Wind is better than solar. Discuss

[Mike OReilly]

Na Mara, I'm sorry if I missed this detail, but do you already have a dodger and/bimini? If so, are you mounting solar there?

My system involves 400 watts of solar mounted in four panels, two of which are part of my bimini (they are my bimini). No additional drag.

I don't have an aft arch, and I prefer not to mount them off the rails or lifelines as some do.

Just a thought/question.

[Na Mara]

My apologies, I wrote in haste. Drag coefficient for a flat plate perpendicular to the flowing medium is greater than 1 (1.28). There are also higher drag coefficients for large complex objects, but for simple shapes this is one of the highest. If anything your correction strengthens my assertion.

[Na Mara]

Na Mara is a pilothouse so we helm and keep watch from inside of the weather isn’t to our liking. Only a cockpit tent otherwise that is lowered for sailing. The pilothouse roof is the only place where semi flex panels might go easily but it suffers badly from shading by boom, mast and dodger and everyone poo poos semiflex.

[Na Mara]

When solar cloth becomes more readily available I will probably get the tent and dodger made out of it. Until then I want them to be easily removable for sailing.

[Mike OReilly]

Originally Posted by Na Mara

Na Mara is a pilothouse so we helm and keep watch from inside of the weather isn’t to our liking. Only a cockpit tent otherwise that is lowered for sailing. The pilothouse roof is the only place where semi flex panels might go easily but it suffers badly from shading by boom, mast and dodger and everyone poo poos semiflex.

All my panels sit under my rather large boom. Some of them are shaded almost all the time. This is why my system is divided into four panels, all run in parallel. One, or more, can go down, and the others keep pumping.

Only you know your boat and the circumstances. I think you can sense from the cautionary comments here that most of us think you might be making a mistake, but only you know for sure.

I really do look forward to hearing the results.

[noelex 77]

Originally Posted by Na Mara

My apologies, I wrote in haste. Drag coefficient for a flat plate perpendicular to the flowing medium is greater than 1 (1.28).

But solar panels are not mounted at right angles to the apparent wind.

The mistake you seem to be making is assuming a surface inclined at 20° has the same total drag as a flat plate mounted at right angles to airflow, but with a frontal area the same as the projected area of the inclined surface.

This is not remotely accurate.

[Na Mara]

The area you use when calculating the drag force on an object in a medium is the presenting area: for solid objects this is the cross sectional area perpendicular to the flow. I assumed this is the case with a solid plane.

Even if it isn’t, imagine holding a door into a 20kn wind inclined at 20degrees. I am stout guy with a low CG, who can lift 80kg but I would struggle not to fall over doing that. That is the sort of force exerted by about 300W of panels laterally over your rudder stock in 20kn apparent upwind. That force increases by the square of wind speed. Solar aches definitely do impact balance in the way I described earlier. As the boat heels in a gust the lateral force over the rudder goes up by sin of the heel angle and the square of the wind speed in the gust. Just when you don’t want more windage aft that’s what you solar arch delivers.

[Na Mara]

Mike. I hear the cautions and maybe I am missing something. I am pretty sure of the math, but the nonlinearity of air flow at 5m off the water on a boat might make more of a difference than I am allowing for. But what’s the worse that happens if I go ahead and am wrong. I’m a little short of generation and add semi flex panels on the pilothouse or a hydro generator. Maybe I’m out 2000 euro for one of the wind gens ( I was always going to have one as I don’t have enough space for sufficient solar without a big arch which I am not prepared to do for already stated reasons). I can live with that.

[rslifkin]

I'm really curious why you'd use semi and not rigid panels on the roof?

[Na Mara]

As an addendum to my early point about windage aft with an arch. Consider also what happens in a broach In high following winds. The bow digs in and the boat pivots and heels over as it swings beam on and the apparent wind increases as the boat ceases to run before. As the boat heels in the Broach it presents the underside of the panels to the wind thereby increasing the turning force on the boat and making it heel even more. Again the windage of the panels substantially increases just at the moment and location you least want it to. Again the windage of the turbines by comparison remains pretty constant through the broaching (it might increase a bit as the apparent wind spikes but not to the same extent as the exposed panels.

I’m not saying that one can’t trim sail to counter the heel dependent windage of a solar arch. I am saying it complicates such sail trim.

[Na Mara]

Originally Posted by rslifkin

I'm really curious why you'd use semi and not rigid panels on the roof?

I stow the dinghy there out of the way when doing short hops and occasionally I want to walk up there. I have thought about it. I could get two 1m by 0.7m panels in front of the main sheet track. Maybe about 250W with good quality panels. They wouldn’t be too much in the way for walking up there, but they would stop me stowing the dinghy there. Also one would pretty much always be shaded. Also holes in my balsa cored deck would be required.

[noelex 77]

Originally Posted by Na Mara

Even if it isn’t, imagine holding a door into a 20kn wind inclined at 20degrees. I am stout guy with a low CG, who can lift 80kg but I would struggle not to fall over doing that.

You are describing carrying a door inclined at 20° directly into a headwind. For this to occur on a boat this would mean that the apparent wind was at 90°. The true wind would be behind the beam. Thus the yacht would be on a beam reach or a broad reach. At this sailing point, for cruising sailboats extra aerodynamic drag is likely to have little impact on boat speed and may even be beneficial.

This does not mean the drag from a solar arch can be dismissed, especially sailing into wind, but the primary drag is produced by the frames of the arch and this should be minimised as much as possible.

[rslifkin]

Originally Posted by Na Mara

I stow the dinghy there out of the way when doing short hops and occasionally I want to walk up there. I have thought about it. I could get two 1m by 0.7m panels in front of the main sheet track. Maybe about 250W with good quality panels. They wouldn’t be too much in the way for walking up there, but they would stop me stowing the dinghy there. Also one would pretty much always be shaded. Also holes in my balsa cored deck would be required.

I put panels on my cabintop with similar concerns. In the picture you can see the size and placement accounting for needing a walkway to access the windshields for cleaning. I've only got one hole in the deck for wiring. The mounts are just aluminum angle held to the deck with 4000UV.

I went with the adhesive mounts based on other people's experience with them and so far they've worked out great. The math says that even if I only got 1/4 of the expected bonding strength I've still got something like a 10:1 safety factor in 60kts of wind. Each of the 4 mounts per panel has 6 sq inches of contact area with the deck. And you're dealing with panels much smaller than mine (mine are 410 watts each).

[Na Mara]

Bonding superstructure for the panels is an option that solves the holes problem. Another is mounting a frame directly onto my teak handrails athwarts. I could live with that and the obstruction to foot traffic as I rarely go up there anyway. But the loss of dinghy stowage is a big one, and unlike you I have substantial shading to contend with.

[rslifkin]

Shading would limit performance, but there's only so much you can do about that. As far as the dinghy, you could probably build a frame to hold it without it sitting on the panels, you'd just lose solar when you put it up there.