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[maw]

I'm about to start building Antje II (Robbe)
I've got the navy geared motor 3,3:1 and will be using 6V 4,5Ah lead acid battery.
Has anyone got any recommendations as to which speed controller I should/could go for.
What Amperage will I need?

[dougal99]

Maw

I like the Mrtoniks range www.mtroniks.net all solid state, no chance of water ingress and fitted with a BEC.

As for amperage I would think you would be pretty safe with the 15amp model. However, I recommend you determine the stall current of your motor and either fit a suitabe fuse or ESC with sufficient capacity.

HTH

Doug

[malcolmfrary]

Almost any ESC should do - what will handle a lot will handle a little.  Electronise 30A is a good start.

[maw]

Thanks for info to date.
How do I determine stall current?

[andywright]

For the extra few quid I would go for the next size up to what you need for safety, and fit a fuse for your rated motor amps, thats being safe, its cheaper in the long run.

[dougal99]

Maw

To determine stall current you need to set up your motor on the bench so that it is cannot move about. Holding it in a vice with padded jaws is a suitable method. Wire up the motor to a suitable battery with an ammeter in series with the motor. With the motor running, squeeze the output shaft, padded pliers are suitable, until it stops rotating. Quickly read the current on the ammeter and release the shaft (do not stop the shaft for more than a second or so). This will give you the stall current which is typically at least 4-5 times the normal running current. You obviously need an ammeter which is capable of measuring quite a substantial current.

HTH

Doug

[Telstar]

Hi With ref to the max. current drawn by a motor when stalled.  If you measure the RESISTANCE of the motor with a multimeter then when stalled ie stopped from turning the current will be the battery voltage divided by the motor resistance. I measured a MFA Hummingbird 15 motor it was 0.15 ohms resistance, so on a 6volt battery is max. current would be 40amps if my maths is right. As for the running current, its roughly proportional to the torque on the motor plus the no load current. This way gives you a ballpark current, without the need to overload the motor in question. As for fuses I use one just below the max rating of the ESC. fuses are not there for normal running but to limit damage when things go wrong. If a fuse blows you may have the inconveniance of having to recover a 'dead' model, but with no fuse, it may be recovery of a burnt up dead model, (is that cremated?)

[Colin Bishop]

And probably to state the obvious, it isn't just the internal setup which could cause problems. If you get something caught round the prop or hit weed it will stop the motor which then draws its full stall current. By fitting a fuse below this rating you protect the motor as well as the couplings and speed control etc. You need a fuse rating which is above the full speed under load of the model and below the stall current. If you run the model in the bath at full bore with an ammeter wired in series then it should be drawing slightly more than the normal full speed on the pond as the boat isn't moving forward. However you do need a fuse with a higher rating than this, preferably a "slow blow" one as if you apply full throttle from standstill or go full astern from full ahead the transitory loads can be quite high, probably approaching the stall current value for a second or two. If you have a twin screw model fit three fuses. One for each motor as described above and another one greater than the sum of the first two to protect the battery from short circuits. It's pretty difficult to beat a fuse as value for money insurance!