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Author Topic: Effect of prop size on motor rpm  (Read 1780 times)

Hotglove

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Effect of prop size on motor rpm
« on: June 22, 2018, 07:08:56 pm »

As a novice to model boats I am learning a lot, mostly due to good advice from forum members and club members, thank you all.
I have changed my Dumas Chris Craft 1930 Runabout to brushless (Turnigy 3648 1450) first of all I have been amazed by the power output from this very compact motor.
I have been trying to assess the best compromise between motor and prop size, when I acquired the model it was fitted with an MFA 850 Torpedo with bronze Dumas 3 blade 1 1/2" prop and performance was a bit disappointing, plus the size of the motor made it impossible to line up shafts accurately and there was a very rough sound at high revs.
With the new motor alignment is spot on, all the roughness has gone (heavy duty rubber coupling)
I found that with no load, on 3s LiPo the motor ran at 18170rpm, with a 42mm X prop in water it ran at 12000, today I fitted a 35mm plastic prop and got 14700 rpm.
I also tried in a brand new test tank (which made my 4yo granddaughter very happy, new paddling pool) to run the boat against a digital scale. If any neighbours were watching it would have been very entertaining, the 3648 is claimed to put out 2.5 bhp and I can well believe it, I will not try that again without a second pair of hands, I was not aware that boats can do wheelies.
one other point is that, for testing purposes, I was using a single 3s 5000 mah LiPo pack ( I normally run two identical packs in parallel) today was the first time that the battery pack felt warm, until now they have never gone noticeably above ambient temp, should I continue to operate them as a pair, even for testing?
Once I have decided on prop size and pitch I intend to fit a balanced alloy prop, both for performance and appearance.
Thus far I am leaning towards 35 or 38mm X type 2 blade.
All comments welcome, thank you.   
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unbuiltnautilus

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Re: Effect of prop size on motor rpm
« Reply #1 on: June 22, 2018, 07:17:20 pm »

Personal opinion, use 's' not 'x' pitch props. Lets the motor spool up with slightly less load on it, equalling slightly lower current drain, longer run time...and a cooler LiPo!
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Hotglove

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Re: Effect of prop size on motor rpm
« Reply #2 on: June 22, 2018, 11:20:25 pm »

Thank you, power certainly does not seem a problem, cool running definitely an advantage.
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red181

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Re: Effect of prop size on motor rpm
« Reply #3 on: June 26, 2018, 10:43:01 pm »


x props have more pitch, they will give more torque, at the cost of higher amp draw, thus shorter run times. I personally only use x props, as I like boats, even scale, to have more torque and speed.
You then bring into play, the size of the blades, most common available plastic x props are perfect for testing, as they are cheap, but they only come in two blade format. It all depends on how fast, and for how long you want to run. I go fast, for short time, 10 mins if going round and round is enough for me, other will prefer slower running, and a much much longer run time.
Brushless and lipo would suggest you want some speed, otherwise why do it, so x prop would be the way to go. Start small, and work up. A warm lipo isn't bad, a hot one is! if you over discharge a lipo, and it swells, its finished. The heat is generated by the discharge rate, the pack will have a c rating on it, the higher the c rating, the better the discharge rate, generally 30c and upwards for a heavy discharge system, remembering that Chinese stated mah, and c rating are generally well over estimated .
The kv rating of the motor is also important, under 1000kv (as a generalisation) will give more torque, and therefore suit a heavier boat, higher kv will spin up more, less torque, more top speed, so lightweight fast electric racing boats will go this way.
If you buy a cheap amp/watt/volt testeer, such as this


http://www.4-max.co.uk/wattmeter-budget.htm


you can plug inline, and test the amp draw. Your motor will have a working max wattage, and more importantly your esc will have max amp useage, exceed these at your peril, you will end up with that smell of electrical death, usually in the middle of the lake :embarrassed: . Its the only way you can really analyse what is going on, your fastest set up might not be the best!
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Tombsy

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Re: Effect of prop size on motor rpm
« Reply #4 on: June 27, 2018, 01:54:32 am »

Easy way to calculate unloaded rpm is kv x volts, you shouldn’t run brushless motors unloaded for very long periods just a blip of the throttle. Going up in prop size will increase your amp draw it’s a balancing act to get the speed you want and not overheating the esc and motor. As previously mentioned a higher “c” rating for batteries is usually needed in boats to supply the amp draw compared to airplanes. Lipo batteries perform better when they are warm, running a cold pack with a high amp draw can lead to the pack puffing. As soon as I get to the pond I put them in the sun to warm up.
We consider “x” props as lifting props on hydros, by lifting the transom they help to keep the bow down to help prevent blow offs.
Your single pack got warmer because it was trying to supply double the amp draw. If the cell voltages are okay it should be fine, they get a lot warmer when you are racing and they are discharging in 2 minutes.
Just don’t discharge them too low, if you can set your cutoff on the esc try to keep it at least at 3.3v.
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Brian

roycv

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Re: Effect of prop size on motor rpm
« Reply #5 on: June 27, 2018, 08:57:30 am »

Hi Hotglove, always interesting to here someone taking actual measurements on performance.  The suggested maximum 2.5 BHP would have drained your battery in less than 30 seconds at 169 amps.  So not much of a runtime but probably out of sight in less time than that!
I go for what looks good against least amount of power.
I was experimenting with a tiny brushless motor with an 11 : 1 gear reduction (ex helicopter) to a 35mm prop and the power was remarkable.  I have a small set up in a 20 inch loa "Bobby" and that goes on for ever!
regards Roy
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Hotglove

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Re: Effect of prop size on motor rpm
« Reply #6 on: June 28, 2018, 09:02:03 pm »

Thank you all very much for the input. I am by nature quite mechanically sympathetic, so am not comfortable with highly stressed drivetrains. The brushless setup is very pleasing because the model lifts quickly and easily onto the plane, it also responds to the rudder much more swiftly.
The original Dumas bronze prop was a dull casting and I have done a bit of tidying and polishing, balance seems pretty good and it will be refitted soon and tested.
I had not realised how much variety and evolution went on with model boats, very entertaining.
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