Why does Sleipner declare thrust at 10.5 volts whereas others state thrust at 12 volts?
Unlike most thruster manufacturers, Sleipner bases its products' thrust (kgf) on a realistic, achievable voltage in real-life conditions rather than on theoretical thrust measured under ideal laboratory conditions.
The electric motor's voltage level will determine the actual thrust output you get from a thruster system. Unless your onboard electrical system is custom-built to maintain voltage under very high amperage loads, resistance will somewhat reduce the voltage reaching the electric motor, resulting in lower thrust output accordingly to the drop in voltage.
Power cables, fuses, main power switches, connections, and the like, create resistance, effectively reducing voltage. If you calculate the thrust needed for your vessel and do not factor in the typical drop in voltage in most systems, you run the risk of a significantly underpowered thruster unit.
Voltage drop and current draw
After years of thorough on-water testing, Sleipner is aware that statistically, on a perfectly installed electrical system, due to voltage drop and current draw, no more than 10.5V will be measured on the thruster 12V system or 21V on a 24V system.
As an example, the Sleipner DC Electric Tunnel Thruster SE100 is tested to provide 100 kgs of thrust at 10,5V. Feed it with 12V, and it delivers 116 kgs of thrust.
Based on decades of experience, Sleipner recommends using 10,5/21V as reference values when calculating the right thruster size. If you choose your thruster based upon the unrealistically 12V thrust level, you will most likely lack the necessary power in rough conditions when needed.
Sleipner lists both values only for comparison reasons, as some other manufacturers only list 12V/24V thrust output.
When comparing Sleipner thrusters with other brands, you can generally go down a size when retrofitting a Sleipner product. Learn more about thruster size here.