# Round Pontoon Boat Stability

Discussion in 'Stability' started by Mattias22, Aug 29, 2022.

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## Could this structure be safe on the water

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3. ### yes she'll be right

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1. Joined: Aug 2022
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### Mattias22New Member

For my hobby I like to make curved ferrocement buildings. In the last floods our house got inundated and I want to work towards a pontoon house. I have worked for a while as a boat carpenter and have got a lot of building experience.

So the question is: "Will this structure be stable?" How can I make it more stable?
I have calculated the floating part for a very similar model and I can increase the buoyancy tanks on the outer part. So making it float is possible. It will be on a dam but we do get high winds here.

So if this is simply impossible please let me know and I will work on a different design. I just don't want to hire a draftsperson to draw something that will not be safe on the water.

Thanks,
Matt

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### bajansailorMarine Surveyor

Welcome to the forum Matt.

Re your sketch above, the structure will be stable if it has a positive GM righting lever - this is the distance between the vertical centre of gravity and the metacentre.
However there is a LOT more to it than just that. It might have a positive righting lever at (say) up to 5 degrees of heel, but beyond that it might decide that it is going to flip over. Nobody can tell you if you structure will be stable based on just a midship section.

What sort of size are you thinking of re this floating building - what is the width and height shown in the sketch, and what is the length?
Is the section view constant along the length?
Why not build a rectangular barge shape hull? This type of vessel is well proven.
Or even a catamaran hull form?

Eric Sponberg's houseboat designs have simple rectangular barge shapes.
Houseboats https://www.ericwsponberg.com/boat-designs/houseboats/

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### Mattias22New Member

Hi Banjansailor,

I could build it rectangular, but I am a huge fan of round buildings. If there would be no other way around it, I could go for square, but that will deff only be the second option.

The plan is a circle with a diameter of 10 m (30ft) and a height of 15 m (45ft).
This is basically just the proportions I am copying from a Neo Renaissance building called Tiempietto. At this size it would hold 4 bedrooms, of course it is possible to make it half the size, in this case it will have just 2 bedrooms. I have to say, I would deff build a smaller model of it first.

If I understand you correctly, it's not possible to just give a guestimation of how possible of building something as high as this.
Would it require me to calculate the dead weight of the building at every level? And then get an engineer to look at it?

Mattias

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### bajansailorMarine Surveyor

Yes.
One of the more tedious aspects of boat design is doing detailed weight calculations.
If you do not know how much the boat weighs, complete with it's structure / house on top, then how are you going to know if you have sufficient buoyancy in your hull to support all this weight?
You also have to calculate the buoyancy (think of Archimedes )

Some years ago a chap here built a power catamaran to his own design.
He never did any weight or buoyancy calculations, it was done purely by eye.
He was especially proud of his very fine hulls, that would cut through the water very effectively, while carrying lots of fare paying passengers on board.
He launched the boat to much fanfare a week before Christmas, after previously boasting about the cruises that he already had booked solid for Christmas Day and Boxing Day.
And when he launched his cat, she barely floated with the deck just about awash, with no passengers or stores or fuel on board......
And this despite me (and various others) telling him long before hand that this would happen.
But he knew better....
He quickly hauled her back out and rebuilt the hulls with more buoyancy, and re-launched her in time for Christmas the following year.

Looking at your sketch, I would hazard a guess that your boat might well float with the main deck awash (if not much worse).
And even if it floats, it might not take much heeling moment (perhaps a 10 knot wind?) to then tip her over.
A rectangular shaped midship section has lots of buoyancy at the maximum width, helping to resist heeling moment, whereas your half circle midship section does not.
Maybe if you design a catamaran with half circle midship sections, then you might be ok re stability - but you still need to do all your buoyancy and weight estimates first regardless.
There is no 'quick fix' in boat design - and especially not when you are involved in something that is very unusual (like this design) - most boat designs have slowly evolved from a basis vessel, for which there is much hard won data available.

Edit - 31st August -
Sorry, I am a bit slow on the uptake, and I just cottoned that your hull form is like a dish - circular in plan view as well as semicircular in section. I was thinking it was square or rectangular shaped in plan view.
I suppose it could behave a bit like a floating navigation buoy - however these buoys have heavy chains underneath them anchoring them to the sea bed, as well as effectively stopping them from capsizing.

Last edited: Aug 31, 2022

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### Mattias22New Member

Thanks for your reply, I will have to do some serious calculations then.

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