“But tomorrow may rain, so I’ll follow the sun…” Paul McCartney sang in the 1964 Beatles song. That’s also what we hoped to do aboard Namani, our 1981 Dufour 35: to follow the sun by leaving temperate latitudes for the tropics in the fall, dropping out of the tropics with the onset of hurricane/cyclone season, and repeating the pattern the next season.
With that itinerary in mind we felt confident that our solar panels would have no trouble keeping up with our power consumption. They seemed a bit out of place in Maine, our U.S. home base, but surely they’d deliver ample power in the sunny tropics — right?
“Yes, but…” would be our answer, and the ‘but’ is a strong one. With the experience of three seasons in the tropics behind us now (first in the Caribbean, then in the South Pacific), we suggest that sailors look at the data before jumping to conclusions. You might be surprised to find that the same solar panel that delivers 100 Watt-hours (Wh) of electric energy per summer day in the Chesapeake Bay will only provide 60 Wh per day around Christmastime in the Virgin Islands.
Stern arch-mounted solar panels on a Tayana 37.
So let’s look at how the numbers that define effectiveness of photovoltaic solar panels play out for voyaging sailors. Whether you’re outfitting a boat for extended cruising or already sitting in an exotic anchorage, trying to squeeze out a few more amps to keep the beer cold, the data that follows will provide some helpful benchmarks for getting the most out of your onboard power plant.
We’ll start with a basic example to understand the variables at play for a boat spending the Northern Hemisphere summer in Chesapeake Bay (between 37° and 38° of latitude), then migrates to the Virgin Islands in November. After spending the winter months at about 18° northern latitude it returns to the Chesapeake at the onset of hurricane season in May.
If we ignore the impact of clouds for the moment, the parameters that will determine daily solar power output at a boat’s location are the length of the day and the sun’s altitude above the horizon. Solar panels receive maximum light energy when the sun is directly overhead (altitude = 90°). The light energy received by the panels drops as the sun’s position in the sky approaches the horizon. This drop is caused by light beams spreading over a larger area and by increased atmospheric absorption as the altitude of the sun decreases.
Figure 1 shows that we actually lose on both counts as we move into the tropics. The number of daylight hours drops from a peak of almost 15 hours during summer in the Chesapeake to an average of about 11.5 hours while we’re in the tropics. At the same time, the gap between our latitude and the sun’s declination widens as the sun crosses the equator on the way to its mid-winter position above 23° south. As a result, the sun will be significantly lower in the tropics than we are used to from our summers in the Chesapeake.
Fig. 1: A graphic representation of peak solar hours from Chesapeake Bay to the Virgin Islands and back again.
To evaluate the combined impact of these two effects on our solar power situation, we use a quantity that is sometimes referred to as “peak solar hours” (PSH). PSH tell us how many hours of direct overhead sunlight would provide the same solar energy as the slowly rising and setting sun at our location over the course of a day. PSH neatly boils down our solar situation to a single number. For example, in January in the Virgin Islands, we have the sun above the horizon for about 11 hours. As the sun moves across the sky during these 11 hours, it delivers the same amount of solar energy that a theoretical stationary sun directly overhead our position would provide in a little more than five hours — five PSH in this case.
It is this “sun vertically overhead” scenario that most of us have in mind when we talk about how many amps our solar panels deliver. It is also the scenario assumed in manufacturers output specifications for solar panels. With this in mind, PSH is what we should be looking at to realistically estimate the energy we can reap from our solar panels each day along our cruising itinerary.
This can be a bit sobering: if I see my solar panels delivering a charge current of 10A during the height of summer in the Chesapeake around noontime, I can assume only 5.3 PSH of that output over Christmas in the Virgin Islands. Take off another 20 percent to 30 percent for cloud cover and I’m at four PSH — on a sunny day and assuming no obstruction from rigging or shore structures.
Figure 2 shows daily PSH values over the year across a range of latitudes. If you know your energy needs during a 24-hour period at anchor, you can use the data from Figure 2 as a starting point for sizing your solar capacity.
For example, let’s assume a moderate energy consumption of 720 Wh (60Ah at 12V) per day that we hope to recover from solar panels. Given the PSH data from above (four PSH per day in the tropics after adjusting for cloud cover), we would need 720 Wh/4 h = 180W of power output from our solar panel(s). Manufacturers provide “Maximum Power Ratings” for their solar panels, but these are based on a maximum voltage higher than the typical battery charging voltage (about 13V on average). To account for this difference, we have to reduce the published Maximum Power Rating by about 20 percent to make it comparable to our calculated power requirement. Conversely, we can divide our calculated power requirement by 0.8 to compare it to the number in the panel specifications: 180W/0.8 = 225W.
Fig. 2: This graph shows peak solar hours over the course of the year for different latitudes. This shows the effect of the annual change of the sun’s declination.
That is a sizable footprint for a relatively modest daily energy consumption. It also assumes that we can mount the entire panel area free of obstructions from rigging and other above deck equipment — a challenge, especially on smaller boats. By way of comparison: we could get away with about 150W of panel power during summer in the Chesapeake Bay, reducing our footprint requirement by one-third (based on a typical PSH value of eight hours before cloud cover adjustments).
