Sport Boaters: Choosing the Right Volt/Amp Setup for Your Speed and Runtime Desires

[CraigP]

Note: This post is directed at sport boaters that do not have constraints on voltage requirements based on racing class rules. Please keep this in mind when replying, so as not to confuse new boaters that may run into the same challenge that I did.

I’m writing this post to try and help others not do what I just had to do, which is replace my motor(s) and batts because of an analysis error I failed to make. I built a twin motor Vortex34 Hydro (its posted on the forum) and selected motors and batteries based on performance expectations and run time desired. For some background, I built this hull before as a single and equipped it with 8S 5000mah battery made up of 4 2s batts wired in 4S configuration. It has a Leopard 4074, 1050KV motor in it and with an 1816-17 prop, has ran 72mph. So when it came to the Dual Motor config, I decided I would use 4S as the primary voltage. I planned on using 4S voltage, using the same batts as the single but wired in a 2S 2P configuration. The 2P ran to a common Cap Bank, then from there each ESC had its power feed wires. I’m using the Turnigy T-120A ESC’s, which have their own Cap Banks located close to the ESC.

I chose to use the Leopard 3650, 2050KV motors to power the craft. This is where my oversight came in. I did not do my homework as I should have and didn’t see that the motor was limited to 600W at 40A. I already knew from the single motor hull that I would need around 2000W to hit the speeds I wanted. First time out was a quick check out trip. I noticed on return that the motors were hot. Batts and ESC’s were fine… I had the watering system hitting the ESC’s first, so I swapped that to water the motor first, then ESC. It has a dual, parallel water system in it. I went back out for run #2, which was about 5 minutes in length. I was planning on the 5 minute mark for run time. As a sport boater, I want more time on the water than racers plan on. When it came back in and I opened the hatch, I was hit with the really bad smell of motor insulation burning. The motors did not fail and short out, but I bet one more minute of running would have flamed them.

So at that point, I knew my math was faulty… Kinda embarrassing, because I really dropped the ball. First thing was to find the actual average amps used in that run. I recharged the batteries taking note that it took 4950mah to top them back off. I ran it using 4, 2s batts rated at 8000mah in the 2S 2P configuration, leaving reserve in the batts. I clocked my run time at just barely over 5 minutes. So using the OSE Calculator, I entered 4950 for the MAH field and 300 (5 minutes in seconds) for the time field and got a value of 59.4A as an average. At 14.8V, this is a power level of 879W. It’s THEN that I looked at the motor data and saw it’s rated for 600W! That’s the mistake… LOOK FIRST!

So I needed to solve the problem and get the right motors in the boat. Knowing that the boat needs 2000W to get to high speeds, that takes the 3650 motor off the table. The 3660 is 800W, so two of them doesn’t get it. The 3674 is 1200W, so two of them has the power capability. So now I need to check the volt/current rating of each motor. I need 1000W off of each motor. If I keep it on 4S, that’s 1000/14.8 = 67.6A The 1900KV motor (specs from Leopard’s web site) is rated to 78A, so it will work. Now I need to check run times… Back to OSE Calculator, I plug in 5000mah and 300 seconds and get an amp draw of 60A, still works. Now, I only got one set of 8000mah batteries, and have multiple sets of 5000mah batteries. I want to use those too for battery changes and extend my fun on the water. So leaving the reserve for the battery, I can use 3750mah on those 5000mah batts. Plugging that in, I get 45A, well below what I’m drawing. So I’m going to have shorter runs on those batts, but how much shorter? Going to the runtime side of the calculator and plugging in 3750 and 67.6 I get 200 seconds, or 3-1/2 minutes. Doable, but I’m not real happy with that… So I start to do some math and see if I can raise the voltage , reduce the amps, and hit my marks. I go with 6S, because I already have two sets of 3S batts for my UL-1 hydro. The single motor Vortex can take the 2S cells I was planning on using. Going that route, I calculate my expected amp draw at that voltage which is: 1000/22.2 = 45A Then, to keep rotation speed the same to the original 4S design (I already bought two pairs of matching, counter-rotating props) I chose to use the Leopard 3674, 1400KV motor. Checking it’s current rating, I see it can take up to 58A, good to go with some headroom for some speed runs! So I got some more 3S batts, exactly like what I use in the UL-1, and ordered two 3674 1400KV motors from Steve (he’s probably wondering what the hell I’m doing with all those motors!) so I can share between those two boats. As it turned out, the Dual Motor Vortex is exactly twice the hardware of the UL-1. Both boats are running 3674, 1400KV motors on 6S, and both will get well over 5 minutes of runtime in that configuration and will be capable of running up to 70mph. I just finished the upgrade, ready for running this weekend!

So hopefully, my oversight will not be others… Check your stuff out BEFORE hitting the “Buy” button!

Note: The 3s batts are 5200mah each.

[785boats]

Wow.
Reads like a bit of a saga.
A useful heads up & instruction for others.

But most motors will comfortably draw more that the 'Max Amp' rating given by the manufacturers, once we add water cooling to them. Obviously depends on your prop sizes.
So you have heaps of head room with those motors.

[CraigP]

Well those 3650 motors couldn’t handle the 880W that they were running. The data sheet says 600W, so that’s 147% of the stated power. I know some racers are over 200%. I think my biggest problem is my 5 minute plus runtime. Practically all racers are over amping their motors, but it’s a short 2 minute heat. So I have to rely on the manufacturer’s spec as a continuous power rating. I’d love to get to 7 minutes, so there is no relation to the typical racer’s setup. I think more sport boaters would love to get more run time, but don’t know how to navigate the numbers towards that goal. That’s why I wrote the thread. Sport boating is a completely different mind set than racing. I’ve seen some racers comment that without the competition, RC boating is a waste of time. But there are far more sport boaters than racers. So suffice it to say, there are different motivations for participating in the sport. But the point is, design goals and usage of the numbers is also very different.

[785boats]

I guess that's true. The max amps ratings are, as you say, for continuous use. Higher amps does equal lower efficiency & run time.
I wasn't really thinking of the longer run time, which was wrong of me. Force of habit I suppose, as I've never run anything over 3 minutes.

[rol243]

Yes it is very critical for sport boaters to keep an eye on run times and not just go out put the model in the water and play around until the model almost stops from a flat battery . Its alot easier for racers that know there only going to be driving flat out for no longer than 2 to 3 minutes. key factors; keep motor kv,s lower than normal / use good size mah lipo packs / keep prop sizes down / time your runs.

[CraigP]

Yes, good points. If your goal is longer run times, then you need to work up to it. Don’t let the amp draw get away from you because you’re itchy to run that new, big prop you got. When changing to a larger prop, cut your runtime in half on the first outing. Then measure what you put back in the battery and go through the calculations on the first post, to get your new run time. ALWAYS remember to multiply your stated battery capacity by 0.75 to leave a reserve in them. Running your batts all the way out will just kill the life. Use a timer!

[Speed3]

Increasing the voltage and reducing the kV will increase the runtime.

Higher voltage setups are likely to be faster.

Example 2 motor similar in size.

Motor 1 has a kV that will allow it to rev 35000 rpm on 4s at no load.

Motor 2 has a kV that will allow it to rev 35000 rpm on 6s at no load.

Motor 2 will be faster and give higher runtime than motor 1.

The only case motor 2 those not run faster is where motor 1 loaded at rpm is almost equal to it rated unloaded rpm.