for every 2 the shark would do.

    The point I am making is a lower kV can be competitive and faster if you setup right.

    Example I can use a leopard 3674 2200kv, 3s an octura x642 in a 26 inches non step hull. Have a handling edge over the smaller mono1 and be a little faster at the same time.

    Example 2 do not know if a 30 inch non step hull would be legal for mono1 but also the revolt hull with a 1730kv leopard 4092 with a m545 on 3s is likely to slaughter any mono1 with the typical 3400kv 3s and 36mm prop. The revolt with that setup would pull less amps so you can run full throttle longer, it will run threw the wake of the smaller boats without being affected like a typical smaller hull would. And at the same time likely having more top speed than the mono1's with all the above listed benefits.

  5. #5
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    Hi again - thanks for the responses - much appreciated

    I am based in the Yew Kay and have no idea what the actual hulls are. Both are non- stepped though.
    They are both second hand and look pretty dated. They were free/ cheap though so are more of a "build it and see" thing rather than an out and out world beater.
    One has the words "Warrior 36" down the side and the other has nothing at all. I will post pics at some point.

    You may be right about living in a vacuum. Its a small club with a focus on sailing, scale lifeboats/ tugs and submarines. There are a few of us who do FE and they all run similar boats so its likely we are still living in the 1980`s. There is almost certainly a reason why they are limiting it to 4s - I just don't know why that is.

    At the moment I have a
    Leopard 3660 2450kv for the bigger boat on 4s - Comments please?
    Leopard 2848 2630kv for the smaller boat on 3s
    I haven't made my mind up about the little one yet. Looking at the responses above I think I may have to look at the smaller boat again. Any suggestions gratefully received

    We run on a purpose built Victorian boating lake. Its 400 feet x 80 feet (ish) and has a brick wall holding all the water in. Many a modern boat has met its end when faced with Victorian engineering.

    I don't have plans to travel about for competitions (yet) and am quite happy to crash my boat 2 miles from home rather than drive for hours to do it in another part of the country. I have a few 4s lipo packs so will stick with those for now.

    Anyway - this has given me lots to think about. Than you so much.
    Andy T Boater

  6. #6
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    OK - quick update
    The 4s limit is a local one - so I will stick with this for now.

    Boat performed perfectly with 2450kv Leopard on 4s.

    Until I put it in the water.

    COG is 28% with 4s and 30% with 3s

    42mm 2 blade prop is going round and round and making the boat go forward, So that's a bonus

    Beautiful in a straight line it rides pretty much perfectly and looks great.

    It cavitates/ ventilates like crazy when starting off so there is a bit of work to do there. Keel on the bench the bottom of the prop tube is about 2mm off the deck.

    Cornering is a different matter though. At about 3/4 throttle it banks and turns and hold the water brilliantly but with a very wide arc. Any attempt to turn sharper results in a pirouette that Olga Korbett would be proud of and quite a lot of sitting upside down in the water.

    Left turns are even worse!!! Any slight left stick just throws it up out of the water and nosedives it back in. Spectacular and very pretty but not ideal. I have a feeling the self righting tube is a bit low and catches all the water on the left side and just hooks the boat in. To solve this I have removed the plastic pipe self righting tube I put in as it obviously doesn't work, patched all the holes and moved things about a bit.

    Rudder is now closer to the prop and also lower in the water.
    Turn fin is slightly bigger and deeper in the water.

    Will update after next Wednesdays run.

    Andy T Boater

  7. #7
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    OK

    Following on from the above I managed to get it all working but with a heavy boat it drove brilliantly on 3s but was uncontrollable/ upside down at 4s.
    I haven't crashed in 2 weeks so its time to move to a lighter, more race orientated boat.

    I have the choice of these to motors to put in my new creation.

    Boat is a mono
    800mm hull length
    4s (but probably 3s as I have more of these.)


    Model KV* No-Load current (7.4V) Max Amps (A) Max Power (W)
    LBA3660-10T 2450 2.9A 61 1100


    LBA32848/15T 2630 0.9 27 560

    So - my questions for you, dear readers are:_

    Is a higher or lower No-Load Current better or worse?

