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Electricity and fuel



Home from work at last, you light up the house, turn on the music, check your answering machine and throw some popcorn into the micro. Itís all easy and convenient and you never give any thought to how much power you use.

Well, welcome to "water-world". A couple of 60W light bulbs at home will use up the same amount of power as your allowance for the entire boat in one whole day .

There are three factors to take into account when dealing with power: How to get it, how to store it, and how much youīll need.

They way to deal with the power problem is to count on it and then go out and try it. Using only your calculator will never measure up to reality, especially on an old boat. The reasons are bad wiring, poor charging and gear that use more power than the manual promised.

Your habits have to be changed. Forget air-con and large fridge (or any fridge at all perhaps).


Power basics

The power mainly used on the boat is DC (Direct current) contrary to the AC (Alternating current) used in your home. DC is low voltage and much safer to work, especially in marine environment.

Voltage (V) is the charge (or tension) you get in a battery. A small AA-battery has a charge of 1.5 volt. Your boat battery will most probably carry 12 volt. If the battery is low on power the charge will go down below 11 volt. If fully charged it will go above 12 volt. You can check how much voltage you have in the battery using the voltage meter on the boat (or if you donít have one , get a digital tester in any hardware store).

Ampere (A) is the flow of the charge. Your battery will be labeled with how many amp-hours (Ah) that may be pulled from the battery at a certain voltage. If for instance you have a fully loaded 12 V, 100 Ah battery , you may pull 1 Amp per hour at 12 V for 100 hours.

Watt (W) is the work the electricity has to do to power up a lamp, a GPS or something else on the boat.

Power need is a combination all of the three above: watt = volts x amps

In the above example (12 Volt, 100 Ah battery) youīll have 1200 W.

To double the amp hours in your batteries, you can do a parallel connection between two or more batteries. Hook up the positive end of one battery to the positive end of another battery and then similarly the negative with the negative. If each battery had 100 Ah at 12 V, they will now still give a charge of 12 V but at 200 Ah.

And youíll have 12 V x 200 amps = 2400 W.

To raise the voltage you need to do a serial connection. Connect the positive of one battery to the negative of the other battery. Then connect the negative of the first battery to the positive of the second. If each battery had 100 Ah at 12 V, they will now still give 100 Ah but at 24 V.

This is useful if for instance working with 6 V batteries, thus raising the voltage to 12 V.



You need two sets of batteries on the boat. The first set will be used only for starting the motor, these are your cranking batteries. The second set will be used to power everything else on the boat , those are your house batteries.

Out at sea you will most certainly run into a situation where you take too much power out of the battery. If you havenít separated your cranking battery from the house battery, you will pretty soon be a real sailor.

Santa Maria is powered by a 12V 60Ah cranking battery and a 12V 180 Ah house battery bank. This is however on the low side.

When choosing batteries youíll choose between wet cell or gel-cell batteries. The gel-cell batteries need less maintenance and wonít have any liquid that can spill out. This whole subject is a science of its own however. When installing and choosing the batteries , shop around and take advice from a good marine electrician.

Imagine your car being lifted 5 feet above the ground, and then dropped. Now, imagine this happening every 30 seconds. Keep doing this for three weeks. This is what is going to happen to your boat during an Atlantic passage. Do we need to say you have to fasten you batteries securely?

Put your battery in a compartment that is easily accessible since you will surely spend some time down here.


Charging the batteries with the engine

The main power source (generator) on your boat will be the engine. This is how it will work: Start the engine (it doesnít matter if your gear is in neutral, forward or back). The engine will start turning the propeller. Simultaneously, your engine will also power an alternator.

When rotating, the alternator will create an electrical impulse. The electric flow will then pass through some kind of voltage regulator (restricting the voltage) and charge the battery. The voltage entering a 12 V battery needs to be at least 14 V for the battery to charge properly.

So, the engine is generating power to the alternator, which is alternating the power to electricity, then pass it through a regulator before charging the battery.

Alternators will generally produce 60 Ah. That indicates that you need to run your engine for one hour to fully charge a 60 Ah battery or 3 hours to charge a 180 Ah battery. Another way to look at it is that if you are using 60 Ah at 12 V, youīll need to run the motor for one hour to cover the power used.

Of course there are some ifīs and butīs to this. First of all, your battery will most probably not be chargeable above 80% of its capacity. Secondly, a battery is often not capable of receiving more than 25% of its rating per hour. This indicates that a 100 Ah battery only can be charged by 25 Ah an hour, even if the alternator is producing more.

How fast should you run the motor for efficient charging? At low speed it might not charge enough and at high you might be wasting your fuel. Discuss this with your marine electrician and learn the specifics of your boat. Donít forget to write it all down , youīll need it when calculating the fuel for the passage.

