Can solar energy power a catamaran?

Discussion in 'Multihulls' started by EscapeArtist, Sep 4, 2016.

  1. EscapeArtist
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    EscapeArtist Junior Member

    Stumble, I think you might be confusing power with energy. At the Earth's surface the peak power is 1050 W/m^2, but to get energy you need to integrate power over the entire day and that's where the 4 kWh/m^2 comes from. I included 15% efficiency (and knocked 20% off of that too) in the calculations. The other big difference is that Solar Planet was running 24 hours a day, and I was assuming only 5 hours of operation for a coastal cruiser.

    Richard, I don't understand your comment about perpetual motion. Solar cells simply convert light energy into electrical energy.
     
  2. Stumble
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    Stumble Senior Member

    Your math is wrong.

    1,000w/m^2 is the immediate power available, 6,000wh/m^2/day is the usable power that can power a solar cell.

    6,000w * 20% efficency = 1.2kwh /day/m^2. You want to dedicate 40m^2 to the panels, so that's 40m^2*1.2kw/m^2=48kwh over the course of the day. If you want to operate 5 hours a day that's 9.6kw or power used per hour. Not very much to move a boat that size, particularly given the amount of windage all those solar panels will cause.

    I hope you never have a headwind.
     
  3. zimbodave
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    zimbodave Junior Member

    Escapeartist....
    There are lots of people already doing exactly what you're interested in.
    Search for the yahoo group electric boats. Lots of useful info on there.
    If Sun21 crossed the atlantic I don't see why you can't build something that you can coastal cruise with.
    if memory serves. Sun21 ave speed was about 5kn. Not far off a sail boat. But without all the hassle that goes with sails...rigging...winches etc.
    I'm not arguing with the math and the fact that solar power is a marginal power source. But it has to be possible. It's already been done multiple times.
     
  4. catsketcher
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    catsketcher Senior Member

    I got into sea kayaking pretty seriously a few years ago but couldn't help think that we were pretty silly paddling around much of the time. Taking away the fact that a kayak is very unstable (we practise rolling back up lots) it is really easy to have problems with fairly benign conditions. I have been worried offshore when paddling with others in a 10 knot headwind - we reached our destination but some paddlers went very slowly.

    Most sea kayakers in Australia now do serious miles with a small sail. It is really frustrating to be out on the sea and not able to use the wind that is usually blowing. Its also really hard to paddle into a headwind, that is when I dream of an outrigger canoe that could sail to windward instead of my kayak. The only time my kayak makes sense is in a calm, and it is not calm that often. I think a low power solar boat would have similar problems in windy conditions. In windy conditions most other boats will be flying along with heaps of power when the solar boat is struggling with insufficient push.

    I think it would be far more clever and useful to try and hybridise a sailing and solar boat. Obviously the sails could be used as solar panels if we ever get cheap plastic solar cells.

    Personally I can't see the point of being pure sunlight driven when the wind is solar powered too. Fiddling with sails is fun, it's what many of us like to do.
     
  5. Stumble
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    Stumble Senior Member

    The average speed over ground was 3.24kn, with an average speed thru the water of 2.75kn (they stayed in the Souther Equitorial Current the whole way which has an average speed of .5kn). Also note that they also were going primarily down wind, so they also had the wind and waves behind them.

    Sure it's impressive that Sun21 made it at all, but an average SOG of 2.75kn isnt. Compare this to the slowest 14m catamaran passage under sail and it's even less impressive given the Sun21 is a pretty high tech construction (all carbon, low drag hulls, etc). Compare the expected passage speed based on Ike purchase price and it isn't even close. Sun 21 price came in around the same cost range as a Gunboat 55 I think.
     
  6. Richard Woods
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    Richard Woods Woods Designs

    I was able to pedal a 35ft catamaran by myself in calm conditions at 2.5 knots. I would have been quicker going downwind in waves.

    One of my regular crew is a kayaking instructor. Even he finds anything over 15 knots to windward challenging and advises to stay under 10 and in flat water for most people.

    My "perpetual motion" comment was because many people don't realise how much energy you need to move things. I could have said "alchemy". What ever your beliefs science and engineering always rule

    RW
     
  7. EscapeArtist
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    EscapeArtist Junior Member

    Here are the details of the calculations for a 12 m (40 ft) boat.

    The available solar energy at the collectors is at least 4 kWH/m^2/day from the NASA website. Assume 15% conversion efficiency and that gives .6 kWh/m^2/day, and over a 5 hour time span gives .12 kW/m^2. Throw in a 20% loss factor for frames and motor efficiency and you get .096 kW/m^2. For the 12 m LWL boat, the beam is 5.5 m and if you cover 60% of the area that gives 39.4 m^2 which gives a total 3.78 kW.

    From Terho Halme's equations the displacement will be 3.5 m^3 and the mass is 7.1725e+03 kg. This gives a hull speed of 8.46 m/s and the required engine power would be 28 kW at that speed. Assuming that power goes up as the cube of velocity, P = k*V^3 where k is a proportionality constant, then k = P/V^3 = 28/(8.46)^3 = 0.0462. Using the available solar power of 3.78 kW we can calculate the velocity, V = (3.78/0.0462)^(1/3) = 4.3413 m/s ~= 10 mph.

    The assumptions here are:
    1) You can store the energy collected during the off-hours and re-use it in the 5 hour sailing period. This is a big question and I realize that it might be a deal breaker, but for the purpose of a quick feasibility analysis I'm ignoring it. If it won't work for other reasons, there's no need to look at the battery question.
    2) Terho Halme's equations are correct.
    3) Power is a cubic function of velocity.
     
