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Old 17-11-2011, 08:35   #1
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How Much Solar Do You Need ?

Ok everyone. Glad to be on cruisersforum, this is my first post and I hope a lot of you can help.

I need to know how large(watts) a solar array would need to be to power a sailboat during long passages with an electric inboard. The boat would be sailing most of the time but motoring when necessary. The boat will house typical electronics. I'm also thinking the boat will have a larger battery bank than typical electric conversions.

From the research I've done I'm thinking 2-3kw would be sufficient for a 35 ft boat under 4-5 hours of sunlight.

I understand that such a large array could be larger than the deckspace of the boat and that it is unrealistic but that's not my question.

I also know that solar-only boats exist but they are custom boats built to fit the panels. I need to know the real world sailors electrical needs.

If you own an electric sailboat your feedback would be critical.

Thanks everyone.
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Old 17-11-2011, 10:46   #2
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Re: How Much Solar Do You Need ?

The Pacific Voyagers are using solar power/electric drive with no backup generators. They are 73 ft double canoes, and have about a 12 mile range between charges.

The Vaka - Pacific Voyagers
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Old 17-11-2011, 11:05   #3
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Re: How Much Solar Do You Need ?

Thanks donradcliffe, I checked out the voyagers and thats pretty cool what their doing but they seem like they're "roughing it". I'm thinking more along the lines of a boat with more electrical like AC, refrigerator, radar, etc. as well as having an electric motor.
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Old 17-11-2011, 12:12   #4
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Re: How Much Solar Do You Need ?

If depends on how much you want to sail your sailboat, alot of people now a days rely too much on their engines for propulsion anyway. During long passages you just need to supplement power you would normally get from running the engine (something you should not do anyway). How much depends on how low a draw system you have. Use of LED lighting, hand pump water, low draw electronics....basically using electrical alternatives whenever possible. Which is a good habit to get into considering the vulnerability of electrical systems on boats.
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Old 17-11-2011, 13:12   #5
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Re: How Much Solar Do You Need ?

OceanArray:

The answer depends entirely on how big an electric motor you have, how much power (KW) you plan to draw from it while motoring and how many hours per day on average that you will motor.

Unless you just use the motor to get in and out of the harbor, the electric motor will use much, much more power than house loads.

For example, a sailboat under sail with an autopilot, radar, chartplotter, lights at night and a refrigerator will use about 200 amp hours each day at 12 V or 2400 watt hours. Your electric motor will use more than 2400 watts in an hour.

FWIW a good rule of thumb in low to moderate (30 deg or less) lattitudes in the winter or high lattitudes (>45 deg) in the summer is that the solar panels will put out 4 x its rated wattage in watthours. This assumes a flat array with no shading on a summer day. So cutting that factor to 3 for cloudy days and occaisional boom shading would mean that 800 watts of panels would keep up with continuous house loads.

I suspect that your motor will produce at least 5 hp and draw about 4 KW. This will require an additional 1,300 watts of solar panels for each hour that you plan to motor on an average day.

And think about it. You probably will only want to motor on light wind days, say 24 hours of motoring out of each 4 days at sea. That means that to motor 24 hours, you will have to store huge amounts of energy- 24*4 = almost 100 KW. Do you have any idea of the weight of those batteries? That is why submarines now use nuclear reactors. The battery size and weight is just too limiting while motoring underwater.

Now if you want to talk about a nuclear powered cruising boat.......

David
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Old 17-11-2011, 14:01   #6
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Re: How Much Solar Do You Need ?

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Originally Posted by OceanArray Inc View Post
Ok everyone. Glad to be on cruisersforum, this is my first post and I hope a lot of you can help.

I need to know how large(watts) a solar array would need to be to power a sailboat during long passages with an electric inboard. The boat would be sailing most of the time but motoring when necessary. The boat will house typical electronics. I'm also thinking the boat will have a larger battery bank than typical electric conversions.

From the research I've done I'm thinking 2-3kw would be sufficient for a 35 ft boat under 4-5 hours of sunlight.

I understand that such a large array could be larger than the deckspace of the boat and that it is unrealistic but that's not my question.