A simple way to check if your engine really is charging the battery, is to switch of all electrical appliances and put your digital tester to the battery. If the voltage is moving upwards when you run the motor - then itís charging. There is a safety switch that stops the charging when the battery is fully charged (at around 14 V).

Never let your battery drain below 10,5 V , that would probably destroy it.

Other power sources

Next to engine power, wind power is the best way to charge your batteries. Wind generators use either a propeller or a turbine to catch the wind.

When crossing the Atlantic you will have the trade wind moving the same way as the boat. This means that the wind hitting the wind generator will be less than the true wind. If your boat is moving at a speed of 7 knots and the wind is 15 knots, the perceived wind will be only 8 knots.

Santa Maria has an Aerogen 6 that is supposed to generate 300 Ah a day (24-hours) at 10 m/s (20 knots). Since we moved at 7 knots, the 20 knots wind only affected the generator with 13 knots or less.

We felt that the benefit of the wind generator was so low, that we actually stopped it after some time.

Sailors are however often very positive about their wind generators, especially the new, turbine units.

We met a couple sailing a catamaran at Los Testigos, of Venezuela. They dragged a water turbine behind their boat and were very enthusiastic about it. We donīt know much about this power source however.

Solar panels are surely the dream of the future. They are yet not as effective as wind generators but quiet. To get 1 Ah at 12 V you need a panel app. 1 feet by 2 feet (33 x 66 cm). Better quality panel gives you a higher output from a smaller panel area. Most panels need however to be mounted on a surface that is not shadowed by the sails to give full power. That can prove hard to accomplish on an Atlantic crossing.



Letís take an overview.

If you use 200 Ah a day and want to replace that you need to:
  • Run the engine for 4 hours a day, or
  • Use one wind generator during perfect conditions, or
  • Use solar panels covering an area of 10 by 3 feet
    (3 by 1 meter)

We recommend that you use all three of these power sources. They all have their pros and cons, and tend to complement each other.


Using power

In the manuals of your electrical appliances it will be specified what effect the appliances need (watts) or how many ampere-hours (Ah) they will use.

Find the information for everything that needs power and put it down in a power scheme, like the one below. Check that all datas applies to 12 V.


Equipment Watts Amp Hours of use Power usage
10 lights
Auto pilot
37.5 Ah
24 Ah
10 Ah
19 Ah
Total power usage 90.5 Ah

To do the calculations, use the formula; watts = volt x amps.

When planning for the passage, our first calculation indicated a total power usage of more than 400 Amps. Just to run the radar all day would count for 120 Ah. We had to cut down the calculations over and over again. We ended up below 100 Ah a day, thus needing to run the engine for around two hours daily.



If you get an old boat like Santa Maria, there will probably be as many sets of wires as the number of previous owners. Try to locate and replace all of them before you set off. We didnít and had a hard time locating the wires when something malfunctioned.

Loose wires might leak current. If the insulation is torn around the conductor there may be additional leakage, and you will end up with much higher energy consumption than expected.

Choose different colors for the wires and note down in a wire scheme how it all connects. Ask for your local official color standard and recommendations at the marine store. Use at least the wire thickness recommended for the appliances you install.


Electrical panel

The current used for appliances should be picked up at a distribution panel and not directly from the battery. We have seen panels that are mounted on hinges, easily accessing to all the wires , ours is screw-mounted and a real bore.

Mark all the switches in English even if that is not your native language. If you need an electrician, he will most certainly understand the English technical terminology.

Mount a couple of DC cigarette outlets on the electrical board. You will need them for charging VHF, computer and more.


DC/AC inverters

There is a way to invert your DC current to AC so you can bring fans, TV-sets or other non-DC prepared equipment on board. There are simple mobile units for low requirements at your local Radioshack or more advanced units for fixed mounting at marine stores.


Dock power

When cruising from harbor to harbor itís great to have the possibility to plug in to the docks supplied AC power. We use it for charging the batteries and have a couple of AC outlet in the boat for use when in harbor. Itís OK to have only one light on when cruising, yet when at a dock itís nice to light up the boat without the worry to flatten the batteries.



Your engine will be using diesel. Before you leave home you need to do a few tests: How much diesel will the engine use at full speed, half speed and at low speed. 

Combine the results with the information you got from the electrician, as of what speed you need to run the engine for most efficient battery charging.

Santa Maria have a 20 year old Universal Atomic Diesel. It generates 44 H.P. At low speed (charging the batteries well) we used just over 2 liter an hour. Our charging need was two hours a day. We left the Canary Islands with 200 liter of diesel but used only 100 liters on our 20-day passage.

The tank took 100 liter. We stored the rest on deck, in tightly fastened containers.



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