Hexagon Pontoon design and cost estimate

(Jake Rosoman) #1

The Drawings

The latest 3D model will always be available here

Top View

Bottom View

Cost estimate:

area = 176 m^2
displacement = area × 1.5m = 264 m^3
density = 2.5 t/m^3
volume = 37.93 m^3
mass = volume × density = 94.825 metric tons

geopolymer = 55.45USD/T
basalt_rebar = $600USD/T
labour = 30USD/h × 40h = 1,200.00 USD

total = geopolymer × (mass-1T) + basalt_rebar × 1T + labour = $7,002.60
total/area = 39.79 USD/m^2

Production would be done using permanent molds which would be made from a combination of steel and fibreglass. These would be pretty tricky to make and would likely cost around $30k. Curing would be best done in an oven made from sandwich panel with an electric heater. This would cost about another $20k to make. The first few would be made on land near the ocean then craned into the water at a cost of about $1k per pontoon. From then on you could use these pontoons to build a floating drydock where you can continue building pontoons without the need for a crane.


The open bottom enables waves to flow freely underneath the pontoon. It is divided into 3 segments so as to stay the right way up when on its own. But the dividers won’t be necessary when the pontoon is connected to other pontoons. So they should have removable shutters installed so that air can flow freely between the segments which will make the pontoon even more stable while a wave rolls underneath.

The open bottom also reduces the cost by quite a bit since it means it’s only one piece.

And by pumping air in and out from the underside of the pontoon you can adjust its draft. The maximum load the pontoon can carry is 169tons. And the minimum draft when unloaded is 500mm. And since the sides extend down 1300mm this means the biggest waves it can handle are 800mm. Or about the size of the wake of another boat. By making the sides taller this design could be made to handle bigger waves. It probably can’t go any further than 5m though so it won’t be suitable for unprotected waters.

Use case

It’s intended for use in protected waters near existing cities. Owners would use the pontoons as a substitute for land. They would assemble a few of them together to form the equivalent of a suburban section and build a normal house directly on top of the pontoon. The deck of the pontoon would act as the foundation and floor of the house which would reduce the cost of the house by about 20%. The main attraction being good location at a much lower price than they would have to pay for normal land.

The producer of these pontoons would take responsibility of managing the community that develops. And once it’s able to look after it’s own citizens independent of the nation whose territory it occupies it will be in a position to bargain for a lower tax rate for it’s citizens using the threat of moving all further development to another country. And someone who is good at that sort of thing could probably bargain a special economic zone out of it, which gets you most of the benefits that international waters offers IMO.

Open Source Project Leadership
(Chad Elwartowski) #2

Great design and thank you for the cost estimate. I know it is hard to get any sort of idea of cost but even an educated guess is better than nothing.

I wish there was a location for prototyping peoples’ ideas like these.

(Jake Rosoman) #3

Yea I asked about places that would be well suited legally to testing these sorts of projects but so far no good answers :disappointed_relieved:


One of my goals is to make an incubator site where people can come model and test in wave tanks, then build.

(Larry G) #5


You’ll need to create a user account there. You can use the same username and password if you like, it’s a separate database of user info.

When creating a new page, just search or the title you would like it to be. If the page does not exist, it will offer you the chance to create it.

Existing pages have an “edit” link at the top right.

(Larry G) #6

Here is a link to a handy tool for calculating hexagon dimensions for purposes of comparing scale:

Hexagon Calculator

(Larry G) #7

So I’ve been working on a similar concept. The major difference is I plan for permanent flotation under the open hull using EPS foam for minimum reserve buoyancy.


Have you considered that you might want a minimum positively buoyant displacement mechanism in addition to your air pumps? I would hate to rely entirely on active machinery. Power outages happen to the best of us.

What is your reasoning on the maximum draft of the skirt?

(Jake Rosoman) #8

Well I was thinking that you would need to reinforce the skirt more and more as it gets longer and eventually you would be better off just switching to a semi-submersible design. But in thinking about it more that might not be the case since there is always air pressure on the inside countering the water pressure on the outside. Perhaps the two forces would be balanced and therefore you wouldn’t need reinforcing. I’ll have to look into it.

