Solar Boat

Two years ago when I bought my Hartley TS16 sail boat I dreamed of converting it to solar power. In January I installed a Torqueedo electric outboard and a 24V, 100AH Lithium battery back. That’s working really well. Next step was to work out a way to mount some surplus 200W solar panels on the boat. The idea is to (temporarily) detach the mast, and use the boat on the river Murray, a major river that passes within 100km of where I live in Adelaide, South Australia.

Over the last few weeks I worked with my friend Gary (VK5FGRY) to mount solar panels on the TS16. Gary designed and fabricated some legs from 40mm square aluminium:

With a matching rubber foot on each leg, the panels sit firmly on the gel coat of the boat, and are held down by ropes or octopus straps.

The panels maximum power point is at 28.5V (and 7.5A) which is close to the battery pack under charge (3.3*8 = 26.4V) so I decided to try a direct DC connection – no inverter or charger. I ran some tests in the back yard: each panel was delivering about 4A into the battery pack, and two in parallel delivered about 8A. I didn’t know solar panels could be connected in parallel, but happily this means I can keep my direct DC connection. Horizontal panels costs a few amps – a good example of why solar panels are usually angled at the sun. However the azimuth of the boat will be always changing so horizontal is the only choice. The panels are very sensitive to shadowing; a hand placed on a panel, or a small shadow is enough to drop the current to 0A. OK, so now I had a figure for panel output – about 4A from each panel.

This didn’t look promising. Based on my sea voyages with the Torqueedo, I estimated I would need 800W (about 30A) to maintain my target houseboat speed of 4 knots (7 km/hr); that’s 8 panels which won’t ft on my boat! However the current draw on the river might be different without tides, and waves, and I wasn’t sure exactly how many AH I would get over a day from the sun. Would trees on the river bank shadow the panels?

So it was off to Younghusband on the Murray, where our friend Chris (VK5CP) was hosting a bunch of Ham Radio guys for an extended Anzac day/holiday weekend. It’s Autumn here, with generally sunny days of about 23C. The sun is up from from 6:30am to 6pm.

Turns out that even with two panels – the solar boat was really practical! Over three days we made three trips of 2 hours each, at speeds of 3 to 4 knots, using only the panels for charging. Each day I took friends out, and they really loved it – so quiet and peaceful, and the river scenery is really nice.

After an afternoon cruise I would park the boat on the South side of the river to catch the morning sun, which in Autumn appears to the North here in Australia. I measured the panel current as 2A at 7am, 6A at 9am, 9A at 10am, and much to my surprise the pack was charged by 11am! In fact I had to disconnect the panels as the cell voltage was pushing over 4V.

On a typical run upriver we measured 700W = 4kt, 300W = 3.1kt, 150W = 2.5kt, and 8A into the panels in full sun. Panel current dropped to 2A with cloud which was a nasty surprise. We experienced no shadowing issues from trees. The best current we saw at about noon was 10A. We could boost the current by 2A by putting three guys on one side of the boat and tipping the entire boat (and solar panels) towards the sun!

Even partial input from solar can have a big impact. Lets say at 4 knots (30A) I can drive for 2 hours using 60% of my 100AH pack. If I back off the speed a little, so I’m drawing 20A, then 10A from the panels will extend my driving time to 6 hours.

I slept on the boat, and one night I found a paddle steamer (the Murray Princess) parked across the river from me, all lit up with fairy lights:

On our final adventure, my friend Darin (VK5IX) and I were entering Lake Carlet, when suddenly the prop hit something very hard, “crack crack crack”. My poor prop shaft was bent and my propeller is wobbling from side to side:

We gently e-motored back and actually recorded our best results – 3 knots on 300W, 10A from the panels, 10A to the motor.

With 4 panels I would have a very practical solar boat, capable of 4-6 hours cruising a day just on solar power. The 2 extra panels could be mounted as a canopy over the rear of the boat. I have an idea about an extended solar adventure of several days, for example 150km from Younghusband to Goolwa.

Reading Further

Engage the Silent Drive
Lithium Cell Amp Hour Tester and Electric Sailing