Motor Setup / Run time / Current Draw

[dazzler]

Hello all,

I've recently started building a C5000 cat (from drawings found on this forum) and while it while take very long until the hull is finished (I'm a slow builder) I'm already figuring out the electronics as well.

The C5000 I'm building will be 115 cm (45") long. I'm not aiming for the ultimate top speeds but rather a good compromise between performance and running time.

But I can't seem the understand why one would choose for example a 2100kv motor on 4S versus a 1500kv motor on 6S
Which is better? What's the difference in amps and run time?

For example, let's assume I can choose between these 2 Leopard motors:
Leopard 4074B/2Y (2150kv - Max 135A - 2600 Watt - Max 23 Volt)
Leopard 4074B/3Y (1400kv - Max 95A - 2600 Watt - Max 35 Volt)

So, with 4S and the 2150kv motor, I would have 31850 rpm at the shaft, and with 6S and the 1400kv motor I would have 31080 rpm at the shaft.... almost the same that is.

Using the same boat, with the same prop and aiming at the same speed, I would assume that POWER (Watt) would be the same (as you need a specific POWER to get a boat up to a given speed)

So, basically, If both motors are rated at 2600 watts, and assuming this is the power that is used to get the boat to a specific speed, I would conclude:

2600 watt / 14,8 volt (4S) = 175 A
2600 watt / 22,2 volt (6S) = 117 A

So, since the setup on 6S draws less amps, the running time must be greater right ?

But then, why do people even consider or use 4S setups? If current (A) drops as voltage (V) is increased (at a given power (W)) then more cells equal less current and longer running times?

Why would I choose a 4S setup? Or is the 6S setup in this scenario always the better choice?

It gets even more complicated if I add for example - the Leopard 4082B/4D in the equation (1450 kv - Max 128A - 3500 Watt - Max 33 Volt)

It has about the same kv rating and voltage rating, but can handle a lot more power (3500 Watt vs 2600 Watt)

am I right to assume that if I want to drive my boat at the same speed as above with the 4074 motor, it would also only use 2600 watts at the specific speed, and as a result would give the same running time? But that is has "extra power" for example it can handle a bigger prop, which in turn increases speed and load on the motor, drawing in turn more amps en giving less running time?

Please show me the light here
Thank you.

[Mike Caruso]

Great Questions Can't wait for the answer. Wish I could help you but I too am learning. Good Luck.

[RandyatBBY]

It becomes more complex then you add the ESC and prop. Most people just want speed the easy way. What works in one body of water changes in another. Light clean aerated water is thinner, dirty high mineral content from early run off form a storm water is thicker. Then there is the amount of funds available.....What you pick out may have different quality ingredients. I run 4S because it is what is most popular in my district and it is not at the top of most ESC ranges. what works on one setup will not work on another due to changes from one to another.

[Tamelesstgr]

I previously was stumped with this kind of question (and may still be)

BUT, what I learned through the fine folks here and with datalog testing is that the motor will continue to work (drawing amps and providing watts) based on your setup. It's not like there is a program in the motor to say, stop delivering watts when you hit 2,600.

I have the Leopard 4082 - 1450kv running on 6s. Depending on my prop, the wattage ranges from 2,200 up to 3,700 and my amp draw average ranges from 105 - 127 with some peaks topping 155 amps.

I don't have experience with a large catamaran, but you might need a larger motor to push a boat that big.

[RandyatBBY]

but you might need a larger motor to push a boat that big.

Not might, get the biggest and highest quality you can.

[NativePaul]

I think the main answer is to fit race rules, unless you are racing or considering the possibility of racing in the future there would be no point in running 4s with the higher amp draws

I think you may be forgetting that more cells also equals more weight, which effects the speed, handling, acceleration, stability and deceleration. If you want the 4s boat and 6s boat the same weight you will have to have lower capacity cells and the 6s runtime advantage vanishes.

The rules I race to used to be 4s but a few years back changed to allow from 4 to 6s with the same battery weight. There has been a migration to 6s and now few people run 4s. The power in is the same but the 6s setup has less wiring losses and is slightly more efficient so you get a couple of percent more power to the prop. In theory you could run a 120A ESC save a little weight and money, and go faster, but in practice it is better to use the same ESC which runs cooler, more efficiently and more reliably.

Take motor power or current ratings with a pinch of salt as all the motor makers have a different definition of what their motor can take and that definition may well not coincide with your driving style. The motor does not set the current, the prop does, so if you run a little 28x36 motor, a medium 40x74, and big 56x110 motor all of 1400kv on 6s with a x448, all the motors will pull around about the same current (say 150A) but the little motor cant shed that much heat and will burn up almost instantly, the medium motor will be running hot at its limits and the big motor will running well within its limits.

Essentially you are correct, if you have 2 equally efficient motors of the same KV but bigger sizes, with the same prop they will pull similar currents, and at the maximum limit of the smaller motor you will be able to prop up the bigger one and gain more speed.

