Prop KV and amp draw help

[Jimmypooh]

I posted this over on RC groups, but this is probably a better place to get good answers:

I have a background in electric planes planes and quads, and once did a lot of nitro boats. I'm working to convert my old gas/nitro boats to electric and need a bit of help getting started since my brain isn't calibrated to boat electric, only air.

I'd like to hear your recommendations for cheap brushless setups for 2 boats:

48 inch cat that once had a 23cc gas engine and submerged drive. I'm not against moving to surface drive, just never set one up myself.

42 inch heavy wood deep V mono that had a .61 nitro airplane motor with water cooled head and submerged drive.

While I'm still just learning brushless boats, I wanted to start cheap so I ordered ZTW Seal 130a 6S ESC, GoolRC 4074 2000KV with watercooling jacket. I'm hoping to run on 6S but will start with 3S. I may have jumped the gun on this one, but wanted to get something ordered and figured I'd use it somewhere (I have several smaller boats I want to work on in the future).

From my reading the KV on that motor is a bit high, so I'll need to start with a smaller prop, any thoughts?

I don't want speeds over about 40mph since I run in pretty rough water. I would appreciate any thoughts you guys have with the power setup I ordered as well as what else you would suggest. Thanks!

A couple more boats I'm planning on updating in the future:
Rigger that currently has an OPS 60 nitro - This is the only one I care about the speed. Would be nice to get 90mph out of this one.
46 inch fiberglass mono running a OS 60 nitro
38 inch heavy wood deep V mono that has never had a motor.

Thanks for the help!

Jim

[srislash]

Well first off, it won’t be cheap converting big hulls. It could perhaps not be terrible since you are looking for modest rough water speeds. You should run a bigger motor in those hulls. Something in the 56mm dia range. If you want to run 6s I think 750-800kv should push through the waves.
So you are looking for more of an ‘offshore’ type running?

I’m just contemplating the same scenario for a rough water boat but smaller hull @39”. But I’m looking at 4s.

Shawn

[Mike W]

You have a power setup much more suitable for a boat in the 32"-36" range with mid-40mms props. 6S may be adequate for the low speeds you are after with the larger hulls but would certainly require a much lower KV in the 1200 range in order to turn a prop of sufficient diameter (mid 50mms) to propel boats that size. A large MA capacity would also be needed for any decent run time.

[srislash]

Here is a setup just for ideas for you. https://forums.offshoreelectrics.com...Endurance-Boat
I think he is running a good bit faster mind you.

[Jimmypooh]

Is it inadvisable to use high kv with a smaller prop diameter?

Also, with the setups you're recommending, what amp draw can I expect on 6s? I.e. what speed control should I be looking for?

Do you think a 4092 1200kv would work for me? What prop size and speed control should I look for?

Sounds like the power setup for larger boats is going to run $350 plus batteries right?

Thanks!

[srislash]

The higher KV and smaller prop gets amp spikes. And that can be hard on esc’s. Smaller prop=less bite in water.
My thought on the 750-800 KV on 6s was that you could run the gas prop. This should help in the rough water. I’m also thinking that the ESC you have should work. After rereading the link on Peters boat you could run a smaller motor too, but just bigger than you have(Gool/rc).

[Mike W]

Is it inadvisable to use high kv with a smaller prop diameter?

Also, with the setups you're recommending, what amp draw can I expect on 6s? I.e. what speed control should I be looking for?

Do you think a 4092 1200kv would work for me? What prop size and speed control should I look for?

Sounds like the power setup for larger boats is going to run $350 plus batteries right?

Thanks!

A certain size boat requires a certain size prop to to provide the required amount of thrust. You can certainly drive the correct diameter prop more slowly to achieve a lower top speed with less power but you can't expect to achieve the same result by reducing diameter.

6S can run on anywhere from 120 - 240 amps depending on the load. You would certainly want to stay on the very high side of that range with a boat this large. Unfortunately power system prices go up more quickly in dollars than they do in amps.

1200KV would be a max in your case and much lower would be even better see the endurance thread linked above. Good info there. $350 would be a minimum.

[Fluid]

It is too bad the OP purchased before he got some answers. I know the desire to get started can be strong, but I see little use for what he has coming. The big cat probably won’t plane off with that motor on 6S, let alone on 3S. A prop much under ~48 mm won’t have the thrust available regardless of the rpm - don’t ask me how I know. 6S and 130 amps is only about 2500 usable watts and that’s max, not sustained. The motor is too small with inadequate torque and will overheat quickly. Most of the above would apply to the 42” mono. Even the 38” mono is too big for his motor.

There are some good recommendations above, but so much depends on what you expect. No one asked you about run time. The words “cheap” and “good run time” mean different things to different people, especially to someone unfamiliar with FE. I’m not trying to discourage the OP - I really appreciate his enthusiasm for the project - but he needs to understand the limitations as well as the advantages to FE power before he invests further.


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[Jimmypooh]

Thanks for all of the help so far! It's been super enlightening.

I'll plan on saving the parts I ordered for a smaller boat down the road.

The reason I'm stuck on 6S is because I already have a lot of 3S batteries I plan to run in series, and don't want to buy more batteries if I can help it.

I've looked at a bunch of threads and wondering if this will work:

Cat - dual drive setup running 4092 800kv on 6S for each screw. I'm hoping to get about 5 minutes of run time from 6S 5000 to each motor. Will each motor work on a ZTW Seal 160 esc?

For the 42 inch mono, would a single 4092 800kv work on 6S? Will this work on a ZTW Seal 160 esc?

[Mike W]

For the twin cat you can safely bump up the KV about 50% to trade run time for speed. The other would be a decent low speed setup.

[Jimmypooh]

Good to know. In that case I'll look for a set of 4092 1200kv and if it want more run time I can just be easier on the throttle. This at least gives me the option to go fast if the water is calm.

Thanks!

[martin]

They have told you what i answered you over on the other forum you asked this question, motor is to small for the size hulls you mention & 2000kv is to high on 6s.

[Jimmypooh]

Yep, thanks. Very helpful.

[Jimmypooh]

One last question for you guys. How do you go about deciding on prop size? Do you get a bunch of plastic ones until you find one you like then get metal? Have any recommendations? I'm planning that the cats drives will be surface and the mono will be submerged.

Thanks!

[Mike W]

Plastic props are only suitable and available for only the very smallest boats. CNC aluminum props would be a more cost-effective option at your power level. Specific recommendations will have to wait until your setup is complete. Provide weight and setup pics at that time. Submerged props are virtually never used anymore even on monos. You definitely want to run surface drive.