User talk:Jesrad/BunkerStead

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Revision as of 21:03, 15 September 2008 by (talk | contribs) (Cargo capacity and being low in the water)
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Stability of split version

With the height that you show I don't think just having heavy stuff in the bottom of the sphere will give you enough stability. I think even wind could tip it. Maybe a hanging ballast would be the way to go. Vincecate 22:48, 14 September 2008 (UTC)

I figured I'd add a ballast on the outside, but then I realized the top half was maybe 2-5% of the total mass, and the truss 10% max. The center of mass stays inside the sphere easily, just having the bank of batteries at the bottom of it, plus scrap metal, would keep it upright.
Of course that means landing aircraft there would be impractical or maybe even impossible... On the other hand, a hanging ballast could help with dynamic stability, if not static stability. I'll look into it.--Jesrad 08:47, 15 September 2008 (UTC)
On second thought, this arrangement may prove sufficient for single stead stability (when moving ?), while multiple ones are supposed to connect and form a semi-submersible platform. Another funny thought is that the top half may pivot and double up as a large sail for migration :) Procedure: move all the furniture to the basement, tilt the house and drive away. I should call that a Sunflower SailStead. At the very least side ballast tanks are needed to compensate for any quasi-static tilting load (persistent wind). --Jesrad 08:51, 15 September 2008 (UTC)
OK, I've decided on the best form of ballast: it'll be hanging from three or four cables attached to the top of the lower half, so they lie on the sides of the sphere and join below it, forming a cone. Adjusting the lengths would allow lowering or rising the ballast as well as move it off-center to adjust. The winches for the cables would be protected from the environment by short sections of concrete tunnels (trapping air when waves wash over) and easily accessible by the crew. Widening gouges along the sides of the sphere would be needed to keep the cables in place. --Jesrad 09:07, 15 September 2008 (UTC)
Follow-up calculations show that an outside ballast will be necessary for static stability, so your intuition is confirmed. Alternatives require changing completely the shape of the lower half (hemisphere ?) or doing away with a top half and stick to a BunkerStead (which would mechanically be best with a saucer shape like your own geodesic vessel). Adjustable hanging ballast as described above is probably the best option. Also, there are a number of features missing on the pictured example I'll have to add next time, too. Let's make a list:
  • hanging ballast on four cables passing through four side tunnels (extending down to the "waist") and joining near the top,
  • central column supported by the truss, covering the access hatch and stairs and/or elevator to the top + air vents
  • docking bridge or transboarding crane, or both
  • your ad here

Cargo capacity and being low in the water

If you figure out the mass of water with the same volume as the part of the sphere above the water, that is the maximum cargo you can put onto your seastead before it sinks. It does not look like very much. Like if I loaded all my books onto your seastead it would sink.  :-)

The sphere is kept near its maximum loading at all times through ballasting, preferably using seawater. The picture shows it at maximum cargo capacity already ;) So the actual cargo capacity is the weight of the emerged volume plus the weight of the total ballasting volume plus whatever built-in load is supposed to remain insaide at all times. Let's say it would be half-way out of the water when empty.
Ok, I get it.

The waves go down some distance, reducing to zero at a depth of about 1/2 their length. But ocean waves seem like 300 feet and longer. Even though you are low in the water I think it will go up/down/left/right with the main waves. So I am not convinced that being low in the water really buys you much.

The point of riding low is to avoid the destructive power of larger waves, which can reach 100 MPa as the wall of water rams the emerged structure more or less horizontally. Because, as you noticed, the actual net positive buoyancy is very very low (about 1-2% of the total weight, if you consider just the sphere) the stead will be subjected to just a few percent of the upwards heaving any other strictly buoyant, permanently emerged structure would. But the downwards heaving remains large - that allows it to ride beneath big waves in tempests by sinking to their valleys and staying there, due to that asymmetry in heaving response. It is still moved left and right, but flattening the sphere vertically, or giving it a more saucer-like shape, would help in this regard.
I think the sphere goes up and down with the wave even if only 1-2% is above water. The water goes up and down with the wave. If you imagine a gallon jug that only has a very small amount of air and the rest is water, it still goes up and down with 4 foot waves. Maybe some water goes over the top, but the total up and down distance is not changed much from a jug that was half full. In my tests I think the ball shape is amazing at making breaking waves go away near it, so it does not get splashed. It moves down the face of the wave enough to kill the break behind it. 21:03, 15 September 2008 (UTC)

The test on my Ball House model show that even models of 50 foot breaking waves don't splash over the top of the ball. So having more of the ball out of the water gives you much more cargo capacity and there does not seem to be a need to build so high up. If you think about the motion in a "crows nest" at the top of a sailboat mast, it is probably worse than on the boat closer to the water level. So while the view could be a bit better up higher, I think it would be less comfortable motion.

Anyway, I like what you have on top (wish I had something interesting on top of my sphere) but probably this concept and the Ball House would end up closer after some Engineering than what we have each drawn now. I am going to link to this from my Ball House page since it seems related. Vincecate 14:05, 15 September 2008 (UTC)

Yes, I understand that my concept of riding low through large waves could be turned into an emergency measure for a BallHouse using water ballasts. It might prove viable as a way to survive hurricanes (and a way to make transboarding to and from boats easier). In calmer weather the BallHouse would float higher on the water and enjoy better comfort ? --Jesrad 16:38, 15 September 2008 (UTC)