The erronious idea that lots of boats were built with a "geodesic grid
inside them that completely obstructs the cabin" has been raised by DSK.

A geodesic structure is usually one which encloses a space -such as a
dome. The term geodesic refers to the great circle in geometry and
typically the circle is used to define a dome which forms a thin shell.
Thin shell hull reinforcing would be of no use in hull design as the key
to stiffness is to increase the inertial moment of the hull surface
without increasing weight. This explains why space frames and trusses
and 'structural grids' (a type of 3D truss) are used quite widely. You
may not realise it, but a foam cored sandwhich is also a 3D truss with
the bubbles forming the space between webs. The racing yacht designs
that incorporate these structures would not allow them to completely
obstruct the cabin as there would be no place to pack sails etc. In any
case, to do that would move hull material to the center of the hull
where it would be of less value in limiting hogg/sag etc. (compared to
increasing material on the far hull side).

It follows that there is an optimum hull thickness for any particulr
construction method. At design extremes where the hull is all truss with
a fabric coating we approach the design of the Zepplin. At another
extreme we see the submarine where closely spaced ring girders reinforce
the pressure hull and help reduce hull compression with depth.

Cheers MC