Friday, June 11, 2010

A bit about energy

One of the key concerns of a cruising boat is resources, chief among them being Fuel, Water and Electricity.  Now on Viking Star we carry 500 gallons of Diesel, about 6 Gals of gasoline for the Honda generator and the small outboard, and 10 gallons of propane for the BBQ and stove.  Diesel will last us 6-8 months of typical cruising, or one trip to Hawaii! Gas depends on Honda use (time will tell that), and propane perhaps 6+ months of use.  So, overall we are OK for fuel.

Water – we have 225 gallons, about 2-3 weeks worth.  Something we do need to consider and deal with, but not on a daily basis.

Electricity:  Those LARGE batteries we installed last winter (all 1,200 lbs worth) are perhaps 2-3x what a typical cruiser might carry.  Even so, these batteries only hold enough electricity to last us 2-3 days, that is it.  Electrical demands on a modern cruising boat is a daily concern.  Modern boats have electric lights, computers, radios, and refrigeration – all of which are typically powered by Electricity.  In days ago, lights were kerosene, computers where fingers, radios were sharing stories around the deck at night, and refrigeration was a dream.  Even today some minimalist folks will take this approach and have just a small car battery to power the VHF and perhaps nav lights.  But we as a people do tend to become used to our modern conveniences.

To get a better understanding of Electricity onboard a boat we need to talk a little about terms.  In your house one is often use to hearing about Watts – mostly around light bulbs.  We know that a 40 watt bulb is ‘smaller’ than a 100w bulb, that it uses less electricity and also produces less light.  We also know that if we leave those light bulbs on for a long time they will use more total electricity – as reflected in our power bill at the end of the month.  Energy for a house is typically measured in Kilo-watt/hours (Kwh), which is the function of Watts * time.  Leave that 100w bulb on for 10 hours, and you have 1 Kwh, which will cost you about $0.10

On a boat we also measure things, but we use a slightly different unit.  We tend to use Amp-Hours.  Like Kwh, Ah is the product of power  used (amps in the case instead of Watts), and the time it is consumed (in hours).  The use of Ah is more common because, frankly, we store electricity in batteries and they are ‘sized’ by Ah.  On Viking Star we have 1,800Ah of battery capacity.  Meaning we could draw 1A for 1,800 hours, or 100A for 18 (See * below for some details) before the battery is totally discharged.  One more background detail:  It is hard on batteries to totally discharge them, on Viking Star I like to go to 700 – perhaps 800Ah max before I want to start recharging. 


So, with all that, let's do some comparisons:  In our house, or even in the boat while plugged into shore power, electricity was practically  unlimited – paced only by the memory of last month’s bill (yes, Dad is going around the house turning off unused lights!).   During the month of May we used 380Kwh of energy while still plugged into the ‘grid’.  That comes to 12.7kWh a day – which translate roughly into 1,270 Ah per day!  So, if we kept up that rate of usage, we would go through our batteries in less than a day…

Clearly a big difference.

This past week or so, we have been using around 200-250Ah / day, where does it go?  Some ideas:
  • Refrigerator:  100Ah / day
  • Freezer:  60Ah/day
  • Lights:  20Ah/day 
  • TV / Movies:  10Ah / hour that we watch (no leaving the TV on all day)
  • Coffee:  25Ah / pot (And we pour into a thermos as soon as it is brewed)
  • Computers:  Perhaps 20-30Ah / day .  More if we use the WiFi repeater.
  • Shower:  100Ah / short shower.  150Ah if we want the luxury of the whole 5 gallon hot water tank (about 10 minutes max)
I know this adds up to more then 250Ah, so obviously we tend not to do all the above in one day.

We are still adjusting.  One big difference between last month and now is we no longer use the space heater – that alone accounts for most of the delta between being plugged in vs. un-plugged.  Even so, we do have to change our habits while out here.  To be honest, this is mostly around heating of water.

At this point, the only way to heat water on Viking Star is via electricity.  Uncompleted projects will tie the water heater into the motor / generator to allow for water heating while those are running, as well as the installation of the Huricane diesel boiler (which will be able to heat water as well as provide space heating).  But for now, Electricity is the only way.  Our adjustments so far?  Taking cold (and very short) showers, and delaying morning showers until we are up and can start the Honda generator.  Will say, this has also extended our water supply from 2 weeks to what looks to be 3 weeks!

Energy systems are incomplete on Viking Star.  I am still tinkering with the Kubota generator, need to install the Hurricane, and perhaps also tie in the Dickinson stove to the water heater.

Am sure will come back to this topic as we learn and adjust more.  Will keep ya updated!


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