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[dodgy geezer]

Gentlemen, your opinion is sought....

I have been starting to test-run my little EeZeBilt Terrier http://modelboats.hobby-site.com/Terrierbuild4.html

This is a boat running on several technical limits. It is light - 140gm + 48gm for the batteries, which are 4 AAA NiMH. I don't want to get it much heavier. There is a total space of around 9 cu in for all the works - batteries, motor, receiver, ESC (with a built-in BEC) and servo, so there is not much more room for anything. It is cheap - total cost is around £5 (the biggest single cost is the on-off switch), and I don't want to spend much more.

The motor is specced at 1.5-3v though it can take 6v for short periods. At 4.8v the boat (unsurprisingly) has a sparkling performance, and could do with cooling. Unfortunately, the TurBORIX ESC is specced at 7.2-12v, and has a motor cut-off nominally at 5-6v, so that an aircraft can still retain radio control after the motor dies. I dealt with this by changing the 'arming button' which sets the ESC going to a switch. So, if the power drops below the cut-off point the system is automatically re-armed. 

The system runs no-load on the bench fine at 4.8v. On trying it in the water, the motor runs fine up to half-power, but when you go up to full the ESC cuts and resets constantly, producing a very jerky motion. Oddly, if you run it for a while, you can creep up to 3/4 power before the jerks begin. Still fast, and the motor is just too hot to touch after a long run. So the voltage under load is dropping just a little too much for the delicate balance I thought I had....



What do I change? The propeller is an obvious candidate - it costs me nothing but time to have a selection of them -  http://modelboats.hobby-site.com/propmake.html refers. 2-bladed would be a good idea? But what shape and area should the blades be?

And is there any way of feeding more voltage to the ESC but keeping the motor voltage low? I could probably fit another AAA cell in if necessary. Is there a clever little voltage regulator I could fit on the motor line? But ideally I don't want to add weight and complexity....

If I have to give the motor more juice, can I cool it better? Has anyone ever tried watercooling through 1-2mm tubes...? Without adding weight....?

Could the batteries be improved? They are Lidl's best at £2.79 for 4, and only 800mAh. Would some of the more expensive brands keep a bit more power flowing?

Can I do anything about the ESC cut-out? Dropping it to 4.5v would solve everything. I'm unlikely to be able to buy a different better-specced one at a similar price  (£1.97!)...

This is a good example of the limit issues you face in R/Cing the original EeZeBilt kits, and a major reason why I went for the slightly bigger 50+ range. But it's also fun to address the problem and find the optimum solution inside the constraints of the original design....

[malcolmfrary]

I would use the board from the insides of a servo.  The output transistors would not appreciate the motor current, being intended for a servo motor, so I would use the output to work an H-bridge of TIP141/142s or similar via a dual opto isolator to drive the motor.  No BEC would be needed.
Of course, hunting for a 4.5-6 volt motor would help as well, or losing the excess voltage by inserting either an appropriate value resistor or pairs of high current silicon diodes in series with the motor leads (silicon diodes, typically, drop about 0.5 volts when conducting).

[dodgy geezer]

I would use the board from the insides of a servo.  The output transistors would not appreciate the motor current, being intended for a servo motor, so I would use the output to work an H-bridge of TIP141/142s or similar via a dual opto isolator to drive the motor.  No BEC would be needed.
Of course, hunting for a 4.5-6 volt motor would help as well, or losing the excess voltage by inserting either an appropriate value resistor or pairs of high current silicon diodes in series with the motor leads (silicon diodes, typically, drop about 0.5 volts when conducting).

An approach I hadn't thought of! That would neatly solve the cutout issue, and though I don't know enough to design the circuit it sounds simple enough. Cost and size would be low with a small cheap servo. Though, of course, soldering might put it beyond the amateur/child audience I was aiming for. And would it be right to say that I would gain a forward/reverse, but lose proportional speed control? I might try this with a smaller boat I have on the stocks...

Would putting a dropping resistor in increase or decrease the load on the battery? If I can just drop the load by a fraction this might make all the difference. I suspect that the improvement after a few minutes in the bath might be because the water was circulating at that time, taking some load off the propeller...

I have 4v motors - next size up - but they weigh 28g instead of 20g. And, surprisingly, seem to be less efficient...