Angling for position
On a small boat it can be difficult to mount that much panel area free of obstructions. Any obstruction of direct sunlight markedly reduces a solar panel’s electrical current output. This would drive up our rated power requirement, further compounding the problem. What we can do is use the available area more efficiently by making the solar panels adjustable, allowing them to follow the sun during the day.
Figure 3 looks at the effect of flexible mounting during January in the tropics and July in the Chesapeake Bay. Let’s assume we can adjust the solar panels’ angle by up to 15° from a horizontal position. Many mounting options will actually allow a bigger adjustment. However, we’re unlikely to adjust the panel’s angle continuously during the day and most mechanisms will only allow adjustment around one axis. Therefore, we’ll stick with the conservative assumption of 15°.
The graphs in Figure 3 show a significant increase in the PSH value with flexible mounting: an adjustment of up to 15° can boost our sun intake by more than one-third (37 percent) in the tropics in January and by more than one-quarter (27 percent) in the Chesapeake during July. Rather than adding another panel in a sub-optimal position, we will often be better off putting our existing solar capacity (or at least a portion of it) on some kind of adjustable mount.
Fig. 3: A look at the impact of flexible mounts on peak solar hours in the Chesapeake Bay during northern summer and in the Virgin Islands during winter. Even as little as a 15° adjustment in panel angle can boost a panel’s energy output by up to 37 percent.
Some boats have panels mounted port and starboard on the stern pushpit where they can be gradually lowered from a horizontal position. Others have a stern arch construction that allows rotation of the panels around an axis that runs athwart ships. On smaller boats, the best option might be a mobile panel that can be moved around the deck and easily tied into an optimal position.
What about wind power?
The lack of moving parts makes solar panels a very attractive solution for voyagers — one less item that can incur mechanical failure in a remote location, and a silent one at that. Thanks to small but steady improvements, commercially available monocrystalline solar panels now approach 20 percent efficiency (the percentage of light energy hitting the panel that is actually converted into electrical energy). However, it seems the majority of cruising sailors still struggle to recover their onboard energy consumption exclusively from solar panels. Logically, one turns to wind to supplement solar power. Wind generators are still a common feature on many cruising sailboats, and may become the sole source of green energy during cloudy days.
As with solar power, one has to be realistic about what can be expected from an onboard wind generator. For example, in 2012, the NOAA weather station in Lime Tree Bay (St. Croix) recorded more than 60 days during which the sustained wind speed never got above 10 knots during the cruising season (November to April). Even Fajardo on Puerto Rico’s exposed east coast had more than 40 of those low-wind days, when wind generators would struggle to produce any meaningful output. While wind generators are great power producers when it blows, they are not a cure all. Sizing our solar capacity under realistic assumptions remains key if we want to reliably cover our onboard electricity needs without burning diesel.
A real-life example
Our 35-foot sloop Namani is probably a “low energy” boat by modern standards. We consume less than 700 Wh per day at anchor without restricting ourselves. We recover this consumption through a combination of solar panels and a convertible wind/tow generator. Our total solar capacity is about 150W rated power output from four panels. Of these four, two are permanently mounted above the dodger. The other two are movable around the deck so that we can keep them free of obstructions and angled towards the sun as the boat swings at anchor. Waterproof sockets at the bow and stern allow us to plug in these two mobile panels where we need them. This configuration allows us to fold back our bimini at night to enjoy a starry sky and also avoids the weight of a permanent stern arch aloft. For us, an ideal solution.
Underway the effectiveness of both solar panels and wind generators is greatly reduced. Sails will often shade the solar panels while apparent wind speeds on trade wind routes are too low to drive significant wind-powered output. That’s when we convert our Aquair wind/tow generator to its tow mode. It’s the perfect alternative for us on passages. The tow generator provides in excess of 1 kWh per day at our typical cruising speed (five knots on average), plenty to cover our modest power requirements underway.
At anchor, we convert the unit to wind mode and hoist it on the inner forestay. We have considered installing a permanently-mounted wind generator on a stern post, but eventually decided against it to avoid weight aloft as well as the expense. Now that our 10-year-old son can do the conversion from tow to wind mode and back, we don’t mind the extra effort anymore.
Fig. 4: A real-life example taken from Schweitzer’s cruising data from two and a half years of voyaging.
To pick up the “follow the sun” theme, Figure 4 shows our solar journey over the past two and a half years. Starting from Portland, Maine, in September of 2011, we sailed down the U.S. East Coast to Charleston, S.C., and then went offshore to Panama, with stopovers in the eastern Bahamas and in Jamaica. In 2012 we followed the Coconut Milk Run via the Galápagos, French Polynesia, the northern Cook Islands, Niue and Tonga. We spent the South Pacific cyclone season in New Zealand at about 35° south. Our second season in the South Pacific had us visit Fiji, Vanuatu and New Caledonia before dropping out of tropics again in November 2013 to spend the southern summer on Australia’s east coast.
Our experience along this route confirms the data from the theoretical example above: the times of abundant solar power for us were indeed outside the tropics — that is, in New Zealand and southeastern Australia during the Southern Hemisphere summer.
Ultimately, your best source of energy is conserving it. Take a critical look at what’s draining the amps on your boat and try to eliminate as many unnecessary consumers of power as you possibly can.
Markus Schweitzer, Nadine Slavinski and their son Nicky voyaged from Maine to the Caribbean and then across the Pacific on their 1981 Dufour 35 Namani.