How do bow thrusters work and do I need them?

A retractable white bow thruster of a boat © Boat Accessories Australia

A challenging day on the water can be enough for many skippers to install bow thrusters to their boat. Less than ideal conditions can make it difficult to get the boat into dock or onto the boat ramp.

Bow thrusters can take the pressure off any skipper whether they're a novice or have years of experience behind them. But there is plenty to consider before adding bow thrusters to any boat. They are a sizeable investment so you want to make sure you choose the right ones and have them installed correctly for the best result.

What are bow thrusters?

A bow thruster is a propulsion device that provides lateral thrust to help with manoeuvrability. Bow thrusters push a boat's bow or stern sideways in either direction through the water. Bow thrusters can come standard on a new vessel or be retro-fitted on boats over 45 feet.

How do bow thrusters work?

The addition of bow thrusters allows a boat to move to the side (port or starboard) without forward or backward thrust from the engine or rudder. Thrusters use suction to draw in water from one side of the boat and push it out the other side to move the boat in the opposite direction. Fitters install thrusters in through-and-through tunnels on either side of the boat or as external or retractable thrusters.

Benefits of having bow thrusters on your boat

Boat owners fit or purchase their boat with bow thrusters for a wide variety of reasons including:

• High winds with thrusters fitted a skipper need not worry about strong winds sending them off course
• Strong currents side thrust helps counter strong currents at sea and in dock
• Crowded dock using a thruster the skipper doesn't need forward motion to turn the boat into position.
• Confidence a thruster can do wonders for a skipper's confidence knowing they can handle their boat in any weather condition. With bow thrusters, a skipper is also less reliant on having a crew to help with docking.
• Fishing aid if an angler is fighting a game fish, a thruster can help position the boat so an angler on the stern has the best chance of landing the fish.
• Return on investment bow thrusters are a sought-after addition and can make your boat more attractive when you put it up for sale.
• Easy to use bow thrusters' operation is as simple as the push of a button. With a little practice, you will be a pro.
• Increased boat usage - having bow thrusters can mean the difference between taking the boat out and staying home because weather conditions aren't perfect.

• Cost bow thrusters are a sizeable investment starting in the thousands of dollars.
• Maintenance like most parts of a boat, bow thrusters need regular attention and maintenance to ensure they work reliably every time.
• Weight when a boat's weight is of concern because of towing or handling, bow thrusters can add considerable weight.
• Professional installation recommended for most boat owners bow thruster installation isn't a DIY job, find an experienced professional.

Types of bow thrusters

The three main bow thruster types are tunnel thrusters, external thrusters and retractable thrusters. You can add bow thrusters to an existing boat or buy a new boat with thrusters already installed.

Tunnel Thrusters

Tunnel thrusters are the most common type of thrusters. The tunnel is installed through the bow and below the waterline. Made of composite material, the tunnel can protect the propulsion unit and concentrate the propeller's thrust. When measuring to find the best location for the tunnel keep in mind, the top of the tunnel should be at least 75% of its diameter below the waterline but ideally 1.5 times its diameter. The fitter inserts the propulsion unit into the tunnel from inside the boat.

One of the biggest difficulties with tunnel installation is to create a radius at the end of the tunnel to allow it to suck water into the tunnel without it cavitating. If air bubbles hit the propeller blades, the thrusters become noisy and lose up to 25% of their performance. High performance boats may use deflectors to minimise speed loss from turbulence.

External Bow Thrusters

Not all boats have enough hull space for tunnel or retractable thrusters so external thrusters are required. They are easy to mount because external bow thrusters don't require holes in the hull. The biggest downfall for external thrusters is the drag they can create which decreases the boat's performance. The thruster is also at risk of snagging a buoy and being damaged.

Retractable Bow Thrusters

As the name suggests, retractable thrusters can be moved back into the hull when not in use so there is no risk of snags or unnecessary drag. Electric actuators extend and retract the thrusters. It requires more space for retractable thrusters compared to the other two types.

Bow Thrusters Power

Bow thrusters rarely connect to the boat's main power system so they need to provide their own source. The power for bow thrusters can be electric or hydraulic. Electric thrusters have an electric motor positioned above the tunnel. While the installation of electric thrusters is fairly simple, the motor creates more weight at the bow. Hydraulic thrusters are lighter however they need more components making installation much more complex and expensive.

Some thrusters come with a remote, but this shouldn't take the place of a hard-wired touch pad or joystick. The risk with only having a remote to activate the thrusters is the batteries may not work when you need or the remote falls overboard.

Comparing Bow Thrusters Single and Double Propellers

The measure of bow thrusters power is in kilogram force (kgf) which is a gravitational metric unit of force. The Quick Nautical Equipment brand of bow thrusters comes highly recommended.

Quick Bow Thruster Single Propeller are ideal for boats sized between 7-9.5 metres. They are available in two sizes 1.5 kW motor power and a 2.2 kW motor power. The 1.5 sized motor provides a thrust of 30 kgf while the larger motor has a 40 kgf. The larger motor is recommended for boats up to 9.5 metres while the smaller motor is better suited for boats up to 8.5 metres. Both are 12 volts.

If the boat requires more thrust, Quick Bow Thruster Single Propellers are available in 55 kgf using a 3 kW motor power. The 4 kW offers 75 kgf and the 6 kW offers 95 kgf.

The Quick Bow Thruster Double Propeller allow for higher voltage, power and thrust for larger-sized boats. The 12 volt with a 3.3 kW motor power has a thrust of 65 kgf for fitting on boats sized between 8-11 metres. The larger 4.3 kW has a thrust of 85 suitable for boats of 10-13 metres while the larger 6.3 kW system has a thrust of 105 kgf suitable for boats sized between 12-15 metres.

The largest Quick Bow Thruster - Double Propeller with a 10 kW motor can offer 240 kgf thrust and is suitable for fitting on a 16-21 metre boat. The thruster weighs 50 kg.

Instead of buying the single propellers on their own, complete bow thruster kits are available which include the propeller, solenoid protection carter, gearbox, zincanode and elastic coupling. The tunnels and thruster controller are also available for individual sale.