motors selection and driveline questions catamaran build

[keesey]

I recently watched a YouTube video going over setting up and designing the electrical side of a Rc boat. it covered 10 topics:

1. hull type (mono, hydroplane, outrigger)
2. calculate wattage
3. determine hull length
4. determine voltage
5. select battery
6. calculate Kv
7. select motor
8. select esc
9. select prop
10. test setup monitor heat

here is my understanding and working through the steps for my build and looking to be double checked and or any input. maybe this might help someone else looking to build themselves!

1. hull type: catamaran
2. hull length: 48inch
3. power=2225.316 watts where power=4.279(hull length)² - 163(hull length) + 190.5, so for me it was 4.279(48)²- 163(48) + 190.5=2225.316 watts
4. determine voltage (wattage=voltage x current), battery standards are 2s=7.4v, 4s=14.8v, 6s=22.2v, 8s=29.6v, 10s=37v. I am planning on running 8s, two 4s batteries in series per motor/esc if I divide my wattage from step 3(2225.316w) by 29.6v I get 75.17amps as a constant
5. going by the video you use (amps)x1.3(build safety factor)x1.8(lipo safety factor)=max discharge current, so for me 75.17x1.3x1.8=175.89peak discharge amps, to calculate c rating you divide the max discharge current by battery capacity, where 3000mah is 3 and 8000mah is 8 as examples, so minimum calculated c rating 175.89/10=17.5 but the higher the c rating the better and longer the life will be of your battery. My selected batteries are 4x 4s 120c 10000mah
6. suggested max unloaded rpm 22500-33000 lower end being mono to the higher end cat, tunnel, hydro and rigger in that order. i picked 28000rpm being a cat, so kv=rpm/nominal voltage, for me 28000rpm/29.6v=945.94rpm/v
7. next it had me figured out the motor watts where wmotor=wattage x 1.4(in runner coefficient) for me 2247.58x1.4=31466.61watts you use this then to figure out motor weight or size, motor size=watts/4.5 so 31466.61/4.5=699.22grams
8. i am running 8s at 75amps so current=75amps x 1.3(build safety factor) x 1.5(esc head room) gives me 146.58amps so need to run an esc that can handle 8s@150minimum
9. starting prop point, pdiameter=11.452 x w^0.1787 so for me 11.452 x 2225.316^0.1787 gives me 45.34mm as a starting point in prop diameter, 1.4pitch if i go smaller i will raise my top rpm if i go bigger in diameter will lower my diameter
10. build and test, too much heat means too big of a prop, if you can data log look for less than 10% voltage ripple, max operating temps 60c or 140f

only thing i really am not sure about is how to do this for a pair of motors, would i just dived everything in half?
looking at my KV value being 945kv would it hurt to get into a bigger motor like a 5680 over a 4092?
sorry for being a pain i am trying to learn and not make big mistakes setup wise from the start and end up chasing my tail.

any input is appreciated here is a link to the video.

[Bande1]

you can put any motor in it you want. just depends on how fast you want to go.

4092 motors are for hot 4s or mediocre 6s systems. 8s would be kinda silly as 100A on 8s is already pushing those size engines. not to mention the ESC's are crazy expensive and need BEC's and everything else. just run 6s per motor with flycolor 150A esc's.

you wildly overthink this. its pretty simple stuff.

[keesey]

you can put any motor in it you want. just depends on how fast you want to go.

4092 motors are for hot 4s or mild 6s systems. 8s would be kinda silly as 100A on 8s is already pushing those size engines.

you wildly overthink this. its pretty simple stuff.

Wiley overthink it? Sorry I am just getting back into rc and lipo wasn?t even around when I last was, it was brushed motors or nitro back then. Up until 3 weeks ago I couldn?t even tell you the difference between 4s and 8s.

I really don?t want to blow up a motor and still trying to wrap my head around motor kv rating, battery setup and motor case size all tie together.

Why don?t I just step up to a pair of tp 5680 970kv and bigger esc?s go bigger on my lipo setup 12s or 14s but I am unsure about it, I don?t have experience with setting it up, why I am posting here.

Thanks for your help though it?s appreciated


Sent from my iPhone using Tapatalk

[Bande1]

The proper KV for 12s is around 400-500kv. you are going wildly too high 970kv is too high for 8s. The bigger the boat the lower the RPM. You are trying to use P hydro RPM ranges on this enormous pig. Gas engines spin 14-18k RPM. That what this hull effectively is. That is the RPM you should be looking at.