Bow/Stern Thrusters: Navigating the subject!

Bow Thrusters, Stern Thrusters. Do you need one? Which one is best?

These are questions we will aim to answer below. There is no right or wrong answer as each variant will have it’s pros and cons. But to begin with, let’s explore what a thruster is for!

Thrusters, simply, are an aid for navigating tighter and more awkward places, such as marinas and small inland waterway rivers. They enable more precise control and the ability to manoeuvre the boat by fine-tuning direction and avoiding obstacles in short bursts to assist your main propulsion. Below, we’ll go through the benefits and drawbacks of each system, but generally, a thruster system is only meant to be used intermittently, and although in the case of a hydraulic system, reliability is significantly higher, it should still only be considered as an aid, not full propulsion.

The vast majority in inland waterways, leisure, pleasure, and small commercial vessels will use what is called a tunnel thruster (Both conventional tunnel and retractable), whilst some larger commercial applications or electrically driven vessels can use a Pod drive. The difference is that a Pod drive can rotate on it’s own, providing more flexibility for propulsion and thrust direction in one installation, whereas a tunnel thruster only provides lateral movement. Finally, there is also a retractable thruster option which, like a tunnel thruster, only provides lateral movement but provides a neater and more streamlined install.

Tunnel systems provide a simple, cost-effective solution and can be installed at two-thirds of their diameter below water level. To achieve optimum performance and reduce the risk of damage from surface debris, an immersion depth of one complete tunnel diameter is advised. The longer your tunnel, the less efficient your bow thruster will be. The majority of tunnel thrusters come equipped with twin propellers, offering equal thrust in both directions. Water cannot be compressed so the amount of thrust attainable is proportional to a thruster’s tunnel or turbine diameter.

Pod Systems are an efficient combination where your propulsion is able to be rotated to give you thrust where it is needed whilst also acting as a thruster due to it’s ability to rotate. Invariably electrically powered, battery capacity will still need to be maintained and specified correctly according to the demand of the Pod drive, but due to the design of a Pod, the demands in power are generally less than that of an equivalent dedicated thruster. It is worth noting, however, due to the pod being lower down than a conventional drive and not being protected by the hull in a tunnel or retractable setup, this is more suited for ocean/sea going vessels and where the waterway is deep. The risk of running aground could severely damage the pod drive.

Retractable thrusters have a neat trick up their sleeve, in that they can be deployed when needed, and once finished with, will retract back into a housing keeping it protected whilst also decreasing the drag-coefficient of the vessels hull. Although not as common as tunnel thruster installations, are ideal for flat-bottomed sailing yachts and have more efficient thrust due to the shorter tunnel the propeller sits in. However, the downsides are the complexity of the system resulting in higher costs and processes to service, whilst also having a larger footprint for install. Although, when retracted is less likely to be smacked into by that shopping trolley, there are more working parts that require correct functionality and even minor impacts can affect the thruster’s ability to operate and more so, retract, correctly.

Bow thruster systems consume more power than most other onboard functions and selecting the correct power source is essential. There are three options:

Electric: Where a dedicated bank of high cold cranking amperage (CCA) batteries are needed to power the thruster. The most common fitment on pleasure and leisure vessels.

PROS	CONS
Generally cheaper than other options on initial purchase.	Not ideal for continuous use (intermittent only).
Relatively simple installation and setup.	Majority do not use proportional (variable) speed at time of writing.
In most cases, take up less space.	Less service life compared to alternatives.

Electrically driven-hydraulic power, an electric DC power system allows you to combine the rugged reliability and lightweight hydraulic power head of a thruster, powered by a quick and easy installation of an electric motor. Other advantages include the ability to select a dry and well ventilated area to install the electric pump and the possibility to centre heavy components such as batteries and pump in a suitable midship section of the boat.

PROS	CONS
Gives the reliability of a hydraulic motor paired with a simple electric hydraulic pump.	Electric-Hydraulic pumps can be costly.
Installation is relatively simple and electrical components located in easier and safer places.	Although not as limited as purely electric, some pumps may have a runtime limitation.
Does not usually require huge battery capacity to run, but this depends on the motor.

Hydraulic Power bow thrusters benefit from unlimited run times. If there is an exisiting hydraulic system on board our boat, or if you are considering installing one, this wil provide a highly cost-effective power option.

PROS	CONS
Can be worked harder for longer.	Initial costs are higher than electric.
Driven by the engine or a take-off from the engine meaning batteries are not a concern.	More complex systems needed which can affect servicing costs when required.
Generally reliable and efficient.