Topic: Motor Advice needed Arun Class 1/12 Scale (Read 8136 times)

klz · « **on:** July 23, 2015, 02:10:51 pm »

I have a Arun lifeboat 1/12 scale it currently runs on 2 x 36v wheel chair motors and 3 x 12v 7mah batteries, run time for this setup is about 20mins and speed is not impressive I think due to the weight of the batteries. I would like to get a little more run time if I swapped the motors out for MFA Torpedo 850’s do you think this would get me a little more speed and a longer run time just on 2 batteries. I don’t really want to go brushless as I have nothing but trouble with them.

Thanks in Advance
Kelvin

Calimero · « **Reply #1 on:** July 23, 2015, 02:13:16 pm »

Brushless and LiPo or LiFePo4.

What troubles do you have with BL motors ?

A decent setup with two outrunners, two batteries and two car ESCs and you'll be all set.

Stavros · « **Reply #2 on:** July 23, 2015, 02:26:37 pm »

Been there had the tee shirt.....The 850's wont give you anymore running time.
Turnigy 700kva Brushless running on 24v for each motor either on Nimh stick packs or lipos and you will see a very impressive set up especially on 60mm 4 bladed props.
There is no way that a brushed motor can compete for power or run time than a brushless

Dave

klz · « **Reply #3 on:** July 23, 2015, 02:39:46 pm »

I thought you was going to say brushless, Calimero I have a MTB running on brushless so far this year I have replaced 2 esc and 3 motors everything seems just to run to hot even though it is all water cooled. Staros I will have a look on Hobbyking for what you recomend I have just had a quick look what do you think of this combo?

http://www.hobbyking.com/hobbyking/store/__25144__Turnigy_TrackStar_Waterproof_1_8_Brushless_Power_System_2300KV_120A.html

Stavros · « **Reply #4 on:** July 23, 2015, 03:16:06 pm »

Right....if you were to run that on 12v you would be pulling revs of around 24000 on each motor and is not what you need....
you need around 700kva/volt for 24v OR A max of 1400kva / volt for 12v

Dave

gingyer · « **Reply #5 on:** July 23, 2015, 03:39:15 pm »

Hi there,
Friends of mine years ago had the big 1/12 scale Arun lifeboats.
They had 2x 24v Bus fan motors in them (smith or similar)
They had 4x 12v batteries (2 per motor) and they ran all day, really good speed and with the
4 batteries laid out properly needed very little ballast

Time Bandit · « **Reply #6 on:** July 23, 2015, 05:59:26 pm »

Just a few quick thoughts:

Half gliding hull, scale speed probably something around 6-8 mph.
Running on usual 60 mm brass props you´d need around 5000-6000 rpm to reach scale speed of around 7 mph.

So kv for 12V would be around 500-600kv and 24V around 250-300kv.
Anything else will increase current and heat on the motor.

Brushless motors have 1 "problem" that´s why they get hot (also in your other boat).
If your brushed motors turns too fast and is overloaded because of that, it´ll just break down in it´s rpm.
By that also the current gets reduced.

Brushless motors are "rpm stiff". They don´t break down in rpm if they are overloaded. So if you let´s say put a 10.000 rpm brushed motor in that boat the rpm will break down to around 6-7000 rpm and it will draw 15A per motor and the boat will run 8mph.
The brushless motor keeps it´s rpm in any case, due to the forced commutation by the ESC.
So the boat will maybe run 9 mph but the motors will draw at least 40-50A

A motor with a matched max rpm (under load!) of around 5000 will make the boat go 7mph but only draw 8A.

Any rpm mismatch is greatly reduced by brushed motors by their feature to reduce rpm if the load get´s higher.
Not possible on bruhsless motors, thats why you fry them much easier.

If you want to stick to brushed motors, go to the next junk yard and get some big fan motors and run them on 12V. They´ll have the correct rpm.
Otherwise and I highly recommend that, get Brushless motors (at least 250g weight per motor) with correct rpm and Lifepo4 batteries (Headway cells).

