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[OMK]

Can nobody explain how a motor outputs power when it has stalled?

Sure can.
But how do you explain why two identical  hulls with identical everything should behave differntly? One blew fuses; the other one didn't.

[Sandy Calder]

Can nobody explain how a motor outputs power when it has stalled?

Sure can.
But how do you explain why two identical  hulls with identical everything should behave differntly? One blew fuses; the other one didn't.

Not without testing.
Motors are a science and not a black art

[OMK]

"Not without testing."

Not even a guess? The fault doesn't necessarily have to be with the motor.
In a nutshell, two hulls built from the same plan. (Said hull can be seen in the attached photo - an early prototype). The main difference being that both hulls now incorporate blade-type fuses. As I said already, they are identical in every way, 'cepting one only runs for a few mins' before it pops the main fuse.
Is the propshaft straight? Yes.
Is the ESC up to scratch? Yes
Good battery? Yes.
Good motor? Yes.

No matter how much swapping around all the gubbins, one hull remains in a corner of the shed since it's no good for neither man nor mouse.

[OMK]

Forgot to attach the photo...

[Shipmate60]

PMK,
What is the coupling? Is it a piece of fuel tubing?

Bob

[malcolmfrary]

......and does it have a sizeable deflection?  This could be putting unwanted side-thrust into the system which is extra resistance to overcome.
Does the motor overheat?  Does the shaft turn freely with the motor attached?  When running, is there movement allowing binding to happen?

[Sandy Calder]

Personally,If I trusted your version of things enough to off my swivel chair I'd put the two motors on the test bench after taking a flat out current measurement in the bath.
It would take minutes to eliminate a motor as a culprit.That would only be a comparison test with two motors using a third motor with resistors as a load.
If the motors are in the clear you can thenprocede with current measurement as a fault detector.

[OMK]

Sorry, I should have been more specific.

Photo is a prototype. Both newer versions employ a proper thru' coupler between the motor and shaft.
Shaft turns freely - no binding.
Motor barely runs long enough to even get warm.
Receiver = 2.4GHz in both models.
Rx batteries = 5-cell 1800mA.
Motor batteries = 12v 7A gel cells.
ESC = 25A
Measured no load draw = approx' 1.5A

Fuse pops in less than 10mins runtime.

Have I tried swapping out the receivers?
What happens?
Same thing.
Have I tried swapping out the motors? Yes.
What happens?
The motor from the bad boat works fine in the good boat; the motor from the good boat works fine in the bad boat before the fuse blows.

[Sandy Calder]

Fuses are warm to hot at their rated voltage and at twice their rated current they take a while to blow.
Melting fuseholders are nothing new.

Use an on-load current measurement then.....calibrate a length of wire and measure the voltage across it and you will get an indication of any problem when the readings change.

[OMK]

.....calibrate a length of wire and measure the voltage across it...

Been there, done that.
The fuses no longer get hot since changing them to spade types. They just blow without warning. All internal wiring remains cool - no heat issues.
Like I said, everything in the good boat works fine - even when I swap all the gubbins from the bad boat.

I gave up on this a long time ago. The answer is probably staring me in the face. But as it stands, the bad boat remains idle in the shed.

[Shipmate60]

PMK,
So is the original "bad" set now in the good hull and operating correctly?

Bob

[Sandy Calder]

PMK,I suspect you must have skipped a bit on the testing lark.

Shipmate,it is hard to follow isn't it!

[OMK]

PMK,
So is the original "bad" set now in the good hull and operating correctly?

Bobbo: You'll have to excuse Mr. Calder's cynicism, but yes, that is exactly how it is.
I'm not trying to send you all round the houses, or even looking for answers. I've tried just about every which way - and more. The fault is just as baffling for me as it is for everyone else.
Like I said already, I gave up with this a while ago, so I'm putting it down to the fact that maybe the hull is just jinxed. I guess I was clinging to straws, hoping that perhaps someone out there would have had a similar situation and might have been able to cast some light this way.

[Shipmate60]

Could the "bad" hull be flexing at either the motor or propshaft causing misalignment when running?

Bob

[OMK]

It's a good suggestion, and would seem the only plausible answer. I have tried flexing the hull by hand (read brute force), but it doesn't budge. By dint of eye, and a few strategic measurements, the bad hull seems to be as true as the good one. Motor alignment is good, so too is the shaft, and no matter how much forced flexing, I just cannot get the fuse to pop on demand. The longest runtime to date was about ten minutes. Average runtime is usually around three or four minutes before the fuse goes blooie.
I was tempted to stick an old ESC in there and link out the fuse just to see what might happen. I figure the hull is expendible by now anyhow, so it might be worth sacrificing the motor and ESC if it means finally finding the answer to the problem.

[stallspeed]

Fuses are warm to hot at their rated voltage

Typo?
Makes good sense if that was current.

J.G.

