Solar Ambition

This is the calculator suite I use to model solar-electric performance on Moonshot, a 53-foot motoryacht carrying about 6.6 kW of solar.

It models four things: deck solar capacity, terminal speed, daily solar range, and voltage sag.

The questions are simple:

• How much solar can I physically fit?
• How fast can the boat move under electric power?
• How far can she go on solar in a given month and location?
• Will the batteries and wiring actually tolerate the current draw?

The answer to the battery question is technically yes. The battery eventually drains. Physics still works. But in normal slow-cruise modeling, we usually do not hit that limit before route, weather, daylight, or common sense interrupts us.

So here’s the walk-through.

Deck Solar Capacity Estimator

The first tool estimates how much solar you can physically fit on the boat.

There is a bit of linear algebra in there, but some of it is also just me eyeing different boats and judging, relatively speaking, how much square footage is available to turn into panels.

There are other benefits too. Solar gives you more shade, protects what is below from UV, and reduces the load on the rest of the system. Of course it also creates new cleaning problems, because apparently every solution needs to bring a small bureaucracy with it.

My practical recommendation there is still: pressure wash.

This estimator is basically the “how much deck can I turn into watts?” tool.

A quick disclaimer: this is based on nominal advertised panel values and a rough estimate of usable deck area. Some of that estimate comes from geometry, and some of it comes from eyeballing boats and judging what could realistically hold panels. So no, this is not a scientific theory. It is an estimator. Please do not cite it in a naval architecture thesis unless you enjoy making professors sad.

Mind you, the result is nominal. And believe me, you need every square inch of it if you want meaningful electric propulsion. Even then, you are still taking the slow route.

That said, power is useful in more ways than propulsion. Refrigeration, pumps, electronics, HVAC, watermaking, battery charging, and the regular house loads all care about the same solar budget. The boat does not care that you made a beautiful spreadsheet. It wants amps.

Terminal Speed Calculator

The most interesting calculator is probably the speed calculator.

Most calculators have a comfortable safety margin built in. This one does not, at least not in the usual sense.

The reason is that I still have a pair of Detroit Diesels onboard. If you do not know what those are, imagine a World War II veteran that started life on a farm: chain-smoking, oil-belching, extremely loud, crude, and stupidly overpowered for its size.

They are supercharged, and as if that was not enough, also turbocharged. Being two-strokes, they make an absurd amount of power.

So yes, they can absolutely get our floating house on plane.

That gives me a real-world baseline to compare against. I had to dig through a bunch of different resources, all written in different units, to get to a usable number. That number is then further adjusted based on what I have actually observed on the water, so the model is not just theoretical hull math. It is also accounting, imperfectly but usefully, for real-world losses like prop slip and current.

The calculator estimates the realistic low-speed electric envelope. Not fantasy. Not brochure math. Just: what can this hull actually do under limited electric power?

And the answer is, roughly speaking: healthy adult walking speed.

Not glamorous. But “confident”.

Not bad for a girthy old gal born in the Bush Sr. era. Moving 24 tons at walking speed takes some swagger. It is amazing she is not moving backwards. If we had to row, she probably would be.

Solar-electric propulsion is not a magic replacement for diesel. It is a second operating mode.

Diesel gives you burst power, safety margin, schedule control, and the ability to get out of trouble.

Solar-electric gives you quiet movement, reduced fuel use, house power independence, and the ability to cruise slowly without lighting money on fire every mile.

That tradeoff is worth modeling.

Daily Solar Range Calculator

The next calculator estimates daily range.

This one helps answer the question: based on the time of year, solar input, battery size, power draw, nighttime draw, and cruise speed, how far can you go without touching the starter?

This is where solar cruising becomes less fantasy and more logistics.

Latitude matters.

Month matters.

Weather matters.

Clouds matter.

Commercial traffic matters.

Ferry wakes matter, because the universe likes comedy.

The calculator estimates daily solar generation against power draw and battery storage. In a good month, at the right latitude, at the right slow speed, solar can meaningfully extend range. In some cases, the boat can cruise for the day and recover a lot of that energy while underway or at rest.

I try not to oversell this part, because “infinite range” sounds like marketing nonsense. But there is a version of it that is kind of true, with terms and conditions very much applying.

The better phrase is: sustainable slow cruising under favorable conditions.

Less exciting. More honest. Still very useful.

Voltage Sag Calculator

The last calculator in this series is the battery sag calculator.

I learned a bit late that pulling 270 amps puts some serious hurt on a lithium battery bank.

These are not your granddad’s lead-acid batteries. Lead-acid is old, heavy, inefficient, and weirdly good at dumping huge current for a short period. They are almost capacitor-like when asked to crank. Terrible endurance, impressive burst behavior.

LiFePO4 is different.

It is cleaner, lighter, and better in many ways, but it also comes with a speed governor: the BMS.

So when you ask it to deliver big current for the thrilling experience of moving a 53-foot boat at a brisk walking pace, it may simply refuse.

Very dramatic.

That is where the voltage sag calculator comes in. It helps figure out whether your battery topology, current limits, cable sizing, voltage, and cable runs are enough to keep the system from browning out.

Which, yes, I learned about the practical way.

A propulsion system is not just “battery plus motor.” It is a whole current path, and every weak point eventually gets a vote. More parallel capacity, shorter cable runs, larger conductors, higher voltage systems, and realistic current limits all matter.

That is how calculators are born. First the boat humiliates you, then you convert the humiliation into software. This is called engineering.

The Pareto Frontier Bit

One detail I like: the motors on Moonshot draw about 6.4 kW combined, while the solar surface is estimated at about 6.61 kW nominal.

That was not an accident.

I am a little obsessed with the Pareto frontier: finding the edge where the tradeoff becomes meaningful, and where adding more of one thing starts demanding too much from another.

For this boat, I think this is close to the maximum sustainable solar-electric speed without abusing the battery bank. The motor draw and the nominal solar surface are deliberately near each other. Not because the system magically runs forever at full output, but because over time, the renewable speed is always governed by how much light you can convert into electricity.

The batteries can buffer. The wiring can carry. The motors can push. But the long-term ceiling is still the energy coming in.

That is the physics. No amount of motivational branding changes it.

Why I Built It

I built these tools because solar-electric boating has a lot of hand-waving around it.

It is easy to say “solar boat” and let people imagine something sleek, futuristic, and self-evidently efficient. And to be fair, other solar boats are often exactly that: faster, sleeker, cleaner.

Mine is not.

Moonshot looks like a cross between a house and a ROS kit.

But hey, she is our home.

She carries 100 gallons of water, can make her own water, has infinite range (terms and conditions apply), and now even has an onboard 31B-parameter AI XD.

So I am not trying to win a beauty contest with a carbon-fiber catamaran here. I am trying to answer a more practical question:

What does solar-electric propulsion actually look like on a real motoryacht, with real compromises, real loads, and a hull that was never designed to cosplay as a spaceship?

That is what these tools are for.

They help me reason about the deck as generation area, the batteries as range, the wiring as losses, and the hull as a drag curve.

Once you can see the numbers, the boat changes shape in your mind.

Speed stops being a personality trait.

It becomes a cost.

So no, this is not a substitute for proper marine engineering. Weather, sea state, installation quality, maintenance, hull condition, and system configuration all matter.

But it is a useful modeling tool.

And for me, it has been the difference between guessing and actually understanding the operating envelope of a solar-assisted motoryacht.

If you are planning your own electric journey, hopefully these tools can help you the way they helped me: https://arpeggio.one/calculator/solar_capacity

That alone makes it worth building.