First, it's critical to choose the right size of thruster based on the length, displacement, and windage of your boat. Nearly all non-commercial thrusters are powered by electric motors, and available sizes range from small, 2-kilowatt units that provide enough thrust for boats in the 25- to 30-foot range (yes, it's possible to get a thruster for your 25-footer) to 15-kilowatt units designed to push the bow of a 90-footer. Bow-thruster manufacturers and installers can offer valuable selection guidance and can usually provide a range of choices to meet your needs. The rule of thumb when making a selection? The more thrust, the better.
Second, since thrusters use leverage to push the bow around the boat's pivot point, somewhere amidships, thruster efficiency increases as the unit is mounted deeper and farther forward on the bow. However, getting a thruster right near the bow and deep below the waterline can be a challenge on many sailboats with shallow bow sections.