We are working on a seastead design that will have a 40 by 16 foot living area above the water.
There will be 4 foot wide columns that are about 20 feet long going out from 
from the 4 corners of living area and down into the water at 45 degrees, which half of
each column under water.   The bottoms of the floats will make a rectangle about 44 feet wide and 68 long.
From the bottom of each column there will be 
2 cables going to the adjacent corners to hold it in place.
There will also be a cable making a rectangle between the bottoms of all the floats so we have some
redundancy in case one cable breaks.
The seastead is about 30,000 lbs I think but this is NOT a normal boat hull shape,
it is more like a tiny oil platform as far as drag.

We expect to use 2.5 meter diameter propellers on two low speed submersible mixers and solar power to move
at around 1 MPH plus any help from using eddies.


When a thruster pushes on water the thrust is proportional to mass of water moved times
the speed the water is given.  It is Newton's equal and opposite reaction for 
the momentum given to the water.  The energy is 1/2 times the mass of water times
the velocity squared.  So looking at static thrust or bollard thrust the
energy needed goes up with the square of the velocity but the thrust just
goes up with the velocity.   So the best static thrust per energy is with
lots of water moving very slowly.

So the low speed submersible mixers with 2.5 meter props turning at low RPM are 
a reasonable idea.

I want to look at another possibility though.

Imagine a sort of underwater quadcopter that gets power from a cord.
Can have 2 opposing propellers rotate clockwise and the other two counter-clockwise,
so like a drone it can rotate by putting more power on one pair than the other.
Then by differential thrust it can turn any direction.   So we can put
this in the water and have it pull on the seastead.
It is interesting because it could be a tug for all sorts of vessels or barges.
The main advantage over just using two submersible mixers is the attachment
is easier, just a cable.  Can also make for less noise or vibration.
Can also be easier to operate 2 big propellers in depth, which mounted
on the seastead might be only 1 level.
It can separate the development of the thruster from the seastead in a
more complete way.  Design iterations could be faster.


I am not at all sure it is worth the trouble but it seems interesting.  What do you think?