NavCad Approach Speed-Dependent Current Draw
Once an electric motor drive is configured in NavCad (see related article on electric motor support), the propulsion analysis outputs electrical input power and current draw at each speed in the analysis grid. Battery range calculation uses these outputs:
- Identify the operating speed or duty profile for which range is required.
- From the propulsion results, note the current draw (A) at the operating speed.
- Divide the battery capacity (Ah) by the current draw (A) to obtain endurance (hours).
- Multiply endurance by vessel speed to obtain range (nm or km).
For multi-speed duty profiles (transit + loiter, for example), weigh the current draw values by the time fraction spent at each speed to compute an average draw and overall range.
Battery Sizing From a Range Requirement
The calculation can also be run in reverse: if a required range is specified, the battery capacity needed follows from the current draw at the planned operating speed. NavCad's ability to run propulsion analysis across a full speed range allows quick sensitivity studies for example, evaluating how much range increases if transit speed is reduced from 10 to 8 knots.
PropExpert Annual Amp-Hour Budget
For smaller vessels such as commercial workboats and pleasure craft with electric drives, PropExpert (2025 and later) includes a dedicated annual amp-hour demand calculation. This tool estimates total electrical energy consumption across a operati profile a useful input for battery bank selection and charging infrastructure planning.
Important Notes on Efficiency
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