Stability of multihull 'raft' design with high CG

Apology accepted but I didn't say you'd lost interest, I said you seemed to have lost interest in your own thread, as you were visiting other threads all week and not answering questions raised in your best interest.

You're welcome.


I think your roof idea is flawed.
It seems based on minimizing the degradation of the rigid panels, their weight, and the vulnerability of glass.
If they are in such good condition why are they being replaced and sold off for pennies on the dollar?
Wouldn't it be more economically sound to simply add more panels to compensate for voltage drops in their systems?
However, you seem fixed on the idea of using them so I propose you make it a sitting vessel instead of standing.
This would lower the heavy roof considerably.

Your pontoon graphics are beautiful for optimizing speed by reducing drag
but it comes at the expense of lost buoyancy.
I believe buoyancy more important in your case than speed.
Tapered bows and sterns on full bodied midsections would better suit your needs and
reduce beam which is more beneficial at your service speeds than optimizing shape.
Full bodied 'toons would also accommodate improved storage, an ideal place for your batteries
while allowing for trim adjustments by moving them fore and aft as needed.
Rocker can still be incorporated fore and aft for easing turning.

What speed do you expect?

Why do you want it so narrow if it's going to be on a mooring year round?

What is the road-height restriction when on the trailer?

 

I have indeed been reading around here and there on my phone, including the replies in this thread. But I didn't get around to actually sit down behind my laptop to write up a comprehensive response including pictures etc.

Speed
About the target speed: Well, I didn't really start this project with a target speed in mind. I went the other way around by starting with how much solar power (power, not energy) I could generate on a 5-6 m vessel. And then used a online hull speed / propeller sizing calculator to estimate what speed I would get with that power. This was a while ago, but I think the result was around 4 or 5 kts with 2 kW of propulsion. But this was with a very rough estimate of the input parameters.

The previous (and only ) boat I have owned (this was 15 years ago) did about 5 kts with a 4 HP outboard at max throttle (that was mounted too high and was sucking in air...). I usually cruised around at a quarter or a third throttle, which made it go 3.5-4 kts. And for my uses (cruising around locally), that was fine. So with this new boat, I'd be satisfied if it did 4 kts @ 1-1.5 kW and 5 or 6 kts @ the full 2-3 kW of power. Of course, I wouldn't say no to more speed and/or efficiency.



Solar powered, not battery powered.
By the way, I think I didn't mention this yet: My goal is not really a battery powered boat with solar panels for range extension, but a solar powered boat with a (relatively) small battery as a energy buffer and "get home" (or at the very least to shore) backup. The idea is to generally have the motor produce the same amount of power that is being generated by the panels at that moment. So the battery charge stays about the same. The battery is just a energy-buffer for periods of cloud cover or for example to go faster to a destination where I know I'll be anchored for a while (to recharge the battery-energy I spent). So I intend to implement an 'auto-throttle' mode, where the motor power is regulated to chase 0 W going in-out of the battery. The battery size I have in mind is one that can drive the boat at for 1 hour without solar power. So probably around 40 Ah of usable energy (@ 48V).
So the proposed slicing the amount of panels and half and doubling the battery capacity does not match my goal. I am considering using 8 instead of 10. Or perhaps the flexible panels if I can find a good deal on them.

Electronics
That brings me to the electronics.
The concept I have in mind is the solar panels supplying a battery bank and motor with a custom built MPPT charger. Off the shelf MPPT chargers (from what I've seen) usually don't have the option to have a variable output current. They will just output the most power they can get from the panels or that they are set to (until the battery comes into constant voltage or float region). This is a problem for my purpose, because I need let's say 40-50A of solar current while cruising around. But when I'm not, a 40-50A charging current would overcharge the (small) batteries. And if I use a 20A charger that is more matched to the battery size, it can't use the full solar power when I need it while cruising. So using a custom-built charger gives me the flexibility to add a current sensor to the battery bank, and have the custom firmware limit the current depending on the net current going into the battery. So let's say the batteries can handle 20A of charging current and the motor is pulling 40A, the charger will output 60A max, even if there is more sunpower available. And then with the boat anchored and the motor off, it will limit the output to 20A. And everything in between. I have not seen off-the-shelf chargers that can do this. Plus, it's a nice side-project. I like making custom electronics.

