Now the the heat over diesel/electric has died down it is time to talk about solar power for boat propulsion. I don't claim to be an expert on solar power although I did live for a week in a solar powered cabin in Vermont when a tree fall took down the powerline from the local electric utility. The cabin owner had installed a few sq. meters of solar panels in an attempt to live a truly green life style. We had electricity for lights, a small refrigerator, and the TV. That's about it. Thank God we had a wood stove. Most of my solar education came after New York State offered to share in the cost for homeowners willing to install solar panels for their home. We studied the literature, visited panel providers, talked to engineers and installers, and even had a Dept. of Energy solar engineer visit our house. In the end we declined because we calculated that we wouldn't live long enough to recover our share of the installation costs. Most of the information was applicable to marine power systems. Solar derived electrical power has come a long way since my aerospace days but it still has a long way to go. Before specifying a particular propulsion system we must calculate, or more likely estimate, the amount of power that is required at the propeller to move the boat at the speed we desire. The rule of thumb that a number of U.S. naval architects use is quite simple. To propel a relatively clean displacement craft, either trawler or sailboat, at hull speed in normal conditions requires one engine horsepower per 500 lbs. of displacement. This is a real world figure assuming driveline losses of 5% and a propeller efficiency of 50%. English naval architects tend to be more conservative of power and estimate that one horsepower per 1000 lb. of displacement is sufficient. Can solar power meet those requirements? Solar power for battery maintenance is one thing. Solar power to run all ship systems is another. The real problem with solar power is that it is delivered in real time. You can only get as much power out of a solar cell as the sun provides. It can be collected over a period of time and delivered in one short burst but the total can be no more than the amount of irradiation summed over the collection time. Diesel fuel too is concentrated solar power but the collection time is the life span of the organic material converted into fuel. Noon sunlight delivers about 1000 watts of power per square meter. Good commercial solar cells are about 15% to 20% efficient so you can extract approximately 150 watts per square meter. Because the solar panels are laid flat on the deck the output is always less than could be achieved if they were tilted directly toward the sun. Commercial quality solar cells cost about $5 per watt. More efficient solar cells which covert over 30 percent of ambient sunlight to electrical power are available but unless you have a NASA type budget you probably can't afford them. So the limiting factor for solar power, other than your pocket book, is the amount of unshaded level surface you have available. A typical 40 ft. trawler has about 50 sq. meters of deck area. If the entire deck is covered with solar cells the entire array will generate 7500 watts of power at high noon. That's almost exactly ten horsepower. In full sun, you will be able to generate full power for 5 to 6 hours a day. Add another 20% generating capacity for the morning and evening periods and you have accumulated about 80 horsepower hours per sunny day. Averaged over a 24 hour day, you would be able to support only about 2400 watts of continuous electrical load. If all the power was used for propulsion with no loses and 100% efficient batteries and motors, an entire day's solar power accumulation would be enough to power a 40 ft. trawler at hull speed for about 1.6 hours. At 3 knots you might power the boat for 16 hours. So how have solar powered boats managed to cross the Atlantic or cruise the Great Loop. First, they travel slowly, well under hull speed. One solar powered boat currently doing the Great Loop speeds along at 4.2 knots. It also cheats by topping off the house batteries at marinas. An Atlantic crossing power catamaran, widely covered in the marine press last year, rarely exceeded 6 knots. Boat shapes were chosen to accommodate large solar arrays. The Looper is a broad, shallow draft craft reminiscent of a river excursion boat. The power cat had a solar cell covered hard top that extended the full length and width of the boat for a total area of 167 sq. meters. It also used expensive 25% efficient solar cells. Power not used for propulsion in the daytime was stored in batteries for the night. On board systems require electrical power as well. To fully energize a well equipped trawler, refrigerator, air conditioner, TV, microwave, etc. would probably require about 20,000 watt hours over a 24 hour period. That might require 26 square meters of solar cells. And, of course, you would have to keep a backup generator for those days when the sun doesn't shine. Passage making sailboats sometimes use solar panels to generate all their onboard electricity but sailors have learned to drink warm beer. The conclusion is that it certainly possible to use solar cells to power a trawler. The present cost of solar propulsion equipment is about three times that of a standard diesel system but solar would deliver a good deal less performance. Of course solar would provide a considerable savings in fuel cost. Thanks to the mass production of residential and commercial solar cell arrays, the cost is rapidly declining. To put it into proper perspective, that solar powered 40 ft. trawler discussed earlier only collected power equivalent to 4.8 gallons of diesel fuel in an entire day. If your 40 foot trawler can cruise on less than five gallons of fuel a day then perhaps solar power is adequate for you. Solar power will require a revision of boating habits as we know them. Slow speeds and/or short trips will be the norm. Limits to available solar power will require boats with easily driven hulls. Power cats with hard top solar cell arrays are a preferred design. Solar power boating is essentially a fair weather sport requiring sunny days and calm seas. Given the requirement for full sunlight, boating in Maine or the Pacific Northwest might be difficult. But then there are always sails, the original solar powered propulsion system. Larry Z