Wind is better than solar. Discuss

[Jim Cate]

Boats with solar arches will tend toward rounding up when sailing upwind because of this. The wind gusts, the boat heels over, the balance in the sails shifts to the main which is exacerbated by the large panels way aft now catching a fair amount of the wind, the force exceeds the turning force of the rudder and the boat rounds up

Well, I can't speak to how your boat would behave, but after some 68,000 miles in our current vessel, one with a substantial solar arch and 400 W of panels, your scenario just does not happen. Not once, and we've been through some strong winds and big seas, upwind and down.

One can decry the aesthetics of solar arches (although some thoughtful design can reduce their impact) but catastrophic changes in balance are not what I've observed personally.

And fwiw, to me the turbine(s) are far worse looking that a well designed arch!

Jim

[chrisr]

imho life is never as simple as either a or b

when purchased our boat was set up 650w solar / 400w wind / 6kva diesel gennie and we very much appreciate the mix (although sometimes we'd like a bit more solar)

no matter what happens we have power & hot water & fresh water

the wind generator has the added advantage that it occasionally gives you a bit more room in a crowded anchorage !

cheers,

[DGH]

It's pretty obvious to me.
If wind generators had low drag compared with other options (solar, Watt&Sea) then you would see them on racing boats, e.g. IMOCAs, Class 40 etc, which you do not.
If you are worried about drag and want power generation sailing upwind then Watt&Sea type seems to offer the best ratio of power gen vs drag. Excellent down wind too. No good when you're stationary of course.
I am amazed that cruising sailors are concerning themselves with minor air drag issues. If sailing performance is your concern just get a lighter boat, deeper draft, more sail area, better quality carbon rig and sails - then see how your wife likes it.

[Nightsky]

Hi Na Mara. Sounds like you've already made up your mind. My experience with a wind gen was not so good, even though I was in an area of good trade winds for about 5 months. I found that my solar setup far exceeded the output of the wind gen (admittedly, not a high end generator). I have recently re-aligned my solar panels so that they will be even more efficient, and am considering not putting my wind gen back up unless I can find a suitable location for it that won't cause any shading on my solar panels. Below is a link to a thread I started some time ago. You will see responses from others with different brands of wind gens, with comments of their wind gen outputs, as compared to my cheapo unit. My conclusion is that solar will provide much more energy per dollar invested. YMMV and your boat's configuration may leave you with few choices.

https://www.cruisersforum.com/forums...ll-231350.html

[Na Mara]

The issue with an arch and sailing performance is not so much that it slows the boat, it will a bit but that is not the main issue. The main issue is that it’s a turning force on the boat that increases substantially with heel. This makes trimming the boat a bit more complicated.

For instance, when cruising upwind you want to reef to the gusts as having someone play the main sheet constantly isn’t on. As a boat with an arch will have a greater tendency to rounding up you will tend to have a little less main and a bit more foresail to compensate for this than you would sans arch. But then outside the gusts, when the boat is flatter the Arch will catch less wind and so the CE will move farther forward than is ideal for balance rigged like this. That’s not too much of an issue as it’s easier for the helm to bare off in a lull between gusts than it is hold against rounding up in the gusts. What you would mostly experience aboard is an autopilot working a bit harder than it would otherwise and a fore biased sailplan. These are hardly catastrophic effects, and would barely be noticed by someone sailing at 80%. I like to push a bit harder, particularly when going upwind, so it would annoy me, but we are all different.

[ohthetrees]

I think Na Mara has already made up his mind, but in the hopes of helping the next guy making this choice, here is my experience...

I have a Hanse 505, with a big stern arch. On that arch is a Eclectic D400 (a very well regarded, high end wind generator) and 750w (nominal) of solar. From Hawaii, to Seattle, to Mexico, and everywhere in between, mostly at anchor or sailing, solar has been our workhorse, and the wind generator has contributed only little bits here and there. I guesstimate that our solar/wind production ratio is about 85/15 at the very best. I'm considering removing our wind generator all-together, because I suspect its shading is causing it to be a net negative. I actually think it is a great wind generator, and would recommend it to people sailing where the sun don't shine. But if you have sun, solar has it beat.

Regarding Na Mara's hypotheses about sailing performance, well, I've had no difficulty balancing my boat over the 6000nm I've had this setup. I think the wind generator is actually a bigger handicap. It is a lot of weight up high (the D400 is hefty), and a lot of wind resistance. My arch (not very well made, I'm afraid) has a bunch of stress cracks, all right around the wind generator mount, which tells me where the stresses are.

I don't know if Na Mara's math is in error, or his assumptions, but something is wrong, because in the real world, it just hasn't worked that way for me. If I had to guess, I would say that he is wildly over-estimating how much, and how clean, the wind his wind-generator will see. He had some optimistic hypotheses that turbulence could be a good thing (!) for production, which just is way off base. What really happens is that when turbulent air comes off the sails, dodger, etc, the generator turns, stalls, starts, stops, etc. Even at anchor it will turn and stall frequently as the boat swings back and forth and dirty air comes off the dodger, bimini, boom, etc.

