night_man

First I would like to apologise if these questions have been asked a million times, but I havn't really found precise answers to a few questions. It could be that I'm just terrible at finding stuff, but anyway...

Background - beginner working on project below (description may sound like I'm rambling...very tired)
At the moment I'm trying to choose a brushless motor, ESC, etc. for an autonomous boat. I figure if I use a 2s lipo, the weight of everything but the boat will be around 700g and maybe 2lb max for 3s. Microcontroller takes 7 to 12 V recommended (6 to 20 limits) so either should be fine. I don't expect the length to exceed 20". I am not worried about speed. I would like around 1 hour run time and 3-4mph would be fine. Is that reasonable with 5000mha battery?

1. Is there a decent way to determine motor and prop given these dimensions? Any suggestions based on experience?

2. Related to first question. Is there a good way to approximate power needed given size, shape, and weight for small RC boat?

3. Outrunner vs. inrunner
I understand how they work differently, and that outrunners have higher torque and lower RPM, but is there any other reason why a boat motor should be an inrunner. It seems like most of the brushless motors labeled as boat motors are inrunners?

4. Cheap ESCs on ebay any good?
I saw this one for a few bucks less, but can't find it right now

http://cgi.ebay.co.uk/RC-Model-Water...ad8971940#shId

I forgot the rest of my questions. Any help would be greatly appreciated.

mfr02

4.5AH at 6V shoves my 6lb boat around at about that speed quite happily for a couple of hours using a 15 volt 385 brushed motor and a 35mm 3 blade plastic prop. Performance for power depends mostly on hull shape - it needs a lot more power to shove a square ended barge than something narrow and pointed at any particular speed. One hour using a theoretically more efficient brushless should be easily possible.
I try, at the start of a project, to work out the power requirements for a scale model from the prototype information - known HP convert to watts (*745), then divide that by the cube of the scale, to give a wattage requirement for the model. You appear to be starting somewhere in the middle of that, and will have to work outwards to what would have been my original question and circle back to your required answer. The wattage figure usually needs doubling for a model because there are greater losses on the model, and the figure is for power into the motor, not power out of the original's engine. Brushless motors are generally more efficient, so the fudge factor could be reduced. So, for me, the empirical way would be to draw up the hull, find a "real" one the same general shape and hopefully with known performance and power, work out what "scale" it would be and go from there.

Tony Fitzpatrick

These are brushless motors which require brushless controllers

That is a typical entry level brushed controller that gives forward and reverse.It won't work with a brushless motor.

I don't know if it is a help but I get the same thing.

Perhaps you were thinking Do I need a controller with low volt cutout if I want to use a Lipo battery?
If so,yes.

night_man

oops I didnt see that was not a brushless ESC.  Thanks Tony.

Thanksmfr02.  I guess my problem is all of the medium sized brushless motorson hobbyking have more than enough power for me, but is there asignificant loss in efficiency if I use a motor that can produce more power than I need?  I was looking at this one

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

Tony Fitzpatrick

You are after precise answers but they will be nearer the mark once a hull is obtained/built and if you describe the project objective is.
You mentioned efficiency power,speed,duration and budget restraints yet I can't venture an opinion because I don't get a picture yet.[sm=confused.gif]
Do you see anything similar about?

night_man

The hull hasn't been purchased yet because I havn't recieved all the electronic parts, and the size might vary slightly depending on certain aspects of the sensors, which havn't been tested yet. Lets say its roughly 4 pounds total, 20" long, deep vee hull, and a target of 4mph. I can't really get any more specific about the dimensions at the moment, but is there a rough estimate for power based on this?

Also after mfr02's reply, I'm not overly concerned with efficiency. I just wanted a basic feeling for the efficiency vs percent of peak power used or something along those lines.

I guess I'll wait until I get the hull to proceed with the motor. I was just being a cheap SOB trying to save on shipping since I was ready to order some of the parts.

Tony Fitzpatrick

Efficiency can go along with peak power.I could suggest a motor giving nearly 90% efficiency,how to get maximum duration,how to fit the hull out for $30 and so on.I am trying to put the requirements in some order of priority.
I don't have all the answers but don't want to go through the process of 10 reply posts before finding none of them are relevant or you then mention it is a landing craft or something.People do that ya know.

