Low KV and high voltage vs high KV and low voltage

[alanr77]

Hello, just wanting to double check some numbers and thoughts. Let's see if I have this correct; it would seem that a lot of builds use low kv motors and high cell count batteries. Example; 900kv inrunner powered by 10s lipos = a no load rpm of 33,300. (3.7v x 900 x 10) or 1800kv inrunner powered by 5s lipo = no load rpm of 33,300. (3.7v x 1800 x 5) The 10s setup will pull less amps due the higher voltage and therefore run cooler than the 5s setup. Is this correct? If this is true, what is the benefit of running a 5s setup? Cost? In addition, if the hull is setup to run 10s, 2x 5s packs, and I wanted to just play with the boat, and ran the 10s setup on 4s most of the time, the components would likely last a long time because they were not being pushed anywhere near their limits. Correct?

[iridebikes247]

Your logic is correct. There is a lot of interest in running hv right now I have hv and high amp setups, I definitely prefer my hv boats. I have small 4082 motors that pull more amps than my 5692s. If the boat is below 40 inches in most cases you might as well run a low voltage setup. A boat that runs a 10s setup on 4s will be slllowwwww lol. The components will last forever but the boat will only go like 20mph. Just wondering what boat and setup are you considering? HV is great but not worth the extra money in smaller boats in my opinion.

[alanr77]

Hello IRB, just running some numbers before I spend any money. I am looking at the 41" FCR Fantasm hull with the intent of just having another toy to play with. Currently building a vac-u-tug as a recovery boat so I haven't bought anything yet. I have plenty of 5000mah 2s packs sitting around for my other toys and figured I would build a 10s hull just for the fun of it. The 5s packs would be used when I wanted thrills but the 2s packs would be used when I wanted to play around. I understand it would be slooww on 4s and the numbers were strictly hypothetical. It would seem that this hull is very stable in the 50-60 mph range and would be a good candidate for what I am trying to do. As with all my toys, real and models, I don't like running things on the ragged edge of their capabilities. This is why I am considering a low kv/ high voltage setup. I am watching a really great build of this hull by a member named Cooper, between watching what he is doing and posting questions like this one I should be able to get in the ballpark- a solid, stable hull that has the capability to fly yet can be run for fun without maxing out the expensive components. So, considering the ampacity of heavy gauge wire, low amp draw of a low kv, high volt setup- I should be able to build a cool running boat; that being the goal. Thanks, Alan.

[iridebikes247]

I totally understand wanting to run a low amp setup, I've adopted this logic when building all of my boats. Just wondering have you considered twin motors? You could buy two leopard 4074 motors 1400kv and play around on 4s with x447 props. Only reason I am throwing this out there is because I haven't built a big single yet

http://www.youtube.com/watch?v=JIjZpm2JRpg

Thats a slightly higher amp setup and the 4092 pushes that boat with authority. Twins are just awesome, and with twin 4074 motors and turnigy 180 escs won't be much more expensive than a 4092 with a HV esc. BTW the boat in my signature runs 5692 730kv motors and I have a small cat that runs twin 4082 1600kv motors. Both setups are kind to the batteries. I used to run 1400kv twin outrunners on 4s with seaking 120s x447 props, boat was so much fun to play around with and it ran forever. The escs/motors stayed ice cold.

[alanr77]

I definately think the Fantasm deserves twins. There looks to be plenty of room in there for the setup. Nice video, that is exactly what I am looking for. I do like the Leopard motors but I have Neu 1527's in mind. I do have to check to see if a 4" long motor will fit though. May have to look at 1521's instead. The general consensus seems to be 20-25k rpm at the prop for a reliable setup. So when looking at the KV of the motors, and keeping in mind a range of 4-10s lipo's, I think the limit should be the KV that provides this on 10s so as to not over rev the prop. I really don't care if the rpm range is low on 4s; provided the hull will still get up on plane. I do have to keep in mind the possibility of overkill on this hull. 1527's are big a$$ motors and they may not be needed. I pretty much have my shopping list drawn up except for the kv/esc amperage selection. As I said before, I would rather have big, low amp motors running cool with a high reserve than little, high amp motors running hard. Is there such a thing as to big a motor- to the point where it is inefficient?

