Technical and Equipment

Energy At Sea!

Energy at sea

  • August 13, 2020
  • by jdo Jörg Domann – SV Aurelia

Jörg Domann has done many ocean miles and has an engineers perspective on the subject of energy, it’s interesting reading and explains some misconceptions!

For those in New Zealand contact Enertec Marine for more info on the products described in the article.

On the first stage, Aurelia sailed about 6,200 mm in 128 days. What energy sources has she used? How much energy was available? How was it used? After a deep look into the logbook, various formula collections and technical documentations, I can give you an answer in this article.

Energy sources

First, let’s take a look at what forms of energy are available on Aurelia. These are wind and solar energy, land power and diesel. The following graphic illustrates their interaction:

Solar energy

One of the first projects was to install a solar system on the Aurelia. On an area of 4 m2,  the solar energy can be converted into electricity. The average solar radiation on the first stage was 250 W per m2. This results in a theoretical potential of 24 kWh per day.

In order to convert as much as possible into electrical energy, I opted for bifacial solar modules from LG (LG390N2T-A5). They offer 390 Wp per module according to DIN. Based on manufacturer info, the additional use of the irradiation on the back (bifacial proportion) achieves up to 30% additional yield. This could really be achieved under optimal conditions and even easily surpassed.

The peak power of the solar system was 1,070 Wp. The highest daily yield of 6 kWh was reached around the summer solstice on a cool, cloud-free day in Locmiquelic (47° 43′ N). On rainy days, about 2 kWh were achieved. Anyone who thinks that the yield in the Caribbean is much higher is unfortunately wrong. This is correct over the whole year, but a sailboat is mainly used on summer days. Comparing them, the yield in the Caribbean is reduced by about 17% due to hazy air, shorter sunshine duration and higher temperatures.

In addition, it is extremely important to prevent partial shading for a good daily yield. Once even one of the 144 solar cells is completely shaded, the power collapses by about 50%. The shadow of a running backstay, for example, hardly matters. However, the Aurelia carries its radar at the rear because the shipyard in Locmiquelic partout did not want to mount it on the mast. As soon as the sun is on the starboard side, the energy yield decreases considerably. Nevertheless, the solar system was able to achieve an average of 3.5 kWh per day.

In order to provide this amount of energy continuously, it is regulated down to 48 V via a solar charge controller (Victron Smartsolar MPPT 150/35) and stored in two LiFePO4 batteries from Pylontech with a total capacity of 4.8 kWh. LiFePObatteries have significant advantages overthe NMC batteries used in the automotive industry:

  • Higher fire safety
  • No use of rare substances such as cobalt
  • A much higher cycle resistance

The higher weight to be accepted does not play a significant role on the Aurelia.

An inverter (partial function of the installed MultiPlus II from Victron) converts the 48 V direct current into 230 V alternating current (with real sine curve). Up to 3 kW of power can be retrieved from this.

The 12 V system with 4 x 110 A = 5.3 kWh gel batteries was already available at the time of purchase of the ship and was taken over unchanged. The capacity sounds very good at first. However, it must be noted that they have a low cycle stability and are much more sensitive to deeper discharge. This is why the 12 V batteries on board the Aurelia are continuously supplied with up to 110 W from the 48 V system via a DC-DC converter. Thus, the state of charge remains mostly in the upper range. Only in a few situations were they discharged deeper. In this cases, the original on-board charge controller with a maximum power of 1 kW was used. It is operated via the detour of the 230 V network (see chart above).

Shore power

The Aurelia can be supplied with land power via the MultiPlus II. Thanks to the solar system, this was rarely necessary. Especially at the beginning of the first stage, we used it occasionally to heat the salon, charge the batteries and supply the 230 V devices directly for the duration of the connection. The conservative estimate use of land-based electricity is 90 kWh. Standardized it is 0.7 kWh per day.