There is a little more to it than that, for instance a bigger motor of a given KV will generally have a higher no load current, will take more power to accelerate the heavier rotor, and will have its peak efficiency at a higher power, whereas the smaller motor will have higher resistance in the windings, and have its peak efficiency at a lower power. the lo load current and rotor weight are not huge differences and have less effect the higher the loaded ampage is, so it is not very important for sport boaters or even racers without power limits, and I would encourage you to buy a motor that is plenty big enough. If however you want to learn more about motors and how sizes, KVs and voltages effect efficiency curves take a look at the Lehner motor configutrator. Not that I would suggest a buying a Lehner to a newcomer unless they have money to burn, but they are the only manufacturer I know of that publish efficiency curves for all their motors, and they have a calculator which while the numbers are not directly transferable to other motors, sticking in different motor sizes, KVs, and voltages can teach you general principles and relationships that do apply to all motors.

[dazzler]

Thanks all for the response, especially Paul, very usefull information!

So generally one can say, that unless class rules require it, there is no real reason to go for 4S, but 6S (or even higher) is more logical, and will generally result in lower amp draw at a given speed (and propeller).

I'm currently looking at the Leopard 4082-B/2Y (140A max - 33V max - 1600 kv - 3500 W) on 6S in combination with the HK Marine 180A (or possibly the HK Aquastar 200A if budget allows) Dual setup off course....

Anyway, I will have plenty of time left to decide, I'm still building the plug, after that I still have to make the molds, so it will take a while before my hull is ready...

[urbs00007]

motor setup.

Thanks all for the response, especially Paul, very usefull information!

So generally one can say, that unless class rules require it, there is no real reason to go for 4S, but 6S (or even higher) is more logical, and will generally result in lower amp draw at a given speed (and propeller).

I'm currently looking at the Leopard 4082-B/2Y (140A max - 33V max - 1600 kv - 3500 W) on 6S in combination with the HK Marine 180A (or possibly the HK Aquastar 200A if budget allows) Dual setup off course....

Anyway, I will have plenty of time left to decide, I'm still building the plug, after that I still have to make the molds, so it will take a while before my hull is ready...

would you take pictures of the plug and mold building process as well as the layup. I'm sure many would enjoy.

[Speed3]

The best choice for a good compromise of speed and runtime would be the bigger motor, lower kV and increase voltage.

Example 4082 1450kv over 4074 1400kv. And 6s over 4s. Because a motor has a certain wattage rating doesn't mean it will consume all that power. Power rating mean, that is the max input power for a given motor, and that's usually for a short burst. (15seconds). The amount of cells, size of prop and ability of motor to rotate freely have the biggest impact on amp draw.

Rotate freely mean no excess friction due to damage bearing cause rotor to rub on stator, or too much friction in the drive line.

[CraigP]

I might add one additional point of consideration. At higher voltages, you have to watch hitting the motor saturation current much closer. This is caused my over propping.. When you hit it, all hell breaks loose! Work your setup slowly and data log your prop changes. You should see nice steady increases in current, big jumps mean your getting too close. Large jumps in current on the data logger can also indicate momentary motor sat conditions... it's very unforgiving as you raise voltage is the main point. Also, use a 30% safety factor on voltage used to stated rated voltage. They are very optimistic figures the manufacturers publish.

[tebby2]

Also keep in mind, especially with the data sheets provided by Leopard, that you should stay within the amp limits of that particular size and winding configuration - regardless of battery cell count. Yes you can over prop a motor and pull more amps, but that will cook your motor quickly. A data logger, with both current and temperature readings is a good tool for prop selection. A GPS read out is also very helpful. Instantaneous telemetry read out is a big help, but the ability to read that information on your computer is the easiest way to make good prop selections. In my experience, going up in cell count with the same prop will also increase amperage and wattage (amps x volts). A larger or longer motor will supply more power, but not necessarily pull less amps. Run time is a function of amperage draw, and I have found that the only way to reduce amperage draw on a given hull is to reduce prop size or battery cell count, which in will in turn reduce over all speed. In all cases, a robust ESC with more capacity than your motor draw is a must. Burst numbers are just that - not a margin to operate in. Almost any motor (if it can fit) will work in any boat with the correct prop selection. Good luck with your motor choice.

[CraigP]

Good stuff Tebby... I have the same mindset. I set a power goal, in Watts. I select a battery configuration. I select a motor that can take the power goal. I datalog to confirm I'm at my power goal using prop. If it's not fast enough, then adjust the power goal and start again. I never run over the set power figure... I do purchase ESC with at least 40% amperage margin. I use higher series, because you can increase runtime IF you don't blow your power budget.

And remember, hull factors can increase your drag factor and blow your power budget. Make sure the hull is worked out before final prop optimization. You could easily leave some speed on the table if you have a sticky hull, bad strut setup or turn fin/rudder issues...

[dazzler]

would you take pictures of the plug and mold building process as well as the layup. I'm sure many would enjoy.

Hello urbs00007,

apologies for the late response... I have decided to start a build log on this forum, and will share my pictures and progress of this build with you all.
This build thread can be found here: https://forums.offshoreelectrics.com...d-from-scratch