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Author Topic: 6Volts or 12?  (Read 1943 times)

geoff p

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6Volts or 12?
« on: August 07, 2010, 11:30:11 AM »

Hi all,
Having recently 'blown up' both my ESC and Motor, I've re-'engined' my tug with an unbranded 500-ish size, brushed motor, geared about 3:1 to the 80mm x ?? four-blade prop, and put in a spare '50Amp' ESC of Taiwanese manufacture.

I thought maybe I should try to get some measurements before going to the pond.

Out of the boat i.e. with no load the motor draws about 1.57 Amps when just starting to turn from 6 Volts or 2.6 A from 12 V.

With the propeller pushing air, on my bench, the current draw is 2.5 A from 6 V at the slowest the radio/ESC will go, ahead or astern.

So today I rigged a 'half-a-test-tank' - my plastic wasn't long enough to get the whole boat in but at the least the stern is submerged to about normal-running depth even though like this she is sitting on the rudder-skeg:

The first sequence of pictures were taken using a freshly-charged 6 V x 3 Ah battery.
At switch-on:


At slow astern - because this jury-rig was a tad unstable, and astern seemed to do OK:


At full astern:


And at full ahead:



Yikes!  That seems like a hefty current drain from only a 3 Ah battery.  It might last 15 minutes.

So how about running from 12 Volts?  I have an old motorbike battery, rated probably 4 or 5 Ah to try.

This sequence was taken at 12 Volts.
Slow ahead:


At half ahead:


At nearly full ahead:


And at full ahead the meter simply showed Overload - more than twenty Amps.  The prop was desperately drawing air, so partially offloading the motor.


At the end of which, the battery cables, and especially the spade-terminal connectors, were extremely hot, whilst the motor and ESC were quite cool.

I guess that 12 Volts might be just a tad too much for my boat!  Unless I water-cool the cables ....

I'll be interested to hear what you guys think about these sort of currents?

Geoff
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Shipmate60

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Re: 6Volts or 12?
« Reply #1 on: August 07, 2010, 12:58:13 PM »

On a 3:1 gearbox a 550 type motor SHOULD pull from 1 amp to about 7amps when going fron full ahead to astern.
Your figures are excessive.

Bob
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malcolmfrary

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Re: 6Volts or 12?
« Reply #2 on: August 07, 2010, 01:53:13 PM »

Quote
unbranded 500-ish size
This could be a clue.  It might be one of those infamous Johnsons that were around a few years ago, or a close relative.  Good for flattening batteries in a hurry, prior to doing a full recharge, and warming up the motor compartment, but not much use for driving a boat.  I assume that the prop shaft spins easily with the motor uncoupled?
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stallspeed

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Re: 6Volts or 12?
« Reply #3 on: August 07, 2010, 02:01:07 PM »

Geoff,I think your measurements are bang on the mark for a water basin test.
In reality you would get more like an hour or more sailing.
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poddy

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Re: 6Volts or 12?
« Reply #4 on: August 07, 2010, 02:04:06 PM »

The Robbe motor/gearbox units employing a 500 motor are 3:1 for 6v and 5:1 for 12v

I agree with Bob and Malcom those currents are excessive. I run the Robbe 12v unit on two 12v 4A batteries in parallel. I've not flattened the batteries on a day's sail yet. Even running at way over scale speed.
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geoff p

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Re: 6Volts or 12?
« Reply #5 on: August 07, 2010, 07:06:51 PM »

I, too, think they are excessive - a trial at the pond a couple of days ago flattened the 6V batteries in about 15 minutes, at mostly pottering speeds.

I took the measurements today because I had thought to go over to the 12V battery, letting the speed controller reduce (average) current to an acceptable level.  I take it they don't work like a PWM (pulse width modulated) motor-control?

My previous set-up was a lovely little 380 motor, geared by approximately 3:1, and from a six volt supply it would run at least a half hour. 

 %% Idiot that I am, I tried it on 12V: bench testing seemed fine but as soon as it hit the water, all hell broke lose, the boat taking-off at full astern.  Whilst I could still operate the rudder, at full astern I think few tugs respond very well to a mere rudder.  Since the ESC was no longer Controlling Speed, the motor succumbed to ten minutes of much-too-much jungle-juice.  Back home, both ESC and motor were dead.

