We are often asked about the impact of wind resistance on solar panels mounted on a pole when the boat is under sail or when it is blowing a “hooley” out there in heavy seas. This is a valid concern. Anytime you put something in the way of the wind on a boat there are bound to be consequences.
It is important to note that none of our customers to date have reported a problem with wind resistance nor have we had a problem on our test boats.
Fortunately, an adjustable pole mounted solar panel offers numerous options to address wind resistance. Some of our customers have reported that they tilt their panel to a horizontal position in a blow to minimize wind resistance. Other customers have replaced the single nut and bolt that attaches the panel to the pole with a locking pin or lynch pin. In a blow, they simply pull the pin, unplug the panel and stow it down below. Other customers just leave the panel alone and don’t worry about it.
The panel is usually tilted at a 30 to 45 degree angle to achieve optimum sun angle. This alone reduces wind resistance by over 50% relative to a panel in a vertical position perpendicular to the wind (not sure why a panel would ever be in a vertical position however). Also, it is important to consider that the boat is moving smoothly with the waves thus enabling a relatively even airflow over the panel. In addition, the panel can be rotated so achieve some potentially beneficial results. With the top of the panel tilted into the wind, the unit will tend to put a downward force on the stern. With the panel bottom into the wind, it will tend to put a lifting force on the stern.
What is the weak point in the system in a heavy wind? Looking at the system as a unit, it is apparent that the pole, the tilt mechanism and the braces attaching the panel to the tilt mechanism are plenty strong. The brackets attaching the pole to the stern rail are plenty strong. Our tests have proven this to be a fact. The only remaining point of potential failure then is the stern rails. We have seen boats with very strong well anchored stern rails and boats with rather dubious stern rail anchoring systems. Each boat owner must assess the strength of the stern rails and determine their ability to support the stresses of a pole mounted solar system. It could be that lateral braces connecting the pole to the deck or to the bimini frame are necessary.
The size of the panel is directly proportional to the amount of potential wind resistance. We recommend mounting a panel no larger than our 120-130 watt panel which measures approximately 49 X 32 inches on a boat with strong well designed stern rails. The Kyocera 85 watt panel measuring approximately 40 X 26 inches is ideal for our smaller panel system. See other blog entries for an analysis of the performance of this panel.
The weather for our cruise in the North Channel of Lake Huron this year was excellent. It was warmer and sunnier than last year. Our Kyocera 85 watt solar panel performed very well. The following are some statistics:
Days cruising: 22 days
Max power generation: 40 amp-hours
Min power generation: 13 amp-hours
Average power produced per day: 35 amp-hours
Average power consumed per day: 51 amp-hours
Our primary use of power was for the refrigerator/freezer. We ran a power deficit each day of about 16 amp-hours at anchor but with our 360 amp-hour battery bank, this not a problem. Moving on every 4 days or so gave the 75 amp alternator plenty of time to bring the batteries back up to charge.
We spent the month of July in the North Channel in Northern Lake Huron north of the 45th parallel. We logged the output of our top-of-pole mounted 85 watt Kyocera solar panel and our daily power consumption. It was an unusually cloudy July. Here are some statistics.
Mostly cloudy 7 days
Partly cloudy 8 days
Partly sunny 8 days
Mostly sunny 7 days
Maximum power produced in a day was 38 amp-hours
Minimum power produced in a day was 12 amp-hours
Average power produced per day for the 30 day period was 25 amp-hours
Average power consumed for the 30 day period was 46 amp-hours
Our travel pattern was to stay at anchor for 2-3 days and then motor/sail to another anchorage. We connected to shore power one night. While we ran a power deficit of 21 amp-hours per day while at anchor, our high output 75 watt alternator quickly replenished our batteries on our travel days.
We ran our refrigeration/freezer every day as well as our computer (chart plotting) and autopilot when underway. We found the combination of our 85 watt panel and our 75 watt alternator provided us with ample power for the trip.
Solar panels provide an excellent solution for obtaining the electrical power needed to support on-board electrical systems. They are quiet, require little maintenance and are reliable.
While cruising, we take a very conservative approach to using electricity on board our 38 foot sailboat. Even so, when considering the solar option, I concluded that using solar panels to supply all our electrical needs would be expensive and would require considerable panel surface area. With this in mind, I set out to use solar power to augment the generation of power with the goal that we could be at anchor for three days without running the diesel to charge the batteries.
The 85 watt Kyocera KC85T is rated at 5 amps (17 volts) and measures about 26" by 40". We can generate about 35 amp-hours on a sunny day on the Great Lakes by adjusting the angle 3-4 times a day for optimum efficiency. Our energy usage is roughly 55amp-hrs a day at anchor. Thus we run a deficite of about 20 amp-hours per day.
Here is how we calculated our energy usage per day at anchor.
Equipment Amps Amp-hours/day
Refrigeration 5 20
Instruments 1 4
Computer 4 6
Lighting 2 (avg) 10
Pumps 5 5
Other 1 10
(propane monitor, etc.)
Total amp-hours 55
When under sail we have an additional current draw of 20 amp-hours from our auto-pilot and instruments.
We have a 75 watt alternator on board and a 360 amp-hour battery bank. Because we typically run the engine several hours on the days we are moving, the battery bank gets charged on those days. This system works out well for us.
Each cruising boat has different patterns of energy usage and different requirements. Calculate your usage, consider your sources of power generation and select the size of the solar array you will need. Based on this, you can determine whether a single panel or multiple panels will be required to meet your needs.
Contact us if you would like assistance in determining the size of panel that will be required to meet your needs.
The top-of-pole mount offers excellent functionality for getting the maximum performance out of your solar panel. The closer the surface of the panel is to being perpendicular to the rays of the sun the higher the performance (amps output). Thus, the panel needs to be both tilted and rotated. I have found that adjusting an 85 watt panel 3-4 times a day will achieve power generation of up to 40 amp-hours on a clear day on the Great Lakes.
A panel mount should be adjustable easily without the use of tools. Our panel mount provides 13 tilt positions by simply pulling a pin, tilting the panel and replacing the pin. The panel, which is atop our crane pole can be rotated 360 degrees. We provide a pole locking mechanism to keep the pole from rotating on a windy day and have found that using the mechanism as a brake to limit free rotation of the pole works well.
As shown in the picture, our pole is mounted to the stern rail away from the boom so there is nothing to cast a shadow on the panel. Shadows can significantly reduce the efficiency of a solar panel. The pole is mounted high enough to clear the bimini yet easy enough to reach to adjust the angle to the sun.
My sense is that the 120 watt solar panel is about the maximum size
(31" X 48") to mount on the top of a pole such as ours. The limiting factor is the strength of the stern rails that stabilize the pole. A larger panel would require bracing struts from the pole to the deck. Clearly this is possible but I would recommend mounting larger panels to a frame above the bimini, on dinghy davits or directly to the stern rail.
Pro and Cons:
Top of pole mount
Pros: Easily adjusted for best sun angle, clear of shadows, out of the way.
Cons: Limited size of panel that can be pole mounted.
Bimini frame panel mount
Pros: Can mount large panels, out of the way.
Cons: Cannot easily adjust for best sun angle, often have shadows cast by the boom, back stay or topping lift.