Electrical Budget and the Virgin Islands | Sailing Blog - Technical Hints and Tips - Sailing Television

Electrical Budget and the Virgin Islands

SNOW SNOW SNOW!! Its blowing horizontally outside the studio window as I write this and tonight its meant to drop to -24C. Tomorrow we are heading south for 3 weeks filming in the Virgin Islands and not a minute too soon. Although we did have fun out on the lake filming a few days ago with iceboats and Bombardiers – more on that later.

The past few days I have been working on the electrical system design for the new boat. Basically all the gadgets and how many batteries we will need to run them all. As any long-term cruising sailor knows, the pile of modern goodies we have all been adding to our boats can drain the batteries pretty quickly. The question is how quickly! What you need to do is calculate how much power each device will use, and how long you will use it each day. I actually do two calculations, one for when we are at anchor and one when we are at sea on passage (assuming sailing 24 hours).

In order to do this calculation you need to assemble all the information on what power is required for each gadget, and this can be the most demanding part. Traditionally we calculate this in “amp-hours” which is how many amps it draws over so many hours. For example, the Autopilot uses 1.5 amps and I plan to run it for 24 hours when we are on passage. So it uses 36 amp-hours in my table. Note you must stick with the same voltage when you do this – so I am using 12V – but the same calculation can be done at 24Volt as well.

If you only have a wattage rating on the device – such as a 60 watt bulb – divide by the voltage to find the amps uses – 60/12=5 amps.

To do my table I used the internet and was able to find specs on a number of the gadgets I intend to put on the new boat. For instance, the cappuccino maker (;-) uses 1400 watts so I calculated 1400/12 is 118 amps at 12 volts. Not really accurate since the inverter will use some as well but this is meant as a rough calculation – after all, the number of hours we will use is a bit of a guess anyway.

Calculating the number of hours a gadget will be used is a fun activity on a winter day! You can visualize the situation and put yourself on the boat. How long would you play (I mean work) on the computer every day? If you are doing this calculation for a boat you already own you can make notes from past excursions. And hopefully you have an ammeter so you know how much the gadgets use as well. (If not its definitely worth getting one! An ammeter is a great tool to help manage your electrical system).

So Here's the first pass through this calculation for sizing the ship's battery bank. And it hasn't even got the washer/dryer in ;-)

Assume windlass is used only when the engine is running so it is not included - budget is only for calculations of battery capacity.
- Radar is in transmit mode for 3 hours. This is a assuming a bad night because we normally leave it in standby mode. Of course in fog it would be on 24 hours.
- Refrigeration is always a big user. I hope this will be efficient but it may be on more than I budget, and may be on more when its really hot.
- Cabin lights don't use much. I am assuming a few reading lights and thats it. If you like it brighter be prepared to have lighting as one of the largest users of power.
- Autopilot use is for moderate sea conditions. In my experience they can use very little if the sea is light or the boat balances well. The Southerly has a very light helm so I predict it will be a low power user in "auto" mode.
- cabin lights are entered twice since we use them differently at sea and at anchor. At sea we just have 2 small lights on for watch the whole night. At anchor we have more but plan it for a typical sailors early night!

If you are planning a budget like this, you might want to splash out on lights, a big deep freeze and other things, but whatever you plan, your batteries should be able to supply the needs without needing a charge more than once a day. I am assuming I will want to recharge once a day, and that I shouldn't use more than 50% of the capacity of my batteries. So I am planning a bank somewhat larger than 400AH. Basically double the "Daily AH on Passage" total of 198 amp-hours).

That's it for now. Got to run and pack!!
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