The expansive deck of an 8m Catalac catamaran proves to be a huge asset for Simon Milward’s solar electric propulsion project


I’ve always been environmentally conscious and with an increasing concern about climate change, converting my new-to-me sailboat to fully electric propulsion was an idea that just wouldn’t leave my mind.

Added to this I used to run a small solar panel company and seven years ago was awarded the Guinness World Record for the fastest solar-powered crossing of the English Channel [Full story coming soon – Ed.]

This was on Dart 15 beach catamaran hulls covered in over 100kg of rigid solar panels connected directly to electric motors with no heavy batteries to slow it down.


Simon passing out of the Traffic Separation Scheme on his record breaking cross channel run in a somewhat smaller catamaran

Boat choice

When I was able to buy my first proper sailing boat for our family three years ago it made sense to try to go electric as quickly as possible.

I was a bit disappointed at how long this took. Nevertheless, apart from a little bit at the start of last summer, I’ve now been fully electric for nearly a year.

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I hope that by sharing my experience, particularly of the advantages of solar-powered electric motors, I can help others work out what could be best for them and help them achieve that as quickly, easily and economically as possible.

Our boat Chateau Cat is a ‘3-tonne’ 8m Catalac and weighs around four tonnes fully loaded. The crew mainly consists of my wife and our three children. My dad and brother also sometimes swap in and out, particularly for longer passages like crossing to and from The Netherlands.

As the world record passage showed, catamarans are a justifiably popular choice for solar electrification on account of the large deck area, perfect for mounting multiple solar panels. The normal engine set up for an 8m Catalac is either a centrally mounted 9.9hp outboard or twin inboard diesels, one in each hull.


Chateau Cat employs a mixture of fixed and moveable solar panels

I bought one with an outboard thinking that would be easier to convert, although in reality it has not made a difference as I’ve mainly used an outboard bracket mounted on the port transom (sometimes supplemented with another electric outboard on the starboard transom).

In fact, in many ways I may have preferred the opportunity to easily convert to inboard electric engines.

First steps

I started off three years ago with one 800W 24V and one 400W 12V Torqeedo motor, both second-hand. These were around five years old and I ran them off three 110Ah 12V lead acid leisure batteries (two in series for the 800W motor and a single one for the 400W).


Electric outboard mounted on the port transom

Together this gave the equivalent of a normal 3hp motor. Not very much for a 4-tonne catamaran although I reassured myself there should be lots of ‘torque’.

For charging I had seven 110W semi-flexible boat solar panels permanently mounted on plywood panels fixed above the doghouse under the boom and up to 12 more 110W roving panels that could be moved around the boat to wherever there was most sun.

I figured this amount of solar would be plenty for all my needs and much lighter than rigid panels. I could then upgrade motors and batteries once I knew what I’d need.


Chateau Cat’s 19 110W solar panels laid out

Frying motors

Within the first week I’d cooked the 800W Torqeedo’s circuit board by running it for long periods on full power.  I bought a second-hand replacement shaft and bulb for it which my dad brought down when he met us in Studland Bay.

This worked quite well for the next three weeks with careful use and we did a number of trips along the south coast and over to France without using the backup petrol motor until the replacement also cooked when running constantly on low power overnight going from Boulogne to Ramsgate.

While rather disappointed, I’d learned a lot from this first year. I had plenty of solar power and just needed more reliable (and preferably more powerful) motors and also a bigger battery.

‘I had plenty of solar power – I just needed more reliable and powerful motors’

The problem is motors are expensive. So I looked for second-hand electric outboards throughout the winter. I ended up only finding another 800W Torqeedo, but my dad did manage to resurrect one of the other 800W Torqeedos by wiring up an external controller.

And I also managed to get a 24V (18.5-25.2V) 5.3kWh lithium-ion Tesla battery from a crashed Model S Tesla (via eBay). This weighs only around 30kg but has about four times the capacity of each 110Ah 12V lead acid battery and about seven times the regularly usable capacity.

Battery care

I’ve since had lots of advice that lithium-ion batteries can go into thermal runaway and combust if overcharged, corroded by salt water or punctured. I’m therefore very careful with mine. I keep it in a waterproof box within a dry locker and am very careful not to overcharge it.


Arm attaches the motor to the rudder, aiding manoeuvrability

Still, when I buy a new battery I’ll probably choose a LiFePO4 lithium battery that has similar energy density but is less prone to thermal runaway and that would give me a bit more peace of mind.

Over the winter I also fitted a rigid 220W 24V solar panel to davits at the back of the boat (one of the panels I’d originally used to win the Guinness World Record). This was because I’d found it much more convenient to have permanently mounted panels than to have to securely rig the movable panels while sailing.

The next year, despite Covid regulations, we were able to sail to Denmark via the Frisian Islands. I now had two 800W Torqeedos powered by a 5.3kWh Tesla battery and a 400W Torqeedo (with 12V leisure battery) in reserve. I was using up to 660W of fixed solar panels and had many more movable ones.


