Engines VS. Thrusters

Tricks for Tight Turns and Perfect Landings

Close quarters maneuvering in a boat can be challenging. Wind and/or current can override your best efforts to point the bow in the right direction. Put confidence back in your boat’s steering success with these tips on using prop wash and prop walk when handling boats with single and twin engines. Better yet, add a thruster and maneuvering becomes a cakewalk.

—Story and Photos by Peter P. Pisciotta

Twin screw boats are legendary for their maneuverability in close quarters. The ability to “split the gears” by placing one engine in forward and the other in reverse to spin the boat in place has always been secretly coveted by confounded owners of single screw boats. Installing a thruster enhances control of a single screw boat and adds a skilled “hand” to a twin engine boat but does it close the maneuverability gap entirely? Are there other considerations?

A helmsperson can only control a boat’s rudder(s) and propeller(s) to maneuver. There are only a few basic tools available for turning: the rudder, rudder-prop wash, prop walk and offset thrust from engines. Turning the wheel positions the rudder to deflect water flow and the boat turns. The slower the boat is going the more difficult the turn because there is less flow for the rudder to deflect. Engaging the prop forces more flow past the rudder than if the boat is just gliding through water thus magnifying the effect of the rudder. This is known as rudder-prop wash. If you want to accentuate the turn while minimizing headway, remember to put the helm hardover before putting the gear in forward. Prop walk is the tendency of a rotating propeller to crawl sideways in the direction of rotation. Though it’s only noticeable in reverse as rudder-prop wash overwhelms prop walk in forward rendering it invisible. This effect varies widely depending upon the boat but cannot be ignored by single engine operators. Interestingly, prop walk can be effectively used on twin engines though few helmspersons do so. Offset thrust only relates to twin engines. When putting the starboard engine in forward, while leaving the port in neutral, the boat veers to port. Likewise, put the port engine in reverse and the bow swings to port. Combining the two maneuvers by “splitting the gears” — starboard in forward, port in reverse — creates a spin (“turning on a dime”).

All these tools have one thing in common. They provide direct control of the stern but indirect control of the bow. The addition of a bow thruster provides control of both ends of the boat, clearly a sizeable advantage for a single engine boat with a thruster.

Prices of thrusters have dropped dramatically while performance and reliability have improved. Thrusters have become easily available in the recreational boat market in the past 20 years. According to Will Heyer, sales manager of Maryland-based Vetus (www.vetus.com), a leading thruster supplier, sales have nearly doubled each of the past five years with no end in sight. Once the sole domain of large, single engine trawler yachts, thrusters are now being installed on powerboats under 30' (9.1m) and cruising sailboats.

Twins Or Thruster?

A twin engine boat is ambidextrous. Prop walk is an immutable force pulling the stern sideways in one direction when in reverse. Single screw operators quickly learn to heavily favor the preferred side. On twin engine installations, the propellers usually counter-rotate to cancel each other’s effect out. In the hands of a moderately skilled helmsperson, prop walk of one engine or the other can be called upon to assist a turn in either direction. The boat has no favored side, either being equally functional.

Clearly, twins allow indirect control of the bow. But in some high wind, high current areas this may not be enough. Thrusters enable very small crews (e.g., an older cruising couple) to handle relatively large vessels much more comfortably and with more confidence, especially in unfamiliar places or unusual berth configurations. If docking is routinely stressful for skipper and crew, chances are the boat will be used more when equipped with a thruster. In this situation, a bow thruster becomes an investment in boating pleasure rather than a cost.

Bow Versus Stern; Electric Versus Hydraulic

A thruster is merely a motorized propeller producing lateral thrust. It’s controlled either by buttons or a joystick and is mounted either at the bow or stern. A bow thruster is more effective than a stern thruster for three reasons. First, the rudder and running gear already give the helmsman control of the stern. What’s missing is control of the bow (more on this later). Second, the bow typically has a shallower underwater profile making it more susceptible to wind deflection. Third, under the influence of a thruster, a boat tends to spin around its deepest section, almost always the stern where the running gear is located. A bow-mounted thruster creates a longer, more effective lever-arm than a stern thruster. The further forward, the better the performance. The single advantage of a stern thruster is ease of installation. A stern unit is less expensive to install because it’s mounted on a bracket, rather than the way bow thrusters always are in a tunnel.