Now the voltages. I've been going back and forth on this for a while now. Having all the panels in parallel would give a maximum open contact voltage of 40V. 30V at the maximum power point. In that case I'd have to use a 24V motor which would pull over 100 A to generate 2.5 kW of power (not even counting efficiency losses). So I'd have to use very thick wires and minimize the wire lengths.
The other option is a 2 series 5 parallel config so I can use a 48 V motor and half the current. But then we are talking about 80V of 'open contact' or no-load voltage between the solar panels and the charger. And combined with the wet and salty (and usually barefoot) environment, I'm starting to get worried about safety. 80 VDC in optimal (dry) conditions is not much to worry about (unless you lick it), but I'm having a hard time finding out how much worse it is in salty, wet, marine conditions. The most relevant thing I found was a document from the fire department, stating 'safe to touch' voltages. For DC, this was 125 V in dry conditions. And 24 V when wet. So a BIG difference. On the other hand, I see OceanVolt has a 48 V drive + battery system that they promote as "no high voltage risk". Any thoughts or experiences on 48 V batteries and 60-80 V solar voltage on a small vessel? Of course I plan to isolate, ground and fuse everything extensively, but I still like to assume the worst case scenario where someone somehow touches the maximum system voltage with a salty and wet body.


Weight
There is a lot of weight to be reduced. What do you think would be a suitable target weight for this vessel?

Current weight of the top 4 heaviest main components:
- Deck/platform (without hulls): 211 kg
- Solar panels: +-200 kg (or 20-30, in case of flexible ones).
- Structure + roof on top of the deck: 171 kg
- Hulls: 100-150 kg (guesstimation)

The platform can definitively be much lighter by for example using a sandwich construction instead of the current solid 30mm wooden deck.
Solar panels obviously can be MUCH lighter by using the pricier flexible panels. This would also shed some weight of the structure on top of the deck.

I like the POL boat! Will try to make a concept structure similar to their's.

Stability
I am a little confused about this. If I understood your input correctly, the current concept is already stable enough (transversally at least) with the current weight and CG. So the weight saving we are talking about is more for range/efficiency/speed than for stability, right? Or did I not understand that right?

Rigid solar panels
The reason people are replacing the 10-year-old panels instead of adding more, is that the average house over here doesn't have much roof surface. Most people cover their entire (south-facing) roof with panels (which is usually 8-10 panels). With the old 250Wp panels, that is around 1700 kWh annually over here, assuming 8 panels. When they replace it by newer, more efficient, and only slightly larger, panels of say 400Wp, that is 2720 kWh annually if they can fit the same amount. Financially, the break-even time is only a couple of years. So it pays off. Especially with the current energy prices (which are only expected to go up even more in the future) and no VAT (thank you government) on already quite affordable solar panels.
On top of that, there are also a lot of farmers who covered their entire stables with hundreds of solar panels and are replacing them now for the same reason. Plus commercial solar farms.
So yes, the second hand market over here is currently flooded with 10-year-old solar panels. I've even seen a batch recently for 20 euro / piece.

Pontoons (when is something called a hull and when a pontoon?)
I came across buoyancy simulator software in the form of a Blender extension. I will play around with that and different hull/pontoon shapes.
Am I correct that 'full bodied pontoon' means a flat bottom?
And by 'tapered', you mean a V-shape bottom?
A previous comment from Alan advised against putting batteries in the hulls. Is that in case of water leakage? Or for another reason?

Width/Height
Trailer width/height restrictions are 2.55 m wide and 4 m high.
I must add, this is a 'wish' and not a requirement. I just thought it would be handy to be able to build the boat at one location and then drive it to sea on a trailer. Or to be able to put it on a trailer and drive it to another city/town and use it there for a while. But it's not a hard requirement.

 

According to the calculations I've made (bare hull, no windage), I agree with your estimation. However, the calculations I've made are based on a "regular" hull shape, not the one you've drawn, which could make you miss your target speed.