Solar panels mounted on boards are ‘portable’ to be set up wherever the sun is strongest

Unfortunately I cooked one of the 800W motors leaving Lowestoft, speeding to make an ad hoc early morning opening of the lifting bridge. But I then continued for the rest of the holiday experimenting further with my 400W connected to a leisure battery and running the 800W (generally not past half power) off the Tesla Battery.

I never ran short of battery power given that the 5.3kWh Tesla can output 400W for over 12 hours and I never needed to use the movable solar panels as the fixed panels gave plenty of solar charging for such small motors.

I also learned that just 200-400W of electric motor allowed us to sail around 10° closer to the wind in Force 3-4 – and helped even more in lighter winds.


The 2kW electric motor is fixed in the ahead position when motoring out at sea

This was because it reduced our side-slip. It also helped us move around a knot faster in light winds, which was particularly useful when crossing shipping lanes and making tidal windows when sailing between the islands.

Nevertheless, we still often used the 9.9hp petrol motor, particularly in the strong currents around the islands, although, when entering the Thames Estuary after crossing back from Holland to England in a northerly gale, I also found the limit of the petrol motor when even at full power we couldn’t make headway into a wind of about 30 knots and rough seas.

We eventually ended up sailing in through a southern channel as the wind continued to ease.

Maturing technology

After our second season we now had plenty of solar panels and quite a lot of battery storage. But we clearly needed more powerful, reliable motors. Over the next winter I scoured websites for second-hand electric outboards. I didn’t find any and ended up buying a new 2kW 24V Torqeedo, said to be equivalent to about 5hp.

This was by far the most expensive part of my setup. But I was pretty confident it would be worth it and hoped I wouldn’t need to use the petrol motor ever again.

For this third summer I kept the 220W of fixed rigid solar panel mounted on the davits and the 440W of fixed flexible solar panels mounted under the boom on the dog house. I still had up to 1,320W of moveable, flexible, solar panels and planned to use these when needed.


Cheateau Cat‘s Tesla battery cells

I attached my new 2kW 24V motor to the 5.3kWh Tesla battery and mounted it on the port transom so that I could either attach it via an arm to the rudder to aid manoeuvrability or fix it in a straight ahead position when out at sea.

I also had the remaining 800W 24V motor but I didn’t attach this immediately as I thought I might not need it.

Range anxiety

Our first passage started at 0400 in the dark, electric motoring out of Swale into the Thames Estuary, heading for Pin Mill. As dawn broke the wind picked up and we started sailing. But when the wind dropped off in the middle of the day I boosted our speed using the electric motor.


Chateau Cat is used as a family cruising yacht

However, I had quite a lot of range anxiety doing this as at 1,000W I would only have about four hours of running time and I was very worried about going down even to 50% charge.

Fortunately the wind picked up in the afternoon and we ended up making good time to Felixstowe, only using the electric motor again for the last few miles up the Orwell to Pin Mill. We ended with about 25% charge in the battery.

And the new 2kW Torqeedo was a joy to use, particularly compared to the older Torqeedos. Our ground speed, voltage and power drain were all shown clearly on the remote throttle. The control, particularly at low power, was amazing. Still, I thought, they should be pretty good given how much they cost.

We solar recharged the batteries for a couple of days at Pin Mill but couldn’t wait around for long because we were meeting my wife in Newcastle in just over a week.

Nauseous re-routing

So we set off towards Newcastle one evening. After about 18 hours the wind and waves picked up off the north coast of Norfolk and the children started feeling sick. I was all for continuing up towards the Humber, but my dad was concerned for his grandchildren so we headed into the wind for Wells-next-the-Sea.

We still had plenty of battery charge. However, given the children’s sickness and my dad’s complaints, I used the petrol motor to get us there more quickly and started to wonder if the electric motor was going to be worth it.

We recharged for a few days in Wells, waiting for good winds to go to Newcastle. I also put the 800W motor in place. Late one evening everything looked good and near the end of a rising tide we electric-motored out and across the bar. The wind was forecast to pick up later that night and be good for a couple of days.


We kept using the 2kW electric motor until night fell and the wind picked up. We then alternated between sailing and using the 2kW motor on low power for the next 32 hours, particularly benefitting from using the motor to cross the shipping lanes coming out of Middlesbrough and Sunderland when the winds dropped during the second night.

As dawn broke after the second night, the electric motor helped us stay close enough to the wind to sail right up to the entrance of Newcastle harbour. Just as the sun was rising I took the sails down and we electric-motored into the wind and the lightly ebbing tide to anchor off South Shields beach.

As I tidied up, a fisherman checking his pots came by and asked if we wanted to buy any fresh crabs. He seemed a bit nonplussed when I declined, excusing myself by explaining I was trying to become a vegetarian. Maybe it was my accent.