Thanks for the response! Gives me a lot more to think about....

[stallspeed]

Dodgy,he is offering to build it for you. 

[malcolmfrary]

Stallseed, no he isn't  .  I was just suggesting that there are many ways of skinning a cat, should you feel the need for some new gloves.
Dodgy, you would gain F/R, you would also have fully proportional control, but at the expense of having a very narrow deadband.
If proportional was not needed, the servo motor could be replaced by a pair of low voltage SPDT relays wired to give F-O-R, the coils fed through diodes so that one would operate to stick forward, the other to stick reverse..
Losing some spare power by putting the wasting elements in series will reduce the total current draw.  Obviously this needs some measuring, calculating, trying and possibly re-measuring and trying again.  Way back, I had an element from a dead electric fire.  I could use lengths of this to make custom resistors by trimming to length, terminating in screw blocks.

[vintagent]

Have you considered the tiny motors from slot cars or railway locos or even those from Nigel lawton..Google him for great drive systems.
All these motors will draw 8-12 volts but not all are current sappers and most are VERY light.

Regards,
Vintagent

[DickyD]

Propeller, 2blade very fine pitch as fitted to my PT boat, just what you need.

[Robert Davies]

As far as motors are concerned I can recommend slot car motors. Else take a wander around the Poundshops and cheap shops for cheap electric toothbrushes (usually around 2->3 quid mark) The motors in those are generally pretty tough but have low current draw - and if they burn out, you haven't broken the bank.

-Rob

ps I used a slot car motor in my Revell 1/72 S-Boat conversion powered by 4AAs - I'm building another shortly, which will have 4AAA power - I can highly recommend Component Shop's AAs and AAAs, but with small ni-mh batteries the charger is very important, it's highly recommended to have a charger that charges each battery individually, like the chargers sold by lidl - quick way to tell a decent charger = one charge indicator per battery - it's not infallible but works surprisingly often!

[tobyker]

7dayshop.com do a charger which takes 8 AAs or AAAs, (NimHs) and charges/monitors them individually. It also gives the option of discharging them before charging. Highly recommended - just a satisfied customer.

[Cklasse]

I built my 1/72 Airfix Severn using 2 micro servo modified to run 2 props and a 2S Lipo for power supply. I wonder how do you guys know when to get the boat back if you are using AAA batteries?

The next project that I am working on now is to convert 1/200 Revell ETV Waker into RC. I wonder how do I know what motor type to use to drive 2 20mm 3 bladed props. I am playing with the idea of using a twin cutter shaver gearbox to drive the 2 prop shafts with 1 motor.

[HS93 (RIP)]

I built a similar sized boat some years ago and the motor and controller I used was from the Late Craig Talbot of Action, it is a small drum type motor very thin he said I think they where out of tapes and one of his micro speed ctr that was all powered from the receiver pack off 4AAs, it was in a Neptune size boat I had and it worked very well as did all off Craig's products it was also quite cheap for the motor and controller at the time.
Ask FLJ if he does them now, they where a good bullet proof unit.

Peter

[FullLeatherJacket]

Ask FLJ if he does them now, they where a good bullet proof unit.
Peter

Thanks for the prompt, Peter.
Certainly do. M7 Micro-cassette motor. http://www.action-electronics.co.uk/pdfs/MOTORS01.pdf
Much better slow-speed control than servo-type motors due to greater internal inertia. Suits P68A Pico ESC, with motor/ESC, rudder servo and Rx all run from one 4-cell pack.
Should be fine for the slightly larger EeZee models (e.g Terrier) but maybe a bit of a shoehorn job in the really small ones?
Suit yourself.
FLJ

[dodgy geezer]

Thanks for the prompt, Peter.
Certainly do. M7 Micro-cassette motor. http://www.action-electronics.co.uk/pdfs/MOTORS01.pdf
Much better slow-speed control than servo-type motors due to greater internal inertia. Suits P68A Pico ESC, with motor/ESC, rudder servo and Rx all run from one 4-cell pack.
Should be fine for the slightly larger EeZee models (e.g Terrier) but maybe a bit of a shoehorn job in the really small ones?
Suit yourself.
FLJ

On to that. See them here... http://modelboats.hobby-site.com/radio.html