Model Boat Mayhem

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: Shaft speeds, Motors, ESCs and other confusions  (Read 2507 times)

Cliff

  • Shipmate
  • *
  • Offline Offline
  • Posts: 2
  • Model Boat Mayhem is Great!
  • Location: Skopelos Island, Greece
Shaft speeds, Motors, ESCs and other confusions
« on: February 23, 2022, 12:16:40 pm »

Good day all,


I am a long retired Brit who has lived on a Greek island in the Aegean for the last 20 or so years. We have no model clubs and no-one to discuss problems with so I looked at the various online forums. I was very impressed with the knowledge base on this forum, and honoured to be allowed to register.


I have been building and flying scale helicopters for a couple of years and have built a couple of static/display steam boats but I am a newbie as far as RC boats are concerned. I have just started building a 1/12 scale model of the Bluebird of Chelsea yacht from the Chinese kit.


So far there are no problems with the build. The Mayhem topic "Bluebird of Chelsea - My Winter Build" by jpdenver has been a great help and inspiration.


Before I start planking I want to specify the electronics. The kit came with a pair of Johnson 775 size brushed motors. The identical unit is on Ebay:


https://www.ebay.co.uk/itm/254604745274?hash=item3b47a01a3a:g:InwAAOSwsxVdflF~


From that site the voltage, speed and current specs are as below:


Voltage:12V  No-load speed:14000 RPM Current:1.7A

Voltage:18V  No-load speed:21500 RPM Current:1.9A
Voltage:20V  No-load speed:24000 RPM Current:2.1A



The propellers supplied are rather nice 3 blade 55mm dia brass units.


I intend to use separate ESCs controlled from my Helicopter FrSky Horus Tx and probably an X8s Rx


My immediate questions are:
  • Where should I mount the twin Tx antenna? - They are rather short pcb types but I could change them for longer single dipoles. Where do you guys mount them, inside or sticking upwards on the cabin roof?
  • What would be the ideal RPM/Voltage for those propellers on a scale boat of this size (1320mm)?
  • What rating ESCs should I specify? Any specific units recommended for boats?
  • What type of battery (LiPo, LiFe. NiMh lead acid etc) do you recommend and what capacity?
Any advice would be most appreciated.


Best wishes to all.
Cliff



 
 
 
 
 
   
Logged

HMS Invisible

  • MICROGYROS.COM
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 515
  • Location: Paisley, Renfrewshire
    • Yes, it works!
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #1 on: February 23, 2022, 12:46:53 pm »

Cliff, are the kit 775 motors identically marked?

The ideal motor would be a 775 but a version with a much higher torque constant, a no-load rpm much lower and they'd be capable of direct coupling to your props.  The ones in the Ebay would need a reduction gear.
Logged
< All manner of contact methods over to the left.

JimG

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,268
  • Model Boat Mayhem is Great!
  • Location: Dundee
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #2 on: February 23, 2022, 01:14:38 pm »

Can't help with the motors but for the Rx its best to mount as high as possible in the hull. Even with short aerials you should be able to put them above deck. Just as with your helis they are best positioned at 90 degrees to each other, probably with one vertical and one horizontal. Best to keep them as high as possible. I also fly and use a Horus X12S transmitter with various receivers but have several X8r ones.
Jim
Logged
Dundee Model Boat club

Cliff

  • Shipmate
  • *
  • Offline Offline
  • Posts: 2
  • Model Boat Mayhem is Great!
  • Location: Skopelos Island, Greece
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #3 on: February 23, 2022, 04:51:13 pm »

HMS Invisible and JimG


Thanks for both for your replies[size=78%]. [/size]HMS Invisible,
Both the motors supplied with the kit have identically markings to the ebay listing.
My understanding of power requirements on model boats is very limited so please excuse my further questions?

If the supplied motors have too low torque rating, then what torque should I be aiming for, and at what maximum RPM? How do you determine them? Is there some rule of thumb?

The scale speed for this model would be about 3 knots. Can I use this to determine an ideal prop RPM and torque?  If so how?

Thanks in advance

JimG
Another Horus fan? Do you use the Frsky OS or OpenTX?
The only above deck area where antenna would look reasonable is the wheelhouse roof but directly below is not suitable for the Tx. If I cannot mount the antenna below the decks and get reasonable reception I guess a pair of FrSky extended dipoles is the solution, or is there a better Rx I could use with longer wire dipoles. The boat has dummy antenna on the roof that could be adapted to functional dipoles.
Regards
Cliff
Logged

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 12,171
  • Location: SW Surrey, UK
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #4 on: February 23, 2022, 05:56:05 pm »

If you can mount the receiver (RX) in the after cabin that should be OK. If the cabin lifts off then just make an internal frame for the receiver that holds it above deck level. That should be sufficient for good reception.

There is no real need for the antennae to be in the open air at the ranges you will be operating at.

