Boat Concept: Solar Cabin Boat, its array in tow

[Possum Bobby]

One of my several interests is boat-building. I've built six. Presently working on this 23-foot cruising cabin boat:

https://boatbuildercentral.com/StudyPlans/GT23_STUDY.pdf

Am considering powering her as a slow-moving houseboat, with a Torquedo electric outboard motor. I would go for their 25HP, 12 kw model. (Boat designer says she'll be happy with a 20hp). Now, a 12kw solar array is not small. Even if you covered her up with panels, say six 400 watt panels, you're only gonna get 2.4kw....I imagine you'd be doing a lot of sitting around waiting for batteries to charge. Plus, the panels are a lot of weight, way up high on the boat, which is problematic.

How about this idea: My array will not be above me, but behind. I will find an old pontoon boat and strip it flat. No need for a motor. I'll cover it with panels, and tow it behind. Inverter and batteries will be aboard the mainship. If I need to enter a tighter space, say a small harbor, I would drop the solar-array-boat on an anchor outside, and pick it back up when I left. This will be for longer cruising. Too much trouble for just a weekend. Our cruising range is the Gulf Coast IntraCoastal Waterway, and protected bays and rivers thereabouts. Quite sunny. I think I could situate ~ 14 panels on the pontoon boat. That's 5.6kw. Prolly would be able to make way with that.....slowly.

Crazy idea?

 

[AnarchyJet]

You are probably looking at 2kw to push that boat at hull speed around 5-6 kts. With your 2.4kw array you could be net zero or even charge batteries while under way in good sun. That would give you a range of around 30-40nm without battery. How far per day do you want to go?
Add in 10kw of battery and you almost double that, but would have to be stationary or have another means to recharge.

 

[Possum Bobby]

Music to my ears. Not sure about 6knots from 2kw. You sure? The mainship is eight feet wide, 23 long. Weight ~1,500 lbs.
Plus some drag from the towed array....10kw of battery is doable.

We are retired, so miles per day? 30-40nm is enough, 60 with batteries plenty. The plan is to cruise marina-to-marina, take a transient slip, and stay at nearby hotels. In which cases she could recharge on shore power. But she is a cabin boat, after all, and if she gets caught out, she has births, a head and galley. She can overnight, awaiting the sun. .......And I've a little 1kw Honda generator as backup.

 

[PVGeezer]

The writeup says 6-10 knots from 25hp. While that sounds very conservative I don't think 2 HP is near enough. You can look at the vicprop website calculator to get an estimate of hp required for a speed target. But what you really need is some actual data on that boat. Has anyone built one yet?

I had a 32ft trimaran (about 3000lbs dry displacement) that would do about 5 or 6kts on a 10 HP Honda at around 75% throttle. Less than 5 or 6kts you can't make headway against any kind of wind or current. That's frightening. And the boat youre building with that tall cabin on it probably has more windage than my tri did and likely will displace more than my 3000 lbs fully loaded with gear, batteries, and a towed solar array. If I had to guess I think youre going to need at least as much hp as I did to make 5 or 6 kts.

BTW my tri was a Searunner plywood and fiberglass constuction similar to what youre building. (I didn't build it tho).

Torqueedo is not being entirely honest when they say a 12kw electric is equivalent to a 25hp gas outboard. 1HP is 746 watts. So 12kw is 16HP. HP is HP is HP regardless of whether the power source is gas or electric.

I like your idea of towing your power source although it might not be fun maneuvering in tight conditions. What id suggest if you proceed would be something like that Torqueedo operated at say 50% during normal cruising to do the 5 or 6 kts. That would give you around 8hp or 6kW at normal cruise with some headroom in bad conditions. So maybe 1 nm/kwh.

Your problem is going to be the length of your towed array. If you want to go 30 nm a day you'll need a minimum of 30kwh. A flat solar array (you'll want it flat for windage) on the Gulf Coast in the summer is going to give you an average of about 6 sun hours. Assume a 20% derate for the PV array and system efficiency losses you'd need 30/6/.8= about 6.25 kw of solar. Id try to go bigger unless you can be more sure of the HP requirements. Id also have at least 1 day of battery storage. Assuming about 20 watts per square foot thats 313 square feet of solar min. Staying at 8 ft beam that's 40 ft long of more.

If you can get comfortable with the weight and put a solar canopy on the boat itself you could get some of what you need there. The boat isn't going to be light anyway because you're going to have batteries, water, etc in the bilge. Weight adds up quick when you're cruising.

 

[Possum Bobby]

All very thoughtful. This boat has been built by others, but I've no access to their experiences. I'm half-way through the construction. Selecting a power source will be a hard decision. The designer, Jacques Martens, died last year. He was helpful. He says the boat is rated for up to 350HP, but that such would be a ridiculous excess. He offers a variety of engine options. Recommends mostly 70-90hp. But the boat actually has a planing hull, and he says to achieve planing speeds, the minimum power is 90hp. I was intending to put a 110hp, or 140hp, gas-engine back there. But the designer also says the boat will be quite happy with 20HP engine for slow cruising. More like a houseboat.

Since I caught the solar bug recently (installing a system for my house right now), I began to think about using solar/electric for the boat, and other things.

I built an earlier boat, a 14-foot skiff, same designer. I tried a "20hp" electric Elco outboard initially. 48volts. I had 4 30ah 12v gel batteries in series.
She would not come close to planing, as she now does easily with her new 20hp Tohatsu gas OB. The company was pretty dodgy when I called them to complain. So I agree with you: Not sure I trust these ratings comparing electric to gas OBs.

I do use a 3hp Torquedo on another boat I built, a 14 foot sailboat. Works great!

Another option might be to put a 20-40HP gas OB back there as an auxiliary, alongside the electric engine.

Anyway, thank you. I will keep y'all posted. Long live DIY.

 

[Rednecktek]

Someone else did that a while back. Might be an interesting read.

 

[PVGeezer]

You might want to check out the UK subculture of canal narrowboat folks. Lots of online info available. Some of those boats are 70ft long. The videos of ppl maneuvering them in the turnaround basins are entertaining.

Many have electrified and solar powered their boats. They're heavy boats but they putter along at 2-3 kts max and generally don't try to go more than a few miles a day. So they can get by with PV that fits on the boat. Most don't move much in the winter if at all, so solar can work for the full timers too if they have an alternative heating system.