The original goal of Sailing-Aurelia was to sail around the world without fossil energies. However, Yanmar’s 3JH5E marine diesel only had 250 hours of operation when it was purchased. A conversion to the desired drive  SD20 with hydrogenerator from Oceanvolt would have been a lot of pleasure for me, but would have been very expensive and time-consuming. The conversion would have ended with the fact that, for safety reasons, I had installed a diesel generator. This is the only way to guarantee an adequate supply of energy at sea in an emergency. So, a changeover made no economic sense, nor could I have done without fossil fuels entirely.

Chemical energy

In the 128 days of the first stage, the diesel engine consumed 609 litres of diesel in 402 hours of operation. One litre of diesel contains approx. 9.7 kWh of chemical energy. This results in a consumption of 5,907 kWh, which is 46.15 kWh per day of chemical energy. This was converted into electrical, heat and propulsion energy by the diesel engine.

Propulsion energy

Based on the logbook and the fuel consumption characteristic of the engine, it can be calculated that the engine was operated for approx. 130 hours in idle mode or with negligible propulsion power. For 272 hours, the engine produced an average of 6 kW on the crankshaft. With an estimated efficiency of the propeller of 75%, this results in an average propulsion power of 4.5 kW. This results in a total of 1,124 kWh. Normalized 9.6 kWh per day were used for propulsion.

Electrical energy

The alternator of the engine produces a maximum of 80 A. Taking into account the speed, the state of charge of the 12V batteries and the active consumer, I estimate the average output to 20 A (I hope I can measure this more accurately in the next stage). This results in an output of 240 W over all operating hours, i.e. 96.5 kWh. Normalized this is 0.75 kWh per day of electrical energy.


The engine was in use on 74 days. At each of these it has each delivered an estimated 2 kWh of heat output to the hot water boiler. This results in 148 kWh or 1.16 kWh per day of heat energy.

The “remaining” 4,538 kWh of chemical energy in diesel was lost as waste heat. Burning the diesel produced 1.6 tons of CO2. If one takes into account the savings of the domestic oil heating system from the same period, the energy balance would be clearly positive, yet the emissions are by no means glory.


The exact calculation of the power that a sailing ship can gain from the wind is almost impossible. Complex currents on the sails, course/speed to the wind, wind speed and many other factors allow the length of a possible formula to grow against infinity. However, the approximate wind energy can be calculated in a greatly simplified way based on the back pressure and the sail surface. The wind power results from the product of the square of the apparent wind, the half air pressure and the sail surface:

A resistance coefficient must be added to the formula. It is also known from the cw value for cars. For an ideal body in an ideal gas, it can be zero. In reality, for example, it is 0.03 for a penguin and 1.3 for a concave half-shell (about a gennaker or a landing parachute). According to literature, the value for reaching courses can also be 1.5 by taking advantage of the Bernulli effect of the sails. With the skills of her recreational skipper, i.e. me, the Aurelia has probably only brought it to 1.1 With this co-value we go into the calculations: If you multiply the force from the above formula by the speed of the ship, you get the power. Some concrete examples from the logbook:

  • On 25.10.19 we managed to get 4.2 kn ride out of 6.2 kn half wind thanks to Sven’s trimming skills, with main sail and the Code Zero at shallow sea. This results in a wind power of 1.65 kW.
  • On the Atlantic, we were mostly only on the poled-out headsail. A fresh breeze about 21 kn pushed us forward. This results in a surprisingly low wind power of 3.43 kW at headsail. The canary current of 0.25 kn, the wave from aft and the surface of the rest of the ship probably helped here too.
  • When we reached between the Caribbean islands at 21 kn in the first reef about 7 kn drive, according to this formula already about 17 kW worked on the sails.

Using the logbook data, Aurelia achieved an average speed of 5.2 kn in sail mode with an average of 16 kn apparent wind. This results in the admittedly very rough estimate of an average wind power of 7.3 kW.