Hence the re-engine-ing mentioned in my first post.  There must be a verb 'to re-engine'  %)

Geoff
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Shipmate60

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Re: 6Volts or 12?
« Reply #6 on: August 07, 2010, 07:19:57 PM »

Geoff,
I think the 80mm prop is a bit large for this set up too.
With that set up you would only need about 2000 rpm and you will be using at least 3000 on 12 volts.
Is there any chance of any pictures of the motor and prop set up?
It might be better looking at a 6:1 reduction!!

Bob
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geoff p

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Re: 6Volts or 12?
« Reply #7 on: August 08, 2010, 05:10:07 AM »

Good thinking, Shipmate.

I have a smaller prop, 70 mm diameter, home-made like the bigger one, so I don't really know the pitch.

As for the motor/prop set up, well it is a little bit cramped.  The shaft is nearly parallel to the keel and only a couple of cm above the planking, so the drive relies on gears to lift the motor above the shaft-line.  These pics show the largest gear I can get on the prop-shaft, and the smallest I have for the motor.



The motor-mounting plate is spring-loaded upwards, against two adjusting screws, so I can tweak the gears' mesh for smoothest - AKA quietest - running.

Ironically, I had toyed with the idea of making a two-stage reduction but balked at the extra work involved.

Geoff
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nick_75au

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Re: 6Volts or 12?
« Reply #8 on: August 08, 2010, 08:13:24 AM »

If you can find an old bubble jet printer, pull it to pieces and salvage the ink carriage drive motor, Its a 550 size usually, designed for 24 volts but will run fine on 12, lower speed so it will handle the larger prop better.

12 volts doubles the amps and RPM the motor wants to spin at (cant do it with big load so amps increase further) and therefore Quadruples the power.

Gear (Ie 6:1) that same motor so at 12 volts is has the same rpm as the current set-up on 6 volts and it will draw half the current of the six volt set-up.

Nick

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geoff p

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Re: 6Volts or 12?
« Reply #9 on: August 09, 2010, 10:40:02 AM »

Many thanks to all of you.

Especially to Nick, who re-introduced me to the forgotten pleasures of taking a printer apart for its 'goodies'.  The two motors I got from an HP all-singing, all-dancing machine are quite tiny, and draw very little current: the carriage motor draws just 0.75A at stall (locked rotor) from 6V or 1.56A at 12V.  Snag is, the revs are also low: 1300 rpm at 12V, so with reduction gear the propeller would turn at distinctly scale speed.

SORRY, no photographs of what I did today.

I put on my smaller prop (70mm) today, twisted the blades to much shallower pitch, at the tips anyway, and went to the pond.  My reasoning being this should allow the motor to rev faster, in line with what you have all been recommending.

And it worked!  With 6V battery and minimum throttle, she moved quite sedately; at half throttle she rode at one wavelength, looking rather good.  At full throttle, the bow wave was curling over, as in, climbing the bow then curling out-and-over.  Impressive, if not very realistic.

I then tried the 12V battery, very, very gently at first.  Hmm, about the same as at 6V, until I tried full throttle.  Have you ever watched a tug coming onto the plane?  Well maybe I exaggerate a little.

The bows came up and the stern dug down so the deck was well below water level, and she just screamed along.  Exhilarating, though not a pretty sight.

I'm going to take the camera and try again tomorrow.

Thanks, all of you,

Geoff
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stallspeed

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Re: 6Volts or 12?
« Reply #10 on: August 09, 2010, 12:21:42 PM »

Geoff,your first test indicated you had a standard,adequate 540 or 600 motor and the prop was just too big.
The props supplied with the 3:1 Robbe Navy Kompact units are a guide to what you should be aiming for.They will give you a whole days sailing at the right speed.
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poddy

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Re: 6Volts or 12?
« Reply #11 on: August 10, 2010, 07:32:38 AM »

I'm running a Robbe motor/gearbox 12v 5:1 with a 75mm 4 blade steam prop through a 15 amp esc. No problems at all, it just gets warm to the touch, not hot. but then I guess the 6v 3:1 would draw double the amps for the same power?
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