Colin
Logged

HMS Invisible

  • MICROGYROS.COM
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 515
  • Location: Paisley, Renfrewshire
    • Yes, it works!
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #5 on: February 23, 2022, 07:35:52 pm »

...
If the supplied motors have too low torque rating, then what torque should I be aiming for, and at what maximum RPM? How do you determine them? Is there some rule of thumb?...
I put in terms of the Ebay motor torque constant (Newtonmetre/amp) being too low to drive a 55mm 3-blade prop. It is a drill motor which needs high rpm to cool the motor which couples to a chuck through a high ratio gearbox.
It's at this point where "choosing a prop no greater than the diameter of motor case" gets trotted out but it doesn't hold water. This is a case in point where there are two motors in a series but this one won't drive the 55mm prop efficiently where other 775 motors will. I dare say that a 555 motor could manage.
If you do a tachometer test in a basin of water you'll see the rpm will drop to a fraction with a 55mm prop.
Do you see all the graphs that pop up with this search are the same shape? https://www.google.com/search?&q=dc+motor+chart
If rpm drops blow 50% you are over the crest of the power curve with that motor. If you then fit a reduction gear the prop runs faster

So yes, if you understand what a lossless reduction gear would do to a performance graph on the same scale then you can understand that you can use no load rpm as a guide to compare two motors and here is how:-
Change a motor's voltage (and the other tabulated figures) so that the V x Istall matches up with another motor from this list: https://www.mfacomodrills.com/motors/motors.html
The motor with the lower no-load rpm suits suits the larger prop.
Logged
< All manner of contact methods over to the left.

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 12,171
  • Location: SW Surrey, UK
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #6 on: February 23, 2022, 07:52:22 pm »

Whilst no doubt useful, I rather doubt if the average model boater could really get to grips with the contents of your last post.

People are really looking for advice that is understandable to the layman. All they want to know is whether a particular combination of motor and prop will work for their particular situation.

In most cases, people have practical setups that work and can be shared without complex calculations.

Colin
Logged

HMS Invisible

  • MICROGYROS.COM
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 515
  • Location: Paisley, Renfrewshire
    • Yes, it works!
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #7 on: February 23, 2022, 08:23:01 pm »

Yes Colin, but context matters too.
As I was sitting in Tesco waiting a few minutes for the rain to go off while checking emails on my phone I wasn't about to start a trawl through Ebay's hundred Chinese suppliers for the right one. I thought it sufficient to tee it up for others with the very direct answer that the supplied motors wouldn't do the job.
Cliff, are the kit 775 motors identically marked?

The ideal motor would be a 775 but a version with a much higher torque constant, a no-load rpm much lower and they'd be capable of direct coupling to your props.  The ones in the Ebay would need a reduction gear.

Can you answer the follow up?
Quote
...
If the supplied motors have too low torque rating, then what torque should I be aiming for, and at what maximum RPM? How do you determine them? Is there some rule of thumb?
...

Logged
< All manner of contact methods over to the left.

JimG

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,268
  • Model Boat Mayhem is Great!
  • Location: Dundee
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #8 on: February 23, 2022, 08:48:18 pm »

HMS Invisible and JimG



JimG
Another Horus fan? Do you use the Frsky OS or OpenTX?
The only above deck area where antenna would look reasonable is the wheelhouse roof but directly below is not suitable for the Tx. If I cannot mount the antenna below the decks and get reasonable reception I guess a pair of FrSky extended dipoles is the solution, or is there a better Rx I could use with longer wire dipoles. The boat has dummy antenna on the roof that could be adapted to functional dipoles.
Regards
Cliff
I use Frsky OS as I don't see any need for the extra programmability of OpenTx for the aircraft I fly. I am moving on to Jeti radio for the more demanding aircraft such as jets.Jim

Jim
Logged
Dundee Model Boat club

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 12,171
  • Location: SW Surrey, UK
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #9 on: February 23, 2022, 08:55:43 pm »

If you refer to Dave Metcalf's original as built article, he used a pair of 285 motors as being sufficient to drive the boat. Personally I think that might be a bit optimistic so a a pair of 385s would be fine. They are low drain with plenty of thrust on a 7.2v or 8.4v NiMh pack. It's not a speedboat!

Colin
Logged

HMS Invisible

  • MICROGYROS.COM
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 515
  • Location: Paisley, Renfrewshire
    • Yes, it works!
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #10 on: February 23, 2022, 09:32:14 pm »

I had time enough to see Dave's plan was 1/24th scale according to Wikipedia and noted 1/12th scale & 55mm brass props in the opener. I considered giving it a mention but assumed Cliff has done the research.

Cliff, if you have a tacho you'll see for yourself the 55mm prop will make the supplied motor run hot and at well below 50% no load revs.
If you are not satisfied with the result you will find sales listings with 775 motors with a lower no-load figure. There will be help to comparitively predict how the more promising motor would do against your result.

Colin, jpdenver thread on Mayhem



Logged
< All manner of contact methods over to the left.

