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Right, you've now got your motor and batteries, some sort of speed control is a good idea. As I said earlier the motors used by boaters are DC. This means if we control the voltage and polarity supplied to the motor, we can control it's speed and direction, useful huh?
Choosing a speed controller - What do I need?

Your speed controller needs to be able handle the voltage as well as the current the batteries are putting out as well as the current the motor may demand. So if you have a supply voltage ( from the battery ) of say 12v and the motor can handle up to 20Amps - the speed controller ( mechanical or electronic ) will need to be minimum rated at 12Volts & 20 Amps - but it's not that simple, (nothing is ever really simple is, is it?).

You could try to work out the true current the motor will require by taking a direct ohm reading using a meter ( set to Ohms or resistance ) but this might not help you very much. The motor is made coils of wire so we should use the word impedance as the resistance will change when voltage is applied to the motor.

What you need to find out is the 'on load' and 'stall' current of the motor. This is easy to do if you have the manufactures instructions, they are usually in the small print or you can look up most motors on the internet.

One way to find the 'on load' current is to use an meter ( set at Amps ), interrupt the supply  between on side of the battery and  motor, then set your model boat up in the bath, with the prop in the water, carefully connect up the motor, meter & batteries ( try to avoid getting drenched ), the meter will then read the current draw on the present set up. If you get a negative reading, just swap around the meter leads around. Also check you motor is turning the correct way.

From this you can deduce what ESC rating and fuse to use... ( some digital meter even have a ' peak hold' where they memorize the highest peak or current draw ), you can also do a stall test this way too and find out the current draw if required - don't do a stall test with your speed controller in circuit, you'll burn it out and don't stick your fingers in the propeller to try to stall the motor, you'll only end up with fewer fingers!!.  If you are experimenting with the boat set up you and are changing props, you will need to measure the current draw again and again as any little change may increase the 'normal' current draw ).

The stall current is the maximum current the motor will take if the propeller gets jammed, this is what the speed controller need to be able to handle this if the worst happens (most people use fuses to protect the batteries, motor & speed controller). I would suggest testing this for yourself but never on a motor over 6volts as you run the risk of damaging the motor, the batteries or yourself in the process! ( You can test higher power motors on lower voltages and creating a graph - if you're a clever person!).

I recommend using an ESC at least twice the 'on load' current and a fuse just below the stall current.

'On load' current = 4Amps 4x2=8Amps call it 10 just to be safe ( ESC are expensive enough with burning them out to save 5 quid! ) Stall current = 8Amps - fit a 5amp slow-blow or car "blade" type.

.What controller you will buy also depend on the type of boat you will be putting it into, i.e. fast electric / race boats or powerful tug or sedate scale trawler etc. Fast electrics don't require much control from start to full power but need to be handle high currents and be able to withstand the heat build up ( some speed controllers are water cooled! ), Big tugs as similar to fast electrics but need full control from stop - full ahead -straight-to - full astern! Scale boats are rarely high powered set-ups but may need easy control when running very slowly ( plus "anti-angry-duck speed!" ).

The speed controller you chose needs to match your requirements because you could end up with a 300Amp 500volt speed controller ( that you could run your whole house on! ) but provides you with very little speed control at slow speeds.

For more information, ask at your local model boat club what speed controllers are popular locally. Ask at you local Model shop but I'll bet you somewhere in the conversation the phrase "That one will do the job but I would recommend this one." will come up in the conversation, guess which one will be more expensive!
   ( - with much plagiarism from Guy Bagley )

Three types of speed controllers.

Mechanical speed controllers

There are three types of speed controllers,
switches, resistive and electronic.
 Switches are the easiest being simple, cheap and robust, but offer the least amount of control. The most you can hope for is stop, half and full forward and half and full reverse, in most installations it's on/off only. Micro switches are most often used because they are small and easily activated by a servo. Use heavy duty micro switches rated above 10 Amps where possible as they will last longer. Some sort of mounting plate and operating arm or cam needs to be constructed to press the switches. An article on switching circuits was published in Nov/Dec 85 of RCMB but here's an extra circuit for forward only control, it only needs one toggle switch so takes up very little room ( diagram below ). A couple of manufactures make commercial units that are fitted on top of a servo, the Robbe and Graupner ones come already wired up and with battery plugs. If you only require simple control for your boat or want to handle a moderate amount of power then switch controllers might be the best answer.