I’m not worried about this because there will almost always be other pontoons connected to the one that fails and they can hold it up. It can be easily added and removed though. Some people would want it for the extra safety and others might want it for underfloor insulation.

(Larry G) #9

Given that you plan to pump air from one open-bottomed chamber to another, and there may be tipping that allows air to escape, I don’t think I would rely on that. But given you have multiple chamber walls involved, and you have 6 x angle 60 degree angles only a few meters apart, you already have a semi-self-buttressing wall to the skirt. You might consider moving the chambers walls to intersect the middle of the skirt walls’ straight length rather than in the corners where the skirt wall is already self-reinforcing. It would also save a percentage on materials for the chamber interior walls because they would be significantly shorter. I’m figuring backwards from your area (176 m^2) to be roughly 8.2 meter straight edges, which makes the point to point diagonal 16.5 meters vs the flat to flat diagonal 14.3 meters. About a 14% materials savings (in the chamber walls, less when considered as part of the overall structure of course.)

Jake's hex

How thick are you planning? I would be more concerned about the skirt walls bumping into the next hex over as a factor in failure more than the water pressure at 1.3 meters depth. Roughly 10 meters is only two atmospheres. 1.3 meters is roughly a 13% point increase in pressure at its maximum. .65 meters is only 6.5% increase. Decent concrete can handle that loading over an 8 meter span.

I would urge you to consider that the cost of installing and maintaining dynamic air pumps is almost certain to exceed the one time capital materials cost of enclosing the entire displacement cavity bottom and making it a water tight hull then maintaining it from time to time. Or as my concept does- provide EPS foam for permanent passive, positive buoyancy in the displacement cavity surrounded by the skirt.

(Jake Rosoman) #10

Good idea!

It’s currently 200mm at the top of the skit then tapers down to 110mm. But the simulation indicates I could slim it a little. I’m not too worried about the weight of the skirt though because it only makes up something like 20% of the overall weight.

They will be bolted together with just a soft shim to dampen vibrations

But then you are no longer transparent to waves. And you lose the ability to lower your pontoon when the water is calm. When the water is really calm you could lower your pontoon to be flush with the surface of the water. Which I think would look really cool.

And I don’t expect the air pump system to be expensive. Maybe $1k to install and no more than $100/yr to run. Which is roughly equivalent to a $2k upfront payment. Putting a bottom on the pontoon would cost close to $7k.

(Larry G) #11

As long as you have the skirt in the water and are floating on buoyancy, you’re not transparent to waves. You have to be below the wave zone entirely, or jacked up on skinny legs such that the skirt is completely clear of water for that. Your air chambers under the platform are still subject to compression by the waves travelling past. Since water is ~800 times denser than sea level air, you get a travelling compression of the air in the chamber: that means movement. Fill a clear glass partway (maybe 2/3?) and upend it in a sink full of water to leave a bubble inside. Now make waves and see what happens.

So now the entire weight of the two platforms combined is trying to flex the vertical walls in a lateral movement. Because the wave won’t hit them both at the same time, the first one will lift (at an angle starting at the leading edge meeting the wave) pulling the skirt wall outward on both hexes. Then as the leading edge of the wave hits the joined edges, it lifts that section up, pushing both vertical skirts walls inward as they strain to lift the middle of two heavy hexes. then the leading edge of the wave hits the back side and the second hex is lifted, pulling the bolted connection outward yet again. That’s a lot of strain on a relatively thin wall. The stress is greatest on the extreme bottom edge, because that’s where the greatest leverage is. That’s simultaneously where the least reinforcement is.

(Jake Rosoman) #12

If the chamber is large enough to span both the peak and the trough of the wave then it averages out and there is no compression change.

So long as the pontoons match the wavelength then they will be transparent to the wave and this won’t happen. If the wavelength is anywhere near the size of the pontoon then this might start to happen but the solution is just to open up ports between the pontoons so the supporting air can flow between pontoons.

(Gordon Hoffman) #13

As long as the pieces are small enough to pass through the locks on dams then pieces could be fabricated inland, floated out to sea, and assembled - I previously mentioned Clarkston, Washington, but it is not near any coal fired power plants - hot and dry in the summer though.

