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Author Topic: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug  (Read 2161 times)

Hande

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I have researched on the universal problem of the title.

Considering
- that no way I'm going to scientifically prove the correct combination
- I'm building a 90cm tug, with two screws in nozzles
- the Billing boats props are rather in the display category - nothing to do with nozzles.
- I will choose anything that is on the safe side and good enough (not perfect and trimmed to the extreme)
- I will not choose the most expensive one

Concluding:
- buy new 4-blade kort-nozzle propellers to fill the nozzles as closely as is reasonable (keeping in mind that the plastic nozzles are not perfectly round => some mm over 50).
- select a fairly high pitch for the nozzle (want the props to turn slowly)
- => select a high torque - low Kv motor to cope with slow spin
        => opting for a direct drive brushless outrunner low Kv
- => select a brushless ESC configuration for genuine tank-driving (twisting) configuration (i.e. not with a mixer)

Remaining problems:
- what are the flaws in my reasoning?
- am I right in thinking that slow turn-high pitch-high torque equals a lot of amps?
- how to figure out the current requirements for the motor and the ESC? [/size](I have Action Electronics' solution for tank driving from their web site - shall I just copy that one?)
[/size]- is the lead-acid battery a poor choice for some reason?- what is the solution for tank driving on the transmitter-control side? Will any two-programmable-stick controller do?- how does one decide between 40 MHz and 2,4 GHz? - How am I going to choose the right single servo for the two rudders?Sorry about this condensed list of questions!  :embarrassed: Can you please help me move forward?


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inertia

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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #1 on: February 10, 2016, 10:21:59 AM »

Remaining problems:

- what are the flaws in my reasoning?  Can't see any obvious ones

- am I right in thinking that slow turn-high pitch-high torque equals a lot of amps? Not necessarily, although you will increase the current as you increase either pitch or speed.

- how to figure out the current requirements for the motor and the ESC? (I have Action Electronics' solution for tank driving from their web site - shall I just copy that one?) Can't help you with choosing the motor/ESC as I've never built a tug like this, but the electronics is easy. Fit a centering spring to both Tx Up/Down sticks and find out which receiver channels correspond to these stick movements (Channel 1 is usually steering, so they will probably be Ch2 and either Ch3 or Ch4). Now arrange the ESC for the port motor to be operated by the LH Tx stick Up/Down, and the Starboard motor by the RH stick.

- is the lead-acid battery a poor choice for some reason? Lousy. SLA batteries are incapable of supplying the current required by brushless motors. Use NiMH, LiPo or LiFe.

what is the solution for tank driving on the transmitter-control side? Will any two-programmable-stick controller do?
- As above. See Fig 4 here http://www.action-electronics.co.uk/pdfs/Twins%20Ver2.pdf

how does one decide between 40 MHz and 2,4 GHz?
- We call it Hobson's Choice in the UK. You will find it very difficult indeed to buy a new 40-mHz set, so 2G4 will be your only choice.

How am I going to choose the right single servo for the two rudders?
Any decent ball-raced standard-sized servo will do. Avoid the so-called "digital" types. They are prone to causing interference and draw a lot more current than non-digital ones. Here's an example of one - there are plenty of others http://www.servoshop.co.uk/index.php?pid=HITHS325HB&area=Servo

DM
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Tug Fanatic

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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #2 on: February 10, 2016, 12:14:52 PM »

I am not sure that I undertsand the you logic of your prop choice argument.

Generally speaking for a working tug the larger the diameter of the propeller, when combined with a suitable pitch, the better.  A big mass of relatively slow moving water will work better than a small mass of fast moving water.

BUT if you size of prop is fixed then to a large extent the thrust available will be determined by the pitch and the revs. Bigger pitch & faster revolutions= bigger thrust, more amps and if you overdo it lots of cavitation.

The correct kv for your motor will be determined by how fast you want the props to turn and what voltage you choose to have. At a given prop size the choice of pitch is often very limited so that tends to be the easy part of the choice.

Can you mix elevator & throttle via a rate switch on your transmitter?  You can then have both throttles working together most of the time on the throttle stick and switch to the elevator & throttle controlling one motor each when you wish. In practice I seem to manage perfectly happily just using the one motor on the throttle stick most of the time and only using the motor on the elevator stick when I want to - but then I am not short of thrust. A lot depends on the model & how far from the centre line the props are located. 