If you like putting your hands in electronics, no one will stop you for doing so. What is fun must be done. In that case, and regarding the safety requirements you have, I would suggest to limit the custom made devices, and consider using a built in converter/inverter/charger, that will handle protection the right way. You could focus on the adaptater, speed controller, and navigation system.
In the same spirit, and because you wish to only use the electric engine battery as an energy buffer, with limited duration, It could be acceptable to skip the solar panels, charge your battery during the night, and plug it any time you want to sail, like a portable fuel tank. Add a service battery for your confort, and you could avoid some harsh time in tests/modifications loops. I should say that, in the scheduled range of power, some outboard electric engines have their battery incorporated, like Torqueedo solutions.

There are quite some missing lines in this BoM. Electric systems+ cables, engine (ICE+E), portable fuel tank + hose, transmission lines, Pulpit with its equipment, deck fittings & furnitures, mooring, safety equipments, crew+passengers, food & water supply, other items. I suggest you complete your BoM, and make another loop on the structure itself. Clever decisions always come under constraints.

Transversal stability would be ok. For the longitudinal stability ( and for a the boat to be pretty ), again, I would make a stepped deck, with an higher aft freeboard. Multihull's plateform are so disgracious.

flat bottom with chine would be just fine. Hulls are made out of wooden developable panels, and built on frames. I adviced against putting the batteries in the hull, anticipating a batteries bank, instead of the installation you describe. Weights are to be centered to the maximum to make light the boat manoeuvers, because it reduces the inertia of the boat. It simplifies also the electric installation, and yes, you won't have any worry of water leakage in the hulls, if there is nothing in them.

 

Quote: The idea is to generally have the motor produce the same amount of power that is being generated by the panels at that moment. So the battery charge stays about the same. The battery is just a energy-buffer for periods of cloud cover or for example to go faster to a destination where I know I'll be anchored for a while (to recharge the battery-energy I spent).

I think you should change your goal to match my previous suggestion. A boat with minimal battery capacity will be unsatisfactory as I have already found out while "cruising" with dying batteries on a shady/cloudy day. If you really want more solar power make a wider boat.

 

"The battery size I have in mind is one that can drive the boat at for 1 hour without solar power. So probably around 40 Ah of usable energy (@ 48V)."

That battery size for 1 hour, might be kind of difficult to predict, because of so many variables that have to be taken into account? Is that "one hour" at maximum speed, while fighting possible strong wind/ current / waves, low temperatures, Peukert, and taking into account the 5 hour solar day plus the maximum charge rate for the battery, Etc?

 

Welcome to the design spiral Istria.

You've got quite a few balls in the air now.
Too many variables to accommodate.

What's the budget and timeline on this project?

 

But that would still leave the relatively high voltage (60-80V) between the solar array and the converter/charger unit. And as far as I can think of, apart of adding a DC GFCI, there is nothing even commercial units can do that makes that risk smaller. The voltage is there. So I guess all you can do is isolate the crap out of it and ground all conductive parts properly. Or perhaps use a step up charger, so the highest system voltage is the battery voltage (48V). But most marine MPPT chargers I see are step down. For example a Victron model I saw can handle 75-100V at it's input. So I assume there are people who use those voltage levels in their marine solar arrays.

That would give me a terribly limited usage time/range, wouldn't it? Unless I give it a much, much larger battery capacity. To be clear, the battery is just a small buffer and 'get home' backup. But the electric motor together with the sun is the main means of propulsion. There is no ICE.
Also, I don't intend to 'cruise' as in going on (day)long trips where a shady day can really ruin the experience or be annoying. It's just for boating around locally on a sunny days. If the forecast is cloudy, then that's not a day to take the boat out. That is how I see my usage of it.

About the BOM. It's not a BOM. Just the weights of the 3 heaviest structural main components to give an idea of how much weight can be saved there still. I will make a complete BOM and equipment list to get an estimation of the total boat weight at some point.

About the 'stepped deck'. It's still not entirely clear to me what that means. Google seems not to know of that term. From the picture you posted, can I assume you mean this (the bottom one instead of the top one)?:


Thanks for the speed/power graph by the way!

