Solar dayboat completes 125-mile Thames cruise on sunshine alone, battery never below 60%

The claim put to the test

Tim Knox, who runs UK electric boat companies Mothership Marine and Torkmar Propulsion, set out to prove a point this summer: that a solar electric boat can cover a multi-day cruise without ever touching shore power. He and his wife Sue took their purpose-built solar dayboat, DB1, on a five-day holiday cruise along the 200 km (125 mile) navigable non-tidal Thames from Lechlade to Teddington.

The result: they never searched for or plugged into a shore outlet, and the battery never dropped below 60% state of charge for the entire trip.

Running hard into a drought, not around it

Even among members of the UK’s Electric Boat Association, where Knox serves as Executive Director, there was skepticism. Most thought seven days was a more realistic estimate than five. The summer then delivered a record-breaking heatwave and drought that made the job harder, not easier.

Dropping water levels forced the Environment Agency to restrict usage on some of the route’s 44 locks and introduce strict timetables. That meant the DB1 sometimes had to sprint between locks at double its normal speed and power draw. Headwinds added load, and a jammed gate at Whitchurch Lock cost roughly two hours on Day Three, forcing a fast run into Henley after dark. The boat still finished every day with charge in reserve.

Where the efficiency comes from

Two numbers explain the performance. The DB1 carries a 5 kWh lithium battery, and its solar canopy produces around 350 W in good sun. At cruising speed the panels can actually outpace the motor.

Knox’s own Torkmar motor is central to that balance. Designed for narrowboats, canalboats and inland dayboats, it delivers high torque so it can turn a large propeller at low RPM. That moves more water with less power at canal cruising speeds, which is precisely where the efficiency lives.

The figures from the trip bear this out. Normal cruising at 3.5 to 3.8 mph draws just 0.25 kW. Rushing for locks at 4.2 to 4.5 mph draws 0.5 kW, and a fast 4.8 mph run pulls 0.6 kW. On Day One the boat covered nearly 30 miles on 1.5 kWh, finishing at 70% state of charge. While underway to Oxford on Day Two, the solar canopy lifted the battery from 72% to 78%.

Each morning the battery was topped up by the rising sun during breakfast. Morning state of charge on Days Two through Five ran 72%, 75%, 60% and 69%.

A boat built to be stress-free

The design choices behind the DB1 are aimed at removing traditional boating hassles. A motorized hardtop canopy raises and lowers at the touch of a button, replacing canvas press-studs and zippers with a rigid, fully enclosed space.

For inland waterways, thick river weed is a fact of life, so the hull integrates a deck-accessible weed hatch. It lets an owner clear the propeller shaft in seconds without leaning over the stern or getting in the water. The dimensions, 5.8 m (19 ft) long with a 2 m (6 ft 7 in) beam, were chosen so two complete units fit inside a standard ISO shipping container for worldwide export.

The argument for solar-only cruising

Knox’s case is straightforward: a boat that can run indefinitely without charge points frees owners from relying on charging infrastructure that may be sparse or absent. He points to the UK’s 7,600 km (4,700 miles) of navigable canals and rivers, and notes that the Dutch-style dayboats currently in fashion are still mostly diesel.

“I’d like to reverse that trend,” he said.

There is a caveat worth stating plainly. These are inland-waterway numbers at low speeds and modest daily mileage, not a coastal or offshore benchmark. The 0.25 kW draw is for canal cruising, not open-water passages. But for the canals, rivers and day-boating use cases the DB1 targets, the Thames run is a credible real-world demonstration rather than a brochure claim.