Comment by mk_stjames | Hacker News Reader

Comment by mk_stjames | Hacker News Reader

Interestingly the author shows a photo of a spot in Cape Town where there are two distinct structures in use - close up, in the photo, are actually Tetrapods - as pointed out in other comments, these are an earlier invention from France and also used quite extensively throughout many parts of the world (including... right here where the Dolosse were invented).

Further down that jetty are structures with the actual Dolos geometry. It would be interesting to know if, specifically, those geometries were chosen specifically to be placed in those two parts of the wall as they are for specific properties, or if came down to having certain quantities made on hand and their distributions matched the shape of the wall required, or what.

You can see what I mean, this distinction in the exact location where that photo is taken on google maps, here:

https://maps.app.goo.gl/Vn3tGhM81oMqPVpm9

-33.899126, 18.412751 Mouille Point, Cape Town, 8005, South Africa

Mathematically, I think they both may be tetrahedons. The Tetrapod models it by 4 lines connecting each of its vertices to its center, the Dolosse by two opposite edges connected by a line through the middle.

I’m saying “may be” because it is possible that the center “arm” of the Dolosse is too large to make a true tetrahedon. That’s a degree of freedom in the design.

> It would be interesting to know if, specifically, those geometries were chosen specifically to be placed in those two parts of the wall

My guess would be that Dolosses interlock better than Tetrapods, allowing for steeper inclines on stacks of them.

That connecting "spine" is there to improve the interlocking of the tetrapods. They can "hook" better to each other. That's the theory at least, I don't know the if it's better in practice.

The Wikipedia article:

> A dolos ... is a type of tetrapod

but also, the Wikipedia article for tetrapod:

> One of the earliest designs is the Tetrapod, invented in 1950. Other proprietary designs include the Modified Cube (United States, 1959), the Stabit (United Kingdom, 1961), the Akmon[W] (The Netherlands, 1962), the Dolos (South Africa, 1963), the Stabilopod [ro] (Romania, 1969),[7] the Seabee (Australia, 1978), the Accropode[W] (France, 1981), the Hollow Cube (Germany, 1991), the A-jack[W] (United States, 1998), the Xbloc[W] (The Netherlands, 2001) and KOLOS[W] (India, 2010) among others.

I have put Ws next to ones with their own Wikipedia page.

Reply on news.ycombinator.com