Electric 12 , 24, or 36 volt trolling motors are really used a lot here in North America, They are very popular with fishermen here, especially bass fishermen.
The power is measured in pounds of thrust.{LBS} 35 Lbs thrust and they go up from there with corresponding voltage increase, requiring 2 or 3 12 volt Deep Cycle batteries tied together
A 55 LB thrust electric would be ideal for an 18 foot Rib.
here is an example of high quality american electric motors.
Minn Kota Motors