1st Electric Mono Build Help

[KB_FSU]

First, thank you for taking a moment to look at my post.

I recently pulled a couple old RC Boats out of my Dad's Garage, in an effort to resurrect them for a fun project with my son. One of them is a 38" fiberglass mono, that we (20 years ago) planned to run as a nitro boat, but never finished. I have a .45 water cooled OPS engine that has never even been started, but I want to set this boat up as an electric. A fast one. However, I have no experience with electric boats, and things have changed a lot since I was a kid fumbling my way through.

As it sits, I have a hull, radio box, some running hardware, but no motor, ESC, driveshaft, etc. At a minimum i recognize I need those things, but I don't have any clue on motor size, battery pack, etc. etc. What I am hoping for here is to start a dialogue related on what I need, what my options are, and ideally avoid some of the rookie mistakes.

I have included a couple photos, but can provide any additional information you need for recommendations. The idea here is to get this thing over the 50mph-ish threshold where possible.

IMG_6707.jpgIMG_6706.jpgIMG_6705.jpgIMG_6704.jpg

[Jesse J]

First things first: welcome to our community!
So, what is your budget? What re your running goals, speed, run time,?
I would recommend selecting motor/esc for 6 to 8s lipo, a 1500 to 1250 kv motor with a 55mm prop.
But for the best info we need to know the answers to the questions.
Sounds fun!

[KB_FSU]

Thank you!

Budget is fairly wide open here, I just want to do it right the first time. I'm also not setting this up to "race". I'll be a weekend warrior at best, but I would like it to be exciting to run while also being reliable. I'm expecting it to be easier to tolerate than these temperamental Nitro outboards I'm also fooling with. Somewhat arbitrarily throwing out the 50+ mph speed threshold, as I wont have a radar gun on it at any point, and I have little bearing on what a run time even looks like for a serious electric craft. I presume I can just bring multiple sets of batteries to extend the time on the water, right?

[HTVboats]

https://www.offshoreelectrics.com/proddetail.php?prod=ose-leo4082-combo

OSE has a nice combo that would give you the results you want. A 4082 motor with a KV of 1250 or 1450 on 6S. I would recommend the lower KV rating as it would be more forgiving to find the correct prop. As for battery I would run a pair of 3300-5000 ma packs 6S in parallel giving you 6600-10000ma of power to draw from. If room allows on a mono I prefer the motor in the center and packs straddled to the outside, allowing movement for and aft to trim and balance the boat.
What part of Florida are you in? Several groups and clubs you may want to talk to near you where you don't need to race but get ideas and recommendations from.
Mic

[jfrabat]

First things first; I also come from Nitro, and here are a few answers to some of the questions I had and that the kind people here helped me out with... DISCLAIMER: I AM STILL A NOOB!

1. Different from nitro, where a prop too big would mean the motor would not spool up and that is about it, an electric motor will do EVERYTHING to reach its operating RPM based on voltage. That includes destroying itself...

2. The Kv in the motors is NOT Kilovolts. It is how many RPM's the motor will want to do for every volt that reaches it. Typically, you want around 20 to 25,000 RPM for a boat of the type you are talking to (30K for racing). LiPo's give out aprox (depends on charge) 3.7~4.2V per cell (I personally try to keep them above 3.8V per cell to avoid damage to the battery). So a charged 6S (6S means 6 cells) would be 4.2V X 6 = 16.8V (and discharged 80% should be 14.8V; you do not want to go lower than that!). So your prop will want to be spinning at 1,250 X 16.8 = 21,000 RPM when charged (and 1,250 X 14.8 = 18,500 RPM when the battery is drained). Obviously, this does not account for inefficiencies, drag, friction, etc.

3. When the motor cannot reach the RPM it wants, it will start getting hot (along with the ESC, the batteries, and everything in between). The trick is to start with a prop that will "get there" easily (starting with a smaller prop), and start working your way up the size chart in props. Then what you do is that you measure temps after 1 minute of run time until you get to the point where the temperature is up to the point where the system can handle it.

4. Speaking of batteries (LiPo's are finicky and they WILL burst into flames if abused or used improperly!), the other thing you need to look at in the battery is the C rating. Typically, you want a 35C or higher. C Rating refers to how fast you can safely discharge the battery. 35C means 35 times the rated amperage, so a 5000mAh with a 1C rating means you cannot pull more than 5A with it. A 10C one you can safely discharge 50A. A boat of the size you are talking about can pull 150A (especially when accelerating quickly), so keep this in mind.

To give you an idea of speed, this is a scratch built 30" cat on a 4074 1800Kv on 4S 6600 on its maiden run. The boat is balsa and fiberglass, so not exactly the lightest boat. It also has a 6600 mAh battery, which, again, is not exactly a light battery.

https://youtu.be/ZmXmY6MpfYk

https://youtu.be/L1nvDpQMfOY

This is with the first prop, so nothing was even close to hot, which means, I can go faster (not sure if I want to yet).