too much weight for my solar powered pontoon styled boat

Discussion in 'Stability' started by Captain Sunset, Mar 5, 2019.

  1. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    This matter should be considered very seriously.
     
  2. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    i think, even if i put the CoG at 1,8m above the hulls, the boat would still stay upright with a 20° heel. And placing the CoG this high is the safe side and not the sorry side for the calculations (batteries and water reserves are placed on the floor, not the ceiling ;-) ).
    I do not have good software, but i do use some simple tools like a trigonometry calculator (i am using this one at the moment: Triangle Calculator https://www.calculator.net/triangle-calculator.html?vc=&vx=&vy=139&va=90&vz=&vb=80&angleunits=d&x=62&y=20)
    I use basic programs like sketchup to help me visualising the problems and mathematics. I know it is not a professional program. But it helps me to think things through:
    I made a sketch of the boat weighing 9 tons (it should weigh two tons less in reality, but i know that margin in weight will quickly be filled up with junk haha). At rest this means an immersion of 50 cm (according to the official dealer site Auftriebsberechnungen https://www.swiss-boats.com/schwimmk%C3%B6rper-pontons-1/auftriebsberechnungen/ ) I put it in an inclination of 20°. If my high school physics still serve me right, after measuring out the triangles, about 75% of the weight will rest on one pontoon. This means the total floating capacity of that hull will have been reached at that point with a small margin of 5cm left unused.
    In real life i will hopefully never have to deal with an inclination of 20° on the inland water channels (as that would mean that the water is 1,5 m higher on one side of the boat than on the other side, only 4 m away, that would be one hell of a steep wave, but possible of course, if an freightcarryin boat exceeds and ignores all rules of the inland waterways i use and intend to use. Also that wave would have to hit the boat in just the right position to make it incline in that way)
    So, I think, that the 5 middle sections extension to the existing hulls would keep this vessel in the safe zone considering that it is classified and only used as an inland waterways vessel (class D)

    I didn 't write down the angles in this sketch as most of them are 90 or 20° and all other angles can be easily derived. Thanks for thinking things through with me and trying to keep me from sinking all my money :D

    I know this is a gross simplification of the calculations that need to be made, but i hope i have always simplified in favor of the safe side and not towards the sorry side!!!

    As always feel free to correct me in my thinking. I still have some time to decide as the local dealer for the hulls seems to be out of reach of contact until monday (i tried ordering the 10 middle sections already earlier on this week)
    20degree inclination 9ton.jpg
     
    Last edited: Mar 14, 2019
  3. Heimfried
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    Heimfried Senior Member

    Your sketch is not very helpful to imagine what a 20° heel would look like. (Stability calculations were made without software for a very long time, so this is perfect possible. But it has done according to the reality and the rules.)

    Weight (in the strict sense it is mass): did you sum up the boat and everything and every person on board? (12 people allowed - means 900 kg.) The buoyancy table of your pontoon elements shows negative numbers e. g. at 5 cm immersion which means the buoyancy is given netto (real buoyancy minus weight of pontoon element). That is appropriate for such a website because it is better for laymen. But for clearness reasons in ship hydrostatics buoyancy it is always the mass (weight) or the volume of the displaced water, regardless what weight the considered part of the ship has. So I will use it in this way. Therefore the weight of the pontoons (1096 kg) has to be included in the boat weight.

    I updated your boat to 16,5 m lenght and 9000 kg DISP. The GZ curve looks not goog at all and a heel of 10° means one kat hull is fully submersed, a heel of 20° means part of the deck is submersed.

    Kat-BE3.jpg Kat-BE4.jpg Kat-BE5.jpg
     
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  4. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    hallo Heimrich, could you show me what the difference would be if the CoG would drop to 1.5 m? In reality the CoG will probably be lower than 1.5m.

    Just to make sure i understand your diagram: a lateral wave, coming parallel to the pontoons with an amplitude of 0.75m and a period shorter than 4m woud lift one side up and completely submerge the other side. Thereby perhaps flipping the boat (since it is submerged meaning as much force is exerted on it as it has buoyancy force?)

    A 20° heel would certainly flip the boat as one pontoon would be more than submerged?

    The maxima of the righting curve on the diagram are the where to boat flips?
     
  5. Dejay
    Joined: Mar 2018
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    Dejay Junior Member

    Heimfried, your software is pretty neat btw. I've looked and delftship free doesn't offer stability curves right?
     
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  6. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

     
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  7. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    about 72% (7020 kg of the total of 9694) I am hoping the boat will weigh about 7500 kg itself, but i know clutter is inevitable when living aboard
     
  8. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    also, i plan to take up gardening on the boat, putting the gardens on the pontoons, they will serve as rainwaterfilters. It is another project i want to test. I am reluctant to share this thought for fear you all are going to run away screaming. It would add quite some weight (at the moment of writing the boat is less than 7 tons, but stuff still needs to be added, the heaviest being the plumbing). But that weight, altough very significant, would also, the more significant it is, the more drastically it will lower the center of gravity. There. I said it.
     
  9. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Words like "hoping" do not instill confidence in me towards your project.
    My last words of advice to you are get a professional to assess your plans.
    Good luck.
     
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  10. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    thank you BlueBell for your good advice. Will do.
     
  11. Heimfried
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    Heimfried Senior Member

    Good to read you will look for professional help.
    Diagramms for a CoG lowered to 1,5 m above top of pontoons are attached. In your case it would help nearly nothing. The maximum righting lever is 1.16 m instead of 1.11 m and the angle of vanishing stability (AVS) is increased from 37° to 41°. But the fact that one hull is submersed in case of a heel of 10° is unchanged and the partly submersed deck at 20° also.

    Kat-BE6.jpg Kat-BE7.jpg Kat-BE8.jpg
     
  12. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    thank you for checking!
     
  13. Captain Sunset
    Joined: Jun 2012
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    Captain Sunset Junior Member

    Again, jsut to see if i got it right: a short steep wave higher than 1,16 meter would submerge one pontoon. What is the difference between maximum righting lever and the angle of vanishing stability? Have I got it right that the point of vanishing stability is where the boat lost all it ability to upright itself again? Whereas as the heel of maximum righting lever is passed less and less energy is needed to increase the heel?
    So, up to 10° it takes a lot of energy to make the boat heel, further than 10° less and less energy is needed, if AVS is reached, the boat won 't right itself anymore, but the points before AVS it might still right itself? Just checking if i understand you and the graphs correctly. Thanks!
     
  14. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    This correct. But the static stability - and this is what I calculate and every common GZ curve is about - presumes all things on board (other than liquid in tanks) are staying firmly at their place regardless of heel angle. This will never be the case in a house boat you live on. Things will fall down and or slide over the floor. This is shifting the CoG to the bad side and will make the stability worse. That means the calculated AVS is a theoretical value, which will in reality only found on sail boats for which a heel angle of say 50° means nearly nothing and all things on board are secured and fixed in their place.
     
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  15. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    No. The caculation of static stability is about calm water, no waves at all, and no motion of the boat (no dynamic forces). So fundamentally a sufficient (static) stability means not a plaining boat is safe, because there the dynamic forces have to be considered. However a slow moving vehicle will be stable if the static stability results say so regardless that the calculations are kind of approximations to the reality.


    But to say it clear: sorry, your planned boat is (with 5 each added pontoon elements) not stable!
     
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