Mechanical or resistive controllers offer a wider range of speeds than switches. There are three basic types of resistive controllers, coil wound, switcher with separate resisters and resistive track. Coil wound controllers are very robust but they are prone to sticking and generate a lot of heat, (see my article on the MFA Piranha). If you have a burn-up, a glass fibre pencil can be used to clean up the controller, check the mechanical contacts are in good order and that the nut is tight. Lubricate all moving parts with a little silicon grease. Switcher type resistive controls are similar, providing three speeds forwards and back and stop. Switcher controllers generate less heat than coil types and seem less prone to burn ups although you can blow the resister. The resister should be in free air as it can get quite hot. Resistive track or wiper board controllers such as "Varispeed" AKA "Bob's Boards" are very simple to install and maintain (it stuck straight on top of a servo) but are only rated up to 8 Amps. The average 540 will easily draw that and most will happily draw 15-20 Amps. Having said that, I've used an 8 Amp Bob's board on a 12 cell - 550 set-up with no visible signs of distress at all, (See Graupner Hydrospeed).

I've used Bob's board controllers many times, so I dwell on them a while. They are most suited to low Amp installations such as scale boat because their limited current range but do offer quite good control. You need to choose the nearest rated board for your installation so as to obtain the highest amount of control. Connect up your motor, battery and a meter (set on Amps) and slow down the motor with your fingers, this will give you an idea of what the current drain for the motor under load will be. If it stalls at 2.5 A, choose the 2 amp board, hopefully you'll never stall the motor in real life. A 3 or 4 Amp board will work but offer less slow speed control. My only criticism of these boards is the shorting out of the motor at the 'all stop' position. This is used I assume for motor braking in car applications. Unfortunately when an electrical motor is disconnected from the power and is slowing down it begins producing electricity. So shorting out the motor will cause arcing, damaging the motor and/or 'Bob's board'. Fortunately this is easily cured by making a small cut across the centre track which leaves the motor circuit open while slowing down. Again a small dab of silicon grease on all the mechanical contacts will inhibit arcing and corrosions. There is another type of resistive track controllers, these have separate resisters mounted on the underside of the board, these have a tendency to wear quickly as the copper track is quite thin.

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Theses controllers are a different kettle of fish altogether. The first thing that strikes you about some ESC are the astronomical price but many are now down to realistic prices, some under £20 (GB) which is cheaper than a servo and mechanical controller! ESC provide the best motor control possible and many have fully automatic setup. ESC's control the voltage to the motor electronically thus varying the speed. Some of the latest microprocessor ESC's have all sorts of useful features such as soft start, variable frequency, speed curves, thermal shutdown, water cooling It's not actually the voltage that is varied to control the motor but we haven't got another 20 pages to go into the intricacies.

ESC eliminate the need of a throttle servo and optionally the receiver batteries as well. Some ESC's are the same size and shape as normal servos so as to facilitate a direct replacement.

The main advantages of an ESC in a sports boat is their phenomenal power handling capabilities, their  small size (in some cases) and accurate control, (the latter is more useful in a scale boat). The disadvantages of ESC are, apart from cost, is the danger of a burnt out if you do something silly, destroying it by getting it wet, a limited voltage range (which inhibits experimenting) and potential volt loss across the ESC. (See upgrading below.)

Terms associated with electronic speed controllers are:


Expressed in Amps and volts, tell you the maximum power the unit can handle. Output current rating will have a continuous value and peak or surge value, the peak value only applies for VERY short durations.

Same as a servo. The speed output of the ESC varies by the amount the stick is moved.

Refers to the type of the power transistors used, sometimes called MOS-FETs. FET transistors are very efficient and so less power is lost as heat compared to normal transistors. Some FET ESC seem powerful enough to handle the output from an electricity sub station!
NB. From Allan -

This is down to the fact that an FET is rated at 25deg c if you keep it at that and the innards about 20 c higher then it will run at the rated current. There are many FET's now that will run 80-100 amps each. So put 3 in a speed controller and you get 240amps…… but only if the FET's stay cool - which without a 6 inch square heatsink each they won't.

As a rule of thumb an FET will run 10% of rated without the heatsink - that brings you down with a thump!

Now if you buy Astec or Electronise etc you will get a true rating of what the ESC will run constantly on. If you buy one of those rated at 200 amps and it is very small with 1 or 2 FET's you ought to start to wonder.

