If you've decided to buy a bow or stern thruster, or if you're planning to upgrade an existing installation, you'll need to determine the amount of thrust output required to suit your specific needs.
Accurate calculation for best thruster performance
Several design characteristics connected to your boat will give you reliable insight into the size of thrusters you should be thinking about as well as the kind of performance you can realistically expect to achieve.
In addition to knowing the waterline length and underwater profile of your boat, an installer will require an accurate assessment of the total windage area in sq.m of the hull's topside profile above sea level. This calculation, along with an idea of "average" displacement in cruising mode, will help an installer advise you as accurately and as realistically as possible. Other factors affecting performance and efficiency is connected to the physical positioning of a bow or stern thruster in your specific boat.
Read more: Pros and cons of different thrusters →
Thrusters positioned as far forward as possible benefit from an increased leverage arm in relation to a boat's pivot point, so the further towards the front you can place the thruster will increase efficiency. Additionally, the topmost part of the tunnel needs to be immersed in water of no less than half the width of the tunnel diameter. If you can achieve a full diameter's width immersion between the top of the tunnel and the waterline as your minimum, then even better.
Thrusters need water for working
The reason why a thruster must always remain immersed is that any thruster that sucks air into the tunnel by being too close to the surface loses thrust efficiency rapidly and emit a level of noise far higher than if it was fully submerged.
Another point to bear in mind is whether your cruising and mooring ground is subject to any swell that might risk your boat pitching up and down to such an extent that the thruster becomes exposed when you need to use it. Then you might have to consider installing an externally mounted or fully retractable thruster, both of which can be placed deeper than possible for a standard tunnel installation.
Strong motor and wide tunnel
When it comes to power, any compromise you might have to make will have an immediate significance on performance. The general advice is always to play safe by specifying the most potent motor with the broadest diameter tunnel you possibly can. Second to the physical characteristics of your boat will be a general feel for the prevailing weather conditions that affect your area, along with an accurate assessment of the maximum strength of tide.
The general advice is to choose a thruster that can push the bow or stern against a minimum of 17.5 knots wind speed. For comparison, a powerful thruster operates in winds of 22-55 knots. The difference between the two comparisons may not appear very vast, but those two installations will give you very different user experiences.
Do you need a powerful thruster or not?
When customers ask me if they need a powerful thruster, I always tell them to ask themselves a question; "what kind of boater am I?"
Do you like the idea of challenging conditions, or do you generally prefer to stay put in a marina when the barometer needle starts to fall and skies above start turning grey?
If you think you need powerful thrusters, then consider upgrading to Proportional Control. In normal conditions, you will find you only need a fraction of the available power, but no one wants to go through to the time, trouble and cost of installing thrusters only to be disappointed with their performance. So try and think about the worst possible mooring conditions you expect to face and plan your purchase accordingly.
Voltage drops may affect available thruster power
A final word of caution is that voltage drop (caused by inevitable resistance from cabling, main switch, fuse, and the like) might have a more significant effect on actual available thrust power than some manufacturers might have led you to believe.
Essentially, a 24V system will never come close to the same voltage as the electric motor. Most battery banks will suffer from a voltage drop directly through having to supply current to a high load unit, such as a thruster. Thus, it is far better to base your requirements on a likely scenario of a 21V and make sure your final choice is rated at sufficient thrust after you have subtracted the average voltage drop.