Camp-Cruising is Where Solar Truly Shines.

Solar power is finally close to serving the needs of camp-cruisers

Article by Joe Grez

Way back in 2011 at the Port Townsend Wooden Boat Festival, I asked an exhibitor with his Nancy’s China about his electric auxiliary outboard. “Never again” was his answer. While he liked the advantages, his motor’s range was simply insufficient to support camp cruising. The earnest disappointment in his response, and what it meant for my fledgling electric outboard business stuck with me all these years; how can electric succeed if it can’t meet the needs of a small-boater? One answer is to focus on applications where a small battery can provide a 100% effective solution like ship to shore. The truth is that electric can do more but not yet; solutions to big problems, even those for small boats, take time to develop. Fourteen years later, I think we may be getting close.

After that conversation, I started taking a personal interest in using solar as a potential solution to the range problem. Anybody can plaster a hull with solar panels, use a huge battery and demonstrate a solar boat can move. But that’s not a functional solution. To me, the challenge was to formulate a system with a small array size, an affordable battery and motor system, and then find out how it could keep up with other boats of the same size and type. By 2013 I found myself supporting solar boat racing teams and saw that, at least with slippery hulls, it was indeed possible to travel on solar power at reasonable speeds. But what I really needed was proof that a typical hull could be powered by the sun without a massive solar array and battery. Modeling is the most powerful tool in the early stages of development, so I brushed off my physics and made a computer model to identify the performance limits of solar- powered electric boating. NOAA’s solar incidence model is able to predict the energy available to a horizontal solar panel any day of the year, each hour of the day, at any latitude. From there I added my own model of sky conditions and then overlaid a speed/ power curve for a boat system consisting of a hull, motor and battery storage. By 2018 I found solutions that show it is indeed possible to provide normal speeds and daily ranges for a small displacement boat using a reasonably sized solar array, a small electric outboard and its small battery. Many times in my life I have seen events, for some reason, line up to produce an opportunity—as if by magic. While the positive modeling conclusion was still fresh in my mind, I heard about the 2019 Salish 100 event and I knew it was time to put these events together.

Swe Pea's arrival on the PT maritime center beach, 2019.  The solar array had three positions- lowered aft it covered the aft cockpit just above the sheer.  Raised like this it allowed me to drive standing up and pivoted forward against the windshield top it provided some protection from the sun and spray when things got rough.

The model showed a solution that could work for my families’ 13-foot runabout Swe Pea. Swe Pea has a wooden (1960s) I-14 hull that is slippery but not too extreme to be considered normal for a small boat, especially at a cruising displacement appropriate for a week’s adventuring. The Salish 100 cruise is well planned for tides and ports of call with no one day’s planned distance exceeding 25 nm. The optimal solution for this trip was an EP Carry outboard package with its tiny 6-lb lithium battery, 12 square feet of solar panels and a solar charge controller, projecting a daily range of 35 nm when it’s sunny out.  Since this was the first time out, I did carry spare batteries just in case. The fleet started from Olympia and I stopped at Boston Harbor for ice cream and to drop off my daughter, who had come along for the start.  But even with that stop, I arrived at the first anchorage mid-fleet. I was hooked. 

With a battery that could provide only one hour of run time, I was cautious with the throttle on this first trip but I could not resist testing the maximum range prediction.  While the Salish fleet traveled up the main Salish channel from Blake Island to Kingston, I plotted a different course from Blake to Port Orchard, then up the west side of Bainbridge on a turning tide, through Agate pass, and then through Port Madison Bay and on to Kingston. The day started sunny and I tried to keep my battery above 80% so I traveled using sun power—at 7:00 a.m. I had 15W of power coming from the solar panels, which produced 2 knots of speed. Speeds increased as the sun climbed higher and I was traveling at 4 knots at mid-day.  The distance was ~30 nm—not quite the 35 nm target—but a surprise storm appeared from the north as I entered Port Madison Bay with strong headwinds and seas and I was happy for the reserve.  Spray from the bow soared over Swe Pea’s low windshield and she hardly made headway for portions of this leg. I had spare batteries but held off using them and by 7:00 p.m., Swe Pea landed in Kingston with overcast daylight to spare. Total run time for this trip was 9 hours for an average speed of 3.3 knots. As I entered Kingston harbor, other Salish boats who took the more direct course were also entering which demonstrated that a small solar micro-cruiser with a modest solar propulsion setup can keep up (on average) with similarly sized camp cruisers using sail, oars and gas auxiliary motors. But I would never take Swe Pea on a trip like that again.  For one thing, she was short on sleeping accommodations, and for another, she offered no protection from the nightly raccoon raids at Blake Island. So another microcruiser followed: Sunnyside. 

