Article by: Martin Henz, CSC member since 2009
July 8, 2025
Sailing in the doldrums
Just one degree north of the equator, Singapore lies in the Intertropical Convergence Zone, also known as “the doldrums”. Thankfully, from December to February, the Northeast Monsoon gives us reliable winds, and in July and August, the Southeast M Monsoon grazes the region, but the rest of the year is hit-and-miss for sailing. You spot some fleeting wind arrows on windy.com, get your sailing buddies and gear together and head out in the morning, only for the wind to die down and the blazing sun scorching your hopes for a day of sailing fun. Sounds familiar?
Sure, yachts have their engines, but those come with noise, grime, smell, and a carbon footprint that deter the next generation of tropical sailors from getting involved. What if we could use the power of that blazing sun to propel us quietly and cleanly when the wind dies down? Imagine solar-sailing whenever you choose, with no fuel costs, noise, or environmental impact!
Bo Bo Cha Cha
The title of my last article in Changi Log, in January 2015 (No. 25, 38-40) asked “Is a Combustion Engine Needed on a Motorised Sailing Yacht?” I described installing solar panels, batteries and an electric outboard motor on my Maxi 77, Bo Bo Cha Cha. To this day, she is Singapore’s only carbon-neutral motorized vessel. After a recent upgrade, Bo Bo Cha Cha has 1260 W of solar panels installed, which on a windless mid-day can get me to a speed of 4 knots without using the batteries at all. With a battery storage capacity of 7.5kWh on board, I can go without sun for more than 7 hours at 1kW and for more than 3.5 hours at 2 kW, the maximal power that my Torqeedo Cruise electric outboard motor can take. On a sunny day, I can go all day even without wind. The catch? The solar panels barely fit over the cockpit, held up by a bespoke stainless-steel construction. Handling and sailing performance is affected, of course, by almost 6 square meters of solar panels on a single-hulled pocket cruiser of 7.7m LOA…not to mention the looks of what some sailing friends call my “helicopter landing pad”.
Optimizing Wind and Solar Hybrid Propulsion
Yes, it is possible and practical to use solar power instead of a combustion engine to get around on a pocket cruiser in our tropical waters. However, with Bo Bo Cha Cha, I’m at the limit of what retrofitting an existing single hull sailing boat can achieve. Bigger and more modern yachts have relatively less space for solar panels, and their performance and esthetics will be even more severely affected. In “Solar-Powered Sailing Yachts in the Tropics” (with Jörg Weigl, Anand Sundaram, Evan Lowell, and Eric Yee, published by UMRAH Press in 2015), we showed that the larger the boat, the more difficult it is to use on-board solar panels to power it. Instead of retrofitting existing boats with solar panels, I’m now exploring new boat designs, optimized to combine wind and solar power.
The Solar Proa
Solar panels work best with direct sunlight; any shadow cast on the panels greatly reduces their efficiency. When we combine vertical sails with solar panels on a boat, we should avoid that the sails cast a shadow on the panels, regardless of our heading and where the sun and wind come from. With fixed masts and solar panels, I can only think of one good solution to this puzzle: Keep the sails on one side of the boat and the solar panels on the other and give bow and stern the same shape. When the sails cast a shadow on the panels in one direction, the boat can switch direction. The bow becomes the stern, the stern becomes the bow, and the shadow disappears.
Fortunately, we don’t have to reinvent the wheel for this. The Pacific proa is an outrigger sailing canoe whose mast is installed on the main hull and whose outrigger is kept on the windward side to minimize the drag created by the outrigger. When the wind or desired direction change, the proa can “shunt”. The crew brings the boat to a stop, and the voyage continues in the opposite direction. As a result, these boats don’t have a bow or stern. The crab claw sail rig of the Pacific proa is optimized for quick shunting maneuvers.
A typical Pacific proa; the outrigger on the windward side, main hull with a crab claw sail
For an optimal wind/solar boat, we simply need to modify the Pacific proa. A solar proa carries solar panels between the main hull and the outrigger and uses shunting to keep the outrigger on the sunny side, avoiding sail shadow on the panels regardless of the direction of the wind.
Building Roti Proa, the World’s First Solar Proa
I was fortunate to guide a team of students at the National University of Singapore (NUS) to build and test a solar proa from December 2024 to May 2025, and here is what we did.
We kept the construction as simple as possible and chose plywood as the building material, reinforced with pine wood and fiberglass, using epoxy glue and coats, and a polyurethane finish. Here are a couple of photos of the construction process.
Main hull (on the left) started; outrigger (on the right): in progress
The hulls completed and painted, outrigger in front; main hull behind
The power management system was the master’s project by my student Loick Le Goff, who tested his setup on the roof of the School of Computing at NUS.
Testing the solar panels on the rooftop of NUS School of Computing
Loick clinched the Best Project award for his work at the STePS student project showcase at the School of Computing, NUS, on April 16, 2025.
Loick’s power management system at the student project showcase STePS on April 16, 2025, School of Computing, NUS
We had our first flotation test at Changi Sailing Club in early May and obtained permission from MPA to conduct endurance and system tests along the floating barrier near Tampines in the last week of May 2025.
MPA-sanctioned testing along the floating barrier near Tampines (blue circle: Changi Sailing Club)
Our endurance tests showed a maximal speed of 6.7 knots with no wind and current, reached at 1.5 kW from the batteries. The solar panels allow the boat to move all day even without using the batteries, at a speed of around 4 knots in the bright mid-day sun. The batteries alone (no sun, no wind) provide a range of 13 Nautical Miles at a cruising speed of 5.4 knots. The picture shows helmsman Abdullah Farid near Changi Sailing Club.
Helmsman Abdullah Farid solar-sailing Roti Proa, with two Optimist rigs from Changi Sailing Club
Energy output from the two solar panels in the late afternoon, enough to reach 3 knots without using the batteries
Roti Proa Data Sheet
LOA: 4.2m, beam: 2.5m
Displacement with solar panels, batteries and motor, without helmsman: 250 kg
Rig: two Optimist masts and sails
Motor: electric outboard (Torqeedo Cruise 2.0) 2 kW
Batteries: two LiFePO4 batteries (Manly) with 24 V and 50Ah each
Solar panels: two 24 V monocrystalline panels (Trina Vertex) 415 W each
Cruising speed at 1 kW (no wind): 5.4 knots
Maximal speed, reached at 1.5 kW (no wind): 6.7 knots
Battery range (no wind, no sun) at 5.4 knots: 13 NM
Acknowledgments
We thank our project sponsors:
- Maybank Green Fund (through NUS Office of Student Affairs) for funding all materials and consumables for building the hulls
- Rezeca Renewables for contributing the two solar panels
- Ascent Movers for transporting the solar panels to Changi Sailing Club
- Changi Sailing Club for providing the safety boat and crew, the Optimist rigs and space for boat assembly
What Next?
We hope to double this design and build an 8.4 m solar proa, following the same principles. We think we can achieve a speed of more than 10 knots and a battery range of 20 NM (no wind, no sun). The boat will have two small cabins for voyages of a few days with two or three sailors. With an electric water maker on board, such a solar proa could go a long way without any fuel, even in the doldrums!
Let me know if you want to get involved!
