Stability of multihull 'raft' design with high CG

Discussion in 'Stability' started by Istria, Oct 16, 2023.

  1. Istria
    Joined: Oct 2023
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    Location: Netherlands

    Istria Junior Member

    Hello all,

    I'm hoping on some input on my (very early stage) design of a low-cost solar powered raft (because calling it a boat would probably be an insult to boats :p). I have a background in mechanical engineering, but I'm quite a novice when it comes to boat design and maritime engineering.
    Basically it consists of two 5.5 meter catamaran hulls with a wooden platform mounted on top. On top of that, a wooden construction. And on top of that 8 or 10 solar panels of 250-300W a piece. The drive will be a 4-5 hp outboard, with the combustion engine ripped out and replaced by a 2 kW @ 4500 rpm electric motor.

    The solar panels on the roof combined with the relatively heavy roof construction push the CG quite high. I believe the roof construction is a bit over-designed at the moment, so there is some weight to be shed there. But the CG will still remain pretty high because of the 200 kg worth of solar panels on the roof.

    It seems that this design will capsize with a heel of only(?) 33ish degrees (perhaps a little bit more, because I didn't account for the slight shift of the center of buoyancy while heeling).


    This seems too unstable for me. I have a feeling a large wake of a passing boat hitting me from the side could already rock the raft over that limit. Especially if the wave frequency matches my rolling frequency. But I'm wondering what critical heel angle is considered 'safe' or 'stable'? And I'm curious about your general thoughts on this design.

    Thanks in advance!:)
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Hello Istria,

    Welcome to the Forum.

    Have you considered flex solar panels?
    They are much lighter and you could build some camber into your roof for water shedding and added strength.
    I like the overhang but it can be vulnerable at the dock and in high winds.

    Tapering your pontoon ends, aft, will reduce drag considerably.
    Zero rocker is going to make turning difficult.

    Your roll analysis is extreme.
    A critical component is reserve buoyancy in your pontoons.
    Design for 1/3 displacement (2/3 reserve) at rest and you should be okay.

    Can you make it wider and/or shorter in height?

    What conditions will your vessel operate in?
  3. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    The CG will not stay at the calculated point if heel sets in. Not all items are fixed in their place. Some gear, food, beverages, crew, ... will "slide down" given a heel of 5° to 10° or even 15° or more.
    that shifts the CG lateral in the "wrong" direction.
    As BlueBell said:
    The pics suggest a nerarly fully submerged semi hull, in case the other semi hull is lifted. In my view this is dangerous already in calm conditions.
    BlueBell likes this.
  4. Istria
    Joined: Oct 2023
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    Location: Netherlands

    Istria Junior Member

    Hi Bluebell, Heimfried,

    Thank you for the advise/remarks.

    I've considered flex panels. But they are much more expensive to get compared to the "normal" type. People are massively replacing their 10-year-old 250W roof panels for newer, more efficient, models. So the second hand market is flooded with the "old" ones for 25-40 euro a piece. That is why I chose this model. But of course, if there is no way to make it stable/safe with all that weight on top, I might have to use the lighter variants anyway.

    The overhang is indeed a bit too much like this. I recently made the "house" narrower in this most recent iteration of the design. I think it looks nicer with the narrow piece of deck all around compared to the sides going all the way to the edge. It just looks like a floating box that way:
    But indeed, there is not enough support left now for the panels. So I'll probably have to make the roof wider and perhaps add some 45 degree support beams on the side.

    The current design is 2.5m wide (without the solar panels), because that is the maximum width that is allowed to be transported on a (normal) trailer in the EU. And the height is 2m, because I want to be able to stand without having to bow my head. I might to for 1.90m (the hell with tall people =p), but much lower than that would become annoying I think.

    The picture falsely suggests that the yellow horizontal line is the waterline, but it is not. It's just a horizontal line meant as a comparison with the heeled angle. Also, the hulls/pontoons are just placeholders. They are not the actual hulls. My plan is to first design the upper part and when the total weight (+equipment + passengers) is known, to choose an appropriate hull size. I will keep the 1/3, 2/3 displacement guideline in mind, thanks!

    What do you (BlueBell) mean exactly by "your roll analysis is extreme"? That the 33 degrees critical angle is more than enough? Or just the opposite?

