Practical sizing for electric boat propulsion
ElectricPropulsion.org is run by TMH and builds free, technically grounded tools and reference content for sailors and installers planning electric repowers. The goal is simple: give you credible numbers to walk into any conversation with a vendor, installer, or surveyor — without paywalls, accounts, or marketing detours.
What we build
The configurator and the content around it
Enter waterline length, displacement, target speed, range and charging — get battery kWh, DC and phase current, cable cross-section, and a vendor-ready PDF.
Articles on cable sizing, 48 V vs 96 V, BMS selection, saildrive vs shaft, range planning, and LiFePO4 in marine environments.
Tailored sizing pages for 30 / 35 / 40 ft sailboats, catamarans, small launches, and trawler hybrids — each one deep-links into the wizard with realistic defaults.
Net upfront cost, annual running-cost saving, and the year a repower breaks even — plotted as a cash curve you can run on your own fuel and electricity prices.
How the numbers are derived
Methodology
Power per tonne of displacement rises with your target speed as a fraction of hull speed (1.34 × √LWL in feet), steeply near hull speed. The curve is fitted to a Nordhavn 40's published fuel burn at 6–8 knots (converted at 0.27 L of diesel per kWh) and checked against other Nordhavn range figures and motor makers' sizing guidance — a calibrated estimate, not a tank test. Multihulls get 0.8×, and a saildrive adds about 4% loss over a shaft. Real boats vary by roughly ±25% with hull form, fouling, sea state, and propeller. Speed-vs-power graphs show your distance from hull speed and your headroom for current.
How accurate is it? The full check against Nordhavn's figures →Pack kWh = power × runtime ÷ drivetrain efficiency, plus a reserve on top, ÷ usable depth of discharge. Ah is then derived from the nominal bus voltage. Defaults follow LiFePO4 practice (80–85% usable DoD and 10–15% reserve depending on the range profile, 90% motor-plus-controller efficiency), and every input is editable.
DC current is power ÷ (minimum battery voltage × efficiency) — sized at the lowest pack voltage so the controller never starves — then ×1.25 for continuous duty, with fixed multipliers for 30 s and 10 s bursts. Phase RMS current is estimated as 1.25 × continuous DC current, a planning heuristic to confirm against the controller datasheet. Cable cross-section is the larger of what your target voltage drop and the calculator's own engine-space ampacity table require. The ampacity values and the 1.25 × fuse rule are conservative planning figures, not taken from a standard, so check them against the one that applies to your installation.
Outputs are guidance for project planning, not engineering specifications. Real installations need installer review against the actual motor controller datasheet, fuse class, and applicable marine standards (ABYC E-11, ISO 13297). The wizard flags conservative ranges where appropriate but does not replace a competent surveyor.
Why free
What costs money — and what doesn't
- • Running the configurator end-to-end
- • All wizard outputs (kWh, currents, cabling, charging)
- • Speed-vs-power and runtime visualisations
- • All reference guides
- • Per-segment sizing guides
- • Vendor-ready PDF you can share with installers and suppliers
- • Includes every computed value plus formulas
- • One-time payment, no subscription, no account
- • Optional — only if the PDF is genuinely useful for your project
Methodology
Found a bug? Disagree with a number?
We take corrections seriously — if you've repowered your boat and the wizard's estimate doesn't match your real-world numbers, that's exactly the feedback that improves the calibration.
Disclaimer: outputs are for project planning. Always confirm ratings against motor and BMS manufacturer datasheets and the applicable marine standards (ABYC E-11, ISO 13297) before commissioning a system.