49 feet Solar Powered Plywood Monohull

I've been thinking about a project and it would be fun with some feedback. Are my assumptions realistic? Cheers!

It originated from the idea of a very simple hull powered by the sun and there I found Bolger's Wyoming/Tennessee. But over many iterations it has evolved to something a bit more complex, but I think more sea worthy and a less rural look.
Usage scenario is not, but easiest described as doing the great loop. But staying in my waters of Sweden/Europe or maaaybe Australia if that opportunity would arrive.

I want to minimize the regulations I need to follow, hence the boat is less than 15x5m. In Australia using less than 3kW may not require registration.

On the 15m hull there is a 5m long and 2.2m wide cabin. Entry is through a sailboat style hatch. Aft to forward, the headroom goes from ~1.9m to ~2.4m. Moving forward along the central walkway, on left hand a kitchen, right hand a head, in front a settee, passenger and driver seat (one or two steps up from the walkway sole), with a fore peak down below and possibly also a bunk aft of the cabin.

The boat would have 1 watertight bulkhead in front of the fore peak and 2-3 aft the cabin. Between cabin and fore peak a bulkhead with an opening. The interior would support the hull.

The boat is designed for 1x1.6m sized panels, 4 of them on the cabin roof, 9 on aft deck and 3 suspended aft. For propulsion I'm looking at one Torqeedo 6.0 RS, one motor for max efficiency.
The 16 solar panels weight approx 300-350 kg and the lead-acid batteries, placed low forward to counter balance them, weight 1-2 times that (cant go smaller or the batteries may get fried from charging).

With 315 Wp panels we get up to 5040 W.
Assuming an average insolation of 50|25|10 %, gives 2520|1260|504 W.
With a 50% efficient propulsion, 1260|630|252 W remains.
Which gives 5.8|4.7|3.5 knots. If we further assumes a 50% power margin (using 2/3 for propulsion) for a dirty hull, wind, charging and other unforeseen losses, we still get 5.1|4.1|3.1 knots.

With a beam to length ratio of ~5.6, a sharp bow and narrow stern, there should be little wake and hopefully a smooth ride even in some chop.

Relative to an Albin 25's weight of ~1.8t, my boat at twice the length, made of plywood, with an area of hull, deck and cabin top of ~130m2 (without bulkheads) I believe a weight of 3-4t may be achievable (20-30 kg/m2, including interior), add ~1t of solar+batteries+electronics and we still have ~1t of capacity for passengers, food and water.

Requirements:
1. 100% solar powered
2. Mono hull
3. Highest possible REASONABLE cruising speed
4. Suitable for doing The Great American Loop or similar cruising
(Cruising in protected and coastal waters)
5. Relatively easy to build out of wood and plywood
6. Shallow'ish draft, for access "everywhere"
7. Liveaboard-able for month's on end, with basic amenities for 1-2 ppls
8. Onboard space atleast similar to an Albin 25
9. Not require license(if possible) to drive/use in Swedish waters
10.Daylight drive only but with a few hours reserve for occasional dark

Approx. Specs, empty hull with solar panels and batteries:
LOA 14.9m
LWL 14.7m
beam 3.0m
BOW 2.6m
draft 0.4m
air draft 2.3m
displacement 5.8t at design waterline, +-1cm equal ~200kg
KM 1.7m
KG 1.0m
LCF 6.3m
LCB 7.1m
wet.surface 28.7m2
block coef 0.669
prism coef 0.533

Following numbers are based on Delft Series ('98)
speed|effective.power|fric.res|resid.res|frode|SLR
1 | 0.01 | 0.0135 | 0.0000 | 0.043 | 0.143
2 | 0.05 | 0.0476 | 0.0000 | 0.086 | 0.286
3 | 0.15 | 0.1000 | 0.0000 | 0.129 | 0.429
4 | 0.37 | 0.1694 | 0.0122 | 0.171 | 0.572
5 | 0.77 | 0.2551 | 0.0429 | 0.214 | 0.715
6 | 1.38 | 0.3567 | 0.0891 | 0.257 | 0.858
7 | 2.28 | 0.4738 | 0.1582 | 0.300 | 1.001
(Torqeedo 6.0 RS has an efficiency of <58% and a maximum effective power of ~2.7 kW.)

 

Can you find 50ft 6t boats going 6 knots with a 4hp OB?

to me your numbers look bit unrealistic. Also I think energy kWh is better gauge for the solar system. Flat panels, summer only 5 solar hours (of course comes over long period but energy equals to 5 full power hours) per day. So 5kw array gives you about 25kWh to use.
Not too shabby but I think the 3kw motor for that size boat is not going to cut it.

I would go with LiFePO4 batteries they are so much superior in performance and still safe. Cost is higher the lead acid but not that much when you account for realistic depth of discharge.

 

Thank you @kerosene I appreciate your concerns!

It's easy to derail when spending too much time in your own bubble, I'm talking about myself here and hence the fishing for feedback.

