Your boat uses electricity for many functions and if you do not have any you may as well be sailing in the 18th century alongside Captain Cook. The principal source of electricity generation on a coastal cruiser is from the alternator on the main engine and is a similar set-up to a car. You need some battery power to start your engine and fuel to run it and once the motor is running you are converting diesel fuels into electricity. You will stock the excess electricity in the batteries on the boat and if you do not consume more than you produce you can continue your voyage until you run out of diesel fuel. There are other methods of generating electricity on a sail boat which include wind generators, hydro generators, solar panels and fuel cells.
     Before installing additional systems for power generation on your boat you are advised to calculate how much power you use on average each day and convert it into engine hours of electricity generation. The normal way of doing this is to add up all the sources of power consumption over a twenty four hour period considering both sailing and anchored mode.
     Let us assume you use 10 amps per hour which translates to 240 ampere hours per day. The first thing you need to check is whether you have sufficient battery storage capacity and a common rule of thumb is that you should have three times your daily consumption in battery storage. This means you need 720 ampere hours (AH) of batteries which is about 8 times that of a mid-sized car or 8 batteries of 90 ah. Many modern cruising designs do not have this capacity as they are designed for short distance cruising and you will need to find additional space to install the additional batteries. In today’s fiberglass production boats with internal molded floors this can be challenging if it was not considered in the initial design. Our modern way of cruising is far more power hungry than in the previous era of sailing and this is due to the presence of refrigeration, radar , autopilots, sailing instruments, navigational and electric internal lights none of which were present in the pleasure yachts in the first half of the twentieth century. Which one of these modern aids would you want to give up by saving on battery storage area or electricity generation capacity?
     Now let us assume that you have a 50 AH alternator on your main engine, you may think that by running your engine for five hours a day that you would put 250 AH back into your batteries and that you have balanced your daily electricity needs. Unfortunately life on-board is not that simple because in the first place your alternator will not produce 50 AH and in the second place the batteries would not absorb it even if you did. My 60 AH alternator would produce around 30 amps for around about 5 minutes and then the charging rate would drop off to 20 amps and after an hour or so a charging rate of 10 amps or less was common. There reason is that alternator will only provide the maximum charge for a few minutes as it will over heat if this is kept up permanently. Alternators can be expected to provide around half of their rated power over long periods and that is only if the batteries will accept the amps. Batteries that are more than 80% charged will accept less and less current (amps) as they become fully charged and it is game of decreasing returns where you are burning a lot of fuel for a small amount of current generation. If you follow this logic through I would have had to run the engine most of the day to keep the batteries full not forgetting we are talking about net positive charging i.e. the excess production after deducting the electricity currently being used by the boats systems. You can upgrade your alternator to a higher output model which is commonly done or you can install a second alternator or you can do both. However be aware that the wiring from the alternator to the batteries via the battery splitter control box may need to be upgraded. There are also available devices called smart chargers that “trick” the battery bank into accepting a higher charge for a longer period of time and you do not experience the drop off in the charging rate to the same extent as mentioned above. There is also have another issue being that an alternator should (rule of thumb) be sized to have a power rating of one quarter the size of the battery bank. So a 60 AH standard alternator should only be used to charge a battery bank of 240 AH which is only a third of my theoretical 720 AH battery bank. If you decide to purchase a higher output alternator be aware that they will be physically larger. A second alternator needs not only additionally space but also another groove on the engine flywheel to accommodate the second alternator belt. In many modern boats with the motor positioned below the main cabin stairs with the flywheels facing forwards you may have a space issue. Blue water yachts are often designed with a walk-in engine room and have traded this against an additional cabin present in the modern coastal cruisers which also have to cater for the rental market and more budget conscious cruisers. Look at the difference between the new two cabin Hallberg Rassy 40 and the new three cabin First Beneteau 40. The former has a walk in engine room where a genset (separate diesel motor that produces electricity and not propulsion) and additional batteries or a water-maker can be easily be added. The latter has lesser access to the main engine and less space to add gear but benefits from two more berths. Both boats have their merits but the former is better adapted from inception to long distance cruising.
      So back to our 240 AH daily energy deficit, we could add solar panels which take up no internal space but need to be exposed to direct sunlight. A 200 watt panel would give a maximum of 16 amps per hour for around 8 hours of daily sunshine (divide 200 by 13 volts) This would provide 128 AH or about half of our energy needs providing it was sunny and that the panel was angled correctly towards the sun. Many cruisers mounted these fixed panels on arches on the stern of their boats and a normal fixed panel of this power output would be around one meter by one meter. Getting two 90 or 100 watt panel of 1m x 0.5m is a viable solution for an arch installation and you will also need to install a voltage regulator to control the input to the batteries. The 15 kilos of weight placed at 3 meters above sea level is not ideal for smaller displacement yachts as it will affect the stability of the yacht.  Other forms of installation we have seen used are guard rail side mounts where the panels are articulated on a pivoting arm on the guard rails. They need to be removed in stormy weather but work well in anchorages. We used three flexible panels installed on the foldable Bimini top; they could be removed if a storm threatened and the Bimini top needed to be stowed away. This system was not ideal as the three flexible panels of 25 watts each provided around 5-6 amps for around the same surface area as the rigid ones and hence were less efficient. There are two types of rigid panels, monocrystalline and polycrystalline, the former is more costly and more efficient but the polycrystalline panels are catching up fast in terms of efficiency. The beauty of solar panels is that they have no moving parts, maintenance or fuel requirements. Their disadvantages are the deck space needed and the fact that they do not function at night or in cloudy weather.
