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For those who are thinking about using solar panels alone to power an electric catamaran across an ocean here's some basic physics that will always apply, regardless of the amount of wishful thinking involved:


Drag, and thus power required to travel at a certain speed, goes up by the cube of length. But area (and thus deck space on which to mount solar panels), only goes up by the square. Thus if you say "we need more panels" that will mean more deck area which implies a bigger boat. But that also means a heavier boat and thus it's a spiral that goes the wrong way.

Take a typical 10m long catamaran. Maybe it has a 5m beam and thus 50sqm of deck area. 1 sqm of solar panel is approximately 100W. So the maximum you can fit is 5000W


Conventionally powered you will need a 10hp engine to motor at 5-6 knots, or 7500W. But an electric motor offers more propulsive drive than a IC engine. So let's say you need 5000W electric motor to cruise at 6 knots.


Thus it seems you can run on solar power alone. However that is for 12hours of daylight only. So at night you will need to run on batteries. Let's assume you use a 48V motor. That will take 100 amps. So you need 1000AH battery of 48V. And you will need to use lithium batteries as they can be deeply discharged. Lead acid batteries must not go below 50% capacity


But if you run for over 24 hours you need to charge those batteries during the day. Thus your solar capacity must not only be enough to move the boat at 6 knots, but also to charge the batteries. So instead of 5000W of solar you really need 10000W, assuming you can charge at the same rate as you discharge.


And all that is assuming you are going downwind in a flat sea with bright sunshine all day, no dirt or shadows on the panels. And no losses due to wasted energy. Nor of course does it include any domestic use - typically you will need at least 500W solar for even the most basic cruising fit out.


Lets compare to electric cars for a moment. An average IC car has a range of about 400 miles before refueling. Which typically takes five minutes. Most electric cars have less range and take hours to recharge fully. OK, you can fast recharge in say 20 minutes, but that doesn't fully recharge the batteries, so your range is much reduced. You need to trickle charge to get back to full capacity. And that means plugging in at night or when you are at work.


We live in a 200 year old fisherman's cottage on the waterfront. So no road access, instead we have on street parking about 100m away. We also own three rental properties, all with on street parking. No one in power (pun intended) has explained how the majority of people who live in similar houses, or flats/apartment blocks, can charge their electric cars at night. One has to assume that they only think of their own situation of having with a garage and large driveway.


In that respect having an electric boat in a marina that is used for weekends actually makes more sense than having an electric car. Most marinas have shorepower at each berth and the week of recharging will ensure there is enough power for a weekend of cruising.


So cost out the motors, the solar panels, the charging system and the batteries. In addition, consider the recycling implications. Add them all up and maybe you will decide that learning to sail is a more sensible option for ocean cruising!