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Old 20-08-2013, 01:21   #1
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Where To Put Solar Panels?

Where are common places to put solar panels on a boat?
Why would you put them there?
How many could I feasibly fit on a 27 foot sailboat?

I also have a slightly unrelated request/question: Can someone point me to a thread that explains voltage, watts, and amp hours?
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Old 20-08-2013, 01:36   #2
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Re: Where To Put Solar Panels?

The most common place is on a stern arch, or on the lifelines.
A 27 foot can usually get about 100-250w. It it a dependent on stern width. Newer boats that carry the beam a long way aft can fit more than older boats.

This is a good free book in PDF format that explains boat electrical problems:

http://www.dawncraftowners.com/12volthandbook.pdf
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Old 20-08-2013, 01:45   #3
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Re: Where To Put Solar Panels?

Quote:
Originally Posted by noelex 77 View Post
The most common place is on a stern arch, or on the lifelines.
A 27 foot can usually get about 100-250w. It it a dependent on stern width. Newer boats that carry the beam a long way aft can fit more than older boats.

This is a good free book in PDF format that explains boat electrical problems:

http://www.dawncraftowners.com/12volthandbook.pdf
Thanks.

So only one solar panel would fit on the stern arch? I've seen some on biminis (or whatever they're called). I'm thinking that if you put that much money in a 27 footer, you'd probably consider getting a bigger boat.
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Old 20-08-2013, 01:47   #4
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Re: Where To Put Solar Panels?

We have two on the davits at the stern and two on the cabin roof, total 320 watts when new, lucky to be half that now.

The two on the cabin roof will be removed as soon as I get a chance, they are a menace under foot when working on the cabin roof (stowing sails etc) and generally one or both are shaded by the sail when under way. If I can find space I will put them on the davits at the rear, otherwise they can go.

The two on the davits are excellently placed, out of the way, rarely shaded by the sail and double as a source of shade for the crew on the rear deck when the sun is behind us.

Matt
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Old 20-08-2013, 01:59   #5
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Re: Where To Put Solar Panels?

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We have two on the davits at the stern and two on the cabin roof, total 320 watts when new, lucky to be half that now.
If you bought the most efficient solar panels to replace the ones you got, how much wattage would you guess it'd be?
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Old 20-08-2013, 02:22   #6
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Re: Where To Put Solar Panels?

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Originally Posted by o_q View Post
If you bought the most efficient solar panels to replace the ones you got, how much wattage would you guess it'd be?
Err.. that's sort of a "how long is a piece of string" question...


I believe, within reason, most panels produce much the same power per area of panel, (but I might be wrong) so I guess the question becomes how large a panel area could I put on the davits, and of course that depends on a few things, as Noelex 77 points out, mostly it comes down to stern width.

In our case, with a 13 foot beam and a fairly round backside, I think I could get two 175 Watt panels up there with luck (at around 1.4 square meters each) if I replaced all the panels, but then thats all my eggs in just two baskets, so that may not be all that smart, still thinking about that one.

And, to complicate things, replacing just two of the panels introduces another electrical problem. If I were to go and buy two new 80 watt panels, and somehow got them to fit next to the best two of the existing four panels, I would have to wire them in parallel with the existing tired old panels, otherwise I would lose wattage to the higher internal resistance of the old panels. And it's a long run from the stern to my battery bank.

So, of course, I am putting that whole side of things off. I am just a day sailor at the moment, so the solar power system is just keeping the fridge cold and the batteries topped up while the boat is in the pen, so I don't have to play around with the 240V shore power when I want to go for a sail. (Oh yes, and my battery bank has already been cooked by the previous owner using the wrong battery charger. It nuked 400AH of AGM batteries only a couple of years old. Sigh! Capacity is now more like 60AH. )

I'm not much help, sorry. There are lots of really much better informed people on CF for these sorts of questions, I must get to reading some more. At the moment, that side of things is not urgent for us, so it is not getting any attention. (More urgent at the moment is getting on the slips to fix a failed skin fitting. Not sleeping too well at night at the moment! Might get the boat on the slips this Friday... fingers crossed.)

Matt
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Old 20-08-2013, 02:24   #7
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Re: Where To Put Solar Panels?

BTW, what sort of 27 footer? Just curious as Dad had a lovely Van De Stat 27 for a few years. What a terrific sailing boat! Lots of fun to sail and really well behaved.
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Old 20-08-2013, 02:31   #8
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Re: Where To Put Solar Panels?