Sunnyside, a small cartoon of a flush deck Lake Union dreamboat, was built in 2020 on a cheekier, but similarly shaped 14’C-lark hull. Like Swe Pea, Sunnyside’s modeling accurately predicted ranges and speeds before she was built and like Swe Pea, Sunnyside travels at similar speeds to other 14 footers in the fleet. Unlike Swe Pea, she has ballast, enough foam to float her upright when swamped, a berth long enough for my 6’2” frame, and canvas that can fend off raccoons and rain. She also illustrates one key requirement for a long-distance solar boat: comfort. You will spend a lot of time enjoying the scenery pass. Sunnyside robbed Swe Pea of her solar panels and motor but we added another 6 sq ft of panels and reprogrammed her motor for higher power due to her much higher windage. Sunnyside has participated in the past four Salish 100 cruises, but last year’s Salish saw a harsh weather report for Point No Point and Foulweather Bluff so we bailed mid-trip. For this season, Sunnyside’s boxy cabin and high center of gravity have been lowered which helped her complete her 4^th Salish cruise. 

Sunnyside on trailer. Photo taken the morning before her Salish 100 trip in 2022.

If Sunnyside were an electric boat with no solar, and if the Salish 100 was a remote cruise without opportunity to recharge at a dock, we’d need a much larger battery bringing the system cost up to ~$6,000+.  But as a solar boat with no need to recharge at a dock, and with the same performance, her total solar-based propulsion system cost is $2640. This includes an EP Carry motor, battery, solar panels, charge controller, and remote steering / throttle, and the main battery also acts as a house bank for radio, nav lights and device charging.  The total weight of all these parts is 43 pounds—pretty much equal to the weight of a 3 hp gas outboard and spare gas can. But to create any gas-outboard-based system with similar functionality requires a larger motor with reverse, remote start, remote throttle, remote steering and alternator. Then you’d add a house/ starting battery, and gas tank. There are, of course, tradeoffs between a gas setup and solar. The cost of a combustion system is higher and top-end speed is also higher. But Sunnyside’s solar system provides 25 nm at 4 knots regardless of the sky conditions and up to 40 nm in a 10-hour day in clear conditions.  Today, and for most boaters, this tradeoff favors combustion. But are we “most boaters”?

This kind of capability has been possible for decades, but several factors have recently made solar propulsion more accessible to small boat owners. Previously, costs were higher, there was a lack of mainstream knowledge on how solar panels work, there were few choices for components needed to balance energy flows through the system, and the same for efficient motor control systems. So in the past, solar boats were too expensive, heavy, slow, and overburdened with solar panels to serve normal boating applications. But today, it’s possible for anybody to assemble a well-designed lightweight solar propulsion system for a small boat that’s fun to manage, looks good, and on a summer cruise consistently provides about 80% of hull speed for 10 hrs/ day. 

I mentioned the square footage of solar panels needed for realistic performance. As someone who favors small boats, this is for a good reason. It turns out small boats are much better candidates for solar propulsion—the larger the boat, the larger the percentage of solar coverage needed to achieve reasonable cruising speeds. This results from a special case of the square-cube scaling principle and can be demonstrated using my solar-modeling software. A 14-foot boat like Sunnyside needs a solar array area of 30% of her hull’s water-plane area to achieve cruising speeds of 80% of hull speed. If she were a 27-foot monohull, she would need 100% of her water plane’s area to provide the same 10 hours per day at 80% of hull speed.  Above 27 feet in length, it’s best to choose a multihull’s greater beam for additional solar panels. Just chalk this up as one more reason that small boats are better.

