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Motor, Hull, Prop choice for large slow boat

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Motor, Hull, Prop choice for large slow boat

Old 02-19-2020, 11:45 PM
  #1  
bcosta
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Default Motor, Hull, Prop choice for large slow boat

Hi all, I am investigating a design for a slightly over-sized RC like autonomous boat run from solar and thought this might be the perfect place to learn some things.

I don't know anything about hull, motor, prop design and was looking for some guidance on where would be best to learn?

Right now I am looking at what sort of motor + prop + hull would be required to move a boat of about 700mm x 300mm carrying 5kg at a speed between say 1 km/hr and 10 km/hr?

Could you please offer any good suggestions on places to learn more about this?

Also even if you can point me in the ballpark of what would be required for a motor to achieve this it would be helpful.

For example, with other components of this system, to have a motor that can run almost 24 hours, it would need to draw less than 10W of power on average at cruising speed.

Looking at one motor example E6EHS-12:
Series_Low%20Voltage%20EC%20Motors_Metric.pdf


Would it be feasible to use this motor to drive such a boat?

It would run at 1820 rpm providing 14.6mNm torque. Would it be possible to push a boat of about 700mm x 300mm carrying upto 5kg?

My first thought is no, but I just dont know and also dont know what would be a reasonable motor for this case?

Thanks,
Brendon.
Old 02-20-2020, 02:50 AM
  #2  
mfr02
 
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[/QUOTE]Right now I am looking at what sort of motor + prop + hull would be required to move a boat of about 700mm x 300mm carrying 5kg at a speed between say 1 km/hr and 10 km/hr?
[QUOTE]
A 11lb, 2 foot boat 1 foot wide is perfectly feasible. 10KPH probably isn't. It's been asked a lot of times before, but google for "hull speed".
These motors have been discussed on another site recently - https://www.aliexpress.com/item/3296...chweb201603_53 - the buyer seems happy with his purchase.
Solar powered might need a lot of resting/recharging time. Even at a sci-fi level of efficiency, I doubt that the small area of solar panel possible on a small hull would gather energy fast enough to do really useful work.
The motor specced in the OP simply would not shift enough water to noticeably move a 5Kg hull. To move a displacement hull forward by its own length in a set length of time, you have to move at least the displaced volume of water by at least the same distance in that time. Moving water requires power that a 3 or 4 watt motor just doesn't give.

Last edited by mfr02; 02-20-2020 at 03:00 AM.
Old 02-25-2020, 02:57 AM
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bcosta
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Thanks for the info. I did some reading about hull speed and design there is a lot of info out there.

I looked up the motor you linked. I am having difficulty finding reliable specs for that particular motor. All websites that sell it say it is a 30W motor, but then some say it draws 0.5A at 12V which is just 6W power draw (others have even lower current draw numbers). I was assuming this current draw to be much higher about 3A for 30W @ 12V with some loss, so this must be unloaded. Also says Max torque is: 2KFG.C, I assume it means 2kgcm which is 196mnm, at least this seems consistent with a few sites. No idea about efficiency. I went looking on the "Guang Wan Motor Co Ltd" website for any graphs with performance curves or reference specs but didn't find anything. Also all sites that sell this motor all seem to have different summary specs documented.

If you think a 30W motor is sufficient, then that gives me a good place to start. I will take a look and see if there is anything similar with better spec sheets (but probably more expensive). Also might just buy one of those are they are not that expensive and give it a try.

It sounds like specs I am looking for then are probably:
* 30W output motor
* about 150mNm max torque
* max efficiency at about 2500 rpm

Thanks again for the info.
Old 02-26-2020, 02:39 AM
  #4  
mfr02
 
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Welcome to the whacky world of electric model boats.
Don't get too hung up on numbers in spec sheets.
Do remember that when a fuel drven engine is being talked about, what is measured is the output power at the shaft.
When talking electric motors, the figures are invariably power IN. How the motor uses this power is dependant on a huge number of variables, mostly unknowable. Start with prop design and transmission shaft losses. An unloaded motor will spin as fast as the applied voltage will let it, current drawn is what is needed to spin the motor. A not quite stalled motor will grind round producing tremendous torque, but also generating lots of heat. The numbers quoted for "max efficiency" will rarely be of any practical use, but merely serve to give a re-producable test bench result.
Us scale boaters, when we need to figure out a power plant, usually start with the original, and its claimed numbers, usually in horse power. Converting this to Watts, then dividing by the cube of the scale gives a base power requirement for a model obeying scale performance. Doubling the figure gives a practical value to allow for motor inefficiencies and other losses.
While yours is not strictly a scale model, somwhere out there is something with a hull form that "could" be a prototype for it. It might even have claimed power/performance figures, which are amenable to having the scaling applied to turn up with a suitable answer regarding the motor.
In your case, probably a pair of RS385s running on a 7 or 8 cell NiMH pack. They won't be happy with a large prop unless geared down. ("rule" - prop diameter must be smaller than motor can diameter, prop should have fewer blades than the motor has poles)

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