Quote:
Originally Posted by GILow
Err.. that's sort of a "how long is a piece of string" question...
Not exactly, but I doubt it's easy to answer. It requires knowledge of the efficiency of your panels, and the efficiency of the most efficient on the market. Then some math.
Efficiency would be defined as output divided by surface area.

Quote:
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BTW, what sort of 27 footer? Just curious as Dad had a lovely Van De Stat 27 for a few years. What a terrific sailing boat! Lots of fun to sail and really well behaved.
Catalina 27

Quote:
Originally Posted by GILow View Post
I'm not much help, sorry. There are lots of really much better informed people on CF for these sorts of questions, I must get to reading some more. At the moment, that side of things is not urgent for us, so it is not getting any attention. (More urgent at the moment is getting on the slips to fix a failed skin fitting. Not sleeping too well at night at the moment! Might get the boat on the slips this Friday... fingers crossed.)
Still more than I know
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Old 20-08-2013, 04:43   #9
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Re: Where To Put Solar Panels?

I just installed 2x225W panels over the top of my Bimini.

They are big panels, 1600x 1000mm but I have a Beneteau 423 which has two backstays about 2.2m apart (above the Bimini), so they fit nicely in between.
I needed to install two 30mm stainless arch supports over the Bimini to support the weight.

I'm very happy with the generation capacity, although there is more chance of shading compared to a stern arch which would give less shade but not allow so much total panel area (less peak generation).
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Old 20-08-2013, 04:59   #10
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Re: Where To Put Solar Panels?

Hiya O_Q! Some boats have custom-made folding "wings" astern. Each wing contains a solar panel that can be opened at anchor or while motoring underway. For a boat under 35 ft, I would not recommend this add-on; it is something to think about for your next bigger boat.

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Old 20-08-2013, 05:12   #11
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Re: Where To Put Solar Panels?

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They are big panels, 1600x 1000mm
Those are very big panels! Even the system I have on my house is comprised of smaller panels, more like 1200 x 1200 each!
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Old 20-08-2013, 05:15   #12
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Re: Where To Put Solar Panels?

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I just installed 2x225W panels over the top of my Bimini.

.
Are they the Mitsubishi mono crystalline type?
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Old 20-08-2013, 06:46   #13
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Re: Where To Put Solar Panels?

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Originally Posted by o_q View Post
Not exactly, but I doubt it's easy to answer. It requires knowledge of the efficiency of your panels, and the efficiency of the most efficient on the market. Then some math.
Efficiency would be defined as output divided by surface area.
Difference in efficiency would not make a huge difference. Unless you buy incredibly expensive or incredibly cheap panels, good grade panels will usually run in the 17-19% efficiency range. For a size panel that would fit a 27' boat that would mean a difference in 0.1-0.2 amps max.

Quick DC101 class (and yes I know the analogy breaks down quickly but it's a great place to start).

Voltage is electrical pressure. You can compare it to water pressure in psi.

Amps are electrical current flow, you can think of it like how much water flows through a pipe like gallons per minute.

Amp hours would be how much amps flow through the wire in an hour. Like how many gallons of water flow through a pipe in one hour.

Some other concepts. You can have a small water pump like you would use in your boat's water supply rated at 40 psi, like a battery rated at 12 volts. Or you could have a 10' tall water pump that might be used in a city water system rated at 40 psi. Obviously the giant pump will move more water than the small one even though both are 40 psi. So a 120 lb truck battery will supply a lot more amps than a 12 oz radio battery even if both were 12 Volts.
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Old 20-08-2013, 06:54   #14
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Re: Where To Put Solar Panels?

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Amp hours would be how much amps flow through the wire in an hour. Like how many gallons of water flow through a pipe in one hour.
Amps and Amp hours causes the most confusion on CF.
A better analogy of Amp hours is the the total amount of water.

So you might have 2 gallons an hour flowing through a pipe and we leave it 2 1/2 hours.

The rate is 2 gallons an hour (2 Amps)
The total consumed is 5 gallons (5 AmpHr)
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Old 20-08-2013, 07:01   #15
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Re: Where To Put Solar Panels?

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Originally Posted by noelex 77 View Post
Amps and Amp hours causes the most confusion on CF.
A better analogy of Amp hours is the the total amount of water.
Besides speed, it's usually the units that factor in time that trips people up.
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