    Also, Heimfried, good point about the equipment/people moving/sliding around when heeling. I'll incorporate that in future calculations.

    The boat is meant to be used in coastal Adriatic waters during spring/summer/early autumn. Close to shore, hopping from island to island, fishing spot to fishing spot, beach to beach. I don't intend to use it in heavy seas, although it has to survive winter(storms) while on anchorage in a (protected) bay.
  5. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Swap out the wood construction for lightweight aluminum for substantial weight savings.
    Could the pontoons be placed outside the structure, possibly folding under it or sliding in for transport? That alone will improve stability greatly.
    “Low cost” and “build a boat” are terms that don’t mix, so unless your design is just an exercise, you’d do well to Google “pontoon boats” and see what’s available commercially.
    BlueBell likes this.
  6. SolGato
    Joined: May 2019
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    SolGato Senior Member

    I would also suggest using flexible panels instead.

    Not only will it reduce the high CG, but reducing weight in general will allow the boat to be more efficient.

    At about 1/10 the weight, flexible panels and the reduced need for a robust support system will significantly reduce displacement, and if you build some nicely shaped hulls with good volume, you’ll reduce wetted surface area which will increase speed and range and allow for an even smaller battery, again reducing weight.

    Multiwall polycarbonate is very strong and lightweight and makes a great support backing for flexible panels.

    Price-wise, consider buying direct from China in bulk or buying wholesale by the case and selling the panels you have left over.

    Now if you must use ridged panels, consider using their frames interlocked as the main structure with minimal support underneath, preferably something like aluminum tube or extruded aluminum.

    I like to use T-slot which makes attaching things very easy and just as easy to change later if need be.
    BlueBell likes this.
  7. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Some good headway being made here Istria.

    If you measure the output of the old, inexpensive panels
    you may find the price of new flex panels easier to justify.
    And you would need fewer to attain the same power.
    However, incorporating the frames of the old ones into the roof may have merit.
    A curved (cambered) roof is stronger than a flat one of the same weight, and sheds rain.
    Do you ever get snow?

    You could employ a sliding system to widen and narrow your pontoon beam for trailering.
    Have you checked on your height limits once it's on the trailer?

    More than enough. I should have said "...roll example..." not "...analysis...".
    Catamarans have high initial stability if designed properly.
    The 1/3 : 2/3 guideline helps a lot in this regard.
    You can also taper your pontoons to give disproportionately more buoyancy the further they are submerged/loaded without compromising a narrow waterline beam.
    A ratio approaching 20:1 (length to beam) at the waterline will give the least drag at your service speeds.
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Fortunately, naval architecture is not a religion, based on feelings or thoughts, it is a science, based on facts.

    So, firstly you need to do a full weights and centres on the design, in the full load condition.
    Then you need to check this condition, against the hulls, in the hydrostatic condition.

    Until you perform these tasks, it will be just...religion based!
    bajansailor likes this.
  9. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    bajansailor likes this.
  10. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _


    You are getting some good input here.

    I will add a few things that you may find insightful.

    Weight is the enemy of speed in boats.
    Calculating your finished weight and it's location is critical to effective design. (Weight and balance)

    When estimating your weights it's easy to overlook many, to your detriment.
    For example: you invite friends out for an afternoon.
    They bring their dog, water jug, water bowl.
    They bring beverages, how thoughtful, cooler, ice.
    Backpacks full of stuff for the day: sunscreen, water-bottles, stuff.
    Then it rains and you accumulate rain on your flat roof.
    You bring extra batteries because you don't want to get stranded out there with all the added weight.
    Jim brings his new girlfriend, nice, and all her stuff.

    The wind blows up, suddenly you have side loads adding "weight" (loading) to the leeward pontoon.

    I think you get the idea.

    Alan Cattelliot likes this.
  11. Alan Cattelliot
    Joined: Jul 2021
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    Location: La Rochelle (Fr)

    Alan Cattelliot Senior Member

    Hi Istria,

    mutihull stability is very different from a monohull stability. First, there is no requirement on the angle at GZmax. Catamaran in coastal navigation condition shall comply with a minimum transversal GZ (roll criterion), and, with a minimum longitudinal stability (pitchpole criterion).
    These two minimum criteria decrease with the boat's weight. So the heavier, the better.