[iridebikes247]

If you run a boat with a really powerful motor and the kv is low and you overcompensate with the prop you'll get a lot of torque roll. Think small car huge motor, the motor is going to whip the thing around.

http://www.youtube.com/watch?v=02i2SF3mGt4

The beg. of the video shows the torque roll I'm talking about. Sorry can't be of anymore help bud I have only build twin cats lol I can sit here and play with a calculator but its your $ I don't want to suggest something I haven't used. Maybe message Keith Bradley on here? He has built more fantasms than anyone.

[Make-a-Wake]

If it were mine I'd throw a swordy 200a in it and spin a 4092 3.5d on 6s2p.

http://www.offshoreelectrics.com/pro...o-4092&cat=148

[alanr77]

But torque roll only applies to single drives if I remember correctly-right? A twin setup with cr props alleviates this. Way back I ran a Dumas Skidaddle Jr with a 550 brushed motor, you had to be careful because the gunwale would dip when taking off and she would take on a bit of water through the vents. Anyway, thanks for your replies. I've got a little time before I start this project so I will continue to research this. Alan.

[iridebikes247]

Yah torque role only applies to singles. The talk of the 1527 had me thinking a big HV single, not a monster twin lol. This is a 40" boat I think twin 1527 would be pretty crazy. An HV twin would get real heavy and wouldn't offer any benefit over a 1400kv/t180 sorta combo. Seaking 180 esc with leopard 4074 1400kv, boat will be able to run 60-65mph on 6s. 1600kv with x447 on 6s you'll almost be at 70. Amp draw won't be bad I run this setup often. The fantasm is a lightweight "small," 40" hull. It won't take much to get this thing going. I can assure you that 4074 1400kv motors in a fantasm will give you a very reliable setup and the electronics will come back totally cold even after WOT blasts. Seaking is a great robust esc for the money and the motors will last as well. To give you an idea here is a boat with 1400kv twins with x447 props on 5s per side.

http://www.youtube.com/watch?v=s1xLP-B27B0

[BHChieftain]

I have a small tenshock mono1 that was initially on 2S with a higher KV motor, and the thing ran really hot. I converted it to 3S with lower KV (same RPMs, same prop, same speed) and it is just right.
Chief

[madmikepags]

Alan I have been running 1527/1.5 y's (850kv) on 10s setups for 6 years. This is one of the best most reliable setups on 10S you can imagine. First off the1572's aren't really that big in size but in my opinion they are very UNDERATED by NEU and will make 40-48" boats fly on a single!!! In a 41" boat it will be perfect. Don't even bother with twins, to get a twin to run great in a mono takes lots of testing and tuning, monos lean in the turns and you will always have one motor unloaded and one loaded more than the other making for a boat that doesn't handle well, remember we run surface drives so most of the time props are only partly in the water, when you turn with a twin mono one prop will be completely submerged and the other almost completely out of the water. HUGE difference in amp draw from one motor to the other especially if you turn one way more than the other.
Next, Don't bother trying to make a dual cell count boat either, you will get a boat that doesn't do either very well. Just build it with a 1527/1.5Y (850kv) use 10s, you can use props from a 445-457/3 and everything in-between to get the desired results, small prop will give you better runtime and lower speeds, bigger prop opposite. BUT don't think you will just change prop and go!!!!! for different speeds you will have to tune the boat differently, just because you're going 15mph slower doesn't mean you're pulling less amps, if the boat runs good at 60mph and you prop it down to do 45, (and don't change anything else) there will be more boat in the water thus you will be pushing more water out of the way and then pulling more amps!!!!! you will need to adjust weight, tabs and probably prop angle to run at the slower speed. Best bet is to build the boat and run it at a speed and runtime that you're happy with and not go crazy with a dual purpose boat. I've built at least 25-30 boats using this setup and it is a great setup. I've also built many twin setups, probably 15-20 but have not kept ONE of them because of the pain they are to get and keep running right.

