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Thread: Prop KV and amp draw help

  1. #1
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    Default Prop KV and amp draw help

    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

  2. #2
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    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

  3. #3
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    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.

  4. #4
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    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.

  5. #5
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    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!

  6. #6
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    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).

  7. #7
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    Quote Originally Posted by Jimmypooh View Post
    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.

  8. #8
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    Cool

    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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  9. #9
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    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?

  10. #10
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    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.

  11. #11
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    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!

  12. #12
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    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.

  13. #13
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    Yep, thanks. Very helpful.

  14. #14
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    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!

  15. #15
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    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.

  16. #16
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    Will do. Thanks. I'm going to install surface drives for the cat and smaller 32 inch mono, but the 42 inch mono I'm going to try the existing drive line before doing a big swap. I have a bunch of props already that I can try, but they may all be too big.

  17. #17
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    It was mentioned that the boats used submerged drive.

    What size prop did you use for the 42 inch mono?

    What size did you use for the cat?

  18. #18
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    The 42 inch still hasn't run on electric yet. I'll let you know when I get it up and running.

  19. #19
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    The twin cat rebuild has just started. Here are some reference pictures and details in case that helps narrow down what props I should be running and an estimated amp draw.

    5kg (11.2lb) bare hull with cowl. I'm still doing some patch and filler work, but I took that into account.
    43cm (17in) wide
    15cm (6in) tall without cowl, 23cm (9in) with cowl
    2.5kg (5lb) hardware. This includes rudder, stingers, shafts, stuffing, motors (two 40x94 1250kv), collets, servos, esc (two ZTW Seal 130A 6S), waterproof servo box (do I need this? Should I be working to waterproof the entire hull?), and some extra for some wires and tubing.
    1.5kg (3.3lb) batteries this includes a 6S 5000 to each motor.

    All up this should weigh about 9kg (20lb). Sounds porky, but I don't expect it to be a rocket, I just want it to run consistently and be able to handle some rough water.

    I'm planning on using the existing holes in the bottom of the back of the hull to mount the shafts. I expect the center of the shafts to be about 13mm above the bottom of the hull. Does this sound right? Should I lower them?

    The rudders will go below the bottom of the hull by about 8-10cm. Does that sound right?

    Should I be mounting the motors as far back as I can? Granted I need to try to keep a gentle curve in the flex. This will allow me to mount the batteries last to balance the boat.

    The existing stringers seem pretty big to me, Do you think I can safely cut them down to about half as tall? Or cut some holes in them to save some weight?

    Thanks for your help!

    IMG_20190625_092213.jpg
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    Last edited by Jimmypooh; 06-25-2019 at 11:20 AM. Reason: Added picture

  20. #20
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    Jimmy, what is ahead of that bulkhead? Sometimes old foam can weigh a ton. I think you are good with your existing holes for stingers and rudder should have the depth. Mounting the blade closer to transom will tighten turns and farther back of course the opposite. Have you got the motor mounting system figured out? Dave at HRC and I designed a dual mount to fit in 5” rails.

    Shawn

  21. #21
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    I'm not sure what's in front of the bulkhead. I'll pull it and see. Do I need the bulkhead at all? It looks to be adding a bit of strength vertically, but might be Overkill.

    The props will be on stingers so it sounds like it will have a larger turning radius which is fine.

    I have a couple mounts in the mail right now, but thanks.

  22. #22
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    Maybe just drill a couple decent holes in that bulkhead. Perhaps if anything is up there you can dig it out. The bulkhead will add strength so try to keep it but cutting rounds will lighten it without sacrificing strength.
    Lighter boat would help. FYI: a Boats weight doesnt matter much once up on plane. Getting it there mind you is where the energy goes. Just food for thought.

  23. #23
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    Yeah, that makes sense. I'll drill some holes with a hole saw.

    I'm so used to worrying about weight since my main focus has been racing quadcopters. It's nice to be in an area of the hobby where other considerations are more important.

  24. #24
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    Yeah I still consider it because the old nicd days are so engrained into me. But have been realizing it doesn’t matter so much because of my last build. Built a lighter version of my BlownAway catamaran and still haven’t matched its runtime yet. The lighter one is 4lbs lighter.

  25. #25
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    Do you have details on that build? Just curious if it's close to this one.

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  28. #28
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    Ok, a few more questions. I've done a bunch of searching for what I would think is common knowledge, but can't seem to come up with answers.

    1. When mounting the stuffing tube to the transom, should I use epoxy or silicone to allow some movement? Something else like a rubber grommet? I don't expect to have to adjust the stinger much if any since this is not meant for racing.

    2. How far into the stinger should the stuffing tube go? I read somewhere that the brass should stick out the back of the hull by 1/4 inch then the liner should go the rest of the way. Is that right?

    3. Thrust bearings don't seem to be used any more. Just leave a 4mm gap from the stinger to the drive dog? My motors are on the cheap end. Do I need to worry about the bearings?

    4. When bending the stuffing, just heat it up red hot and bend it with a screw driver in there? Do I need to fill it with salt or anything so it doesn't kink?

    5. Do I need a servo box to keep the radio gear dry or should I focus on waterproofing the entire cavity of the boat?

    6. When making a cowl, are there any good tutorials? Most of my boats never had a cowl and I will need to make a few.

    Thanks for all of the help!

  29. #29
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    1) I use epoxy. Using a flexible sealant is not the solution to an adjustable stuffing tube height. Something called a flooded tube is.

    2) It depends on the hull and hardware. I run the brass into struts just enough to get a good joint, but not too far so there's room for the prop shaft. On a stinger, the solid brass tube cant protrude too far into the assembly or you won't be able to adjust the stinger trim angle. I usually stop the stuffing tube flush with the rear of the hull and just extend my teflon liner into the stinger. Sometimes I'll install rubber O-rings over the liner between the hull and stinger to help keep the water out.

    3) I don't run thrust bearings on my boats. Most people who do, run them between the collet and motor can. For basic sport boat setups, a gap at the drive dog will be sufficient.

    4)I find the easiest method for bending brass stuffing tubes is to first anneal the brass with a stove or torch. Heat the area that needs to be bent to a cherry red, then quench in cold water. The annealed area will bend easily with finger pressure. To harden that part of the tube after you're done bending, heat it up again to cherry red, then allow it to cool on its own to room temperature. Clean, polish, or sand the black, oxidized area that will be in contact with epoxy to ensure a proper bond. The sand/salt method is cool, but I've never had to put such aggressive bends in a tube that required a filler to prevent kinking. The above method has worked well for me for basic bends.

    5) No need for an electronics box. Use a waterproof servo, or, waterproof your non-waterproof servo. I conformal coat the circuit boards of my receivers with epoxy or liquid tape. Corrosion X spray also works well.

    6) Can't help you with cowls. I've never had to make one.
    Vac-U-Tug Jr (13mph)

  30. #30
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    Thanks Fweasel! That's extremely helpful. The last thing I need to figure out is what size props to order. I'm thinking of starting with x440 or x442. Thoughts? This will be a dual motor setup with 6s to each 4094 1250kv motor. I don't need blistering performance. I'm hoping to keep the amps at or under 75 at each motor. Does that seem doable?

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