Model Boat Mayhem
Mess Deck: General Section => Tugs and Towing => Topic started by: Darren007 on May 30, 2022, 06:31:12 pm
-
I have just bought the above tug, the builder fitted brushless motors- hobbyking
Is a brushless set up ideal in a tug?
I’ve not tested on water yet but I’m pretty sure it will be to fast?
-
It all depends on the type of motor and the Kv rating of the motor. The Kv rating gives the number of revolutions per volt when off load.If they have used low Kv rated motors then they will be low revving and high torque which is what you want to drive a large prop on a scale tug. As long as you keep the battery voltage down then the speed shouldn't be too fast. If necessary then if you are using a modern transmitter you can set the throttle endpoint to less than 100% and reduce the speed like this.If they have fitted a high Kv motor then you might have a problem, these are high revving and suit small props better, using a large prop leads to overloading the motor and high currents. In this case a low battery voltage may be essential to keep down the speed and current.I have the ST Cruiser tug which has a low Kv rated brushless driving quite a large prop. I did find it overpowered as standard using a 3S LiPo pack but used the programming ability of the esc to reduce the maximum revs of the motor making it much more scale like.
Jim
-
Thanks for your reply! I’m not familiar with brushless. The motors have 2213-935kv on them?
Is this a ideal motor?
-
Thanks for your reply! I’m not familiar with brushless. The motors have 2213-935kv on them?
Is this a ideal motor?
Depends on what size battery you use . So a 2s lipo is 7.4v and a 3s lipo is 11.1v ,Sealed lead acid SLA are 6v or 12v
So the battery voltage and times it by the KV. So a 2s lipo 11.1X935Kv = 10378 rpm. a 2s Lipo 7.4X935Kv=6919 rpm.
6V SLAx935Kv= 5610 rpm.
Some motors will only run a minium voltage. So if its a 3s min it wont like a 2s or 6vsla. My self I think they are a to high KV for a tug.
Give it a go before you buy any think eles.
John
-
Yes, good advice from JimG & cos918. :-))
Kv does sound a bit high for a tug... but you could use a belt drive reduction if needs be.
As you have the motor, it's worth connecting it all up to get a feel for it / get a sense of the power, especially with your chosen propeller.
NB. Don't be too worried about dunking the whole motor in water, they are pretty robust, make sure you blow / air dry it afterwards!
This one: https://www.unmannedtechshop.co.uk/product/brushless-motor-emax-mt2213-935kv/#
-
Thank you very much for the sound advice.
And yes that’s the exact motor!
How would a belt drive help? I understand it would lower the speed of the prop but could this not just be done via the transmitter ie not go full throttle?
Forgive me I’m new to model boating!
I’m struggling to understand the whole torque thing- surely the prop spins faster and that gives more torque?
-
If anyone has any answers to my newbie questions that would be appreciated sincerely
-
Drive reduction - reduces the high motor RPM but increases the torque, 'turning power' of the output shaft, i.e. the propeller.
Think of it like a car gearbox or bicycle derailleur chain drive.
You can rev the engine the whole range / pedal as hard as you can, in each gear, thus utilising all the available power
to suit the speed and load of the car / bike, for pulling off, cruising, hill climbing, high speed, etc.
You select the gear according to the current needs.
We don't need to change gears much in boats, especially model ones, once is usually enough.
So it's useful to be able to spin the propeller at it's most efficient speed, even if the motor spins MUCH faster,
thus gaining useful mechanical advantage.
NB: Belt drives are usually a lot quieter than gear boxes.
.... you can do all this .... or just get a better suited motor to start with and eliminate all of the above,
which is what JimG & cos918 were saying below.
Further reading: https://en.wikipedia.org/wiki/Gear_train
-
Thank you very much for the sound advice.
And yes that’s the exact motor!
How would a belt drive help? I understand it would lower the speed of the prop but could this not just be done via the transmitter ie not go full throttle?
Forgive me I’m new to model boating!
I’m struggling to understand the whole torque thing- surely the prop spins faster and that gives more torque?
The speed of the prop determines the THRUST, and as you go up in speed you require more TORQUE.
I'm assuming the Al Khubar has props in the 40-50mm range... they should be just fine at 5600RPM running on a 6v battery. If you can't fit one, try a 7.2v NiMH battery.
-
I’ve just looked at the spec of the brushless motor and it requires 3s or 4s lipo’s.
I’ve got a 3s lipo but will this be overkill for a tug?
Ideally I want to tow with it at my local lake
-
With a 4s, 14.8V, you'll probably be able to tow the whole lake! ok2
-
So do you think I’m better off staying with the brushless motor and a 3s or changing the running gear to two T12 brushless motors for towing power
-
I’ve just looked at the spec of the brushless motor and it requires 3s or 4s lipo’s.
I’ve got a 3s lipo but will this be overkill for a tug?
Ideally I want to tow with it at my local lake
As long as you realise these specs will be for turning an aircraft propellor and don't necessarily relate to a boat. For the size of prop needed for the plane the 3S pack will be the minimum to give enough rpm. When mounted to a boat it will probably work well enough on a 2S pack or a 6V gell cell. Contrary to what many people think a brushless motor doesn't stop working on voltages below the published specs just as a brushed motor for 7.2V may still turn on a single pencell although with little power. My ST Cruiser uses a brushless motor intended for a 6S pack, I use it on 3S and is still slightly too powerful. I don't use 2S as I don't have high enough capacity 2S packs. (I originally used 2 8000mAh packs in parallel o make sure it had a long run time)
Jim
-
On the 6v battery the motor cuts out when under load, I’m guessing it’s because the motors need more voltage, hence the specs say 3s or 4s.
Sorry for all the questions I’m new to boating
-
On the 6v battery the motor cuts out when under load, I’m guessing it’s because the motors need more voltage, hence the specs say 3s or 4s.
Sorry for all the questions I’m new to boating
Again, 2S!
If the esc works on 6v Pb battery without loading a motor then it will work on 2s Lipo. The question I have is does the esc have a 2S voltage cutout at about 6.0 volt to protect the Lipo battery?
This link applies equally to brushless and brushed permanent magnet motors.
https://islproducts.com/design-note/how-to-read-dc-motor-gear-motor-performance-curves/
You can see the point made about gear reduction if you move the vertical dashed line past the peak in the power curve and do number crunching. If the motor shaft load was at 12 Nm and you slapped on a further 1.5:1 reduction to take it back to 8 Nm you can see the prop shaft rpm and torque both increase.
That is the case on the right hand side of the power peak and why it is prudent to take tacho or current measurements.