What Are Bow & Stern Thrusters & How Do They Work?

Imtra thruster expert Peter Nolet explains the basics of bow and stern thrusters and how they work.

 

Since the 1990s, recreational boats have increasingly been equipped with extra propulsion devices that are commonly called bow, stern, or side thrusters. Thrusters are equipped with small propellers that help maneuver a boat sideways rather than forwards or backwards, typically while docking or mooring.

Thrusters can only be used when the boat is moving fore and aft very slowly or not at all. In their most common application, they are located below the waterline in the bow to push the bow of the boat one way or the other, and are therefore called bow thrusters. Additional thrusters are sometimes located in the stern, giving the captain total control of or maneuverability for the boat.

 

How do thrusters work?

In simplest terms, thrusters are designed with propellers facing in a sideways direction so when they are turned on, they push the bow or stern of a boat sideways through the water, in either direction. If only one thruster is used, for example in the bow, then the boat will also turn and change orientation. If bow and stern thrusters are operated simultaneously, a boat can be moved sideways without changing orientation.

The majority of thrusters have simple on/off controls and deliver full power either to port or starboard as long as they are engaged. Some thrusters are capable of delivering varying degrees of power, called proportional thrusters. These allow you to control the speed at which the thruster pushes the boat sideways. Proportional thrusters are sometimes connected to joystick-operated helm controls, thereby linking the main engine and propeller systems to the thrusters for improved low-speed maneuvering.

 

What different types of thrusters are used on recreational boats?

Tunnel thrusters are the most common, with a tunnel installed through the bow below the waterline and the thruster is inserted into the tunnel from inside the boat. The tunnel is usually constructed of fiberglass or aluminum tubing, depending on the material of the hull, which both protects the thruster unit and concentrates the thrust from the propeller.

One of the challenges of tunnels is creating a proper radius at each end of tunnel to allow water to be sucked into the tunnel without cavitation (bubbles of air hitting the propeller blades). If cavitation occurs, thrusters can become very noisy and lose up to 25 percent of overall performance. In higher-performance boats, deflectors on the forward side of the tunnel entrances also become important to minimize any speed loss due to turbulence.

External thrusters are typically used when a boat doesn’t have enough space for a tunnel to be installed. Carrying a thruster as an external appendage does create a small amount of drag, dampening performance, particularly on faster boats that reach speeds of 30 knots or more. It can also be a challenge to fit a boat with an external thruster on a boat trailer, and there is, in addition, some potential for damage if you snag a buoy.

External Thrusters

Retractable thrusters, as the name suggests, may be retracted into the hull when not in use, and deployed into the water with an actuator when you need them. This eliminates additional drag, which is something that sailors are often concerned about. It also solves limitations due to draft. For example, if the bow section of a boat is too shallow, any tunnel would be too close to the waterline to be effective. More space is required inside the hull to install retractable thruster, however, along with the need for a mechanical installation.

 

What are the different types of thruster propeller configurations? 

There are three common propeller configurations on thrusters—single, twin tandem, and twin counter-rotating. They are:

Single propellers are the simplest, cost the least, and are well-suited to applications with shorter, smaller-diameter tunnels.

Twin tandem propellers move water more efficiently by using two propellers, making them a better choice when higher thrust is desired.

Twin counter-rotating propellers provide the most efficiency in moving the most water in a given tunnel diameter. As the costs to produce are higher, they are mostly used on yachts 50 feet and larger

 

What are the different characteristics of thruster propellers?

Thruster propellers are a complex subject as they come with varying numbers of blades, are made of composite and alloy materials, and work within a variety of tunnels. Those with minimal prop-tip clearance – the distance from the end of the prop to the surface of the tunnel – tend to make a thruster more efficient and powerful. Propellers with more prop-tip clearance are quieter but produce less thrust. In general, prop blades are fairly rigid; if the material is too flexible, there are losses.

Prop Tip Clearance

What are the different types of thruster power sources?

DC electricity is the most common power source for thrusters on recreational boats and is relatively easy to install for what is normally low-intensity usage when leaving and arriving at the dock. DC-powered thrusters may have limited run time due to the power draw on batteries and the heat generated in the electric motor, as thrusters with on/off configuration are always run at full speed.

Proportional controllers for DC thrusters developed by Side-Power provide the ability to apply power on a graduated basis, extending run times while reducing both noise and power draw. Coupled with joystick products that link thrusters and primary propulsion and maneuvering systems, the popularity of DC power aboard recreational and commercial boats has steadily increased.

Hydraulic power has the advantage of unlimited run time, making it a favorite in many commercial applications and on large yachts that use hydraulics for other purposes. It can be relatively expensive to install, however, especially on a boat being refit if hoses need to be run to the bow.

AC electricity is the third common power sources for thrusters, and is found mostly in commercial applications. AC requires a large, heavy motor and draws a large amount of power. This requires significant generator power aboard. Advantages of AC electricity are that run time is unlimited and smaller cables are needed. Another is that the motor’s variable frequency drive allows the operator to apply only as much thrust as needed.

To learn about choosing bow and stern thrusters, read “How to Choose the Right Bow Thruster”.

 

Written by John Burnham & Conrad Taylor

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