A boat needs a hull skin to be tough enough to take some kicks and stiff enough to hold its shape when impacted by waves. By the time you get the strength in the skins to take this abuse you are close to having enough strength to take the rig, foil and wing loads. You then add some framing/bulkheads to the hull to give all the strength you need. Properly designed the framing strengthens the hull and the hull strengthens the framing. If you isolate the framing from the hull into a space frame you duplicate structure and weight.
If you look at the way gantry’s are built, they do not need any skin to work so most are built using tubes, however they can be built just as stiff and just as light using foam panels. This is a case where monocoque is as good as space-frame even if the skin is not required.
It is bad if the hull deflects when you pull the rig tension on, but also bad if the skins are deflected by the waves, this uses energy and if significant changes the shape, probably in a bad way. It is not too difficult to build/reinforce a ply boat in such a way that it will take the rig loads, however the skin stiffness is more problematic.
The stiffness of a panel is a function of the cube of its thickness so doubling the thickness of a panel makes it eight times as stiff, Typically you would build using 4mm ply against 8 or 10mm foam sandwich. Foam is softer and more flexible than ply but once you have put glass or carbon skins on it becomes incredibly stiff and the weight is low because the heavy but strong skins are held apart by the foam. A layer of glass on a 4mm sheet of ply will only make a small difference but an extra layer on 10mm foam sandwich sheet will make a large difference for the same extra weight in both.
If you can’t get the panel stiffness through skin stiffness you need to get it somewhere else, one way is panel curvature if you take a piece of paper it will bend in every direction, hold it so it has a curve in it and suddenly it is quite strong in one direction. This has been done in ply boats like the recent [link=
http://www.internationalcanoe.yachting.org.au/?Page=24752&MenuID=How%5Fto%2F13919%2F0%2CHow%5Fto%5FBuild%5Fan%5FIC%2F13920%2F0] Hollow Log[/link] DC and a Moth by the same builder. He uses very low density foam blocks to shape the hull stiffen the panels in areas where there is low curvature. If you can abuse the ply into a double curvature then you have loads of stiffness, think Morris Minor bonnet.
A different way of getting panel stiffness into a hull is to use a large number of panels that are joined together along curved edges, the panels are curved along the length of the boat forming the shape. The joints between the panels give good stiffness along their length and the panels are not that wide so do not have a large unsupported area. Think Enterprise or Wayfarer both originally designed to be built in ply. This can only be done to a certain extent in a cherub as there are restrictions on the placement of chines.
If you do not use panel curvature or multiple chines to stiffen you panels you can simulate skin thickness by using frames and stringers and battens (think aircraft fuselage) each frame or stringer reduces the un supported span of the skin panel and so stiffens it, this, however takes time to do and adds weight.
A well designed structure will use parts of all of these techniques to get the required skin stiffness and hull strength and will do this whether it is built from strip plank, ply sheets, Glass/carbon/Aramid foam sandwich or carbon nomex or something new that no one has thought of yet.