Key stats

This extended trip had proved to be an informative fact finder, and I was able to bank the following useful information:

  • In calm seas, with little wind we could make: 2 knots with 400W, 3 knots with 800W and 4 knots with 1,500W. Subsequently, I now know on a dead calm surface we can make 5 knots with 2,500W.
  • A 10 knot adverse apparent wind slows us down by about 1 knot.
  • Smooth seas vs calm reduces speed by about 25%.\
  • 400W of motor when close-hauled in light winds and slight or smooth sea enables us to sail around 10° closer to the wind and adds 1-1.5 knots of speed up to about 4 knots.

In Newcastle we swapped my dad for my 5-months-pregnant wife and the four and a half of us set off for Holy Island. Here I started to appreciate how the electric motor could really help us to speed up our passage.

We started at about 0130 with almost no wind so kept the electric motor on for the first few hours. Winds stayed quite light throughout the day and when we seemed at risk of missing the tide and getting a strong adverse current past the Inner Farne Islands, I turned on the electric motor for a few hours to make sure we would make the window.

We were at less than 50% capacity in the Tesla battery. But by now I was quite happy because I knew I wouldn’t need more than that 50% charge to get to Holy Island and also the sun was out meaning we were charging the batteries at 150-300W (although of course knowing I had the backup of the petrol motor also helped).

The electric motor is almost silent and turns on or off simply by pushing the throttle. Somehow, by not having to start it, having almost no sound or vibration, no fumes and knowing you’re not producing CO2 and using up petrol that’s going to need to be replaced, this means you don’t disrupt the equanimity of sailing. I therefore felt much happier to turn it on at low power to give that extra push.

We ended up making the tide window and as we passed the Inner Farne Islands the wind picked up. For the final entry into the harbour I took the sails down and used the electric motor to head dead into a wind gusting to 20 knots. At 1,500W of power, in calm seas we still made about 2.5 knots.

Whale tale

We continued in a similar way to Inverness via Stonehaven, Peterhead and Whitehills. I became better and better at using the electric motor to make tide windows and we passed Rattray Head at exactly slack water.

However, my wife – by now 6 months pregnant – still felt sick in the swell and ended up taking the bus from Whitehills to Inverness with the children the next day. This meant I did the final 70 miles from Whitehills to Inverness solo, seeing dolphins on the way and even the distant tail of a whale.

At Inverness we entered the Caledonian Canal and crossed through the whole length to Fort William, using only the electric motors, as well as sailing on some of the larger lochs.


Silent propulsion makes it easier to get close to marine mammals without spooking them

Near the end of the holiday we went back up the canal to moor for the winter. At dawn on Loch Lochy, with no wind and flat water, I maxed out the 2kW engine (which actually runs at up to about 2.1kW) and also ran the 800W engine at about 400W. We made 5 knots under electric power only.

At the end of the holiday I realised I’d fully weaned myself off the petrol motor and decided that two of the 2kW engines (effectively 10hp) would be perfect, particularly alongside 880W of fixed panels, 440W of moveable panels and twice my current 5.3kWh battery capacity.

For this summer I’ll put in place the 880W fixed solar panels and keep the 440W of moveable panels. I’m also keeping a look out for another 2kW engine. Although if I don’t get one I think I’ll be quite happy just to continue this year with my total of about 2,800W.

I will also get more batteries at some point, probably LiFePO4. But given the price is falling every year and the energy density increasing (I’ve heard by an average of around 8% per year).

I think I’ll hold off another year – especially as this year we’ll probably just be pottering around the Hebrides with our new baby. But then next year I’m hoping to sail to the Faroe Islands

How electric motors changed my life

  1. For me I don’t think I’ll ever need to use petrol power again and certainly wouldn’t ever renew the current petrol engine. It helps to have a catamaran to cover with solar panels but I think a monohull of similar weight could quite easily accommodate my ideal of 880W fixed and 440W moveable panels.
  2. Because it’s so easy and pleasant to use and it helps us to sail closer to the wind and faster in light winds, having an electric motor makes it easier to make tide windows and ultimately significantly reduces our transit times and increases our enjoyment.
  3. Getting stuck in irons when going about, always a concern with a cruising cat, is now a distant memory. I even got quite close to hitting rocks tacking into Helford passage when I first bought Chateau Cat. A small burst of push button power is all that’s needed now to help her round. It’s so much easier than getting an internal combustion engine started and into gear.
  4. For me, it’s been really useful to have the back-up of the petrol motor while making this transition. I’m sure you could move straight to electric from a normal combustion engine – and ever more examples of people doing this will help. However, for me, to get to the right size of motors, panels and batteries and push through my range anxiety, it has really helped to do this incrementally, while having the peace of mind of a back-up fossil fuel motor just in case.
  5. The potential range under power, at a reasonable speed, has significantly decreased versus internal combustion engines (although with the solar charging, I could probably travel at approximately 1 knot perpetually).
  6. If I started again I’d consider choosing a boat with an inboard motor and also be likely to go for a 48V system with LiFePo batteries.

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