The required effective power feels low indeed, even after doubling it for the typically 50% efficient propeller. It is the reason I supplied a lot of data. Conclusion: I need to verify the drag calculations.

The 25kwh, it's the reason for the 50|25% insolation numbers chosen, the 10% was added to see the effect of even worse weather. Meaning, I did the same 5h estimate as you but tried to simplify the report for an easier read. Sry, my bad. All things considered, I think the 4.1-5.1 knots range will be normal cruising speed, if the inputs are valid.

LiFePO4 vs lead acid, looking at up front cost isolated from everything else, lead acid is a no brainer. But that is not how it works. Relative to the boats total cost the difference is much smaller, while the added performance of LiFePO4 would triumph lead-acid. Point taken.

 

I also think that your numbers are too optimistic for power available and when, and it is misleading you to believe you can get away with a big monohull. If you do an optimization with a realistic power budget you would likely find that the hull gets shorter to reduce wetted surface, and though faster, it's still awkward and unsafe. For safety, I would suggest you do some calculations for windage and waves -what power is required just to keep a boat that size under control, and multiply that by the time those conditions could exist. Tides are another major concern -I can't figure how you could navigate to both the tide table and the sunlight schedule. My opinion is that you need to add a substantial pressurized gas tank and a fuel cell -even if to only be used in emergency.
Try applying your simple assumptions/calculation to the solar cats that are real boats. Does it say they should be ripping around reliably, or does it show they need diesel backup to assure the expensive yacht isn't lost when it can't get out of it's own way? If it's not the later your analysis needs work.

 

Okay, let's back up to square one - you are fine with a 3-5 knot boat. What does one of those look like?

As you get slower and slower, the water gets less effective at resisting the wind and wave perturbation forces, so the volume of boat above the waterline must be made proportionately smaller relative to the volume below the waterline. For the boat you describe with very low power, the above-water-line volume needs to be no more that about twice the below-water volume. And in profile, the projected areas should be pretty similar. You need a built-down design with a decent keel and big rudder. If your ship displaces 150 cubic feet of water, the total ship interior volume would be around 450 cubic feet. If the hull's block coefficient is about 0.5 (you made a mistake in your block coefficient calculation, it can never be bigger than the prismatic coefficient), your boat should fit in a box of 900 cubic feet. If the beam is 6 feet, your profile box is 150 square feet. For a 50 foot hull, that leaves a depth of hull of 3 feet. If you want a six foot deep hull, you are right back to the Albin's 25'. Which shouldn't be a surprise, really. The Albin is a decent little 3-5 knot boat that will do fine with power you intend. The vessel you show isn't just inefficient, it is plain impossible to handle with the power you are suggesting.
__________
So what to do about it?

If you want a longer boat intended for low power, look at the historical sharpies. A 35' sharpie that has a few square feet of seated headroom under the coach roof is possible. The hull depth would be about 30-34 inches keel to deck at midship.
If you want more volume above the waterline, you have to provide the power to move the boat fast enough to control the boat while steaming and you have to provide the thrust to control the boat when docking. Your described hull needs about 40hp because I think it will have real problems in the handling department. Basically, you are out by one order of magnitude. And you need to provide energy storage so you can operate at that power level. Hunter 50 has about 160 gals of fuel capacity. If 100 hundreds pounds of batteries is equivalent to one pound of fuel, you would want about 45 tons of batteries to do what it does (but then it would sink). You would need about one ton of batteries just to have a decent chance of being able to run for two hours after dark in iffy conditions.

Scrap the Torquedo and drop in a Beta 50 hybrid (and a 100 gal diesel tank) and feed it with your solar setup. Either that or reduce your proposed hull's volume by half, give it a decent keel, add some ballast, and be satisfied with enough seated headroom below for four friendly people of average stature.
__________

I use the following formula for primitive living off solar. 1 panel per person, one for the boat, one for refrigeration, and one for modern electronics like laptops. The rest would be available for propulsion. Hotel battery storage requirements are similar based on average lead acid deep cycle batteries, however, you want enough total battery capacity that the peak solar current is around 2 amps per battery. This lets you get the batteries to a high level of charge before throttling the power from the panels. It also saves the batteries when the solar controller fries and shorts the panels to the batteries.
__________

Your 50% efficiency panel-to-prop is not realistic. At three knots, 50% propeller efficiency is dream. You might make 50% at 5 knots. The motor controller and motor together might manage 85% at modest power levels, and I think there are parasitic cooling losses. When running off batteries, subtract another 10%. Wiring will eat 3% or so. So 3 knots under batteries might realize about 35% panel to prop. At 5 knots and 4.5 times the power draw running straight off the panels, you might make 40% efficiency panel to prop.
__________

You seem to be confused about length/beam ratios and length/displacement ratios. For transport efficiency, which has to do with commercial goods and the operating margin of the boat, a L/D of around 60 is optimal but very seldom seen (there is always excess shipping capacity, and that drives the design towards higher numbers to preserve margins) . For your boat which is underpowered, a L/D of around 350-400 would be about right. A length/beam of about 2.5 - 3.5 would be fine. So something like a Beetle Cat at the fat end or a 35' sharpie at the skinny end.