(Theodore M. Amenta) #14


I understand your post to describe a platform with an area of 176m2 or 1,894 square feet to be built at a cost of $7,002 or $3.69 / SF. This is 10% of the lowest cost ($30./SF) I have been been able to demonstrate myself. I applaud you. Please price this out with local contractors and confirm in a second post. I am working with concrete not geo-polymer.

Your cost is low relative to land values of land adjacent to water. The result is high probability of financial feasibility. That is investors will be motivated to move onto the water rather than land. This raises an additional topic for you to consider. Land value (cost) is directly correlated to the economic intensity to which the land is placed. $3.39 is above most agricultural land but in line with some residential. A suburban shopping center with at grade parking might yield $6.00 / SF land value and a several level office building $20 / SF.

Consider your displacement: At 1.5 meters in draft (5.0’ rounding) you are providing buoyant for 300# / SF (all rounding). A conventional land-based building might weigh 150#/SF for dead load and #50/SF for live load — So you can support an imposed load of two levels — assuming open space.

Consider a vertical support system on a columnar grid or partitions that will support the building structural grid above — as part of the platform.

Consider a double bottom to the platform “hull” for additional lateral structural design.

Good show! Ted

(Jake Rosoman) #15

I’m a contractor. What I have given though is just a cost estimate. There are no profit margins added on. Portland concrete costs about the same as geopolymer concrete.

Yea well in NZ residential land is the most valuable type of land so it’s the one I would compete with first. You can’t get it below 13 USD/ft² near any city here. And its about an order of magnitude more expensive than that in prime locations like on the coast. In countries like Tahiti prices are even more crazy. So there is definitely a clear economic case to be made for these pontoons. I think the only type of land they won’t be able to compete with is farm land which is 0.13 USD/ft² in NZ. Cheap as dirt because that’s all it’s allowed to be.

And yea for high load use cases you could add reinforcing to the pontoon. You would probably want to extend the skirt too. I’m not sure how far you can take this pontoon design but I’m sure it could be designed to cater to most use cases apart from high rise buildings. Column support is probably better done by the contractor that puts the building on the pontoon since its requirements will differ a lot between buildings. If I was starting a company building these pontoons I would probably offer a few versions differing on their load rating with this design being the lightest. And people can add reinforcement themselves where it’s needed.

(George Hawirko) #16

What exactly is the purpose of the Mold You should actually be able to build these without a Mold like you donèt need Molds to build a house.

(Jake Rosoman) #17

It holds the liquid in shape while it turns into a solid.


Geopolymer cement passes through a very runny phase once the reactions begin, which is much more watery consistency than OPC, and will not hold a shape, unlike slump-testing OPC.

(Tom Schaefer) #19

I am a professional cost analyst. I don’t want to throw cold water on this topic and analysis, but I feel the cost estimate is unrealistic for two reasons: 1) Comparison to current offshore construction costs and 2) A proof by lack of existence. I infer the author of this post thinks it is much less than $1,000/Ton.

For 1) I refer you to this article ( http://www.offshore-mag.com/articles/print/volume-72/issue-7/rig-report/reviewing-rig-construction-cost-factors.html ) that one can infer gives a cost of gives a cost of ~ 20% of ~$20,000/Ton or ~ $4,000/Ton for very large floating platforms.

For 2), I would think that river front restaurants and bars would have forced the development of this if it could be done so cheaply, as would lake front property owners for patios, decks, and docks. Perhaps even the houseboat industry.

I’m asking that the Seasteading Institute consider creating an on-line cost database and ask projects to submit planned and actual costs for their projects. Such costs would have some “Developmental First Item” or “Prototype” designation that could be extrapolated to quantity and rate in production. I’d be glad to help.

(Jake Rosoman) #20
  1. This design is very different to those designs so that article isn’t very helpful at all.
  2. Well in my country its illegal. People have done it but even when they get consent it’s only temporary. But anyway proof by lack of existence is an extremely weak form of proof.

People should figure out the cost of something by adding up all the fundamental inputs. It’s more accurate than comparing to market rates of “similar” projects and probably easier too. It didn’t take me long to come up with the figure I gave and I can iteratively improve it if I want.