I have not thought this through in detail yet. But what I approximately have in mind is a useable amount of energy (so including Peukert-effect) to motor for 1 hour without solar power at an energy-efficiënt speed/powerlevel in average conditions. So longer with the wind in my back, little waves and no or positive current. And shorter with headwind, current in the wrong direction, waves, etc. The "1 hour" is just a ballpark number. It might just as well become 1.5 or 2. The most important thing is I have reserve energy to get back to shore (which I'll probably never be more than a mile away from). And if I do decide to go further out for some reason, I'll probably bring my Yamaha F4 outboard and some fuel with me, just in case.

I sure do. Really with all the feedback I got and things I learned in the meantime, I will probably restart from scratch.

There is really no clear budget nor timeline. I'm still just exploring options and possibilities. About the timeline, let's carefully say I'd like to start the actual building the summer after the next one. About the budget: It's flexible. It depends on what kind of quality level of boat or raft I end up deciding on building. For my original "raft" idea (the Hobie 18 hulls with a simple platform on top, a couple lead-acid batteries, some cheap solar panels and an old outboard engine with the ICE removed and replaced by an electric motor), I probably wouldn't want to spend more than 1000-1500 euro. And I wouldn't expect it to have a long life.
But the more I learn and read, the nicer and less "junkyard wars", I want it to be. So if it's going to be an acutal "boat" that will last for years and years and not just a short-term "raft", I'm willing to spend (much) more. But I find it hard to put a number on it. It just depends on the quality level boat the final concept is going to be.

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I've been reading a but about hull shapes today. I wonder what the benefit is of a (multi)chined hull compared to a right-angled, flat bottomed, hull with only some rocker (like my sketch).
From what I understand now, a round or (multi)chined hull reduces the wetted area and thus drag. And adds more counter-force as well when heeling (more displacement the deeper it submerges). How big of a difference in drag is a round hull vs for example my sketched 'airfoil' hull?
If anyone can recommend a good "hull design 101" book or website, I'd appreciate it. To understand the basics.

 

One of my concerns is the windage of the panels on the sides and the podium for the controls. With only 2kw available a strong wind may render the boat uncontrollable. Maybe consider a different approach that serves the same purpose.

I second the flexible solar panels. That would make a huge difference in stability, and some difference in overall efficiency. May offset some of the extra cost.

 

-with an athwartship curve to flex panels, part of the array might not be at a good angle for soaking up the rays. When you think about it, unless the array can be aimed at the sun, it’s going to be sub optimal at some point.

- I second the concern ^ about windage - wind has a bigger effect than you might think- with a Torqeedo travel 1003 on the L 7 tri, without the mast up (I was going from launching ramp to slip), I could paddle faster than the motor could get me away from the lee shore launching ramp in 10-15 knots of wind without full throttle, and with full throttle, range anxiety becomes a bigger, and shorter term whiff of panic. (Which made my paddling strong, like bull) It took me a whole battery charge to make a 1/4 mile trip mostly upwind. Docking upwind I was staring at my ‘distance remaining’ number way too much. Multihulls have a lot of windage anyway- add a square house, and it gets worse.

Maybe more beam between hulls (as was mentioned above), make the array wedge shaped at the bow, gradually rising up to a low sitting height, say shoulder height, 50-65% from the bow, and make sunshade deployable, or wear hats….. If you steer with a tiller, you can stand or sit. Arrays on the back shoulder high sloping towards the stern. You might be able to put some arrays on the side, but again, streamlined.

electric boats looooove going off the wind, and even a small sail will help you there a lot- & if you can go fast enough downwind (~5 - 10 knots), some motors (like Torqeedo) have a regen function to charge batteries.

 

I did a lot of research on this(and I used to work in solar). You basically take a 20% penalty in overall output when flat mounting solar modules, as opposed to mounting them at the optimal permanent angle. But, flat mounting I would say is the most efficient for a boat because the boat changes its relation to the sun. Or you could make a tilting mechanism, but that would add weight and complexity, you might as well just add another module.

I like your drawing, efficient to windward and very hard to capsize.

 

One way to get a wider catamaran on a trailer- hinge deck longitudinally in the middle, hulls on platforms that move in and out, so catamaran is folded /unfolded on trailer, quick, proven, and easy, esp with no mast to set up…

Sailboat - Gibbs Shark Catamaran (1969) https://www.nextechclassifieds.com/listings/1552697/

 

An MPPT will compensate for curved panels and mis-perceived solar losses.