    Is 61A better than 27A and why?

    Do I need 1100w or will 560 do? (this based on the more of something the better it is.)

    Which one do I spend all my time and money putting into a boat?

    Please help

    Thanks
    Last edited by Andy T Boater; 06-26-2019 at 11:27 AM. Reason: Trying and failing to get the numbers lined up

  8. #8
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    We need more information in order to help:

    - exactly what is the boat, brand model and hull type (deep vee, shallow vee, etc.). “800 mm mono” tells us very little.
    - what speeds and run times are you looking for?
    - what cell capacity are you using? Is there a pack weight limit?
    - what voltage is most important to you? A single motor will not be optimum on both voltages.

    Focus on motor size and Kv, not on amps or watts (since these are pretty much arbitrary depending on the manufacturer and you can easily exceed them anyway by using too much prop). But to clarify, 61 amps will be more powerful at the same voltage but shortens run time. Example: 61 amps with 4000 mAh packs will give a bit over three minutes run time but will be quite a bit faster than 27 amps. On this side of the pond, a 31” boat averaging 27 amps will be pretty slow on 4S. It were me - and it isn’t - I’d choose the larger of the two motors.


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


  9. #9
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    Thanks - I have put more info below. All these numbers must mean something _ I just cant work out how to rate one motor again another

    Quote Originally Posted by Fluid View Post
    We need more information in order to help:

    - exactly what is the boat, brand model and hull type (deep vee, shallow vee, etc.). “800 mm mono” tells us very little.

    Its a lightweight glass fibre deep vee of an unknown make. It was taken from a mould owned by one of the blokes at the pond

    - what speeds and run times are you looking for?
    As fast as possible for a 5 minute race

    - what cell capacity are you using? Is there a pack weight limit?
    No pack weight limit and I am planning on either 3 or 4 s.
    I have a few 3s and only one 4s so will probably stick with 3s.
    Its a local thing so I can run in either 3 or 4S class
    They are all 5000Mah 60C packs


    - what voltage is most important to you? A single motor will not be optimum on both voltages.
    3S is probably more important

    Focus on motor size and Kv, not on amps or watts (since these are pretty much arbitrary depending on the manufacturer and you can easily exceed them anyway by using too much prop). But to clarify, 61 amps will be more powerful at the same voltage but shortens run time. Example: 61 amps with 4000 mAh packs will give a bit over three minutes run time but will be quite a bit faster than 27 amps. On this side of the pond, a 31” boat averaging 27 amps will be pretty slow on 4S. It were me - and it isn’t - I’d choose the larger of the two motors.

    So given an 800mm deep vee mono running 3S for 5 minutes what Motor size and Kv would you suggest?


    .

    Cheers for the help.
    Andy

  10. #10
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    As suggested the focus should be on motor size and kV rather than no load current and rated watts.

    But to help you understand a little better smaller motors have lower no load current, lower max current rating and lower total power rating, when compared to a larger motor of similar kV.

    Lower no load current because the rotor is lighter therefore easier to turn.

    Lower max amp rating because the winding in the stator have motor turns, and higher internal resistance, so the current handling ability is lower because of higher resistance. The limit rating is because the manufacturer know the motor will burn up if too much current runs threw the motor.

    Same goes for the power rating.

    Back to the boat now, the bigger motor holds the rpm better underload, it can handle a bigger prop, can dissipate heat better.

    If it was me, a 4074, 1800kv would be my choice at minimum for a 800mm boat.

    I run a 4092, 4200watt rated motor in a 750mm boat just for reference. I can tell you from experience that size motor is much better than the smaller motors, on 2s, 3s, 4s in a boat of 700mm and up.

    I have ran a leopard 4092 1730kv on 3s with an octura m545. That would get that 800mm boat moving decent enough. At 4s I hope your hull is good because a lot more speed awaits.

