You want a more maneuverable boat? Thrusters are the answer, and they're becoming more common on all kinds of boats from 30 feet on up.
With today's electronic controls, thrusters are easy to operate and add the ability to move laterally, which makes docking a much simpler drill. Try them once on a sea trial, and they'll soon be at the top of your wish list.
But choosing and installing thrusters can be a big job, requiring holes in the hull, significant fiberglass work, the placement of major equipment and a dedicated power source to run it. Generally speaking, you'll want to have a pro do the work. And it is not cheap.
SCOUT BOATSBow and stern thrusters have found their way into smaller boats, like this Scout 350.But you can trim those costs, and minimize problems, by understanding how to choose your equipment and how the installation process works.
THRUST OF THE MATTER
Traditionally, thrusters are installed in the bow to allow direct control over the front of the boat (twin props allow direct control of the stern, but move the bow indirectly). But these days, stern thrusters are in the mix, too. They're easier and can be less expensive to install than bow thrusters, because they are often bracket-mounted. Tunnel mounting is also an option, keeping them hidden and therefore out of harm's way.
But stern thrusters aren't appropriate for all applications. On smaller boats, for example, the stern is often too cramped by running gear and mechanical items. Stern thrusters also have less overall effect than bow thrusters. That's because the farther forward a thruster is, the longer the lever arm it gains when it tries to maneuver the boat. The stern is also deeper in the water and has running gear under it.
VETUSChoosing the right thruster isn't a choice of horsepower, but of pounds of thrust.There is an exception to the rule, however: boats fitted with both bow and stern thrusters. This allows complete lateral control, which can't be attained with the engines alone.
Of course, these factors assume you can choose the location of your thrusters at will, and on many boats, this is not the case. A tunnel-mounted thruster requires a large, unused area for mounting, and needs to be accessible so the motor can be serviced. Water or holding tanks, mechanical items or other things occupy the area that would otherwise be ideal for the thruster's location. In addition, on planing boats, it must be submerged several inches (six is ideal) when the boat is at displacement speeds, yet fully out of the water when the boat's up and running.
Then, there are batteries to consider. Most thrusters require a dedicated D.C. power source, which must be relatively close to the units themselves. Long wiring runs require heavy-gauge wires and can result in voltage drops. They also add weight and expense. That means you must find space to house the batteries nearby, too.
DO THE MATH
Once you finalize the location(s) for your thruster(s), you can determine power requirements. According to Vetus, a leading thruster manufacturer, this has less to do with actual horsepower than it does with pounds of thrust and the design of your boat.
The company has a formula that helps you figure out exactly how much juice you need for a bow thruster, expressed in pounds of thrust. You first need to find out the required torque, then run a second equation to turn that into required pounds of thrust.
Determine the torque as follows:
T = wind pressure x wind draft x 0.75 x ¾ LOA
To obtain wind draft, use the boat's square footage of exposed surface above the waterline. To determine wind pressure, use the following table:
16 knots (moderate breeze) = 1 ft/lb of pressure
22 knots (fresh breeze) = 1.9 ft/lb of pressure
28 knots (strong breeze) = 3.1 ft/lb of pressure
The second equation is as follows:
Pound Thrust = T/distance between center of thruster and transom
Here's an example: You have a 30-foot boat with 150 square feet above the waterline, which you want to maintain control over in winds to 22 knots. The formula is: 1.9 x 150 x 0.75 x 15 = 3,206.25. The farthest forward point you can install the thruster at allows 28 feet between the center of the thruster and the transom, so the second formula is: 3,206.25 / 28 = 114.5. So, you'll need a thruster with about 114.5 pounds of thrust to control your boat in winds to 22 knots. 
Check the Vetus table (below) of average thrust force for boat lengths, and you'll discover this fits right into the norm.
What about stern thrusters? There are no hard formulas to use, because thrusters mounted in the stern aren't fighting to move around a distant pivot point. In most cases, fairly small thrusters are up to the job.
The big question is, how fast do you want to be able to move sideways? Though it may be tempting to install big thrusters at the bow and stern, there's a danger to putting in oversized units. Not only do they require more battery power, but they also can move the boat faster than advisable and build up momentum.
If you're going too fast, why not simply reverse thrust? You can – to a point. Thruster motors are designed only for brief bursts of power, not sustained use. A shot here to get the boat lined up, and a shot there to slide it into the slip. That means seconds at a time, and even top-quality units can't run for more than a few minutes an hour. Lean on one for too long, and you could pop a breaker or burn up a motor with heat buildup.