Of course, installing both a stern and bow thruster enables the boat to move perfectly sideways, a feat even twin engine boats cannot duplicate. Since bow thrusters outnumber stern thrusters by a wide margin, the rest of this article concentrates on bow thrusters.

Power to the thruster motor can be either electric or hydraulic. Since few recreational boats under 50' (15m) have hydraulic systems, electrically powered units are the most popular choice because they are less expensive and less complicated to install. Hydraulic thrusters do have some distinct advantages. Unlike electric thrusters that are either on or off, hydraulic thrusters have proportional thrust. Barely nudging the joystick induces mild thrust; pushing it all the way achieves maximum thrust. Plus, hydraulic thrusters are continuously rated whereas electric thrusters are duty-cycle rated, generally around 5 minutes per hour (formerly 2 minutes per hour), after which time they may overheat and trip the circuit breaker, never at a convenient time.

Thruster Control

Thrusters are simple to operate. Move the joystick left and the bow moves to the left and vice versa. Below are a few simple rules for using a bow thruster.

Rule 1: The faster the boat is going, the less effective the thruster becomes. This means it’s best used when stopped or nearly stopped, not underway.

Rule 2: Electric thrusters have a limited duty-cycle, around 5 minutes per hour, more than enough unless the thruster is woefully undersized. Once tripped, the unit is inoperable until it cools and resets. (Manufac-turers promise continuous-duty electric units are on the market horizon).

Rule 3: Thrusters are usually best when “pulsed” in several second increments rather than continuously activated, otherwise too much turning momentum may be built. It’s easy to add more.

Rule 4: Joysticks are more intuitive than button controls.

Rule 5: For safety purposes, thrusters turn off automatically after a predetermined period of time, usually around 30 minutes.

Rule 6: Test your thruster in both directions before you need it.

Figure 1

Figure 2

While there are probably several unique ways to use a thruster, almost all are intuitive. One notable exception is pulling away from a side-tie (Figure 1) . While simply rotating the bow out until it’s pointed in the desired direction works, a bow thruster when used in conjunction with the engine enables the boat to veer away from the dock, almost sideways. The key is putting the helm over toward the dock and using alternating bursts of thruster and engine. This gets both ends of the boat moving away from the dock.

Learn to use the thruster and the engine simultaneously. The leftgraphic shows the rudder hardover toward the dock. Synchronized use of both engine and thruster enable to boat to move almost sideways.

Backing a boat is where a bow thruster really excels, especially when adverse influences are in effect (Figure 2) . To control the bow, the twin-engine boat operator needs to split the gears (shown in the right side of the illustration) to lever the bow around. Rather than pivot near the stern as the thruster-controlled boat does, the twin pivots much farther forward. The result is the twin tends to creep sideways much more. This alone is a compelling advantage for a thruster, especially in areas where docking is traditionally stern-to or Med-moored.

With a thruster, the boat will pivot close to the stern making backing procedures very intuitive. Even when the wind comes up the thruster controls the boat where the twin will creep sideways.

The Maneuverability Gap

A single engine boat with thruster and twin-engine configurations each have close-quarter strengths and weaknesses. Singles still have a favored side due to prop walk whereas twins are equally nimble, though few twin operators really maximize their capabilities. Thrusters are probably easier for the average helmsperson to master than twins because they are so intuitive. Thrusters really shine in backing a boat, so a slight nod in maneuverability may be their due.

Low cost and improved performance of thrusters has solved one of the primary disadvantages of owning a single screw boat — maneuverability. Controlling the bow is an important advantage for all boats, not just singles.