Stavros · « **Reply #7 on:** July 23, 2015, 08:08:01 pm »

Sorry beg to dissagree big time as I used to own a 1/12 scale one and I used to get it up on the plane as she should have been and get around an hour running time on 24v without water cooling and the motors never got hot.

Dave

Calimero · « **Reply #8 on:** July 23, 2015, 08:09:19 pm »

Yep. I'd try a couple of 600kv outrunners with 3S LiFePo4.

HK has a 4240-620kv outrunner in their SK3 range.

I'd power them with 3S LiFePo4 batteries like these ZIPPY Flightmax 4200mAh 3S1P 30C LiFePo4 but of course you could first try with your current 12V SLA batteries (although I suspect the LiFePo4 batteries would probably give you slighty better run time).

ESC wise, a couple of these should do the trick : HobbyKing® ™ Brushless Car ESC 100A w/ Reverse (keep the fans on !)

klz · « **Reply #9 on:** July 23, 2015, 08:19:39 pm »

Thank you for your replies, looks like this winter she will get a motor upgrade. Calimero thanks for the links.

Kelvin

Time Bandit · « **Reply #10 on:** July 23, 2015, 09:52:22 pm »

Quote from: Stavros on July 23, 2015, 08:08:01 pm

Sorry beg to dissagree big time as I used to own a 1/12 scale one and I used to get it up on the plane as she should have been and get around an hour running time on 24v without water cooling and the motors never got hot.

Dave

If you use motors big enough, they don´t get hot. Doesn´t mean they run in a economical useful range.

700kv @ 24V and a usual 60 mm prop would mean, the boat would theoretically go 23.6 mph

that would have been faster than the original in 1/1.
This is not possible with this kind of hull. It´s not made to get on the plane, it is half gliding which stops at around 2.5 times the hull speed, which would be around 9mph for this 1/12 hull.
Since power consumption increases ^3 with the speed the amps drawn must have been horribly high.

15000 rpm are definetly completely wrong for this kind of boat.

klz · « **Reply #11 on:** July 23, 2015, 10:00:45 pm »

from what I have been reading it looks like mine runs at scale speed so a better run time is what I will aim for.

furball · « **Reply #12 on:** July 23, 2015, 10:08:05 pm »

Top speed of an Arun was around 18/19 knots (when they were new and at their original displacement), but they didn't 'plane' at this speed, as the hull was the wrong shape.

Lance

Stavros · « **Reply #13 on:** July 23, 2015, 10:20:02 pm »

Time Bandit i bow to your superior knowlage and calculator skills........guess what YOU are so wrong in your assumptions of current were horribly high as they defo wernt. But you obviously know better.
Please stop trying to incite typo wars as I FEEL YOU Are trying to make me out to be a liar which I take great exception to.

Dave

Time Bandit · « **Reply #14 on:** July 23, 2015, 10:37:13 pm »

Sorry about my typos, I´m not a native speaker, hope that counts as an excuse.

Wasn´t my intention to make you look like a liar or idiot or whatever, maybe the current drawn was limited by current capacity of the batteries,you used low pitch props or whatever. So I hope you take that as an excuse as well.
Just saying if you use that setup on modern types of batteries it´ll fry something (wether it´s the cable, motos, ESC...) within a few minutes.

Calimero · « **Reply #15 on:** July 23, 2015, 10:47:07 pm »

I agree with Time Bandit, 700kv * 24v = 16800 rpm (no load) on a 60mm propeller. That's huge.

An 800kv BL under 11.1V (3S) turning a Raboesch #147 40mm 4-blade prop draws 20A, that's around 230W for a theoretical speed of 8900rpm - I didn't measure the actual speed. A much larger prop spinning twice as fast is going to draw much much more.

Are you sure you're not mis-remembering one of the numbers, Stavros ? Maybe it was running on lower voltage ?