[madmikepags]

Alan I have been running 1527/1.5 y's (850kv) on 10s setups for 6 years. This is one of the best most reliable setups on 10S you can imagine. First off the1572's aren't really that big in size but in my opinion they are very UNDERATED by NEU and will make 40-48" boats fly on a single!!! In a 41" boat it will be perfect. Don't even bother with twins, to get a twin to run great in a mono takes lots of testing and tuning, monos lean in the turns and you will always have one motor unloaded and one loaded more than the other making for a boat that doesn't handle well, remember we run surface drives so most of the time props are only partly in the water, when you turn with a twin mono one prop will be completely submerged and the other almost completely out of the water. HUGE difference in amp draw from one motor to the other especially if you turn one way more than the other.
Next, Don't bother trying to make a dual cell count boat either, you will get a boat that doesn't do either very well. Just build it with a 1527/1.5Y (850kv) use 10s, you can use props from a 445-457/3 and everything in-between to get the desired results, small prop will give you better runtime and lower speeds, bigger prop opposite. BUT don't think you will just change prop and go!!!!! for different speeds you will have to tune the boat differently, just because you're going 15mph slower doesn't mean you're pulling less amps, if the boat runs good at 60mph and you prop it down to do 45, (and don't change anything else) there will be more boat in the water thus you will be pushing more water out of the way and then pulling more amps!!!!! you will need to adjust weight, tabs and probably prop angle to run at the slower speed. Best bet is to build the boat and run it at a speed and runtime that you're happy with and not go crazy with a dual purpose boat. I've built at least 25-30 boats using this setup and it is a great setup. I've also built many twin setups, probably 15-20 but have not kept ONE of them because of the pain they are to get and keep running right.

[alanr77]

Hello MM, thaks for the detailed reply. The Fantasm is a cat though, by looking at the hull it seems to be designed to carry twins. I had not thought about the wetted area increase on lower cells though. That would increase the amps. I guess what I am trying to do is cheat the compromise. Would it be a better idea to narrow the cell range to accomplish what I am trying to do? Instead of a 4s and 10s boat, have a series and a parallel harness and run either 5s or 10s? I am getting the advice I want, now I am just trying to fine tune it. The reason I am looking at twins is for no other reason than why not. This boat is being built for fun, I like to tinker with things and twins sound really good when running. I am trying to get a setup that has the potential to scream and yet can still be able to play around without hurting the components. As you know, if the boat is always running 10s, I will always bury the throttle. So, the stingers are adjustable, I could mark the measurement on them for both 5s and 10s and easily move them depending on which harness I am using. Keep the advice coming, you guys have done this before so I am all ears. I have time before buying anything because I am still building my recovery tug.

So, is there a good compromise to be found in a dual setup boat? I can see how a 850kv setup on 10s would be fast and reliable. But, what about a 1000-1200kv setup that would pull 44,400 unloaded on 10s and 22,200 unloaded on 5s? Would 4s and 8s (17760 and 35520) be better or maybe 6s and 12s (26640 and 53280)? How many rpm's are usually lost between unloaded and loaded rpm? Looking at the numbers, it would seem that I should shoot for 20k-35k unloaded.

[madmikepags]

If you're looking for a twin I have a Twin aeromarine conquest RTR w 1527's and Hydra HV240's.

[Fluid]

Instead of a 4s and 10s boat, have a series and a parallel harness and run either 5s or 10s?....is there a good compromise to be found in a dual setup boat...what about a 1000-1200kv setup that would pull 44,400 unloaded on 10s and 22,200 unloaded on 5s? Would 4s and 8s (17760 and 35520) be better or maybe 6s and 12s (26640 and 53280)? How many rpm's are usually lost between unloaded and loaded rpm? Looking at the numbers, it would seem that I should shoot for 20k-35k unloaded....

It is tough to have the kind of dual-voltage boat you are seeking. Some motors have the ability to change their Kv, and these may be your best solution. Too, a lot depends on how you will run the boat - general running around in circles or SAW-type runs. The latter can use loaded rpm in the 40,000 rpm range, but it can be hard on components if you run very much. High rpm also means it is easier to accidentally over-prop and burn down.

With the common 4-pole motors in use today, expect around 90% of calculated rpm when loaded reasonably and 82-85% loaded for SAW running. Example: Kv of 2200 x 14.8 nominal voltage x 90% = 29,000 rpm. This is pretty close to what most decent quality motors/decent packs will do in a well-running boat. You'd have better luck with a smaller voltage range. If you use 6S and 4S packs to make 10S, then motor the boat for 6S and 10S. It isn't perfect but it is closer, especially if you only run the 10S version occasionally.


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