Small boat electrics - demand and supply

[wgarnier]

I wonder if you will apply your humble opinions to this one. i have a 24ft sailboat being refitted to cross the pond. My power demand will be radio, gps, nav lights, internal lights, camera/mp3 battery charging point and a power take-off point for such things as mp3 player and spotlight. I am fitting an outboard (5hp) with power generation capacity (as 'emergency backup') but otherwise think I will rely on PV to supply one or two deep cycle batteries (not sure which type).

Does this make sense? What is the best way to calculate the demand-supply figures? Can anyone suggest the size and type of PV I might need to satisfy the proposed demand and batteries? I am not keen on wind generation or dragging anything through the water.

Thank you in advance.

 

[William_H]

I think you will find that the PV (sollar panels) are limited by both the space you can find to mount them and the money you are willing to spend.
Your current usage will be dictated by what they can supply especially how much sun you get.
First thing you need is an amp meter so you know what they are supplying. Acumulative charge/discharge meter would be very valuable but again the cost may better be spent on yet another PV panel. (If you have room.)
Be aware that more deepcycle batteries will have more inherent leakage so if you are on a shoestring of current available more batteries could be a bad thing. So only have capacity you need don't go OTT.
Now a 80 watt rated panel will give about 4 amps. NB that is 80 watts divided by 20 volts (not 12v) The angle of the panel to the sun is important especially at higher latitudes. The actual time of sunshine producing current also depends on time of year and latitude. Let us say 10 hours will give at best 40 Amp hours. By the time you put that into a battery and take it out again you can expect about 20 amp hours available per day for use. Nav lights are your biggest killer depending on hours of darkness Assume 10 hours per day(night) then you can only provide 2 amps for nav lights ie total of 24 watts. Consider 1 mast head light or LEDs or a strobe light in lieu of nav lights when far from traffic.
Various radios MP3 etc depend on period of use and are calculated on current times hours.
I would suggest your 5HP outboard be checked for current capability with an ampmeter as it is likely to only give half amp or less when running. it may do better, but certainly older ones don't.
I would suggest you have more than 1 PV panel as they can be easily damaged or fail so 2 small ones are safer than 1 big one.
I hope that gives you some ideas. regards olewill

 

[jeremyshaw]

You might want think about switching to LED nav lights and internal lights as this will be one of your main current drains. There are some fittings with LED now available or you can buy conversion bulbs for many fittings from suppliers like ultraleds.co.uk.

Calculating the consumption is just a question of adding up all the stuff you are using and the hours you will have it for. For available amps the rule of thumb is 30% of the rated capacity. Then of course you need to put it back.... There are lots of threads here which go into more detail if you need it.