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January 8, 2010

Engineering talk

The talk I held at the 2009 Seasteading conference is up! Thanks everyone who was there, for the interesting discussions we have had afterwards.

Some comments on the then and now: I unfortunately no longer have the confidence in this concept I had back then, for the reasons explained below:

 

Connecting seasteads in such a manner is more difficult than I supposed, because the horizontal forces involved are much larger than I anticipated on an intuitive basis. To get a bit technical: the horizontal forces on a submerged body can be estimated by Morrison’s equation. Not only does it contain a drag component, being a function of the relative velocity, but also a component proportional to the acceleration of the water. It’s a little bit harder to see why this must be so, but if one considers the boundary of the displaced water, and the fact that this is the acceleration the displaced water would have experienced had it still been there, a body of non-water on the same position would either undergo the same acceleration, or would have to be held in place by a force proportional to it.

I had looked at these issues before, but committed a fallacy ‘ad linearisatum’: the effect is small for short waves, and the effect is small for long waves, so at the time I supposed it was probably reasonably small for all waves. Unfortunately, not so. Under worst case conditions , these horizontal acceleration forces are of the same order of magnitude as the weight of the entire structure itself. the components required to withstand such thousands of tons forces get a little bit out of hand; perhaps it is not impossible, but taking this effect into account, it simply no longer feels like a conceptually elegant solution.

Secondly, I underestimated the effect of heave motion on comfort. I had been reading some literature that suggested heave is not that bad as long as it is not combined with any significant roll motions, but it appears the rest of the world does not agree. Given that heave suppression for this concept is not very strong, its comfort characteristics will not suffice in the open ocean.

 

I am somewhat disappointed by these discoveries, as it nudges up the minimum scale of open-ocean seasteading which I believe to be plausible, but as someone commented, ‘that’s progress’, and I suppose it is. Its unfortunate that progress doesn’t always come in the direction you wish it would, but such is life. There are still other options which are being explored at the moment that I feel confident about, so no lack of work ahead!

 

A happy new year to you all, 

Eelco