  8. EscapeArtist
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    EscapeArtist Junior Member

    Richard, your experience with pedaling a 35 ft catamaran is a great example. The power required at hull speed of 8 m/s is 19.8 kW so the proportionality constant k is 0.0389. You were able to move it at 2.5 knots = 1.29 m/s, so the power required at that speed is 0.0389 * 1.29^3 = 0.0834 kW = 83.4 W. Over an hour period most people can produce 50 - 150 W continuously, so you're right in the expected range. The setup for pedaling a 35 cat is probably not optimal, so you're probably in pretty good shape.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    Power by solar is a lot like rain water catchment. There has to be a balance between the size of roof and the tank.

    Our 28ft powercat has 350W solar power. That is enough to run the domestic supply, fridge/freezer, lights, instruments etc. But the whole cabin roof is covered with solar panels. There would be room for another 400W over the bimini. But any more than that and we would end up with a very impractical cruising boat

    Our boat needs about 30hp outboard power to do 10 knots (7 knots needs 20, 16 knots needs 40. And that is proven amounts over 4000 miles or more)

    So do some more calculating based on that, and a 10 hr cruise time. How much deck space will you need. How long before you can cruise for 10 hours again?

    It requires very little effort to go slowly in flat water.

    RW
     
  10. Richard Woods
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    Richard Woods Woods Designs

    This is a good video to watch (I'm taking the video not pedalling BTW!)

    https://www.youtube.com/watch?v=Q5Oq45JQbXw

    It shows the boat sailing in normal wind then at 20secs one person pedalling at 2.5 knots and then sailing at 3.5 although the sea is still glassy calm

    But I do pedal in this one!

    https://www.youtube.com/watch?v=IaAp-AT84Ac

    The 1.5m dia surface piercing props worked really well, except they picked up weed and threw huge spray over the crew in reverse

    RW
     
  11. catsketcher
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    catsketcher Senior Member

    Equation don't relate to real world

    I like the maths, Escape, and from what I can follow it looks good until you get to the point of using the equations to find hulls speed and then slow down from that.

    My 38ft cat is pretty similar to the design here in length but much lighter - 4000kg, just over half the 7125kg in the hypothetical design. My boat has a 25 hp single outboard and at max throttle can do 9 knots with a clean bum. Power = 18.6 kW. If my prop has an efficiency of 50% then it is putting out a useful power of 9.3 kW to get to top speed. That is only if the 25 hp is measured at the prop and not at the crack. 3.78kW would certainly not get my much lighter cat up to 10mph. The final third of the throttle does little more than push us from 7.5 to 9 knots.

    I didpush my cat around with an 8hp once. It could do about 5 knots quite easily but I would have hated to see any wind with this configuration. I also once pushed the cat with the dinghy 3.3 outboard. I did do this once and I could get it to go about 3 knots. Of course it was a total calm, any wind and I would have been blown around (or sailing but this did happen inside a coral atoll) . Remember my boat weighs about 60% of what the hypothetical boat weighs.

    A solar cat would be a really unsafe boat. When the weather gets nasty I want to have lots of power, either big enough motors or sails. I can't see how going totally solar can get around the problem of being at risk in bad weather with anaemic motors and low run time. In bad weather the sun won't be shining much either.
     
  12. champ0815
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    champ0815 Senior Member

    Well, wouldn't it be a more realistic setup to use the wind as primary energy source for transportation and store the additional energy supplied by sun and wind even when not passage making for the hopefully rare cases when there is need for an auxiliary engine.
    Then you can calculate the battery size based on the assumption of using the engine just for harbour ballet, resulting in a lighter and more environmental friendly ship.
    For travelling extended periods in tight channels where sailing is not possible, an additional wind generator would be far more efficient in providing electricity as relying solely on photovoltaic energy conversion.
    Sail while you can, motor while you must...
     
  13. Timothy
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    Timothy Senior Member

    While I think a hybrid sail and solar configuration is the way to go, if
    the design length were to be increased the weight and displacement (foils?)reduced and every effort made to reduce windage employed perhaps using today's flexible panels light weight construction lithium ion batteries and Torqeedo like motors it could be done.

    Like Richard I use solar panels (6 100 watt flexible 4 lbs each) for all my house loads as well as the auto helm. I have a fridge and freezer each with their own compressor. I saved 700 lbs by converting to solar from diesel.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    A great post as always Catsketcher

    I towed my 32ft Eclipse catamaran behind a dinghy with 2hp outboard after being hit by lightning. Maybe at 3 knots in a calm. In days of old "ships of the line" were towed by rowers in the ships boats.

    30 years ago I did some testing on a 35ft Banshee catamaran (3.5T) I used 4hp up to twin 90hp outboards. The 4hp did 4 knots in a calm, with the 180hp, 22 knots towing a waterskier.

    How about this for perpetual motion? Tow a water generator that charges the batteries while motoring under electric power?

    RW
     

  15. EscapeArtist
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    EscapeArtist Junior Member

    Richard - Great videos and that pedal powered propeller is really cool, too. It's fun to think about how much you could get from solar power, but when I watch the videos I think - could I ever really give up sails?

    catsketcher - The power - velocity function was the biggest assumption. Here's a completely notional plot of required power at different velocities following a cubic curve. At V = 2 the power is 8, but if you drop back to V = 1.6 the power is cut in half to 4. So a 50% reduction in power only reduces velocity by 20%, and that's where the big advantage comes from. I'm just not sure how realistic the cubic function is, though.
    I completely agree that actually building a boat that relied solely on solar would be completely nuts. The suggestion by champ and Timothy to use it as part of a hybrid setup seems more sensible.

    If anyone wants it, I can post the code to GitHub. I mostly write in Matlab / Octave but it would be easy enough to re-code in Python or even as a spreadsheet.
     

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