How to Choose a Bow or Stern Thruster

Thrusters used to be something you found on large yachts. That has not been true for years. Today a thruster fits almost any boat with a rigid hull and room for a 110 mm tunnel, including plenty of boats well under eight metres.

What has not changed is how easy it is to choose the wrong one. A thruster that looks right on paper can be too weak on the day you need it, or far too strong on a calm afternoon. These are the things we go through with customers before recommending a system.

If you are still deciding whether you need a thruster at all, start with Why every boat should have a bow thruster.

Compare thrust, not kilowatts

The number that tells you what a thruster will actually do is thrust, given in kilograms-force (kgf). We often get asked for “a 3 kW thruster”, but 3 kW of electrical power can produce anything from about 45 to 70 kgf. The difference comes from the propeller: its size, its shape and how efficiently it moves water.

How much thrust you need depends on the boat. Length and weight matter, but windage matters most. A flybridge cruiser needs more than a low open boat of the same length.

Check the voltage the thrust is rated at

Most manufacturers give thrust at the nominal 12 V or 24 V. In use, a thruster never sees that. It pulls a heavy current the moment it starts and the voltage drops. On a 12 V system you will have 11.5 V at best, and more often around 11 V.

So if your boat needs 60 kgf and you buy a thruster rated at 60 kgf at 12 V, you will not get 60 kgf at the quay. You might get 55, which is not enough for the boat you sized it for.

Where the manufacturer gives them, compare thrust figures at 10.5 V (or 21 V on a 24 V system). That is much closer to what you will actually have when you need it.

Tunnel position matters as much as size

A bow thruster works best as far forward as possible, because the further it is from the boat’s pivot point, the more leverage it has. The tunnel also has to sit deep enough, ideally at least one tunnel diameter below the waterline. If it is too shallow it draws in air and loses thrust.

On most hulls the bottom rises towards the bow, so these two requirements work against each other. The tunnel often ends up further aft than it should be.

The way round this is to get more thrust out of a smaller tunnel. An efficient thruster can deliver the same push from a smaller diameter, which lets the tunnel go further forward. A small tunnel well forward will usually do more than a larger one set back.

Stern thrusters follow the same logic at the other end. On many boats they can also be mounted externally on the transom, without a tunnel through the hull.

Tunnel or retractable

A tunnel thruster is the usual choice: simple, reliable and the most affordable. On some boats it is a compromise. A tunnel adds drag at speed, which matters on fast planing boats and sailing yachts, and on some hulls it simply cannot go far enough forward.

A retractable thruster drops out of the hull when you need it and folds away when you do not. With no tunnel, there is no drag under way. It can also sit further forward and deeper than a tunnel, so it gets more out of the same power. The trade-offs are price, the space it needs inside the hull and more moving parts to look after.

We compare the two in more detail in Tunnel thruster vs retractable thruster.

On/off or proportional

A standard thruster has two settings: off and full power. That works, but it has two limits.

The first is runtime. On/off thrusters are built for short bursts and cut out to protect the motor if they run too long. Most of the time that is fine. In a strong crosswind, when you need to hold the boat in place for a while, it may not be.

The second is control. If you size a thruster for windy days, it will be far too strong on a calm one. Touch the button and the bow is thrown sideways like a potato in a bucket.

Proportional thrusters solve both. You choose how much power you use, from a gentle nudge to full thrust, and they are designed to run for far longer than on/off models. They are also quieter, and their control panels usually show motor temperature and battery state. The main downside is the higher price, and few people who have used one would go back.

On boats with a Volvo Penta joystick, a proportional thruster can be connected to the same lever.

Batteries and charging

Most leisure boats use an electric (DC) thruster. Hydraulic systems mainly make sense on boats that already have hydraulics on board. With DC, the thruster is only as good as the battery feeding it.

Size the battery for peak current. A thruster draws a very large current for a short time. For example, a small 20 kgf thruster on a 6 m boat draws around 150 A, which needs a battery rated for at least 300 A CCA (cold cranking amps). A 40–50 Ah battery usually covers that.
Choose a battery that can take it. AGM batteries handle heavy current and deep discharge much better than ordinary lead-acid. If the boat runs on lithium, check that the battery management system can deliver the thruster’s peak current without shutting down.
Capacity sets runtime. More Ah means more thruster use between charges, but it also needs a bigger charger.
Use a charger of at least 10% of battery capacity. A smaller one charges slowly and can shorten the battery’s life.
Fit a battery monitor. Even a simple one tells you whether there is enough power before you find out the hard way.

Cable size counts too. The further the thruster is from the battery, the heavier the cable needs to be, or the voltage drop gets worse.

Get it checked before you buy

None of this is complicated, but a wrong choice in any one area leaves you with a thruster that does not do what you paid for.

We are the official Sleipner distributor for the Baltic states. Sizing, installation, joystick integration and service are covered on our Sleipner page. For a first estimate, Sleipner’s Thruster System Builder suggests a setup from your boat’s dimensions. Send us the result and we will check it against your boat.