Battery Range Explained: How to Read an Electric Boat's Spec Sheet

The single most misread number on any electric boat spec sheet

If you ask ten electric boat dealers what the "range" of their boat is, you will get ten different answers. All of them are technically correct. None of them tell you the full story. This guide is the decoder ring.

Range on an electric boat is not a fixed number — it is a curve that depends on speed, sea state, load, and the state-of-health of the battery pack. A manufacturer-published "60 nautical mile range" is almost always measured at the most flattering point on that curve: the "economical cruise speed" for a displacement hull, or the optimal cruise band for a planing or hydrofoil hull. Real-world use rarely sits exactly there.

Understanding why will let you compare two boats apples-to-apples instead of being swayed by whoever quotes the bigger number.

The physics: why range collapses with speed

Water resistance on a boat hull scales roughly with the cube of speed. Double your speed, eight times the power. This is the hard physical ceiling that no amount of clever engineering can avoid for displacement hulls. If a 60 kWh displacement catamaran does 60 nm at 6 knots, it will do much less than 30 nm at 12 knots — and it probably cannot even reach 12 knots sustained.

Planing hulls work differently. Below planing speed, they are very inefficient (high drag, pushing water aside). Above planing speed, they are in a much more efficient regime (riding on top of the water). The curve has a minimum-drag speed — typically somewhere between 15 and 25 knots — where the hull is most efficient. Range is highest there, not at lower speeds.

Hydrofoils rewrite the curve entirely. Below foiling speed (typically 15–18 knots), a hydrofoil behaves like an inefficient planing hull. Once foiling, hull drag drops to near-zero and only the foils themselves resist motion — a 70% drag reduction compared to the equivalent planing hull. The Candela C-8 is the poster child: it cruises at 20+ knots on foils for 50 nautical miles while most competing day-cruisers achieve similar range at half the speed.

Once you understand which hull type a boat uses, you can read the range number correctly.

Reading the manufacturer spec sheet honestly

Every electric boat dealer quotes range at the speed that flatters the product the most. For a displacement cat it is the "eco cruise" speed. For a planing day-cruiser it is the minimum-drag speed. For a hydrofoil it is the foiling cruise. All of these are real, honest numbers; they just do not all describe the same scenario.

A professional spec sheet will include a speed-vs-range table — multiple data points covering displacement, planing (or foiling), and wide-open throttle. If the dealer cannot produce this table, ask for it. Do not accept a single-point quote for something that varies by a factor of 3–4 across the speed range.

When comparing two boats, always align on the same speed. If Boat A quotes 50 nm at 8 knots and Boat B quotes 40 nm at 20 knots, you are not comparing them — you are comparing two different use cases. Decide which speed matters to your usage first, then compare.

Battery health and capacity fade

Lithium batteries lose capacity as they age. A well-managed marine battery pack will retain 80–85% of original capacity after 1,000 cycles, or roughly 5–8 years of average private-owner use. Commercial charter use ages packs faster because the duty cycle is harder.

This matters for long-term range planning. If you are buying a 10-year-old used electric boat, the pack is likely at 70–75% of original capacity — so the "60 nm range" on the spec sheet is realistically 40–45 nm. This is not a defect; it is the physics of lithium chemistry. Check the state-of-health (SoH) reported by the battery management system before you buy used, and factor it into your range estimates.

Most modern battery management systems expose SoH clearly. Older packs and some low-end systems do not. Walk away from any used boat where the dealer cannot show you the current pack health.

Charging efficiency: the range number hiding on the other side of the socket

A less-discussed number: charging losses. When you put 30 kWh into the battery from shore power, you consume 32–34 kWh at the pedestal. The extra 2–4 kWh dissipates as heat in the charger and inverter. This matters for cost calculations but also for real-world range planning — you cannot plan your day assuming lossless energy flow.

For DC fast charging, losses are lower (around 3–5%). For slow AC charging at lower voltages, losses can be higher (8–12% in worst-case cheap chargers). If you are choosing a boat for minimum per-mile cost, charger efficiency is worth asking about.

