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Author Topic: Counter Rotating Props.  (Read 5754 times)

Glyn Roberts

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Counter Rotating Props.
« on: April 14, 2015, 02:44:13 PM »

Hi Folk

I am soon to start building the Dumas Dauntless boat and I need to decide on the merits or drawbacks of counter rotating props.   Also as it has 2 motors I will have to decide on Brushed Or Brushless and what size motor.   I was thinking of a pair of MFA 850 or 800.

My take on this is that Brushed will drive boat at Scale speed cheaper and with less equipment IE cooling, 1 ESC instead of 2.  Plus acid lead battery will serve as ballast for such a big Boat 49 1/2 inches.

I welcome any thoughts on this subject as I have not committed to anything yet,  if my thinking is flawed please tell me.

Regards and thanks Glyn.
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Stavros

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Re: Counter Rotating Props.
« Reply #1 on: April 14, 2015, 03:19:07 PM »

Right if I was you i definatly would Ditch the thoughts of using a MFA 850 motor simply for the reason they are very current hungry.................Secondly ...Personally I wouldnt use a single ESC as with 2 motors you could well be in excess of the working capacity of the esc....certaintly with the 850's you would  O0 O0 O0 O0 O0 O0
 
A Pair of 900 series motors or similar would suffice ....a lot depends on what size of props you are envisiging on using let alone 2/3/4 bladed ....
 
Dave
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Calimero

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Re: Counter Rotating Props.
« Reply #2 on: April 14, 2015, 03:27:22 PM »

Quote from: Glyn Roberts
My take on this is that Brushed will drive boat at Scale speed cheaper and with less equipment IE cooling, 1 ESC instead of 2.  Plus acid lead battery will serve as ballast for such a big Boat 49 1/2 inches.


At the same performance (ie: speed) a brushless setup is likely to draw a little less current and decently sized motors probably wont require cooling at all. BL motors would indeed require an ESC per motor but electronics are cheap these days.

SLA batteries don't look like the best choice for such application. Although SLA batteries can withstand high Amp draws, capacity is then very limited. The capacity rating is usually given for a 10hour discharge (C/10). And if the amp draw happens to be higher, capacity can be just a half or a third of the rating.

What props (brand, dia, pitch) ? What target speed ?
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #3 on: April 14, 2015, 03:49:02 PM »

Hi Dave,

Thanks for your reply,  I take it that the 900 series you mention is the Graupner 900 I found on the net westbourne-model.co.uk.  They are temp. unavailable.  It did not say whether they are brushless or brushed.

My knowledge of modern motors and equipment is limited to the 3 months of my return to modeling,  so lets face it I know Nowt.
I have not considered size of screws as I need to find a starting point.  You have helped by steering me away from power hungrey motors.

Regards Glyn
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #4 on: April 14, 2015, 04:01:14 PM »

Hi Calimero,

Thanks for reply,  I see your point that brushless would be best for power consumption,  and I need to re-think the battery type,  would NiMH be OK with brushless motors?

As for target speed, I would require at least scale speed for an offshore Cruiser,  and some spare speed in hand would be nice.

Regards Glyn


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TomHugill

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Re: Counter Rotating Props.
« Reply #5 on: April 14, 2015, 04:12:02 PM »

I would definitely run counter rotating probs, torque roll is a pain!  As to your power train a brushless setup is the way forward, it will be significantly cheaper than a brushed setup, they generally perform better than brushed on lower current. Power wise I'm a firm believer in Lipo batteries, as long as you follow the safety advise and respect them you shouldn't have any issues.
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Calimero

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Re: Counter Rotating Props.
« Reply #6 on: April 14, 2015, 04:20:12 PM »

BLDC can work with NIMH batteries, yes. But these days LiPo are pretty cheap (and so are chargers).


From what I found about the Dauntless, she seems to be in 1:16 scale. What propellers are you going to use ?

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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #7 on: April 14, 2015, 04:29:07 PM »

Hi Tom,

Thanks for your reply, The torque problem was on my mind when asking about counter rotation and you have confirmed what I have heard.

I must admit feeling a bit uneasy about Lipo, having read some accounts of mishhaps IE (boats going up in flames)   Having said that I will bite the bullet and go for Lipo.

Regards Glyn
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #8 on: April 14, 2015, 04:47:09 PM »

Hi Calimero,

Thanks for info. I cannot say what size props untill the motors are chosen.  I cannot even order the running gear for the Dauntless as Cornwall's are out of stock at present.   The kit should arrive this week but will have to wait for the running gear.

Regards Glyn
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Stavros

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Re: Counter Rotating Props.
« Reply #9 on: April 14, 2015, 09:00:59 PM »

Glyn Give Steve tranter a ring at Steve's Model Boat Bits a ring He Stocks Caldecraft 900 motors these will do you nicley
 
Dave
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #10 on: April 14, 2015, 09:41:58 PM »

Thanks Dave,

Cannot get Graupner 900 for love or money,  some one paid over 70 for one on ebay.  Will phone him tomorrow.

Lovely place Bethesda,  I used to live in betws y coed and Llandudno but was born in Cardiff. 
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TomHugill

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Re: Counter Rotating Props.
« Reply #11 on: April 15, 2015, 12:05:36 AM »

I would also check out a brushless setup, you will probably find total cost less than a brushed setup as brushed Esc seem to be rather expensive these days!
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Calimero

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Re: Counter Rotating Props.
« Reply #12 on: April 15, 2015, 03:49:57 PM »

Thanks for info. I cannot say what size props untill the motors are chosen.  I cannot even order the running gear for the Dauntless as Cornwall's are out of stock at present. 