In the real world, solar is better for almost everyone. In the theoretical world, I guess it is the other way around. I know where I live.

[Na Mara]

So I have been hunting around for something that might explain the discrepancy between the math and peoples lived experience and I think I might have found it. It’s not turbulence. This does reduce turbine efficiency but can’t account for the discrepancy people are talking about between actual performance and predicted performance. What seems to be really going on here is wind shear. I did not know this but it turns out that wind shear increases as wind speed decreases. The wind speed measurements I used were taken at various heights but were centered around 12m. In light winds (ones that leave a smooth sea state F1-2) the wind speed at 5 m will be only a third of what it is at 12m. On average seastate conditions F3-4 the wind speed at 5m is 80% of what it is at 12m. One can almost ignore wind shear in average or fresher conditions, but I missed that you can’t in lighter conditions.

This makes a very big difference to my early calculations as the mean wind speed in light air regions is 10 kn at 12m. Assuming a wind shear of about 50% the mean wind the generator seas is then 50% of 10kn, or 5 kn. That is the cut in wind speed for a silentwind. Half the time the generator won’t turn at all. The other half it will produce about 25W on average, so then the average output in light wind regions is not 36W per unit as I supposed but rather 12W and further, generation will be zero 50% of the time. That puts a whole new complexion on the issue and I may have to think again.

To summarize ignoring wind shear effects in light airs I calculated average performance of 36W per generator and so 72W for the pair with less generation than consumption occurring 5 days a month even in light wind regions. But if you include that wind shear is at its worst in light winds then I calculate an average generation for the pair of just 24W with 0 generation occurring 50% of the time.

That is too little generation and to long periods of zero generation for my needs. Ok this is a worst case scenario assuming light southern Baltic summer conditions, bu still. The disproportionate wind shear in light winds is a real bi*ch for deck mounted turbines.

I’m going to have to rethink this.

[Na Mara]

The lesson here maybe that if you want decent output from wind generators in light airs they need to be sited way up the mast. Problem there is that on a sloop that implies VAWTs which are only 70% as efficient as HAWTs and have a higher cut in speed. All of which negates their advantageous placement. If its light enough for wind shear to be a problem for HAWTs then it’s too light for the VAWTs to function.

I realllllllly don’t want an arch on my boat. It will look awful, add windage, be in the way. My cockpit is so high and aft that the panels would have to be about 3.8 m above sea level for head clearance. On a 14m boat it will look hideous.

[Anders]

Sounds like you are planning to sail south to the sun?
When you get to the sun you will need a bimini. Consider designing an arch that's useful for the bimini and solar panels.

[Na Mara]

Two reasons why a Bimini isn’t on the cards:

1 Na Mara is a pilothouse sailboat and a pilothouse roof makes a great low windage rigid bimini underway. At anchor I have a fold up cockpit tent with stooping room under it that works as a bimini for alfresco activities but I like the fact I can get it out the way easily for sailing.

2 The cockpit is raised to get a master cabin under it and a view over the pilothouse. A bimini high enough to give headroom under it in the cockpit would be so high that it would look weird and it would interfere with the boom.

I’m thinking that the best bet here may be davits but I’ll need to look at it closer.

[Adelie]

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). 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.

A flat plate held 20* from edge on to flow is going to have a Cd significantly less than 1.0.

Or people would not biminis on their boats.

[Na Mara]

My curiosity peaked I went searching for what the CD of an inclined solar panel is. According to the formula found

https://mdpi-res.com/d_attachment/en...s-08-02438.pdf

and assuming an aspect ratio of 2 ( that the panels as twice as wide at hearts as they are long for and aft) and heel angle of 20degrees the Drag coefficient is 0.48. That is about the same as a wind generator. Lower than the value of one I first assumed but certainly not negligible given the presenting area of a panel array will be about a third of its actual area at 20 degrees of heel (wind abeam). A 4.5m2 array would then present 1.5m2. The silent wind is 0,95 m2 swept area. The drag force on heeled panels of any substantial size can indeed be greater than that on a wind generator..

[Na Mara]

Addendum . At 40 degrees apparent upwind the presenting area of a 3*1.5 m panel heeled 20 degrees is 0,95m2. I.e exactly the same as the swept area of a silent wind.

[MicHughV]

the only real issue with solar panels.....imo....they obviously don't work at night or in cloudy/rainy conditions...and a lot of panels don't like any shade over the panel...even a partially shaded panel generally does not provide much.....
wind generators are noisy....no two ways about it...but good to have....obviously they don't work in light or no air....sailing downwind is troublesome for a wind gen.

If I had to pick only one over the other....I'd opt for the panels...

[Na Mara]

For interest I calculated the turning force exerted by 20 degree heeled panels on an arch using the above figures

Wind abeam

20kn apparent. 88N
40kn apparent. 352N

Apparent wind 40 degree off the bow

20kn apparent. 33N
40kn apparent. 142N

Recall that these forces are acting at the end of a long lever arm. On my boat the turning moment would be about 6 times these values in Nm. That’s a lot of torque to trim out, particularly at 40kn.