Solar Cruising Part 3
Purpose-built low-power cruisers, multihulls, and DIY electric propulsion
My previous columns on solar electric cruising focused on adaptable monohull designs (and mainly those with readily available plans) and ready-to-buy electric outboards. The next two columns will focus on purpose-built low-power cruisers, multihulls, and DIY electric propulsion. And hopefully it will provoke even more discussion and debate about small boat “solar sailing.”
But first, since there are many questions and concerns about the real-world performance of electric outboards, I felt it would be useful to get a better idea of their potential. It turns out that the ePropulsion website has a collection of Performance Bulletins showing the speed and range for their electric outboards on a wide selection of boats, from an 8-foot inflatable dinghy to 32-foot catamarans. I chose several examples of boats with the best efficiency and plotted their power-versus-speed data, using semi-log paper. Power, in watts, ranges from low idle for the 1-kW “Spirit” up to full-power on the 6-kW “Navy” outboards (and in the case of a catamaran, two 3-kW outboards). Speed in knots is shown across the bottom axis. Notice that, instead of the typical power-speed curves where the power rapidly increases as the boat approaches “hull speed,” on my semi-log graph the data looks more linear. Just for comparison, I also added a theoretical speed-power curve, assuming that the power increases by the cube of the speed, as predicted by extensive empirical data.
Across all of the ePropulsion speed-power data, the most power-frugal boats are (of course!) sailboats. And not surprisingly, the smallest, lightest, and skinniest boats use the least amount of power. The slick-looking 20-foot Liteboat LiteXP20, weighing 330 pounds (dry) uses about 800 watts to reach 5 knots, and it easily beats the Nomad 17 sailboat (660 pounds, dry weight), which needs more propulsive power than its 1-kW outboard can generate to hit that speed. It’s also notable that the Olga 33 cruiser, with a displacement of 6 tons, only needs about 2 kW of power to reach 5 knots. Since the theoretical hull speed of a boat with a 30-foot waterline is 7.3 knots, the Olga 33’s boatspeed appears to be still following the cubic speed-power curve at its outboard’s maximum output of 6 kW.
But the overall winner for “most power efficient” electric outboard-powered boat (among those provided by ePropulsion) is a 14-foot fiberglass skiff called e.Balin Túra produced by Csonak Epito, a small boatbuilding company in Hungary.
The “Tura” dinghy was designed to be electrically propelled; and the hull shape looks like it would also make a good sailboat. Although the dry weight is about 600 pounds, the rated displacement is 1,300 pounds, more than enough for a lithium-ion battery bank and a small outboard. If the boat is lightly loaded, the length-to-beam ratio could be over four, and maybe even approaching five. The Tura’s power-speed follows the theoretical curve up to “hull speed,” about 4.5 knots. Beyond that, the power increases faster, and it’s a good demonstration that, for a displacement hull, more power just “digs a bigger hole” in the water beyond hull speed.
Previously, rather than fixating on the specifics of planing, semi-planing, and displacement hull design, I pointed to naval architects and boat builders like Thomas Firth Jones, who have an appreciation of boat design, history, and practical experience, for guidance on efficient, low-resistance boat designs. The design "number" that made the most intuitive sense is the hull's waterline length-to-beam ratio -- and this is the metric used by Weston Farmer, Phil Bolger, and Jones. And there are many monohull designs, older and perhaps less well known, that can speed along at more than theoretical “hull speed” while using relatively modest amounts of power. All of these boats tend to be long and narrow, with typical length-to-width ratios that are greater than five. More recently, Nigel Irens, famed multihull designer, focused on designing slippery, low-power “low-displacement/length” (LDL) monohulls. His explanation of what constitutes a power-frugal LDL hull is described in a decade-old article in Professional Boatbuilder magazine (link in Additional Resources, below). And he defines “LDL” to be when the unitless displacement-to-length ratio (D/L*) is less than 100. And where “...wavemaking drag starts to reduce significantly, and hybrid or electric power can potentially meet the propulsion needs.”
* D/L is calculated from the displacement (in long tons, 2,240 pounds) divided by the cube of 1/100th of the waterline length, that is (0.01 x LWL, in feet)**3.
Apparently Irens’ LDL concept is now gaining real traction, with recently published articles about his GB8 skiff in Professional BoatBuilder and in Woodenboat #289 (Nov/Dec 2022). By the way, Irens’ 24-foot LDL “yacht tender” launch called ELECTRA, mentioned in Part 2, scores a D/L ratio of 68 and can run at 16 knots – or more than twice “hull speed” – for nearly an hour using a pair of 12-kW electric motors and power from her lithium-ion battery bank. And it has a claimed range of up to 60 miles when throttled back to a cruising speed of 10 knots. Recently Irens formed Clara Boat, in Devon, England, to commercialize the LDL concept, and the website has a summary of the development of his “GB8,” an 8-meter LDL skiff (whose prototype was apparently called GRETA). It has a displacement of 1,825 pounds, and a D/L ratio that is incredibly low – less than 50.
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