     Some of our cruising friends use wind generators that are mounted on long poles at the rear of the vessel. You need to check carefully the output of these devices because in many cases they work best in 25 knots of apparent wind which is fine if you are anchored in a windy bay or pounding upwind in a force 5 but less useful if your sailing downwind in medium trade winds were the wind across the deck in a light boat maybe around 10 knots.  An example is the well-built Rutland 913 which gives 1.5 amps at 9 knots of wind and 6 amps at 19 knots. You need to have 26 knots of true wind if sailing downwind at 7 knots to produce half of the output of your 200 watt solar panel. The advantage of these wind generators is that they work not only during sunny periods but in all weathers and at night. The installation of these windmills is more onerous than solar panels due to the large and long pole required to safely support the device especially in high winds. We did not install a windmill because our route was mainly downwind and the visual aspect of these devices was unappealing. Besides the constant noise of these devices another issue I had with the wind generators was the safety aspect of a three or six blade high speed propeller one meter above our heads and I could imagine the result of landing a playful tuna fish whilst putting the end of the fishing rod through the rotating blades.
      We preferred to use a hydro generator which is a propeller on the end of 18 meters of rope attached to an alternator mounted on the push pit or back rail of the boat. The propeller turns as the boat moves through the water and the faster you go the more amps you produce. We had an Aquagen 4 that produced 2 amps at five knots and 8 amps at eight knots of boat speed. It would come out of the water in very choppy conditions or if we did more than 10 knots. We had a larger set of blades for the propeller that would produce 4 amps at four knots and 11 amps at eight knots but would surface at eight knots. Crossing the Pacific we averaged around 7.5-8 knots and the hydro generator produce almost all of our electrical needs. This piece of kit is simple and worked without maintenance for twelve thousand miles at a cost of 1,200 euros. A major advantage was that the alternator is on-board and if we lost the rope or propeller I could fabricate a replacement with a steel tube and some rope.
     We looked at the DuoGen system which is a windmill that can be converted to a hydro generator and is pivoted on the back of the boat to drag through the water having removed the wind blades. This system is more efficient than the Aquagen as there is no 18 meter rope to absorb part of the energy created however the mounting is more complex and should ideally be on the boat’s center line. We have a wide and extended swim deck on the rear of the boat and apart from the aesthetics of the device we would have had to construct an external platform to support the pivoting structure. We met two boats that had this equipment and the owners seemed happy with their choice. The manufacturer claims 16 amps at 8 knots that we have no reason to doubt as long as the yacht benefits from a good installation.
     Another system that will become more popular is the Watt&Sea system that is a flip down mini rudder with an alternator and propeller at the bottom of the rudder. The efficiency of this system is quite remarkable and at 8 knots it can produce 40 amps or 10 amps at 5 knots. It is about five times more efficient than the Aquagen and about four times the price without installation. This device was not commercialized for cruising boats when we left for our voyage and only the carbon racing version at twice the price was available. If we installed this system in the future then I would make sure that it was in line with rudder as the danger of losing the system to debris or other object would be minimized.
     We also looked at fuel cells which burn methanol and produce up to 9 amps per hour with a daily consumption of 2 liters for the 2500 watt/day machines. The machine costs the same as the Watt&Sea but has the additional cost of methanol of 5 euro/ liter which is not readily available outside Europe and is difficult to ship from Europe due to the explosion risk. The price of the fuel cells has decreased over the past few years due to their use in camping cars however they are not an economical option for an extended voyage. Eleven months times sixty liters per month would have a methanol cost of over three thousand euros and where would you stock 660 liters of methanol on your boat?
     We thought about a genset which produces a large amount of energy and costs around eight thousand euros for the small 5kw system but they take up considerable space, require maintenance, fuel and connection to a fuel tank. They also involve making a hole in your hull for the exhaust and sometimes the cooling system and lastly they weigh around 200 kilos including the insulation case. We passed on the genset but we did however carry an 1800 watt petrol portable generator to charge the battery and start the main engine if all else failed which we never used. We carried a small amount of petrol/gasoline for the outboard engine for the dinghy.This is the production side of the energy equation however there are many things you can do to reduce electricity consumption. The major consumers of energy on-board are radar, electric windlass (winch that pulls up the anchor chain), electric winches, pressurized water in the taps, SSB radio, television screens and PC computers, lighting, fridge and freezer, autopilot, chart plotter and navigation instruments.