Practical sizing for electric boat propulsion
TMH 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.
Short briefings on marine batteries, motors, charging infrastructure, and propulsion technology — kept current so you're not relying on stale forum threads.
How the numbers are derived
Methodology
Estimated from waterline length, displacement, and target speed using a hull-resistance model calibrated against published curves for displacement sailboats and small power craft. Speed-vs-power graphs show your distance from hull speed and your headroom for current.
kWh = continuous power × runtime ÷ usable depth of discharge, with reserve subtracted explicitly. Ah is then derived from the chosen bus voltage. We default to LiFePO4 chemistry assumptions (80% DoD with 10–15% reserve) but every input is editable.
DC current is computed from continuous and peak motor power divided by nominal bus voltage and an inverter efficiency factor (90% by default). Phase RMS uses the standard 3-phase relationship plus a small motor-loss term. Cable cross-section is then computed from current, run length, and target voltage drop percentage.
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 blog and news content
- • 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
Get in touch
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. Open an issue or drop us a line.
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.