[stallspeed]

I thought it was a simple typo but I checked back with Johnson motors.

It possibly explains the chart at the foot of this page. 
http://www.johnsonmotor.com/Performance-Charts.266.0.html

Sandy,
The chart doesn't mean much without figures!!

Bob

That chart is all to pot.

Speedy

[Damien]

The graph is quite simple and no figures are needed to understand it, simply states at point of stall Motor speed is ZERO, Torque is at MAXIMUM and Current is also at MAXIMUM.
On the subject of fuses there are Fast and Slow blow Fuses, the Blade type used in cars are Slow blow meaning they will tolerate running at rated Amps for some time, ie; a 3A Slow blow fuse will tolerate 4 or 5A for short periods 5mins tops. And YES they will be HOT
A fuse rated at 3A is designed to blow at 3A Slow will allow occasional increased A's but not for extended periods, a fast blow gives no leeway it will blow instantly at it's rating these types are often used in sensitive electronic equipment especially those with CMOS chips.

Is it possible that the boat that's having problems has a close fit shaft to bearing that gets warm/hot after 10 mins and pinches the shaft causing the amps to rise and blow the fuse.

As an unrelated example In the '70's i built a race version of a 1969 Renault 1100cc motor every time I started it, it would run until it built oil pressure then stopped suddenly. I spent months on it and finally took it to a specialist engine builder, who told me my main bearings were locking the crank under pressure. I was amazed to be told 5 thou was acceptable at the time.
Unrelated but the question needed asking.

Damien.
 

[Sandy Calder]

PMK,
So is the original "bad" set now in the good hull and operating correctly?

Bobbi: You'll have to excuse Mr. Calder's cynicism, but yes, that is exactly how it is.
I'm not trying to send you all round the houses, or even looking for answers. I've tried just about every which way - and more. The fault is just as baffling for me as it is for everyone else.

Me a cynic.....guilty as charged but "Bobbi" already knows why I'm that way.
PMK,I credited you with by thinking you had the experience and electronic knowledge to stop arsing about and sort out the problem logically.If the problem is overcurrent then ammeter tests go straight to the heart of the matter so if you stuck with them you could not argue with the readings.

I borrowed a single knuckle coupling out a fibreglass boat and when I went back to it after a year the prop shaft and motor were one eighth inch misaligned.The same thing happened to a ply skinned boat.
Never used a 2-knuckle coupling?

The chart is completely and utterly "to pot" :- Power is Torque x Rotational speed.
The chart is on a page where Johnsonmotor try to highlight that you can characterise and compare DC motors with four key data figures:-No Load Speed--No Load Current-- Stall Torque--Stall Current
If you leave out just one of those figures and use meaningless terms like torquey and low drain you can shift palletloads of useless motors at 80% markup to poor saps who have no comeback.
Back to the chart......Maxpower should coincide with 1/2 no-load rpm and ~ 45% efficiency.It doesn't happen on that chart as the power curve is out of scale.
Regards
Sandy Calder

[Sandy Calder]

Wait a minute did you just call me a cynic? 
Right then i'm out off here cause there's nothing but rudeness in this forum.
ˇArriba! ˇArriba!"ˇÁndale! ˇÁndale!

[DickyD]

Now look what you've done PMK.

[Colin Bishop]

Don't worry Richard, he's still got several other IDs on the Forum. 

[Peterm]

I know this shows my age and level of technical knowledge ( or lack of) but whip out the electronic speed controller and try it with a Bob`s Board just to see what happens.  Pete M

[OMK]

Pete M,
A mechanical speed controller might cure the problem - I don't know, I haven't tried.
I'm at the stage where I'm not really bothered about getting the boat on the water anymore. I'm more interested in what is causing the fuse to blow all the time. An ammeter shows not even the slightest hint of anything untowards... no glitches, no burps, no sudden tell-tale meter deflection... nothing. Everything checks out fine with the motor unloaded and also fully loaded.
Something is causing the fuse to pop after a few minutes, and the answer HAS to be something blindingly logical.

It seems this thread has rucked a few feathers.
Bummer. That wasn't supposed to happen.

[andrewh]

PMK,

Your pair of boats have me intrigued!  (I shall try and leave feathers smoothly preened)

The one that blows a fuse after 10 minutes or so - I have two inklings which might point in a useful direction, but can't claim that they are the answer.  But they might help

Inkling A - "after 10 minutes"  This might suggest an element in the problem which includes time.  I know that you have swapped everything between hulls, but I can imagine a scenario where the prop shaft (or possible the entire running line) binds heavily as it heats up.  It could be quite free when cold, so the measured currents would be low, as you found, but under running conditions it heats and throws big loads on the motor - popping the fuse

Inkling B - motors, and al the other electrics in the system can suffer glitches  - possibly time and temperature-related - which could cause a huge current surge and blow the fuse.  I can't make any guess as to why this would not swap with the equipment{:-{

intended to try and help
andrew