Changes

* Solar Impulse HB-S1A airplane left on a transcontinental flight acorss the USA on May 42013.

There are only two possible renewals for powered boats:

According to NASA, the earth's surface receives a maximum of 137 Watts per square meter (W/m2m²) at noon at the equator when the sun is perpendicular. At other latittudes and sun angles the energy is less.

In practice, for an 8-hour summer day, 40 degree latitude, the sun delivers an average 600 W/m2m². Currently the best commercial solar panels are ~16% efficient. At an efficiency of ~16% on a perfect sun day this is a yield of 96 W/m2 m² averaged over 8 hours (0.768 kWh).

Now, metric horsepower, widely used in the auto industry, is defined as 0.73549875 kilowatt (kW). Assuming we need 200 horsepower (hp) to drive a 70-ft boat, this is ~147kW. At 96 W/m2 m² this equals 1,531 m2 m² (16,472 ft2) of panel acreage [1 m2 m² = 10.76 ft2ft²]. And this is just for propulsion during daylight hours. There is no extra for storage in a battery for night operations.

The highest density commercial solar cell has a capacity of 174W/m2 m² or ~16W/ft2ft². Assume this is feasible output, even though we only receive 137 W/m2 m² at the equator and 96 W/m2 at 40 degrees latitude. For our 147-kW motor we would need 845 m2 m² (9,092 ft2ft²) of panels. At most a 70-ft yacht would have less than 1,000 ft2 ft² of surface area, so you can see why most boats using solar have hybrid diesel-electric propulsion systems.

The huge carbon-fibre catamaran PlanetSolar MS Tûranor has 537 m2 m² (5,780 ft2ft²) of 38,000 photovoltaic panels with an 18.8% yield. These are mounted on deck and on large fold-out wings [http://www.planetsolar.org/]. The panels feed six blocks of lithium-ion batteries, like those used in the Boeing Dreamliner. The reported maximum daily yield during one 24-hour period was 661 kWh. According to the logs, recharging each day typically took until noon. Build cost was of the order of USD $16 million.

The Solar Impulse HB-S1A airplane can carry only the pilot. Using high-technology materials it weighs 3,527 lbs and has a gigantic wing span of 208 ft -- as much as an Airbus A340 or a Boeing 747. It has four 10-hp electric motors driving propellers. These are powered by 11,628 monocrystallane solar cells spread across every available surface. "With 200m² of photovoltaic cells and a 12 % total efficiency of the propulsion chain, the plane’s motors achieve an average power of 8 HP or 6kW," according to the design team. So if we had 100% efficiency the plane would have 67 hp average available. Build cost is reported to be 90 million Euros (USD $118 million).