We are working on a seastead design.

The goal is to design our seastead such that all the parts can pack into a single a High Cube 45 foot container which has:
   width 7.7 ft
   height 8.9 ft
   length 44.6 ft
   max weight:  62,000 lbs  (rated bouyancy at desired waterline is 27,500 lbs and we hope structure is enough under this that humans and their stuff can fit)

Above the water there will be a big equilateral triangle frame, 44.0 feet on a side. 
The triangle frame is also the wall of the living area and will be 7 feet high (floor to ceiling).
It will be enclosed and the whole inside the living area.
Around the whole outside of the wall, except where the dinghy is in the back, will be a 3 foot wide walkway and railing that 
is bolted on and has some diagonal supports from below bracing to the wall (so walkway is 1 food higher than bottom of the wall).
The walkway will have an aluminum grating that would let a wave pass through but you can walk on.
Also two doors on the back side,  one two feet in from left and one two feet in from the right side. 

There are 3 legs/floats/foils/wings/keels that provide the buoyancy, so it is a bit like a trimaran but with a very soft ride.
Each leg/wing will 21.5 feet long and have a NACA 0035 foil shape with 8.5 foot chord except that the last 0.5 feet of
the thinnest part will be cut short, so with foil does not come to a point at the trailing edge and fits within 8.9 feet
hight of container.  But the buoyancy is very close to that of an 8.5 foot chord foil.
Each of the 3 legs will be attached to the underside of the big triangle near one of the 3 points.
The center of the thickest part and going 1.5 feet in all directions from there will be within the area of the triangle,
but within that constraint, each leg will be as close to the point of the triangle as possible.
The legs will go down so that the lower half is in the water.
This makes for a bit of "small waterline area" similar like a small oil platform but one that can move through the water easier because of the foil shape.
It is not an extreme SWATH design as a 1 foot change in water level is about 1/7th of the total buoyancy, so still significant change.
The 3 legs will all be parallel with the blunt or "leading edge of the wing" side facing forward so it is lower drag when moving forward
than a typical cylinder on a semi-submersible platform.  
Each leg will be 50% under the water (so 0.5 * 14.5 feet) and the top 50% out of the water.
On the top half of the front of each leg, so the top half that is out of the water, will be a built in ladder.

The reason for these sizes for the triangle and legs is so they can pack into a container nicely and shipped to 
a shipyard anywhere for assembly.
Two of the legs will be packaged together with one having the round edge down and pointy edge up and the 
other having the round edge up and pointy edge down so they are just over the thickness of the foil together
and loaded along the right wall all the way to the back.  
The 3rd leg will be along the right wall with the round side down between the first 2 and the container opening.
So the right few feet of the container (width of legs) is used by the 3 legs.
Then the 3 frame/wall sections will be upright (so 7 feet high) next to each other along the left side of the container.  
I am not sure the width of the walls but if they were 10 inches wide then 3 widths is 30 inches and some extra is 3 feet on the left side.
There should still be lots of room in the center of the container for all the other parts.

Connecting the mid points of the walls both at floor and ceiling level will be structural beams that
make another triangle 22 feet on a side.  Then all the remaining spans will be less than 22 feet.
The rest of the floor and ceiling will be small pieces that are bolted in.

On top of the roof there will be solar all over.  With batteries and electric thrusters as the main propulsion system.

There will be 6 RIM drive thrusters of 1.5 foot diameter, one on each side of the 3 legs/wings about 2 feet up from the bottom.
These RIM drives will be all be fixed orientation to provide forward thrust.  It will use differential thrust to turn.
For slow movements in tight areas like harbors it can reverse thrust on one side and forward on the other to turn in place.

There will be a conduit/pipe welded to the back of the trailing edge to take electrical wires down to the thrusters.
There will not be any "through hulls" in the legs.  The legs will also have multiple
airtight compartments each for safety.


Behind the back near the center will be two supports going out and 2 ropes going down to a dinghy.
The dinghy is a 14 foot RIB boat (deflated for shipping) with an electric Yamaha HARMO outboard.  It is sideways against the center of the backside of the living area.
When the seastead is moving forward the dingy is shielded from the wind by the living area.

On the lower part of each leg will be several bolt on heave plates.  These will help dampen the response to waves.

About 25% of the displacement will be for LiPo4 batteries which will be put low in the 3 legs.
Each leg will have its own charge controller and inverter so there is triple redundant power on the seastead.
Also, the thrusters for a leg will get power from that leg's inverter or batteries.  So
the 3 pairs of thrusters will have independent failure modes as far as power.

When the seastead is going to be staying in one place for awhile, we can put down 3 helical mooring screws and give the seastead tension legs 
so it becomes nearly stationary when parked.  Near each corner there will be a pair of helical mooring screws with a motor unit between them.
We only plan to do this in the Caribbean where tides are very small and in protected places where the saves are small,
so pulling down 3 feet will be sufficient to never go slack.


Two seasteads will be able to connect together with a walkway, one behind the other, so that while underway 
people can move between seasteads, enabling a real community.  The two computers for the two seastead will both work thrusters
to minimize the movement of the walkway, particularly when warned that someone will be on it.





Now imagine a catamaran tender/dinghy with an electric outboard and tracks around the two pontoons with
an electric motor on each.  This dinghy can move along the sand using the tracks.
The tracks will have sufficient groves/bumps in the tracks so that when in shallow water they will provide
some thrust.  So you can lower the outboard only after the water is deep enough.
And going ashore you can do the reverse.  Raise the outboard and either let 
momentum carry you to the beach or have the tracks turning to give some propulsion.
Then when you get to the sand you can drive up the beach.  Can use differential
track speeds to turn, like a tank.

This would only be for beaches with small waves in a harbor.

Have you ever seen anything like this in a tender or dinghy?   I know some larger boats that can go onto the sand 
but has anyone made an amphibious boat 9 to 14 feet that works as a tender?
How long/heavy are the other amphibious boats currently for sale and what do they cost?

Is there a better way than some sort of tracks?  Maybe some sort of walking mechanism?
We don't need to go far, 20 feet up the beach is probably good.

Please flesh out a design.  Try to keep it simple and keep costs down.
What materials would you recommend?   
Seems like the floats could be rotomolded with HDPE plastic with whatever shape works well.

The hard part seems to be engineering tracks/rollers that are not too heavy or costly
but can handle sand without jamming and can be driven by a motor and be reliable.
What exactly would you recommend?

What does it cost to get a custom rotomold mold made?   What is sort of a common
first order size for making things with rotomolds?  

I almost wonder about just having a tiny anchor and a winch.  The HDPE catamaran could slide 20 feet without trouble.
Would have to go ahead and place anchor but then you don't need any strength to move the tender up the beach, it would
do that itself.  Could be very cheap and simple.  Just need shape that rides up over the sand.  What do you think?

For a dinghy that holds 4 people,  say 11 foot catamaran, how cheaply could one be made in China if we ordered 20 sets of everything?  (list
tooling costs seperatly, not in the batch costs)

Do you think it could be a reasonable product that could sell with a reasonable markup?