Practical range planning: a working framework

Here is a simple mental model that will keep you honest about real trip planning on an electric boat:

1. Take the manufacturer's cruise-speed range and subtract 20%. This accounts for sea state, wind, passenger load, parasitic loads (fridge, lights, instruments), and battery pack aging.

2. Plan the day to return with 20% reserve. This is the equivalent of not running a car to empty.

3. Identify two backup charging locations within your planned radius. Even if you do not need them, knowing they exist changes how far you are willing to go on a given day.

4. Match the speed to the trip. If you have 30 nautical miles to cover and all day to do it, 7 knots is comfortable and will use roughly a quarter of a large battery's daily capacity. If you need to be there in an hour and it is 30 nm, you are planning to arrive with very little reserve — do the math before you commit.

5. Reconfirm the numbers annually. Battery health changes. Your personal driving style changes. Do not plan a four-year-old boat's day the same way you planned its maiden voyage.

A worked example

Consider a Soel Senses 62 — a solar-electric catamaran with a 252 kWh pack. Manufacturer range at 5-knot cruise: 100+ nautical miles. At 10 knots: roughly 30 nautical miles. At wide-open throttle (12+ knots): 15 nautical miles.

A practical day plan for this boat might be: 30 nm at 7 knots for 4 hours 20 minutes — consuming roughly 40% of usable capacity — returning with substantial reserve. Attempting the same 30 nm at 11 knots consumes close to 90% of usable capacity, which is technically feasible but leaves no margin for a headwind or a detour. The same 30 miles becomes a totally different day at the two speeds.

Good electric boating is about making these speed/range trade-offs consciously, not by accident.

Environmental factors that eat into range

Range degrades with conditions buyers rarely ask about upfront. A 20-knot headwind can cost 15–25% of cruising range on a planing hull; a 30-knot one makes the trip untenable for an electric boat that was quoted at calm-water conditions. Choppy seas (half-metre or more) similarly cost 10–20% at cruise as the hull slams through waves. Warm water carries slightly less density than cold, marginally helping the prop; cold water slightly hurts. These effects compound in shoulder-season cruising in exposed waters.

Passenger load matters less than new owners fear. A fully-loaded six-person cruise typically costs 3–5% of range — well within the margins you have already built into your planning. Heavier loads (full fuel-equivalent, water tanks, cargo) degrade range proportionally; this matters more on liveaboard catamarans than on day-cruisers.

Battery chemistry and what it means for range

The two chemistries you will encounter on modern electric boats are LFP (lithium iron phosphate) and NMC (nickel manganese cobalt). LFP offers longer cycle life (2,000–3,000+ cycles to 80% capacity), better thermal stability, and a flatter discharge curve. NMC offers higher energy density per kilogram (typically 20–30% more range per battery weight) at the cost of shorter cycle life (1,000–1,500 cycles) and stricter thermal limits.

Most production electric boats favour LFP in 2026 — the weight penalty is acceptable on a boat, the thermal stability is a safety win, and the 3,000-cycle lifespan exceeds most owners' use horizons. Hydrofoils and performance boats where weight matters more sometimes favour NMC. If you are comparing models, note the chemistry; it affects resale value, battery replacement cost, and real-world calendar life.

Closing thought

Range on an electric boat is not a number. It is a curve. Once you learn to read the curve — to understand that speed, hull type, load, sea state, and battery age all shape how far a given battery will take you — you stop comparing boats by a single spec-sheet number and start comparing them by how well they serve the way you actually use water. That is the mental model every successful electric boat owner eventually develops.

Come back to your own spec sheet after a full season of use. The numbers that looked abstract at the boat show will have real meaning once you have lived with them — your boat's real-world curve will be slightly different from the brochure curve, and you will know exactly how. At that point you are a confident electric boat owner, and you can help the next buyer interpret the same numbers correctly.