If having 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.
Several basic design characteristics connected to your boat will give you a reliable insight into the size of thrusters you should be thinking about and the kind of performance you can realistically expect to achieve.
Accurate calculation for best thruster performance
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 will be connected to the physical positioning of a bow or stern thruster in your specific boat.
Thrusters positioned as far forward as possible benefit from an increased leverage arm in relation to a boat’s pivot point, so the further forward you can position 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 and 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 allowed to suck air into the tunnel by being too close to the surface will lose thrust efficiency rapidly and emit a level of noise far greater than if it was fully submerged.
Another point to bear in mind is that if your cruising and mooring ground is subject to any kind of swell that might risk your boat pitching up and down to such an extent 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 positioned deeper than is possible for a standard tunnel installation.
Strong motor and wide tunnel
Regarding thruster power, any compromises you might be forced to make will have an immediate bearing on performance, so 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. To give you an idea of what that means, a powerful thruster would be able to operate in winds of 22 – 25 knots. The difference between the two comparisons may not appear very wide, but those two installations will give you a very different user experience.
Do you need powerful thrusters or not?
Ask yourself the question, "What kind of boater am I?" Do you positively relish challenging conditions come what may, or when the barometer needle starts to fall and skies above start turning grey, do you generally prefer to stay put in a marina?
If you think you need powerful thrusters, then think about 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 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 might be expected to face and plan your purchase accordingly.
Voltage drops may have an effect on available thruster power
One final word of caution is that voltage drop, caused by inevitable resistance from cabling, main switch, and fuse, etc. might have a more significant effect on actual available thruster power than some manufacturers might have led you to believe!
For instance, in a 24V system, most thrusters will never see close to that voltage at the electric motor. Most battery banks will suffer from voltage drop directly through having to supply current to such a high load unit as a thruster. Far better, therefore, to base your requirements on a likely scenario of 21V and make sure your final selection is rated at sufficient thrust after you have subtracted average voltage drop.