The same basic rules apply to pretty much all alternate energy projects:
How practical are solar powered boats?
You have to be hard-nosed and realistic about:
1) how much energy is really available
3) understanding how well the two match up.
The latest edition of the Solartanic uses two 24 volt, 54 pound thrust Minnkota trolling motors.
• At full power the motors draw about 45 amps@ 24 volts = 1080 watts while pushing the boat just under 5 MPH.
• Cruising speed is defined (rather arbitrarily) at 20 amps@24 volts or 480 watts which pushes the boat just under 4 MPH.
• Note that 1 HP = 746 watts so full power works out to about 1.5 HP, although in practice the motors are not 100% efficient.
• I currently have four 12 volt marine duty batteries (arranged in 2 pairs to produce 24 volts) with a realistic capacity of 100 amp hours. So I can get a bit over 2 hours at full speed
or 4 to 5 hours at normal cruising speed out of a charge. The boat still moves along nicely at
10 amps in which case the batteries last about 10 hours. The boat could easily hold two or
three times as many batteries if I want, but Lake Boon is not very big so the current battery
1) How much energy is available?
2) What are the energy requirements?
I found my solar panel by cruising the web. The going rate for new PV (Photo-Voltaic) panels seems to be a bit under \$4 a watt. I wound up buying a 70 watt panel for \$269.00. This can produce a maximum of 70 watts IF full, unobstructed sunlight is shining at a right angle onto the panel. Note:
• if the sun is more than about 30 degrees from a right angle the output falls off fast.
• if it is an overcast day, the PV panel will only produce about 30% of rated output.
• a "12 volt" panel is typically 36 solar cells in series to produce the 17 or 18 volts needed to charge a 12 volt battery. Even slight shading from a cable stay (for example) on a few of the
cells will significantly reduce output.
Directly powering the motors from solar panels is an expensive and inconvenient proposition:
• Full power would require at least fifteen 70 watt panels (at close to \$4000) and then only under the most favorable conditions.
• Even cruising speed would require 7 panels, again under ideal conditions.
• If we wish to maintain cruising speed when it is cloudy, we need three times as many panels. Even then, what do you do when the sun goes down? Or when shaded by trees on a small lake
or river?
• Mounting 7 panels that are roughly 2x4 feet (let alone 15 or 21 panels) on a 22 foot boat is a bit of a problem (particularly if your trying to maintain the profile of the Titanic!)
3) How well do power requirements and availablity match?
After initial investigation, I quickly disregarded the idea of mounting the panels to the boat. It just didn't seem practical: it was too expensive and certainly didn't fit the "look" I was after. I've seen boats on the web that do run directly from solar panels, but they tend to have large flat surfaces for mounting them. Catamarans are often popular for this type of boat since they provide a lot of square footage and can provide a somewhat higher cruising speed at low power.

The obvious alternative is to run on batteries and mount the solar panels to the dock instead of the boat. It is (usually) much easier to orient the panels to maximize the energy collected. Since I live right on Lake Boon, my boat, like many pleasure boats, will spend most of its time at the dock where the panels can charge the batteries. My estimate on how I typically use my boat:
• an hour after work a couple of days a week for a sunset cruise if the weather is nice.
• four hours on each of the weekend days
I picked 10 hours a week as an average utilization.
Two 70 watt panels (in series to provide 24 volts) can provide a maximum of 140 watts. Making some allowance for less than perfect efficiency, it takes 4 or 5 hours of good sunshine for each hour of cruising at 480 watts, or between 40 and 50 hours a week. Given the long days of summer, an average week should provide enough sunshine to support 10 hours of cruising. My experience at the end of last summer (with the Solartanic's original 12 volt motor and single solar panel) seemed to confirm this estimate. I've recently received my 2nd solar panel and will be observing the new setup as soon as I get it installed.
Is it worth doing this? The panels cost about \$540 and will be generating about 50 cents of electricity per week. However it would cost several hundred dollars to have an electrician install a GFI protected line down to the dock so I decided the solar panels were worth it. So the short story is:
• \$700 worth of solar panels, controller, wiring etc will support 2/3 of a HP for about 10 hours week. If this meets your requirements, it may be cost effective if you don't already have 120 volt power at your dock.
• As with most forms of alternate energy, you must be willing to live with the statistical averages unless you have a backup power source.
• The Solartanic will most likely only exceed the averages if I trailer it away from Lake Boon to show it off. If the boat is in my drive way on the trailer its easy to hitch up the battery
charger and charge the batteries using house power.
Analyzing a specific design case helps illuminate some of the issues with AE. The Solartanic can serve as a useful example. I had already decided on electric power for my boat. Electric boats running on storage batteries have a history over 100 years long and continue to be successful in niche markets where peaceful, sedate cruising rather than speed is the priority (water skiing will have to wait for higher density storage). The question was: do I run an AC line down from the house to the dock to allow convenient recharging of batteries or can I do it with solar panels?
Summary: My two 70 watt solar panels, along with a controller and wiring cost about \$700. This will support about 2/3 of a horse power for 10 hours a week during the long days  of summer. I decided this met my requirements so I installed the panels on my dock. But people expecting a couple of solar panels and 4 storage batteries to be a replacement for a 200 horse power outboard motor will be greatly disappointed.