The Boat
My starting point was the boat that I already had - Corvelia - a Thames launch built in 1950. Length: 11 feet 9 inches (3.60 metres) Beam: 4 feet 7 inches (1.40 metres). Corvelia is clinker built with oak planking - the diagram below illustrates clinker (on left) with carvel techniques for wooden boats. And the picture is of Corvelia during the messy business of stripping her bottom and repainting during the spring!
The disadvantage of clinker boats is that they always leak, but the advantage is that the leaks reduce the longer they are in the water as the wood swells and effectively closes the gaps. For Corvelia it took just about 3 weeks for her to almost stop taking in water after sitting on a trailer out of the water for nearly 18 months.
The other disadvantage is that traditional boats like this are broad and heavy (in the image of their owners, perhaps, or vice versa) and not ideal for being driven by small electric motors with minimal torque. A lightweight glass fibre boat - long and thin - would have been better. On the other hand Corvelia is relatively stable and sturdy, important when facing large wash on the tideway and for getting in and out. The catamaran, with lightweight aluminium hulls, is ideal as with the boat designed by Monte Gisborne in Canada - The Loon - illustrated below.
Monte was the inspiration for this trip as he has completed two solar journeys in North America. See www.tamarackelectricboats.com
The shape and weight of Corvelia puts a design limit on the optimum speed in terms of efficient drive power. No doubt it would move along nicely with a 40hp engine, but with under 1 hp it will not get up on a plane! My estimate is that its optimum speed is about 5 knots (nearly 6mph). In practice with my two electric outboards on half power I was making 3.5 knots (4 mph) through the water, with occasional bursts up to 5 or 6 knots when heading a very strong current. Downstream (or no-stream as in the canals) this would have been quite sufficient, but definitely not quite enough against wind and current.
The Engines
My choice of two very basic Minn Kota electric outboards (trolling motors is the proper description, which are in the main designed for fishermen in small dinghies - both quiet and slow) was based partly on economy (£195 each) and partly on the need to have duplicate systems and run each engine normally at half-power. These engines are not designed for continuous use at full power.
In retrospect I should have taken advice on the choice of motor and gone for a model that is slightly more powerful and uses solid-state switching technology to control speed, rather than the simple 'switched resistor' method on my motors which is inherently inefficient. Also it would have been possible to obtain an upgraded propeller that could have added up to 10% more oomph!
I lost one engine to fisherman's tackle that somehow got right into the submerged motor, but other wise they performed well and survived a number of bumps and weed events!
Solar Panels
Each solar panel on Corvelia is rated at 80 watts maximum output. These are linked in pairs to charge each battery. In theory this means that at around midday when the sun is overhead I could get 160 watts of power for each side which converts to 13 amps (Watts = Volts x Amps). In practice I get about 10 amps maximum. If the solar panels could be moved to point at the sun then I could keep this level of charge for more hours in the day. Alternatively the glass covering the panel could be made of small prisms which optimise the collection of sunlight and focus it on the photoelectric (or more correctly photovoltaic) cells that make up the panel; I gather this is one innovation on the cards for the near future.
You can get lightweight 'amorphous' solar panels, of the type used in calculators; but these need a much larger surface area for the same power as the heavier crystalline panels I used.
Each set of panels needed a regulator to ensure that the batteries did not over-charge; not an issue for me in practice! I blew up one of the two and the simpler, cheaper replacement proved sufficient. Good advice and friendly support from Mike at Sunshine Solar (http://www.sunshinesolar.co.uk/) where I also purchased my panels at a fair discount.
There is an excellent discussion on the whole science of solar energy at
www.solarnavigator.net/solar_power.htm
The Batteries
Here is a picture of the batteries! The key to successful electric boating is have decent 'deep-cycle' batteries of the type used in fork-lift trucks; these are different from a car battery, which is designed for high starting loads and then to be kept at full charge most of the time. The basic measure of the capacity of a battery is the ampere-hour, which is the number of amps it will provide per hour from full until empty. My batteries are rated at 225 ampere hours which means approximately 20 amps for 10 hours as you can never get the last bit out! At the speed I was trying to maintain I could only guarantee 15 hours continuous running from a full charge.
My experience is that these batteries proved their mettle on the many days when I was trying to extract the last dregs of power. I probably should have cycled (full charge - 30% charge - full charge) them a few times before the trip.
Overall Conclusions
The fact that I was able to make sensible progress against current and wind with meagre hours of direct sunlight proves the concept and basic validity of the design. A lighter slimmer boat, a lighter slimmer skipper, slightly more efficient engines, a simple means of tilting the panels to face the sun and some decent weather and it would have been an easy ride! Even with the configuration as it is at the moment there are many pleasant days of cruising ahead on canals and downstream stretches of our waterways.
And Corvelia is a happy lady. Landlubbers may be cynical about the spirit of a boat or ship, but there is no doubt that a neglected boat is a sad boat and after a year left on dry land Corvelia is pleased to be on the water again.
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