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Old Feb 18, 2010, 08:52 PM
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Questions about RC boat brushless motor, ESC, Props, and building boats.

I am looking for boat parts for a school project where we have to build a boat (theres a price limit so the cheaper the better), and it has to go fast. I was looking at this brushless motor,
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8595&Product_Name=2040SL _4800kv_Brushless_Inrunner_(WaterCooled)


would that work with this prop, if not, which one would be good?
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=7358&Product_Name=Turnig y_4mm_AquaProp__31x22x14mm.

and this ESC if not, which one would be good?
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9299&Product_Name=TowerP ro_H40A_Brushless_Speed_Controller_


Also that brushless motor's site says the shaft is 2mm and the prop is 4mm, what would I need to connect them (which shaft, coupling, etc)?
For the boats hull, would wood or foam be better and which kind?
Thank you for your help!
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Old Feb 18, 2010, 09:12 PM
I feel the need for speed!
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All the gear you just listed cant be factored in to a working boat without hull type and size. The hull type and size will basically dictate your running gear. What length can you build in your guidelines.............do you want a vee hull, a catamaran, a hydro........or maybe just a flatbottom type job? How fast does it need to go.......does it just need to run with no speed in mind to attain? This info will help greatly
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Old Feb 18, 2010, 09:30 PM
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The hull type doesn't matter for this project, but I was thinking a monohull or it could be as simple as this shape, x|________/ <-- front ( x is the prop)
For the size I was thinking 1ft to 1.5ft long and about 6 inch wide.
I don't know that much about building boats, so if size doesn't work I don't mind changing them. The boat also has to carry a small orange so it needs to be big enough to accomodate that.
How fast? As fast as possible without costing too much. I used a speed calculator online with a similar size and pitch prop and it was around 35mph.
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Old Feb 18, 2010, 10:01 PM
I feel the need for speed!
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Just based on length and the fact that you have to carry an orange i think the motor you listed is too high of kv..........needs more torque........maybe something like this motor on 4s lipo. You probably have to go lipo due to weight and the voltage you will need.

motor: https://www.hobbyking.com/hobbyking/...hless_Inrunner

Lipo: https://www.hobbyking.com/hobbyking/..._20C_Lipo_Pack

ESC: https://www.hobbyking.com/hobbyking/...eed_Controller

Hope this helps a bit................i think you may need to consider a hull that sits up a bit on the water.........shallow vee or cat.
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Old Feb 18, 2010, 10:22 PM
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My fear was it being too high of an RPM and not enough torque :/ but with that lower RPM I hope its fast enough. What prop would be good for that? And due to our price limit of $80, would the ESC that I posted 1st be okay because its $12 less.

Do you know what I would need to connect the motor to the prop, like a shaft and coupling or something like that?

If I did get that Turnigy 1800mAh Lipo, I would have to get a cell balancer (which adds to the cost) and my Lipo charger does 1700mAh is that okay to charge it at that?

I was thinking about getting this battery http://www3.towerhobbies.com/cgi-bin...&I=LXHUJ8&P=ML
but a 4s Lipo for $13 thats nice

Thanks for your help (sorry for so many questions) and you don't have to use hobbyking.com you can use which ever RC vender that you prefer. I just was using that because the motor and electronic speed control were cheap. (the orange is small too, ~ baseball or tennis ball size?)
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Last edited by R/C_dude; Feb 18, 2010 at 10:33 PM.
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Old Feb 18, 2010, 10:55 PM
I feel the need for speed!
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The ESC you listed is only good for 3s lipo............you may be able to try a 3s setup with a little higher kv motor.........cheaper esc and a cheaper battery too.
And yes you can use your lipo charger.

If the contest is fairly short water cooling probably wont have to come into play, this motor could work on 3s........much cheaper too......then you could use the ESC you were thinking of.

https://www.hobbyking.com/hobbyking/...hless_Inrunner

Heres a 3s battery: https://www.hobbyking.com/hobbyking/..._20C_Lipo_Pack

EDIT: You also have to consider the drive.............subsurface or surface piercing............that will determine the prop.
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Last edited by scorpion1; Feb 18, 2010 at 11:08 PM. Reason: add info
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Old Feb 19, 2010, 02:37 AM
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given the budget, you'll probably want to go with a submerged drive (solid shaft) and a flat bottom boat. in that size range 30-35 is un-realistic(unless there is no penelty for putting the fruit on the bottom of the pool ). think more on the order of 20-25, a 28-60 motor(2500-2800kv) on 2s lipo w/ a 31mm prop (octura x431 or octura 531 plastic) should work nicely.
you forgot to mention what sort of water you'll be running on and the size of the course
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Old Feb 19, 2010, 06:11 AM
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By " size range 30-35 is un-realistic" do you mean cm? and if you do isn't 20-25cm a bit small?
Do you have an estimate on how fast a "motor(2500-2800kv) on 2s lipo w/ a 31mm prop (octura x431 or octura 531 plastic) " would go? My goal is around 30mph (~49km/h), but if its at least over 20mph (~32km/h) thats good.
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Old Feb 19, 2010, 06:15 AM
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30-35 is the size of the motor... 30mm diameter / 35 mm length. This is one of the accepted coding for brushless motors. Rex is right, try to find the plan on the net for the crackerbox and resize to around 350-400mm.
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Old Feb 19, 2010, 06:17 AM
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okay thanks. And for the prop "subsurface or surface piercing" which ever one would work better for the motor and boat speed wise, I'll build it that way.
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Old Feb 19, 2010, 06:57 AM
I feel the need for speed!
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quote: "in that size range 30-35 is un-realistic"

I'm thinking Rex means speed................

What about an air boat..........flatbottom with a big ole Fan.....easier to setup most likely.
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Old Feb 19, 2010, 11:52 AM
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I was indeed refering to speed an 18" boat is not going to be controlible at 30-35MPH doing anything other than a straight line on perfect water. even that would bust your budget w-i-d-e open. speed on the water takes power, lots of power.
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Old Feb 19, 2010, 12:55 PM
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Ooops, did not read carefully....
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Old Feb 19, 2010, 08:17 PM
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I made a model of the boat I could do, I used a motor and battery pack that was on this page. The boat is flat on the bottom and catamaran like in the front (my boat may not be, I don't know yet.) The big orange thing is an orange, its kinda floating there because there isn't a cover on; and I know that prop is a bit small, it was a pain to make. The boat size is in LxWxH 18x6x2 inch or 44x15.5x5 cm.
boat
My model I made is at:
http://www.yousendit.com/download/Rm...SU9IcWMwTVE9PQ
and it will be gone on Feb 26th

Is the size good for the motor/battery below? Tell me what you think, Thanks



This is the motor in the picture
https://www.hobbyking.com/hobbyking/...hless_Inrunner

This is the battery https://www.hobbyking.com/hobbyking/..._20C_Lipo_Pack


and does anyone know where to get a prop such as the one Rex R said to get? "a 31mm prop (octura x431 or octura 531 plastic) should work nicely"
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Old Feb 19, 2010, 08:24 PM
I feel the need for speed!
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Dude! This is what i meant by airboat..............this is exactly 18" and it hauls........its mainly due to sitting on top the surface...............i'm convinced that this is what you need if there are no rules about a prop "in" the water.

Electric Airboat #2 (0 min 41 sec)
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