John here: Phyllis and I are excited to publish Eric Klem's first article for AAC. That said, Eric is far from new here. For years, in the comments, along with Matt Marsh, he has been helping us all understand the engineering that does, or at least should, drive every decision we make about how to equip and operate our boats. Over to Eric:
One of my favourite gear additions to our cruising has been a solar panel. Prior to adding solar, our batteries did not last long and we regularly had to ruin the peace of quiet anchorages with hours of engine charging.
At the same time, I know that the addition of solar has negatively impacted our boat’s performance, and I’m conscious of the need to avoid overdoing it.
Often, sailors focus on the question:
What mix of energy storage (batteries) and energy generation (alternator, solar, etc.) will meet the energy needs of my boat?
But if we simply solved that problem, we would be ignoring the higher-level problem statement:
How do you optimize the boat as a system to make it comfortable while maintaining performance and seaworthiness?
The major trade-off here is sailing performance against electrical performance.
Of course it’s impossible to define the “right” trade-off for every boat and sailor. Racers might be willing to replace their batteries every season in exchange for lower weight and no windage from solar panels or wind generators, while others may be willing to give up performance in favour of increased comfort.
This article examines how to optimize this trade-off for people who do not want to run their engine or generator exclusively for charging and are willing to make the trade-offs to do so.
For people who run a boat with large electrical demands that necessitate a generator, John already has an article on optimizing that—see Further Reading.
Why Not To Oversize
Because everything on a boat is a trade-off, there are negatives to oversizing a system that can impact boat performance, your enjoyment of your boat, and perhaps even your safety.
For example, say that you are sailing to weather in your 40’ cruiser at 6.5 knots on lovely flat water with a 20-knot breeze and 20° heel.
The total force propelling your boat forward is going to be surprisingly low at around 1000N (225lbf).
Then let’s say you add a 1kW solar array on a tall arch. In the relatively undisturbed air back aft, that solar array is going to add around 200N (45lbf) of drag, being about 150N from the panels themselves and the rest from the supporting structure.
This 20% loss in net propulsive force is enough to slow your boat down from 6.5 to 6 knots, ouch—likely a bigger hit to performance than downgrading from a feathering propeller to a fixed one.
Windage isn’t the only thing to watch; weight is also important.
Batteries are not too bad if installed low and near the centre of the boat—only racers will be likely to tell the difference of increasing the bank say 200 amp hours (Ah) at 12 volts here.
But solar panels are a different story, as they are usually placed higher where they decrease the righting moment, and far aft where they will also increase the fore and aft moment of inertia, thereby causing the boat to pitch significantly more.
A 1kW array of good, rigid panels will weigh around 50kg, and the mounting structure will likely be at least as much again, so call it 100kg (220lbs) in a bad place.
Take It Offshore
Let’s go back to that lovely upwind sail we were doing but now put it offshore.
20-knot winds mean something on the order of 8-foot (2.4m) waves with a reasonably developed sea state.
Between that extra weight and windage on the stern, I would guess that we might find ourselves dropping from 5.5 knots average to 4.5 knots or worse, and averaging a few degrees lower on our course made good.
I suspect most AAC members have been in the situation where the boat is slow and wallowing, which is rapidly fixed by just untucking a reef, showing how sensitive boats can be to drive force. But we can't so easily change the weight and windage of a solar array.
So on a passage with a lot of windward work, this 1kW array might make our passage 25% longer, and the results will look even worse as the wind increases further.
Another noteworthy consideration is sight lines; solar panels often obstruct your view of the sails and/or the ability to see other vessels.
Finally, there are the safety aspects if a wave sweeps the boat. John has covered this in the past (Further Reading) so I won’t cover it again here.
A Balancing Act
So the key questions that we must balance are:
- Is this solar array:
- hurting performance too much?
- adversely affecting our ability to sail the boat safely.
- Is it big enough to keep up with the electrical loads?
Optimization should always start and end with conservation. Powering refrigeration, autopilots, watermakers, computers and other large loads is very challenging and minimizing these loads is critical.
Related to conservation is making sure that components are as efficient as possible. On the demand side, this means doing things like re-insulating your icebox and investing in LED lights.
Looking at efficiency in electrical generation:
- Having a powerful alternator that is properly regulated is key, I like ~3A/engine hp.
- With solar, I look for a minimum of 22% and prefer 24% efficient panels.
- Install a quality MPPT controller.
Arriving At The Right Balance
So now we have the problem defined, how do we solve it in the best and most balanced way?