What makes up the 4lb?
Sensors?
I would hate to take you on at poker.

night_man

Hmm I'm actually decent at poker, but I'm pretty sure that's not why you wrote that

"I am trying to put the requirements in some order of priority."
OK. The project is an autonomous boat that measures water data based on GPS points.  There are really no other requirements.  This is not a landing craft.  I am interested in designing it to get decent run time.  I picked an arbitrary speed that seemed reasonable without going through the battery very fast.

The 4lb I guessed was total.  I have no idea how much a 20" or so hull+accessories weighs cause I'm a newb.  The sensors hardly weigh anything.  Maybe 50g total.

So is the power of an RC boat always determined by scaling from some larger boat, or is there some formula somewhere for a rough estimate if you know the weight, size, shape, speed...?

Tony Fitzpatrick

OK now you are speaking my language.
Try to keep the hull as narrow as possible,say 4 inches at the expense of longer length.With that you can use the aforementioned brushed 385 motor and get 4mph and one hour with a 3 amp hour 12 volt lead acid battery. See the como drills website for 385 info or higher power motors.

Your craft is autonomous.Do you know the speed control input is a 1-2ms modulating pulse every 10 to 30 ms? The particular brushed esc you linked to requires that too.
You will now get a suitable forward only brushless esc and motor for less than $20 but most have them have the annoying characteristic of requiring an arming sequence.Are you familiar with that and rc electronics?An engineer friend favours brushless motors purely for the reduced radio frequency interference but a 385 brush motor can take a 6 amp filter from a 120 volt system.

If you are worried about it rocking from side to side then avoid vertical sides.I guess you are in open water so you would get that anyway.Look on youtube and you will see that anything other than smooth water will rock a 2 foot boat considerably.
You could build a lightweight hull to your own design in a few hours with 1/8 1/16 ply and skin with 1/32 or 1/64 ply or you could purchase a destroyer or corvette hull from Ebay or model supplier.
There should be plenty on line build logs.
Are you considering a radio control set?

night_man

Thanks. I'll check out that stuff later tonight.

I'm using an Arduino and the servo libraries can be used to send PWM signals to the ESC. It can handle up to two PWN signals at once, so the other should work for controlling the servo for the rudder. I was aware that ESCs had to to be armed by sending the minimum throttle signal in most cases.

"If you are worried about it rocking from side to side then avoid vertical sides.I guess you are in open water so you would get that anyway.Look on youtube and you will see that anything other than smooth water will rock a 2 foot boat considerably."

Noted.

For radio control I am first using a cheap RF tranciever just to stop the boat in case it isn't working properly. I might have it so I can toggle between being autonomous and remote controlled later on.

I'll also check out the hulls on ebay. I've considered building from scratch. It would be easy to set up things to be mounted if I did that, and it would be pretty fun. I've seen pictures and videos on youtube of boats built from scratch. I'm just not convinced that I will pull it off Do you know of any good basic tutorials. I understand the overall process, but I'm worried about the specifics (materials, techniques ...)

stmjxxi

What your time is worth,I'm not sure.
Ebay might turn up a ready-to-go battleship complete with toy like radio control.
What could be more fun than that?

mike_o

Battery/run time:
Run time on a 5000mAh (=5Ah) battery depends on current drain. It's straighforward: 5Ah is 5 amps for one hour, or 1 amp for 5 hours, or 500milliamps for 10hours, or 50amps for 6 minutes, or.....

Motor selection
Given that you only require 3-4mph, you won't need much power. I'd go for traditional quality brushed motor. If your hull is reasonably streamlined, you wouldn't need more than 10-20 watts to drive it. That could come from a Speed 400 Motor, and that would draw only a few amps (14,8watts=7,4V x 2amps).

Prop selection:
A 400 motor runs some 8000-18000rpms depending on load. Use rpms and prop pitch to calculate speed. Remember slippage (40-50%, so prop speed should be 6mph, or some 300cm/sek. I think. Experiment with diameter. Perhaps 25mm?

keep us posted!

Mike  

mike_o

By the way, "Autonomous boat" - what do you have in mind? And why does the weight matter in a relatively slow boat??

mfr02

Quote:

Experiment with diameter. Perhaps 25mm?