For Yacht Builders, Buyers, and Owners: 🔐DIY-Q&A

What If Solar Won't Keep Up On a Cloudy Day?

Phil Friedman — Fri, 27 Mar 2026 12:03:00 GMT

At least once a week, a question comes across my computer screen about how to back-up solar power on cloudy days or at times during the year when there just isn’t enough sunshine available. This is not a question about the merits of solar power on a boat. It’s a question about how to create a sensible back-up alternative source of electrical power when moored or at a dock.

The other day, I noticed an interesting post on Facebook in which a sailor/DIY-craftsman was speculating about assembling a small diesel engine to run a standard alternator to provide 110vac current at times when the solar power system on his sailing auxiliary doesn’t keep up with demand. He opined that such a rig would be preferable to using his inboard propulsion engine in lieu of a stationary genset in such circumstances. He said,

Here is a 5 HP gas engine hooked up to an alternator 120 amp 12 volt, but I want to build one that is the same alternator but with a small diesel engine on it to put on my sailboat. My sailboat does have solar on it but there are days when it is overcast and the solar panels cannot keep up. I think this is a better alternative than running the engine. Would like some comments on this what do you think of it.
— Bruce Gripp, (https://www.facebook.com/groups/Sailing/permalink/3645167569056556)

I commented that while it might not be a bad idea, I thought there was a better solution — one which avoids adding the weight of a second engine, yet still avoids the potential problems a propulsion engine might suffer if used for extended periods of time as a stationary electrical generating plant. Here’s what I wrote:

If it were me, I’d look at the possibility of adding a second high-output alternator with a simple clutch to the propulsion engine for stationary battery charging. With a small engine, a high output alternator will by itself provide sufficient load to prevent the engine from running under-loaded and possibly coking up. While clutching out that load when running under power will avoid potentially overloading the engine when using it for ship’s propulsion.

The advantages of solar power are numerous. But there are times and circumstances when a more-or-less average solar power system won’t generate enough current to meet demand. So it makes sense to have a back-up available. A wind generator would be, IMO, too mechanically complicated and have too little output for the application in question. Moreover, such units are pretty bulky and almost as costly as a stationary liquid-fueled genset.

Against that, the back-up generating system outlined above could be economically fabricated using mostly off-the-shelf parts. The only slightly thorny issue might be finding an appropriate clutch to enable the high-output alternator to free-wheel when not being pressed into service. But since a simple mechanical clutch would do, I that issue should be relatively straightforward to resolve. And my guess is the cost to fab and install the setup envisioned would overall be less than building a makeshift genset. It would also— be significantly lighter and more compact than other alternatives since it would require only enough space for a second alternator.

I don’t at this point know if there is a proprietary manufactured system of the type currently on the market. But if there is enough interest among our readers, we’ll follow up with a search. And we’ll see if an engineer or two among our membership might draw up some plans and specs for a custom fabrication and installation. So do let me know what you think.

— Phil Friedman

Image credit - Bruce Gripp

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DIY or Pro Question? Post it for the community in the comments here ...

Phil Friedman — Thu, 01 Jan 2026 19:06:00 GMT

The central objective of this forum is to facilitate the free and open exchange of key industry information, expert opinion, and relevant professional and lay experience, for the mutual benefit of all our subscribers.

To ask a question or initiate a discussion, enter it in the comments to this post.

We receive real-time notices of comments that appear, and we will keep track of the responses, which should also be posted in the comments thread here.

We’ll moderate and track the discussions.

Those which are short or relatively simple to resolve will be moved to a DIY Archive that will be kept to serve as a help resource going forward.

Those which are more complex will be moved to our Chat section, where exchanges are real-time and it’s easier to keep the conversation organized and updated. When resolved or talked out, they will also be moved to the DIY Archive.

Please, let’s keep the questions and discussions reasoned, rational — and relatively significant in a boatbuilding and refitting context.

In other words, please avoid the trivial. If you want to know what the best waterproof cockpit speaker in the world is, that is best asked on Facebook or researched on Amazon. But if you want to know, for example, how to layup epoxy carbon fiber or where it’s worth the extra labor and expense, or the differences are between linear polyurethane and acrylic modified urethane coatings, well, those are the kind of questions we hope to be able to answer for you here in the DIY Forum section of For Yacht Builders, Buyers, and Owners.

That said, feel free to join the conversation. Cheers!

— Phil Friedman, Editor and Publisher

And if you have another minute or two, please share this post with some of your friends and connections, or leave a comment — or both.

Cost-Effective Boatbuilding With Sheet Material for Hulls and Decks

Phil Friedman — Wed, 02 Oct 2024 20:37:16 GMT

Preface: This article is nominally about boatbuilding materials and processes, but in reality, it's also about a cost-effective approach to small craft manufacturing, recreational marine marketing, and perhaps even the future of the small-craft industry itself. The materials and methods discussed here represent a pathway for returning to an era when own…

To Convert or Refit ... Or Not

Phil Friedman — Thu, 22 Jun 2023 16:14:00 GMT

Sometimes, nostalgia or raw gut attraction for a particular vessel or form or type leads one to spend more in labor, time, and dollars on a conversion, refit, or refurb than makes cold financial sense.

Not too long ago, I received a query from a potential client who wanted to talk about refitting a partially-built motor yacht he was considering buying. H…

Small Solutions... Big Results

Phil Friedman — Sat, 29 Apr 2023 12:00:42 GMT

Value-engineering is an approach to boatbuilding that seeks to deliver maximum utility value per unit cost. Not the cheapest boat per se, but rather the most advantageous ratio of benefit to cost. Value-engineering isn’t, however, simply about the design of a product but, just as often, about the planning and execution of its manufacture.

For example, s…