How does a solar boat work? Simply put: Solar panels collect energy during daylight hours that powers the motor, charges the battery, or both at the same time. Energy flow is automatic, making a solar-propelled boat simple enough for a novice to use. And even if you’re a little too throttle-happy and discharge your battery mid-day, solar continues to provide useful propulsion levels even when the battery is exhausted. For example, this graph shows real-time solar speeds throughout the day for Sunnyside without tapping her battery capacity. It assumes that the array is flat and unshaded. 

Though overcast power production is only ¼ or less vs. sunny conditions, boat hulls become a lot more efficient at lower propulsive power levels.  This results in a smaller speed reduction in overcast conditions than you might think.  Sunnyside’s max speed with our tiny EP Carry is 4 knots, which explains the full-sun curve flatness between 11:00 a.m. and 1:00 p.m..  At these times and conditions, Sunnyside can charge the battery while also traveling at full-speed.

You can see that on a sunny day at 7:00 a.m., Sunnyside can travel at 3 knots from sun power alone. If it’s fully socked in, solar speed drops to just under 2 knots. So unlike an electric or a gas powered boat, range anxiety with solar is based on the fear of going slower. It’s not about being dead in the water.

A quiet anchorage at Penrose showing Sunnyside's dinghy and rope steering arrangement.  Besides steering, this rope-pull direction also allows the skipper to raise and lower the motor by pulling on both lines.  This same motor has now pushed Swe Pea and Sunnyside over 700 nm.

Taking solar racing boats out of the mix, I think of practical small solar boats falling into three categories: slow cruisers, fast cruisers, and auxiliary sailboats. Here’s a description for these categories in sizes up to 25 feet.

• Slow cruisers like Sunnyside are budget cruisers with a modest array, small battery, low cost systems, and with accommodations focused on comfortable enjoyment of nature while traveling. These types require trip planing to leverage currents and do not lend themselves to tight arrival schedules due to the unpredictability of wind and sky conditions. Due to a small amount of storage capacity, these types can only use a large portion of the daily allotment of energy from the sun when traveling throughout the day at speeds that more-or-less match the rising and falling of solar power generation.

• Fast cruisers are designed for both hydrodynamic and aerodynamic efficiency; see S19C below.  They have a much larger solar array, more powerful propulsion systems and more battery capacity than slow cruisers so their systems cost can exceed the slow cruiser cost by 4x or more.  When used for cruising, fast cruisers can travel for 10 hrs in a day at hull speed or higher.

• Probably most potentially popular type of solar propelled boat is an auxiliary sailboat. These have a smaller array because of space limitations and most travel will be a mix of solar and wind, especially when motor sailing. The solar panel array rating needed for meaningful solar propulsion on a boat up to 16ft is 100-200W of solar—or 1-2 panels—each a little smaller than 2’ x 4’. The problem of where to put these panels remains a challenge but it is not unsolvable.

Today I am not aware of any ready-made solar camp cruisers to purchase. So for now, if you are interested in this boating style you will need to build it yourself. But before jumping into a build, you’ll need a design that provides the range and speeds that work with your style of cruising. To get there successfully, there’s simply no way to avoid doing some math. But for those willing to put the work in, this is the best option today. If you’re not ready to take on a project like this, I suspect that small turnkey solar cruisers will appear in a few years’ time.

My S19 concept mentioned in the article. This higher performance solar design is a little heavier than Sunnyside, but has a longer waterline, 4x the motor power and almost 3x the solar area. Modeling indicates 5.8 knots steady over 10 hrs on a sunny day. Its layout is similar to a small sailboat with a tiller (tiller not shown). 

The path to understanding the potential of solar propulsion has been fun and rewarding. While it’s not for every boater, I do think it’s finally close to serving the needs of camp-cruisers who value reliability, predictability and independence from liquid fuels. •SCA•

Joe is the founder of the EP Carry, a US-made outboard motor designed to provide lightweight, easy-to-use, yet rugged and survivable propulsion for small boats. His body of work highlights a commitment to leveraging technology to reduce environmental impact while improving simplicity, user experience, and functionality. Joe has a B.S, in Physics from West Chester University of Pennsylvania and has decades of experience as an innovation team manager with brands like Philips Sonicare and L’Oréal Colorsonic as well as with startups in marine technology and personal care products.