  11. #11
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    Hi Speed3 - thanks for the info. I think I am beginning to understand some of the numbers now

    You say an 1800kv motor on 3s would be a suitable choice but plugging the numbers in (Kv x V) gives unloaded RPM of
    3s - 19980
    4s - 26640

    at about 80% efficiency that's
    3s - 15984
    4s - 21312

    This is the bit that really confuses me. All perceived wisdom is that an RPM of around 30k is good
    That would suggest an 80% efficiency of:-
    3s - 3500Kv - 31080rpm
    4s - 2630Kv - 31139rpm

    I understand that theory and practice are two very different things but the disparity between what is recommended by the theory and what people actually run is melting my head.

    Are you saying
    Lower Kv - Lower revs - bigger prop
    Higher Kv - Higher revs - smaller prop
    Would give the same (ish) speed?

    Sorry for my ignorance
    Andy

  12. #12
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    OK - just calculating stuff so could somebody please check this and let me know I am on the right track please


    Given that
    Amps = motor watts/ voltage - for the 2 motors I have
    My 2450Kv motor gives 1100/11.1 = 99
    My 2630Kv motor gives 560/11.1 = 50.4

    Battery Mah rating (times) 0.06 (divided by) motor load in amps = run time

    For the 2 motors I have
    A 5000MAh battery would give a run time of

    2450Kv - 5000 x 0.06/99 = 3.03 minutes
    2630Kv - 5000 x 0.06/50.4= 5.95 minutes


    With an 1800Kv that Speed3 suggests (I used a Leopard 4074/1800 because its the first one I found on Google)

    Amps = 2600/11.1 = 234
    Runtime = 5000 x 0.06/234 = 1.2 minutes

    What am I missing here?

    Andy

  13. #13
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    The reason the lower kV performs so well is because of the efficiency of the bigger prop versus the smaller prop.

    A boat that is 800mm long is 32 inches. That size boat need to use a prop from 40mm to 47mm in diameter.

    The 2848 2600 ish kV would have to use a prop of 35mm on 3s. That size prop might not plane the 800mm. So that is not a practical setup for that boat.

    Then you have the 3660 2400 ish kV leopard. The motor would be suited for a 37 to 38mm prop on 4s. Maybe the 37 to 38mm prop might plane the boat.

    The motor might rev that size prop at 28500 rpm.
    But this is what happens with that setup. That prop is only 35 to 40 percent efficient on the 800mm boat. So the prop should have 2 inches of pitch but at 35 percent efficiency it only translate to .7 inch pitch per turn.

    That will give you 18.9mph even though you are reving 28500rpm.

    At 40 percent prop efficiency you would get 21.6mph.

    Now the 4092 1730kv leopard would rev a m545 at around 18100 at 3s and 23500 at 4s.

    But this is where it gets interesting the prop efficiency for the m545 maybe well over 80 percent in your case.

    So let's say the prop has 2.5 inches of pitch and it 85 percent efficient on your 800mm boat, that give 2.125 inches per turn.

    On 3s at 18100 rpm the m545 would have your boat doing 36.4mph.

    On 4s at 23500 rpm the m545 would have your boat doing 47.3 mph.

    That's the difference there. So even 3s at 18100 rpm is given you 36.4mph versus the 2400 ish kV 3660 reving 28500rpm but on given you 21.6 mph.

    And do not think about trying lil motor an big prop. That's a smoked esc, and melted solder on order right they.

  14. #14
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    Speed3

    You are my new hero and possibly the greatest living human on the planet.

    That sounds perfectly reasonable. I understand a bit more now. Thank you.

    A new motor seems in order.

    Looking at what is available in the UK it seems we are a bit limited.

    4092 1730kv leopard is not available in the UK as far as I can tell.
    Google is not my friend (:

    4092 seem to be either 1650Kv or 2000Kv

    4074 I can get an 1828Kv
    Its an RCECHO® RCS Model 4074/4D 1828KV so I imagine it will explode or collapse into a black hole within minutes of me using it.
    But at £48 a go I may just risk it.

    Or should I go for the 4092 2000Kv?

    Unless anyone can tell me of a better one available in the UK?????

    Thanks

  15. #15
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    Forgive the garish colours
    This is due to an abundance of rattle cans

    20190627_085200.jpg
    20190627_085215.jpg
    20190627_085233.jpg

    Also went with a very slight glitter in the white gelcoat. This does not show up well in the pics

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