The Aurelia was under sail for about 875 hours in the first stage. According to the above projection, 6,388 kWh of wind energy was consumed. Normalized this is around 50 kWh per day.


Usually a gasoline engine is used for the dinghy. In my case, this would have been a very small 2.3 hp drive with only one litre of tank capacity. A 10 litre canister would have served as a reserve. Assuming that I would have used the canister 2 times during this time, it would have been 21 liters * 8.6 kWh/litre, i.e. about 181 kWh total or 1.41 kWh per day.

Since I replaced the gasoline engine with the electric 1 kW engine (Travel 1103 CS) from Torqeedo, I could completely do without this fossil energy and the typical problems of an outerborder. However, the electric drive is not without disadvantages. I will discuss this in more detail in a separate blog post.


Sailboats usually have a gas stove on board. This is operated with butane gas. Aurelia originally had 2 bottles of 3 kg on board. Assuming a bottle change every 2 weeks, about 2.7 kWh per day are available. I have also replaced the gas stove with electrical alternatives. So I could also completely do without this fossil energy. I was particularly happy about this during the curfew weeks in Curacao. Also the time-consuming fiddling with the many different connections in this world is not exactly the kind of pastime you want on a sailboat.


After subtracting the conversion losses of primary energies, Aurelia consumed just under 66 kWh per day. More than 90% of this was invested in propulsion. Thanks to wind and sun, the share of green energy was 81%.

70% of the electrical energy generated on average 5 kWh per day came from the on-board solar system. In addition to the typical consumption on a yacht, it was also used to cook, generate drinking water and drive the dinghy.


Already during the first days it became clear: autopilot and refrigerator are the biggest consumers. These two alone consumed in the Bay of Biscay in bad weather more energy than the solar system could provide. Therefore, on the 3rd day we let the engine run for a few hours. So we didn’t have to give up cooking. After commissioning the wind control, the situation relaxed. However, we deactivated them on the Atlantic (see blog entry). Here, the better weather helped us to achieve a higher solar power yield. During the use of the water maker, we let the engine run along for safety reasons, so that the 12 V system remained well filled for the night driving.

In all the remaining time, the energy situation was not always lavish, but always sufficient. The fridge supplied cold beer throughout and 230 V were always available at sea when we needed it.

When the battery level approached 100%, we also used the energy to heat up the hot water boiler.

An overview of consumers with partly estimated call-on times is shown in the following table:

The very different operating times of the devices bring to light some, sometimes surprising findings:

  • Almost 40 kWh were used for the permanent provision of 230 V with a true sine curve. With a different configuration of the inverter, this consumption could be reduced by about 20%. However, with low loads, sine quality will be lost.
  • The DC-DC converter also has a fairly high power dissipation. Here, another smaller solar system with a 12 V solar charge controller would be more efficient.
  • Another energy source, e.g. a hydrogenerator or a second solar system, would significantly improve the security of the energy supply while sailing. Unfortunately, this assessment is relative when considering investment costs (e.g. Watt&Sea Cruising 600).
  • When you combine a stove, hotplate and water heater, they are the biggest consumer. We never had to limit ourselves to using them, except in two situations. Once the energy becomes scarce, this block can be used for a more even load distribution with low comfort limitations.
  • Powerful winches and the bow thruster do not have a significant impact on the energy balance during normal use.
  • Consumption at sea is more than twice as high as at anchor or in the marina.
  • The wind vane not only offers more resilience, it is also an important factor in energy management at sea.


I have selected the components mentioned in this post to the best of my knowledge, provided that they were not on board when the sailing yacht was purchased. I did not receive any benefit for naming the products.

The numbers above are measured, calculated, extrapolated or estimated to the best of our knowledge. They are still without a guarantee. Any constructive feedback to improve these values is welcome.

English is not my mother tongue. Since the post is very technical, there may be misunderstandings. Don’t hesitate to improve me and the post. Just send me a personal note.