Lead acid batteries can supply outrunner motors but only if they are big enough. I would suggest if you are going to draw max amps more than the rated capacity of the battery then you are better off with another type of battery. eg 7amp max draw from a 7amp battery. In reality you are unlikely to be near 7amp draw most of the time. Whilst tugs can hold weight better than many other types of similarily sized model a lead acid battery will almost inevitable lead to a higher centre of gravity and a less optimised weight dispersal than could be achieved with a more energy dense battery (lighter) and lead or other ballast. How much this matters depends on the model & usage. Others will tell you that a lead acid battery is in essence free ballast. Your choice.
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Hande

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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #3 on: February 10, 2016, 01:31:34 PM »

Thank you Dave, TF!
Great response. (I'm sorry that Tug Fanatic was already taken for alias, when I joined ok2 ).

Prop choice is based on the nozzles being given and my wish to be "realistic".

I have seen that in the Kort nozzle the clearance between the propeller and the nozzle is minimised. In my model, I can safely fit a 55mm propeller - if such a thing exists.
Smit tugs of the class that I am building (Smit Poland, e.g.), seem to have 4-blade props.

TF, I'm sorry, but I don't understand, what elevation means in this context. I have studied various vendors' products, but didn't notice elevation being mentioned. - Lifting the stick up? - Your suggestion seems re-ally sensible and I would like to follow that as long as I have the possibility to practice as realistic twisting as possible. I am still in the process of selecting the R/C system.

Thanks for advising me away from lead-acid!

And I would never have believed that I would be advised away from digital servos (being a computer-engineer my self). The mantra has been:"digital is the way to go", "digital is the future!" ...





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inertia

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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #4 on: February 10, 2016, 02:32:10 PM »

And I would never have believed that I would be advised away from digital servos (being a computer-engineer my self). The mantra has been:"digital is the way to go", "digital is the future!" ...

I've been told that they are superior in every way to non-digital servos when it comes to speed, power and precision. It's just that I have personally handled four different types and I wouldn't have put any of them into one of my models. Two of them interfered with the other servos in the circuit, despite having ferrite-ring suppression coils, and the other two sat and twitched/trembled all of the time, which I found unnerving! That said, they were all from the cheap end of the market and I suppose if I were to pay a premium price then I would get a premium product - but I'm not inclined to shell out thirty or forty quid for something that I don't really need.

This is my opinion only - I don't want to start a war. Suit yourself, Hande!

BTW TF's reference to 'elevation' is to the usual function of the two sticks in a model aircraft. Depending on the stick mode, one will operate the throttle when moved up and down, while the other will operate the elevators. Conventionally the throttle stick will have a ratchet rather than a centering spring, but for use in a model boat (where the motor stops at centre-stick) it's much better to fit a spring to that stick. You will almost certainly find the corresponding receiver outputs labelled with 'Thr' and 'Elev' or similar.

Dave M
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Hande

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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #5 on: February 10, 2016, 02:53:23 PM »



BTW TF's reference to 'elevation' is to the usual function of the two sticks in a model aircraft. Depending on the stick mode, one will operate the throttle when moved up and down, while the other will operate the elevators. Conventionally the throttle stick will have a ratchet rather than a centering spring, but for use in a model boat (where the motor stops at centre-stick) it's much better to fit a spring to that stick. You will almost certainly find the corresponding receiver outputs labelled with 'Thr' and 'Elev' or similar.

Dave M




Got you on elevation. Elevator as in planes... I have already seen that there really aren't systems made especially for boats.





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Tug Fanatic

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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #6 on: February 10, 2016, 03:11:00 PM »




Got you on elevation. Elevator as in planes... I have already seen that there really aren't systems made especially for boats.

Yes sorry to confuse you - I am mainly a flier so it just is what I call it - the other up & down stick!

For 90mm props you are going to be looking at a very low kv motor. Personally I have always used car heater motors (brushed) for props of this sort of size although I have never gone this big!
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Re: Hypothesis about propeller-motor-ESC; 90cm-twin screw-kort nozzle-tug
« Reply #7 on: February 10, 2016, 04:03:22 PM »

Oops. 90cm model 55mm props.
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