    Here is an example of ISO 12217 minimum criteria, based on the principal parameters you gave (Lh = 5.5m, Bcb = 1.98m, frontal surface area 10m², lateral surface area 20m²)
    minimum operating conditions (Moc) : 1 crew + safety equipement + anchorage + outboard engine + empty fuel tank
    mass in Moc : 350kg | minimum transversal GZmax : 1.02m | minimum area under the longitudinal stability curve up to the limit angle (longitudinal deck flooding) : 2.91 kN.m.rad
    mass in Moc : 500kg | minimum transversal GZmax : 0.95m | minimum area under the longitudinal stability curve up to the limit angle (longitudinal deck flooding) : 2.71kN.m.rad
    mass in Moc : 900kg | minimum transversal GZmax : 0.87m | minimum area under the longitudinal stability curve up to the limit angle (longitudinal deck flooding) : 2.50 kN.m.rad

    Given the dimensions you gave, I would say that the total weight should not be the principal concern, at least regarding the transversal stability. However, the weights shall be as much centered around the centerline as much as possible. Installing batteries pack inside the hulls is not a good idea. Better in the console, or in a coffin, on the plateform. The freeboard at the fore perpendicular shall be sufficient to ensure an sufficient area under the longitudinal stability curve. With your configuration, i would consider making a stepped deck.

    Also, I would take good care to the advice of Bluebell and Solgato, regarding the weight. Due to the important windage of the plateform arrangement, and because the choice of an hybrid propulsion system, the manoeuvrability could become difficult in certain current/wind conditions. For safety reasons, and for you to enjoy sailing without troubles, weights shall be chased to the maximum. Besides reviewing the overall structure and materials, the re-boarding ladder that is represented is far look far too heavy. A combinaison of a good positionning of handles and a smaller ladder will do the trick. I would remove the aft enclosure to facilitate access to the working deck.
  12. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Istria seems to have lost interest in his/her own thread.

    Too bad.
    Unanswered questions, lost dialogue, but (s)he is an engineer and we're not known for our social graces.

    Good luck Astria!
    Post pictures of your progress.

  13. Istria
    Joined: Oct 2023
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    Location: Netherlands

    Istria Junior Member

    Istria (he) has definitely not lost interest!
    My apologies for being absent this week. It was a busy week with work and family stuff, so I got a little sidetracked. I should've at least posted a short message saying "stand by, I'll be back" =).

    Again, thanks a lot for all the input I've received. I've made some changes to the design today.

    Current design state

    The current total weight is 770 kg, assuming 50 kg per hull. This is the weight of the components seen on the picture, minus the person, plus the battery bank (which is hidden inside the console thingy). So excluding fasteners, electronics, wiring, people, equipment, still to be designed storage compartements, seats, Jim's girlfriend's stuff, etc.
    Also, the CoG is a bit lower than in the previously posted picture, because I forgot that I had the hulls set to 0 kg of mass.:)

    Current CoG:

    The roof
    I did some stress/deflection analysis on bolting the solar panels together and using them as a rigid, load bearing, construction for the roof. But it seems I'd still need some supports in the middle. Especially if I incorporate some wind and/or snow loads and the fact that solar panels are not very keen on being bent even slightly.
    On the positive side, it seems the 680 mm overhang is less of a problem than I suspected. Assuming a 1 m thick and fairly compacted layer of snow (100 kg per m2), the deflection on far end of the overhang is about 15 mm. But I still have to figure out how much panel deformation is acceptabel to not break the solar cells or their internal connections.
    I'm also still deciding on if I'll make the roof slightly angled towards both sides (like a house roof), or to keep the roof flat, but putting it on a 3-5 degree angle for water shedding. In any case, it will not remain completely flat and level like in the current design.
    So the current roof design looks like this:

    If needed, some side supports can be added:

    The hulls
    I came across this video on youtube which gave me some inspiration.

    My initial plan was to reuse hulls of a 18ish ft catamaran, like the Hobie 18. But it seems that is not going to work out weight/displacement-wise. Then I saw that video which actually made it seem doable for me to design and build my own hulls out of plywood and GFRP. But I'll have to read up a lot more on that (hull design) subject. For now, here is just a quick sketch I modeled up that should have a good displacement (1350 kg of displacement when fully submerged) to keep my raft afloat with a couple of people (and Jim's girlfriend's stuff) aboard and around 1/3 of it being below the waterline. A tapered design like BlueBell suggested seems a good idea both for optimizing the waterline beam and also for cutting through waves instead of falling down on top of them with a flat(ish) bottom. But like I said, I still need to read up a lot on this subject.