 

A good set of napkin level rules to get to a starting point for overly complex analysis. I am wondering what is the basis for the first paragraph rules? Empirical? If it is empirical and based on inboard diesel, a vectoring electric would be far more capable than it's power rating. BTW we haven't even gotten to positioning, which must be under the hull to stay submerged through waves. For the low speed he is talking I think a fully shrouded prop may be best to maximize thrust. I don't think he has the luxury of higher weight for headroom. He needs to minimize weight to keep the design from blowing up.
It seems to me that he needs to start with thrust and move on to power requirements before any talk of speed. If you add the weight and size of adding power you may well find the design requirements are non-convergent. The L/B in the last paragraph looks about right but I don't see how it came from L/D. I tend to just look at length as a function of power available keeping wave loss down. Does your reasoning also comprehend comfort factor (roll natural frequency)?

On the brighter side EB gear is getting better and cheaper fast. The investment surge is the biggest in my lifetime.
(102) Why GM Is Getting Into Electric Boats - YouTube
Chase Zero catamaran to bring a dash of hydrogen to the America's Cup (newatlas.com)

Considering the economic and geopolitical importance I am surprised how little discussion there is here on BD.net.

 

For comparison purposes here are some links to a vessel that has been converted to 6kW electric with performance similar to what you are seeking. At 35ft loa and 30ft lwl it is shorter and has much less windage although it probably has more displacement having a deeper draft.
Restoration of motor launch “Nancy” – Wooden Boat Association of Queensland Inc https://woodenboat.org.au/restoration-of-motor-launch-nancy/
Nancy AWBF Reference Information - lbt http://lbt.rforster.org/about-us/boats/nancy/nancy-awbf-reference-information
Nancy is home on the Huon https://docs.google.com/document/d/1Jtilxumz9poUnEn7MyE9Jx45oC5YynEvo6-4P9LRwxI/edit

 

Thank you @philSweet, your input is everything I ever could had dreamed of. I appreciate the time you have taken to write the feedback!

I will elaborate a bit while trying to reflect on your feedback.

One of the things I looked at was a 5kW electric inboard swap on an Albin 25 and add solar panels, but it seemed impossible to fit enough panels

So figured I need a longer hull for more solar real estate and make the plans suitable for plywood, which I have sufficient experience working with. For that the Phil Bolger Wyoming seemed a good starting point, but 'if only' I could make it my own design and here we are. I also wanted to stay with a mono hull, as a cat will be at least twice the work and connecting the two hulls would add a lot of complexity, plus a proper working solar mono hull have not been made yet!(?)

For Swedish regulations, staying below 15m will make things easier and a ~6 length to beam ratio, should give even a sub optimal hull decent characteristics, so 3 rows of solar panels i.e. 3m beam was chosen. Oh, and ofc that long narrow hull looks cool!

I had to go without a keel to reach the drag numbers needed, so a centerboard to grab on to the water has crossed my mind. The free-board was simply drawn to look decent with the cabin top while offering about 2m head height inside, the high sides also helps with longitudinal strength. The gunwale arch'es down towards the ends to lower wind-age, hull weight and get the deck placed solar panels as low as possible, they are currently ~at Metacenter height.
Meaning the hull volume was never considered, it just happened.

I tried to save the hull as coordinates for stability testing in another software, but the reverse bow ended up shopped of. Maybe something similar happened inside DELFTship with the block coefficient calculations?

Your input makes it clear why I have never seen anything similar, it's just not a valid design as it stands. Without a proper keel and rudder my design would be like a balloon on the water, big volume, low displacement and hence be chauffeured by the wind rather than me.


@Skyak, ty for voicing my optimistic numbers and hinting about inter alia windage, which philSweet detailed elaborated on. In my local waters we are blessed with near non-existing tides, but I think philSweet's post included enough info about how design affects handling that we can skip that here.
The existing solar cats you mentioned, like Solarwave Silent 60 are CE-A certified and the diesel gen set may be required for safety there, but if only cruising in sheltered waters I believe one could survive without a gen-set.

@KJL38 that boat seems to have exactly what philSweet pointed out that mine lack's.

 

Can you help me to design the passenger ferry ship with
Payload 3500 T,
speed 16 knots,
Maximum draft 4m,

Requirements
Main dimensions,
Engine power,
Fuel consumption,
Steel weight,
Zinc anode in kg.

 

John, you should really start a new thread with your question, rather than simply asking in the middle of another member's thread.

 

This is an interesting thread. Has the OP decided the inevitable, yet?

It sounds like the tradeoffs are not there.

But.

Is this going to change if we see a 25% pv yield? Like if we get to a place where solar panels are that much better; is it then attainable?

Like, at what point does it become possible OR is it always going to need to be supported by diesel?