Time Bandit · « **Reply #16 on:** July 23, 2015, 11:07:09 pm »

Moderated

We must miss something important like a gear reduction or whatever.
The usage in direct drive on 80mm props on a displacing hull with 20A fuses sounds just plain impossible.
I´d expected the current in such a application to be at least 100A.
So something must limit rpm and current drawn in your drive train.

Stavros · « **Reply #17 on:** July 23, 2015, 11:12:42 pm »

will take pics of motor And esc and batts tomorrow not going to shed tonight I Also use the same set up in a lesro javalin and a Models by Design hull ....ok admittidly these 2 models are on 60mm 2 bladed props......YES obviously they are smaller props and obviously lighter models and yes they are bananas in terms of speed.
The motor i will take a pic of will be of the models by design hull...simply as it is on the bench and easy to get at .....the ones in the Happy Hunter are buried and is at rhe back of the shed

Dave

Stavros · « **Reply #18 on:** July 23, 2015, 11:19:35 pm »

Further more no gear reduction No tricks

Moderated

Dave

Time Bandit · « **Reply #19 on:** July 24, 2015, 03:19:09 pm »

I wouldnt write anything if it was pure guesswork. In my case its more than 25 years of building model boats and planes by myself and at least 10 years of blowing up all kinds of motors, ESC... due to bad decisions. Main reason for these decisions in the beginning were the completely stupid motor recommendations by Graupner. So I started to test and measure.

What I write now, is not my personal opinion, I believe that 95% of that, can be counted as "common sense" amongst model boat enthusiasts.

15.000 rpm are good for fast boats. Deep V racing boats with fully submerged propellers use those rpm, Zenoah IC engines run on rpm like that. Depending on motor size you can energize any fast full gliding! boat with such rpm and a 60 mm prop. BUT you will most likely not reach run times of 1h on these boats, runtime is limited due to battery capacity. Typical run times are 10-30 min with modern batteries mainly depending on full throttle speed.
Higher rpm are only used for half submerged propellers and boats at very high speeds. Run times of these boats are typically 2-10 min depending on speed.

15.000 rpm are definitely too much for half gliding hulls like lifeboats or fast pilot boats. The hulls are not made to come to full gliding, they "stick" in the water what makes it impossible or very power hungry to bring them to gliding. Usual rpm used always depending on hull and prop size (smaller prop --> bigger rpm) are between 4000-8000 maybe 10000 rpm on very small props.
Anything else makes the boats only a little bit faster but a lot more power hungry, as the current increases by ^3 with the speed. Double the speed and you´ll need 8 times the current. 80% of these hulls are limited to around 2.5-3 times their hull speed, for a 4 foot boat that's 7-8 mph.
You do not necessarily fry something if anything is oversized enough, but current increases a lot if rpm are too high compared to the speed the hull is capable of.

15.000 rpm are definitely a motor and/or ESC killer (in best case a fuse killer) if used in displacing hulls with 80 mm brass propellers. That is not my personal opinion, that is what happened to a few thousand motors and ESC in the last few years.
And it doesn't matter whether the motor is 200g or 500g, it will blow up in direct drive of an 80mm brass prop.
Common sense is to run those boats on rpm between 1500 and 4000 depending on prop size and pitch. 1500 would be a 100 mm+ prop on a very big boat, 80mm maybe needs 2000 rpm.
These hulls are limited to their hull speed (2.8 mph for a 1m hull) and it´s most likely impossible for them to glide if you dont massively overpower them. So any rpm above a certain level (and with this power) is not creating any speed faster than the hull speed, but is fed into waves which consume the power.

80% of ESC´s and motors that have been fried in the past, died because of too much rpm or too big propeller.

Moderated

Stavros · « **Reply #20 on:** July 24, 2015, 07:58:18 pm »

Moderators I would strongly advise closing this thread as it is getting personal and I for one will only take so much before I explode.

Dave

Martin (Admin) · « **Reply #21 on:** July 24, 2015, 08:13:43 pm »

Topic Locked and moderated.

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Author Topic: Motor Advice needed Arun Class 1/12 Scale (Read 8136 times)

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