Smallboat electrics - demand and supply

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wgarnier

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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.
 
Make sure you know how to use a sextant, cause I doubt your electrics will last unless you have a solar panel or wind generator of some type.
 
I'd assumed that PV refers to photovoltaic, a solar panel.

If this is the case then, I'd suggest you do an audit of your gear and find out what demands each bit of kit has, make some assumptions about how much you will use each bit of kit. Then add up all of the amps then you'll have rough idea of how much power your solar cell will have to provide. Then add some addtional comfort to your figures, personally I'd go for at least 50% more but you will have to make your own up about that. Then find some panels , I'd not want to rely on one, that fit the bill.

Best of luck, 24' across the Atlantic is not my idea of fun but each to their own.
 
By all means, do at least a rough demand-supply balance:

- for each device, determine their consumption in Amps and multiply for the number of hours of expected daily usage.
(example: a tri-colour nav-light is 2 Amps, presumably used for 10 hours/day, so it's 20 Amp-hours per day)

- sum up everything and determine your total daily consumption (my guess is anywhere between 30 and 50 Amp-hours a day, in your configuration).

- your battery capacity should be at least 3 times, even better 4 times, your daily consumption, assuming you want a 1-day reserve.

So in your case, anywhere between 100 Ah and 200 Ah: considering the small boat size, I would assume 2x50 or 2x60 Ah to be a practical maximum.

Recharging: you would need to produce as much as you consume, so something between 30 and 50 Ah per day.

Consider that the practical output of solar panels is in the range of 30 Ah per square meter per day (or, to put it in another way, you need a 100-watt panel to produce 30Ah per day).

So, if you cannot fit such a large panel on your boat, either you work out how to limit your power consumption (for example: fit a 12watt lamp in the tri-colour light: it would not be as visible as the regulation 25-watts, but would still be much better than sailing without lights!), or you have to add some generation capability.

For trade-wind sailing, the common wisdom is that a towed generator would be much better that a wind generator (and much less conspicuous as well!).

My recommendation would be to keep a towed generator as a backup, and use it according to need.
 
From experience I can say your outboard charging capability should not be relied on, especially in an emergency.
I had a 9 hp OB charging a small 80ah battery on my previous boat and it made very little difference, only putting in amps when at close to full throttle.
Outboards don't have alternators, only "lighting coils" designed to power a small lamp and not much else.
You might be better with a small portable generator as back-up to your solar panels, given you will have to carry fuel for the OB anyway.
 
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So, if you cannot fit such a large panel on your boat, either you work out how to limit your power consumption (for example: fit a 12watt lamp in the tri-colour light: it would not be as visible as the regulation 25-watts, but would still be much better than sailing without lights!), or you have to add some generation capability

[/ QUOTE ]Doing this may result in invalidating your insurance should you be involved in an accident, the light has a regulation value to ensure that your lights are visible, if they are not and a ship runs into you, your insurance will not pay up if it is found that your lights do not meet the required regulations.
 
You can cut down on power consumption a lot by switching to LEDs. There are downsides too (do a search of this forum, the subject has cropped up a few times), but the energy saved is quite spectacular compared to filament bulbs. Nav lights are available in Europe from "Lupo" and "African Cats".

I don't see a problem with using solar as your main charging method. I'm planning to do the same, but will be doing coastal sailing for the first few months during which time I can hopefully iron out details without being left electric-less in the middle of the pond.

Good luck though, should be quite an adventure!!
 
If you choose to save energy use by changing globes to LEDS you'll not incur the cost of the LED Nav Light Units either.
I'd sadly echo the negatives on outboard charging. We attempted this some years back and also ditched it after trials.
I'd also be more hesitant to rely soley on solar as I would on wind or towed generation - and possibly suggest a mixture of all three might be better (if it can be afforded)?
Whatever you do - good luck on the trip.
JOHN
 
Maybe you should talk to people on the mini transat sites, I know some have used generators, but most seem to get by with just a solar panel. What they do do is have it on a tilting bracket so that it can be angled to the sun. I'm assuming this will give a better output.

SteCa_Mfastnet03_Mini.JPG
 
Totally agree with Gian. Just a 25w trilight alone is 25Ah daily. We have a similar range of kit to WilliamG's, using small inverter for recharging MP3 player, We've abandoned the rechargable spotlight as too demanding, unless mains is available.

Our demand can be kept down to about 60Ah daily when in economy mode on passage or at anchor in port, but that is very tight. A towed generator delivers this easily when sailing, but this much is not practical with solar panels (we haven't the area) or a wind generator alone. I thik WG needs at least 2 100Ah batteries, 3 would be better.
 
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Just a 25w trilight alone is 25Ah daily.

[/ QUOTE ] ????
batteries are measured in Ah not equipment 25Ah means the battery can supply 25Amps for one hour, your 25W trilight uses 2Amps, if you run it for 12 hours it will use 24Amps not Amp hours. So your total demand would be 60Amps not 60Amphours unless you really do use 60Amps an hour which I doubt.
 
[ QUOTE ]
[ QUOTE ]
Just a 25w trilight alone is 25Ah daily.

[/ QUOTE ] ????
batteries are measured in Ah not equipment 25Ah means the battery can supply 25Amps for one hour, your 25W trilight uses 2Amps, if you run it for 12 hours it will use 24Amps not Amp hours. So your total demand would be 60Amps not 60Amphours unless you really do use 60Amps an hour which I doubt.

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[/ QUOTE ]Yes, me too!

"An amp-hour is a measure of the quantity of current. One Ah is the number of electrons that will pass a given point if a current of 1 amp is sustained for 1 hour. It says nothing about the rate of flow, since the same number of electrons could be supplied by a 60 amp current for one minute or a 3,600 amp current for 1 second; in the same way, 100 gallons of water coule be supplied by a 1 gallon/minute pump running for 100 minutes or a 100 gallon/minute pump running for 1 minute". Nigel Calder, Boatowners Mechanical and Electrical Manual, 2nd ed, page 9.
 
Actually AndrewB is more or less correct. Batteries are rated in amphours, which is a quantity of electricity that it will give out under certain conditions. An Ampere is a measure of flow, not quantity. So a 100Ah battery will sustain a 10A current for 10 hours. This is not entirely true thanks to certain complications like the rate at which you discharge it, internal resistance etc, but it is a good rough guide.
Importantly, you should never let a battery discharge deeper than 50 per cent, or you will considerably reduce its lifespan. So calculate your daily requirements, then double the amount and add 20 per cent (for inefficiencies etc) to get the minimum battery size for daily charging.
 
African cats LEDS look interesting - has anyone any experience of them? The tricolour/anchor/strobe looks good on paper. Yet to do an ocean crossing but (though technically illegal) mid ocean I would think there would be more chance of someone on watch on a ship noticing a strobe rather than a tricolour. Any thoughts?

Padz on Conachair.
 
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