That's a bit of an egg-or-chicken situation.

Motor choice depends on the props: ie getting the right RPM (power probably won't be much on an issue; BLDC motors exist in various sizes).

I guess the Prop Shop's General Purpose props could do the job. Maybe a pair of 4514 (45mm x 63mm) props would do the job. Though I don't know how/whether they would fit on the Dumas shaft if you go that way.

There are a few Dauntless topics on RCgroups but not much info on the props used.

If we do some basic calculation for one of these props: RPM * pitch * slip rate = speed

Pitch is 63mm
Slip rate on a planing hull: let's assume 20% (maybe less)
From what I read these commuter boards were apparently capable of doing 35-40kt!
As the Dauntless is 1:16, scale top speed would be around 19kph.

Let's do some basic calculation:
19*1000 (speed in meters per hour) / 60  (60 minutes per hour) / 0.8 (20% slip rate) / 0.063 (prop pitch in meters) = 6283 (shaft RPM)

With 3 cells LiPo batteries (11.1V) here are a few options:
- Turnigy Aerodrive SK3 - 4240-620kv
- Turnigy Aerodrive SK3 - 3548-700kv

I don't think either motor would require cooling. Car ESCs wouldn't require cooling (you may keep their little supplied fans though to be safe).

But as you can see on the Prop Shop's site, prop pitch can vary greatly between props and hence the motor speed.
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #13 on: April 15, 2015, 04:05:44 PM »

Hi Calimero,

Wow!  the maths went over my head, but thanks for the result.   I will research the possibilities although I have more time now as Cornwall's have realized that they do not have a Dauntless in stock. 

Regards Glyn

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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #14 on: April 15, 2015, 04:42:37 PM »

Hi  Calimero,

If I go for the Turnigy 700's what ESC would I need to match Motors and your suggested 3 cell Lipo?

Glyn
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martno1fan

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Re: Counter Rotating Props.
« Reply #15 on: April 15, 2015, 05:45:02 PM »

A friend used 600 size outrunners on one of my 58" Apaches she runs 50 + mph on 6s per motor,how big is this hull you have?.
Mart
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Calimero

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Re: Counter Rotating Props.
« Reply #16 on: April 15, 2015, 06:22:43 PM »

If I go for the Turnigy 700's what ESC would I need to match Motors and your suggested 3 cell Lipo?

Keep in mind that those motor recommendations were based on a specific prop and may not be suitable for other props (dia/pitch).

I would probably go for car ESCs. Car ESCs have heat sinks and usually even small fans which should suppress the need for water cooling.

The downside is that when the low-voltage cut off kicks in (to protect your LiPo battery from deep discharge) the ESC might cut the throttle. Boat ESCs usually reduce the throttle so that you can get back to shore.

I have a couple car BL ESCs from HK (but use them on tugs with SLA batteries and have the low voltage cut-off disabled) and I'll check their behavior.

One work around is to plug a little stand-alone low voltage alarm on each battery which will emit a loud beep whenever any cell gets to 3.3V or below, letting your know time's up.

Also it is unlikely that both batteries would hit the same low voltage value at the same time but well ...

Regarding batteries one of these for each motor would probably give you decent run time. This one is significantly cheaper even though run time would be a little shorter (probably better value).

By the way what radio are you going to use ?
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #17 on: April 15, 2015, 08:47:58 PM »

Hi Mart,

The boat is 49 1/2 inches long.

Regards Glyn
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #18 on: April 15, 2015, 08:53:58 PM »

Hi Calimero,

I have the planet 2.4 Ghz

Glyn
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martno1fan

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Re: Counter Rotating Props.
« Reply #19 on: April 22, 2015, 11:42:55 AM »

If shes 49" long then the 600 size out runners would push her very well maybe too well  :embarrassed: ,you could probably get away with using 4-5 s to get a more scale speed,maybe go with 880 kv and 4-5 s lipos,these are the motors he used he then switched to the Hi model versions in 910 kv,same or slightly better performance.Two turnigy 140 or 180 esc should work well.These motor have bags of torque,one would push her but i know you want a twin setup.I would most def use water cooled esc,s the motors work much harder in a boat than in a car and you will need them cooled,the motors should be fine but it wouldnt hurt to water cool the mount maybe.Cpl pics of them in the large 58" Apache,they look tiny lol.
Mart

http://www.hobbyking.com/hobbyking/store/__39873__Turnigy_T600_Brushless_Outrunner_for_600_Heli_880kv_UK_Warehouse_.html?strSearch=t600

http://www.himodel.com/electric/HiModel_PRO_Series_530KV_Outrunner_Brushless_Motor_for_600-700_Helicopter_Type_HG3740PRO-7T.html
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #20 on: April 22, 2015, 02:03:14 PM »

Hi Mart,

Thanks for that info, it will be very useful if  I should ever recieve the kit I ordered.  Seems that if the importer does not have them to supply Cornwall's, then  will have to be shipped from the States.   I will follow up on the leads you gave me. 

Regards Glyn
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Glyn Roberts

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Re: Counter Rotating Props.
« Reply #21 on: May 15, 2015, 08:22:58 PM »

Hi Stavros,

Finally got the dumas dauntless kit and running gear.  I have decided to go with your suggestion for a pair of Caldecraft 900's from Steve's Model Boat Bits.
That leaves me with a problem of what size props and how many blades?   I will have to leave that decision with some one with the experiance to say.
I don't mean to put you on the spot but a suggestion would be very useful at this time.

Regards Glyn
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