Solar Cruising
Solar cruising—or “solar sailing” accord- ing to Joe Grez—is using solar energy and electric power as the primary mode of propulsion. While solar cruising is not new, it’s getting much more attention as a result of rapid developments in high-ef- ficiency electric motors and outboards, along with high-capacity/low-weight batteries, and better solar modules and battery charge controllers. Possibly also because of higher gas prices. And, per- haps, due to our aging bodies… Solar cruising boats tend to be long
and narrow, reminiscent of the low-resis- tance hulls originally developed for steam launches and for the early “one lunger” gas engines. Unlike the high-powered planing (and now foiling) electric boats that are headlining the power boat shows, solar cruising boats operate at less than the theoretical hull speed. And solar cruisers are typically capable of traveling considerable distances—as long as there is some sunlight. For example, Joe Grez, the founder of PropEle Electric Boat Motors (
www.electricpaddle.com), com- pleted the inaugural Salish 100 cruise in a salvaged Jet 14 hull powered by one of his EP Carry electric outboards and a couple of solar modules. Recently Doug Wade described building and operating his solarized 18-foot Nomad cabin cruiser in Small Craſt Advisor #120 (Nov/Dec 2019). And during the Covid summer of 2021, David and Alex Borton completed an epic 100% solar-powered trip up the Inside Passage to Alaska in their 27-foot Sam Devlin-designed “lake boat.” Since the Canadian border was closed, they were unable to land anywhere for the twenty days it took to cruise from Bell- ingham to Ketchikan. Wayward Sun was propelled by a 4-kW Torqeedo pod drive with 21 kilowatt-hours of lithium-ion batteries—and 1,700 watts of solar pan- els to charge the batteries. According to Alex, they had only two clear, sunny days during their voyage, yet they managed to
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Devlin’s long-distance solar cruiser, “Wayward Sun” (Solar Sal photo)
average 3.7 knots using whatever sunlight they got. Besides a slippery hull, lithium-ion
(Li-ion) batteries are the common de- nominator in today’s solar cruisers, and they have many advantages over tradi- tional deep-cycle lead-acid batteries. Li-ion batteries can be discharged nearly 100% without damage. They can be charged at least twice as fast as lead-acid batteries. Tey can accept all the power that solar modules produce; and they can accept a partial charge if the sun de- cides to be uncooperative. Tey do not need to be fully recharged or require a lead-acid battery’s current-tapering “ab- sorption” stage that wastes solar power; and they do not need to be float-charged during off-season storage. And they are lighter—a lot lighter! While more expensive than deep-discharge AGM batteries, prices for Li-ion batteries have been decreasing year by year, and if you aren’t in a big hurry to buy one, they will become even more cost-competitive as the supply chains recover and manu- facturing volumes skyrocket. Some of a Li-ion battery’s higher cost results from its sophisticated “battery management system” (BSM), which provides charge control and protection from unsafe usage conditions. And many Li-ion batteries now also include a Bluetooth connection for wireless real-time monitoring, several features that are rarely, if ever, found in a lead-acid battery. Most of the electric outboard suppliers
mentioned in my previous Tech Bights column sell proprietary Li-ion batteries designed to play nicely with their motors and controllers. For example, Torqee- do’s (
www.torqeedo.com) 3-kW Cruise outboards run on their 24V battery,
while the 6-, 10- and 12-kW motors use Torqeedo’s higher-capacity 48V battery (or two 24V batteries connected in series). Both batteries can be connected in parallel to increase storage capacity. Like the batteries in electric cars, where high output current, fast charging, and heat control are critical require- ments, Torqeedo designed theirs around hundreds of small, cylindrical NMC (nickel-manganese-cobalt oxide) Li-ion cells instead of “pouch” cells (like those used in computers) or higher-capacity “prismatic” cells. Torqeedo’s batteries all have internal thermal and overcurrent protection, and the packaging is rated IP67, that is, they are protected against dust, splash and even immersion. Te metrics used to compare different
battery technologies are based on energy density— specifically, “storage capacity per pound” and per cubic-foot—and bigger numbers are better. Torqeedo’s 24V battery has 3.5 kilowatt-hours (kWh) of storage capacity in a compact, 1.1 cubic-foot rectangular enclosure that weighs 56 pounds. The energy density works out to 63 watt-hours (Wh) per pound and 3.2 kWh per cubic foot. For comparison, the energy density of deep-cycle lead-acid batteries is only about 18 Wh per pound—or less than one-third, and around 2.9 kWh per cubic foot. One last metric is a battery’s cost per kilowatt-hour of storage capacity (that is, dollars per amp-hour rating times the voltage). At a list price of $3,000, Torqeedo’s 24V battery sells for $860 per kWh. For comparison, deep-cycle AGM batteries cost $250 to $300 per kWh. However, if you derate a lead-acid battery’s nameplate storage capacity to avoid reducing its working lifetime
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