".....brushless motors the main question is speed controller as I would need forward and reverse...."
You can get reversible brushless ESCs. However, watch out, as boats are a minority interest and many cheap reversible brushless ESCs are designed for car use. These may have a 'braking' feature, which means that the controls work like this:
= motor braked
quick forward and backward again = motor goes in reverse
which is optimum for cars, but cumbersome for boats. A proper boat designed brushless ESC will be fine, as will be a cheap car one where you can turn off the 'brake'. RNinMunich recommends the Quicrun series of Chinese brushless ESCs, but of course products change all the time.
So check that the ESC you are after is reversible, and has a 'no-brake' setting....
Rather more complex background explanation.
This is my understanding of how one particular 'brushless' system works - there may well be other technologies out there... :
'Commutation' is the process of switching each coil in an electric motor on at the right time to get attracted to the nearest magnet and off when it passes the magnet, so that the armature will rotate in one direction.
In a brushed motor this happens mechanically, and we don't need to worry about it. If you want to know a bit more, get a Taycol motor and start reading papers like this: http://what-when-how.com/electric-motors/commutation-electri...
In a brushless motor, commutation is done by software
inside the ESC. So the ESC is essential to a brushless motor - it won't run without one.
What the software
in a brushless ESC does is start the motor with a pre-programmed series of activations of coils. Once spinning, the software
detects the generated back-emf as a magnet passes each coil, and uses this to work out the motor speed, timing, and hence which coils to activate next, depending on the throttle signal. Obviously, this happens very fast.
If the motor went round slowly, the generated back-emf would be very low, and the ESC would have difficulty detecting it. So a typical brushless motor has a minimum speed which is quite high compared to a brushed motor.
In a sensor motor, the ESC can be much more precise about timing because it reads it off the Hall-effect sensor. It can also run at much slower speeds. But the extra kit, and the low production levels mean that these are going to be more expensive motors.
For a brushless ESC the starting process, and the reversing process, are much more complicated than they are in a brushed motor. In each case the software
has to try to detect what the motor is doing and adjust the timing to get it running in the right direction 'blind', with possibly odd feedback from the coil back EMF. For a brushed motor, the ESC just reverses the polarity and the mechanical commutation automatically applies a force in the direction you want.
The result is that brushless starts can be a bit jerky
, and brushless reverses need 'setting up' with a process of signalling from your transmitter
or using a program card. Since cheap brushless ESCs are often turned out with little documentation, making them reverse can sometimes be difficult for an inexperienced purchaser.