Perkasaman2

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 943
  • Model Boat Mayhem is Great!
  • Location: North East
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #11 on: February 23, 2022, 11:37:39 pm »

This is not a planing hull by any means. The large 55mm prop will need a high torque motor of at least 700-800 size in brushed guise.
Logged

Martin (Admin)

  • Administrator
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 23,365
  • Location: Peterborough, UK
    • Model Boat Mayhem
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #12 on: February 24, 2022, 12:42:50 am »

 
This is still a good start, Dave Milbourn’s comprehensive guide to model boat motors and electrics: 

http://www.modelboats.co.uk/news/article/but-i-don-t-understand-electronics/18054
Logged
"This is my firm opinion, but what do I know?!" -  Visit the Mayhem FaceBook Groups!  &  Giant Models

canabus

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,347
  • Boats a hole in the water you pour money into!!!
  • Location: Tasmania,Australia
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #13 on: February 24, 2022, 03:41:38 am »

Hi Cliff


Three Blade 55mm prop recommended max rpm is 6400.


ESCs two Hobbywing Quicrun 60Amp.


These have a link setup for Lipo/NIMH and other for forward/ brake/reverse, forward/brake and forward/reverse.


The forward/reverse works very well with little delay between the two.


No programming card require.


As for motor a pair of rock crawler 540 ones (7000 rpm no load on 7.2 Volts).


I have this setup in a Smit Nederland Tug on tank steering.
Twin Lipo 2S (7.2 volt) batteries.


Also using a single setup my Eileen and Hellen fishing boat on 60mm 3 blade props.


Welcome to the forum.


Harry


More power than it requires, but, I can limit this on the transmitter setup.


[size=78%] [/size]
Logged

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 12,171
  • Location: SW Surrey, UK
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #14 on: February 24, 2022, 10:35:09 am »

Quote
I had time enough to see Dave's plan was 1/24th scale according to Wikipedia and noted 1/12th scale & 55mm brass props in the opener. I considered giving it a mention but assumed Cliff has done the research.

Apologies, I missed that but I think J P Denver's kit is 1:18 not 1:12 unless there are two versions.

Colin
Logged

chas

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 587
  • now in los montesinos Spain.
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #15 on: February 24, 2022, 03:10:35 pm »

I'm not sure why no one has mentioned this, but there is a simple answer. If driving sizable props at displacement speeds, as Bluebird of Chelsea needs, a pulley drive or similar is a simple, cheap easy and very efficient way to do it. I've been using pulley drives for over 40 years, and it's often featured in the magazines by people like Glynn Guest, Colin Bishop and others.
  With a pulley reduction the power draw of the motor drops, run time increases and a hefty esc isn't needed.

Logged

canabus

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,347
  • Boats a hole in the water you pour money into!!!
  • Location: Tasmania,Australia
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #16 on: February 24, 2022, 11:19:10 pm »

Hi Chas
The hefty ESCs in my post was because I can get two of these for the price of one from the local hobby shop.
As for the motors a third of the price !!!!


I would not use pulleys, but would use MFA geared motors far simpler set up.


Canabus
Logged

Martin (Admin)

  • Administrator
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 23,365
  • Location: Peterborough, UK
    • Model Boat Mayhem
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #17 on: February 25, 2022, 01:30:51 pm »


Try not to use gears or pulleys if possible, pulleys slip, gears are noisy!
( Personal opinion - don't shoot me! )

There are so many motors around these days, that you should be able to find a close match to your desired Prop pitch / size,
But you could build kit as supplied just allowing for the motor mount to be removed, replaced, adjusted to fit a different motor as required.

1. One Motor / shaft or 2?
2. Length of model?
3. Kit props?

 Martin  :-)
 
 

 
Logged
"This is my firm opinion, but what do I know?!" -  Visit the Mayhem FaceBook Groups!  &  Giant Models

HMS Invisible

  • MICROGYROS.COM
  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 515
  • Location: Paisley, Renfrewshire
    • Yes, it works!
Re: Shaft speeds, Motors, ESCs and other confusions
« Reply #18 on: February 25, 2022, 03:53:18 pm »

I wrote this on February 1st so I am in broad agreement with that sentiment
Quote
It makes me scratch my head when I see bog standard motors, on 2:1 gearing for reduction rather than mounting convenience.
There are 555 motorson Ebay and Amazon with twice the torque constant of 545, 540. and 550 and those "lathe" motors.
Do people not know that?

My post with the published spec of 555 motor used to turn 55mm seven bladed scimitar props.

Cliff, you asked
...If the supplied motors have too low torque rating, then what torque should I be aiming for, and at what maximum RPM? How do you determine them? Is there some rule of thumb?...
I'll take a second attempt at this part of your question, not for you but mainly for the "average model boater" who isn't going to follow a google search link or ask for clarification.

A current or tacho measurement will reveal where in your motor's performance curves you are operating.
The attached image (universal curves...or.jpg) shows a suitable motor for the given load point at an input voltage of about 10volt.

If the prop is too big for the motor you still get the rpm and torque with a higher supply voltage. There isn't a torque rating as such.
The four dots (rpm, current, power, efficiency) will be away to the right with a short run time and red hot motor.
Loading the motor too much makes the input power, P=VxI (proportional to current, supply voltage fixed) go into heating the motor and not turning the shaft.
Logged
< All manner of contact methods over to the left.
Pages: [1]   Go Up
 

Page created in 0.095 seconds with 21 queries.