Full bridge ESC's have four legs each run by FET's (Mine and Electronise ESC's still use relays - well it works fine) In this case you need at least 1 FET per leg and 1 FET  is about 10 amps.

The ESC uses the main batteries to supply 6v to the receiver and rudder servo as well as the motor. The ESC plugs in where you would normally plug in the throttle servo. A servo has three wires, +ve Red, -ve black & signal or control wire white. The BEC circuit supplies the voltage to the receiver via the red & black and in turn the receiver controls the speed controller via the white wire. The plug can be changed on the ESC to match your receiver but make sure you wire it up correctly, it should be the same as the rudder servo. When running with BEC, as soon as you notice a drop off in motor speed, it's time to bring the boat in as the boat will soon become very erratic when there is not enough voltage to keep the receiver in control. Some ESC shut down when the voltage drops too low, very annoying when the boat is the other side of the lake!  
Self explanatory, not much good for boats as you can't back out of the weeds, but does reduce the price of a ESC. BRAKE, not much use with boats. Forces the motor to slow down when throttle stick is in neutral, not much use on a boat. (See Bob's board above).
Not much use either. Uses the current produced by the motor slowing down to top-up the batteries. In theory, if you allowed a radio controlled car to roll down a long and steep enough hill, this feature would re-charge the batteries. Only trouble is, most boats don't have wheels and very few lakes are on a slant. TORQUE ADJUST. Limits the maximum current to the motor. Useful when experimenting.
Basically offer much smoother response for  Electronic Speed controllers
These ESC are known as "squeelers or whistlers". When you hear one, you'll know why.

If your ESC has a forwards and reverse mode, it is very important to get the boat going forward when the controllers is in forward mode. Many ESC do not handle as much current in reverse as forwards. If your motor/ESC is connected up the wrong way round, the model will work but you can overload the ESC causing a burn out. Some ESC have a LED that lights up when the ESC is in reverse mode which tells you straight away. If your ESC doesn't have a LED then the ESC's wiring diagram will indicate the +ve and -ve wires to the motor. The motors on all speed boats should rotate anti-clockwise (unless you're using a pulley drive but no one ever does!).
Touch the battery wires direct to the motor so that it runs anti-clockwise and mark the motor terminals corresponding to +ve & -ve. Once you know which way the motors need to turn, set the boat up on it's stand, connect up the receiver, ESC and motor.


Turn on the transmitter then connect main batteries to the ESC. If everything is OK, the motor will twitch or spin for a second or so and then stop. If the motor continues to run adjust the transmitter throttle and trim until the motor stops. (Some ESC's have a centring control that may also need to be set-up, also check that the throttle stick is in the middle position if equipped with a two stage neutral.) If you can't adjust them so the motor stops, unplug everything and re-read in makers instructions and try again.

When everything is OK, connect your volt meter to the motor and battery -ve wires ( diagram below ), push the throttle stick forward and check the motor is running forwards (can you feel a draft from the propeller at the back of the boat?), then check the voltage on the meter, it should 0v or very low at full throttle. If it is then everything is OK. If the voltage is 0v but the motor spins the wrong way, then reverse the two motor wires. If the meter reads +7.2v (or whatever your battery voltage is) and the motor runs backwards, then just flick the servo reverse switch on the transmitter and check again. If the meter reads +7.2v but the motor runs the right way, then the wires to the motor need to be reversed AND flick the servo reverse switch over on the transmitter throttle (channel 2) and re-check. There are other ways of checking the mode of the controller but the above method should work in most cases.

If your radio Tx doesn't have servo reversers this will involve swapping the wires around on the inside. Unfortunately this option involves tampering with the transmitter and if you don't know what your doing then it's best left alone and asking someone to do it for you. To reverse the servo via the transmitter, remove the cells and open up the back. Look on the back of the rudder stick control, you should see three wires going to it. Make a diagram of the wire and colours, unsolder the two outer wires and swap them over and re-soled. Reassemble the Tx and try it out. The movement on the rudder servo should now be reversed. If it doesn't work, your in trouble and don't blame me! Put everything back as per your diagram and ask someone else to do the modification for you. You may find that the servo output arms have to be re-positioned after the servo is reversed.
If you're not sure about all this, DON'T DO IT!!!

Well all of this is just my opinion, but what the hell do I know!

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