    About some if the input/comments
    • I looked into flexible panels a bit. Also on Alibaba. But unless absolutely necassery for weight saving or CoG lowering, I don't think the benefits justify the (times 10) cost per Watt compared to used 'houseroof' panels. The degradation curve is quite acceptabel in my opinion. After 10 years they should still output almost 90% of the rated power. And the degradation becomes less and less with time. They degrade the most in first couple years of the lifespan.
    • The ladder is also just a placeholder I found a CAD model of online. The actual one will indeed be a lot more compact. Also, I was planning on making a door on the aft enclosure. But maybe removing it alltogether and adding a different kind of shear-reinforcement would be even better. I'll think about it!
    • Thanks for pointing out the existance of ISO 12217 to me! I found 'part 2' online, which is for >6m sailing vessels. I'll have to do some more searching for 'part 1', which is for non-sailing vessels.
    • Aluminium vs wood construction: I plan on making an alternative design using aluminium and comparing it to the current wooden design. It will be interesing to see how much weight can be saved that way and what the difference in cost will be.
    • Extendable hulls: Great idea indeed. I'll look into that possibility when I design the hulls/deck connection. If needed, because according to the ISO 12217 critera that Alan shared, it should already be stable enough (transversally) it seems. The GZmax is 990mm with the current design.
    • About the stepped deck suggestion: What is a stepped deck? I couldn't find much when I googled it. Do you perhaps mean an elevated deck? Like the design in the YouTube video?


    So, that's it for now. Still a lot of work to do of course. Exploring more weight-saving methods. Reading up on hull design. Reading up on ISO 12217. Designing the connection between hulls and the rest. And much more...
    I will probably continue in a new topic in the 'projects' departement, which I believe is more fitting for the direction this thread went in. Or maybe a moderator could move it?

    Thank again all for your comments and input! And again my apologies for going dark for a week. :)

    Attached Files:

    Last edited: Oct 21, 2023
  14. SolGato
    Joined: May 2019
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    SolGato Senior Member

    Still looks and sounds a bit heavy to me.

    Weight is definitely your enemy with a build like this. It will affect safety, performance, maneuverability, range, efficiency, etc..

    IMO, you ideally want to build the boat like a performance sailboat using strong lightweight materials, however when it comes to the hull shape, you want to think more about load carrying like a pontoon boat.

    Yes you want the hulls to be able to move through the water efficiently, but you don’t want to shape them so that they are severely lacking in volume and unable to support the stern, and ideally they would have some bulge in them (not flat sided) and be semi-planing so as you add more weight onboard, you don’t bury the hulls creating increasingly more wetted surface area and less freeboard.

    You are wise to ditch the idea of using beach cat hulls with the weight and structure you plan to add. Lots of examples of heavy half sinking builds out there that started with beach cat hulls as the base floatation structure. Some work well, but you really have to keep everything very light and well balanced.

    I understand your desire to use the ridged second hand panels due to cost, but it penalizes you in so many other ways is it really worth it?

    You’re going to have to make bigger floats, and you’re probably going to have a boat that feels underpowered and slow, so then you’ll probably end up upgrading the motor, and then the battery capacity, and so on.

    It’s a slippery slope, especially with small boats like this.

    Have you thought about how you will configure your array wiring-wise? Sometimes “house” panels and their voltages are not a good match for small Boat/RV type DC MPPT chargers.

    Not trying to discourage you, just giving you some stuff to think a bit more about.

    You want to build something more like this:

    POL - On electricity | LUX

    Rather than this::

    Last edited: Oct 22, 2023

  15. alan craig
    Joined: Jul 2012
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    alan craig Senior Member

    My thoughts: Use a beach cat such as a Nacra with removable daggerboards for a head start in the build process and the efficiency of the rounded section low surface area hull.
    Use only 5 of your proposed 10 domestic panels to save 80kg to 100kg on the roof, and add about half the weight saved in battery capacity mounted low down. You say the boat will be on a mooring so it will be charging up all week for you to enjoy at the weekend!
    ....and you can always tow the panels....
    portacruise and BlueBell like this.
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