I also know that solar-only boats exist but they are custom boats built to fit the panels. I need to know the real world sailors electrical needs.

If you own an electric sailboat your feedback would be critical.

Thanks everyone.
"Real world" it won't work, simple as that. A daysailor or weekender can get away with electric, recharging batteries back at the marina when they get home, but trying to live on the boat, recharging solely with solar.... with current technology it's just not practical.

For a 35 foot sailing mono you'd need at least a 4kw motor, so even if you could fit a 4kw solar array onto it, you would only get about 4-5 hours fairly slow motoring worth of solar per day at best. Maybe 15 miles. Then there's the fridge to run... the lights, the autopilot....
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Old 17-11-2011, 14:31   #7
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Re: How Much Solar Do You Need ?

Tough part I've encountered with solar is that when it's cloudy for a few days straight the whole thing falls apart. It's just not realistic to carry sufficient batteries to make up for that much lack of input.
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Old 17-11-2011, 15:15   #8
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Re: How Much Solar Do You Need ?

My friend: the wind is solar powered,think 20th (early part) century not 21st and you could be sailing soon.
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Old 17-11-2011, 15:36   #9
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Re: How Much Solar Do You Need ?

It seems solar powered wind can be more effective at times than sun powered solar power.
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Old 17-11-2011, 19:11   #10
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Re: How Much Solar Do You Need ?

My take, from the perspective of owning 3 electric vehicles is this, if you stay in the 5~6 kt speed range AND have long, narrow, efficient hulls like a light displacement cat, you might be able to cover one mile on 500 w/hr. With the new batteries on the horizon, a 500 Kw/hr bank will in the near future come down in weight to 3000 lbs. So if your solar panels (which now have come down in price to $1.30 per watt. thanks China) can produce enough to meet "hotel loads", your fully charged (via the last marina) batteries could give you a 1000 mile range without sails. Of course the marina, not knowing for the two weeks you had the slip, you sucked down $75.00 of electricity, just helped your cruising kitty. Currently I like the diesel-electric hybrid systems. Instead of a 350 lb lump of diesel engine aft in both cat hulls, there is a 60 lb brushless PM electric motor with the genset in the center where that weight belongs. Quiet motoring in port, nice weight balance, and with reverseable pitch props, one heck of an underway hydro-generator for hotel loads and charging a large bank under sail.
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Old 17-11-2011, 21:24   #11
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Re: How Much Solar Do You Need ?

How long is a piece of string?

Do your homework, decide what equipment YOU want and work out the power budget for it. Then you'll find out how much solar you need and how much battery you'll need for YOUR selection.

You may also want to factor in engine time and battery capacity along the lines of the major ocean racing rules, which typically require entrants to have an engine capable of maintaining way in storm conditions AND enough fuel to power it for 48 hours.

If you give that any consideration, you're going to have an incredibly large and expensive lithium battery bank, or a diesel genset onboard.
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Old 18-11-2011, 05:16   #12
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Re: How Much Solar Do You Need ?

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Originally Posted by deckofficer View Post
Currently I like the diesel-electric hybrid systems. Instead of a 350 lb lump of diesel engine aft in both cat hulls, there is a 60 lb brushless PM electric motor with the genset in the center where that weight belongs. Quiet motoring in port, nice weight balance, and with reverseable pitch props, one heck of an underway hydro-generator for hotel loads and charging a large bank under sail.
I probably won't need 1,000 mile range under motor ........50 would be more than enough - and I would probably settle on 30 (in anticipation that would increase in the future)........and for that I am thinking the genset solution is my answer - even if it is wildly inefficient .....I might never fire it up (in anger!) but would be nice to have "juice" on tap should the need arise .
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Old 18-11-2011, 13:06   #13
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Re: How Much Solar Do You Need ?

Wind is caused by the uneven heating and cooling of the earths surface. Which makes any sort of non-fuel powered generator solar (even the one you tow behind your wind powered sailboat).
Actually you need less power for your electrically powered engine on long passages than short coastal stuff because on long passages you spend more of your time sailing.
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