Gabriel,
Answers must be tailored to the type of constrction you are doing . Ribs have slightly different functions in different types of constuction. It would help if you would describe what you're building.
On a wood and canvas canoe the ribs form an inner layer of planking to which the outer planks are nailed. The ribs are very wide to accomodate the many nails that are used. The ribs are every bit as important as the planks.
On an all-wood planked hull, ala J. Henry Rushton, the planking and the method of joining the planks form a fairly strong unit so the ribs can be quite thin. The ribs don't contribute much strength to the hull but form a buttress that helps maintain the cross sectional shape.
On an old-fashioned stripper the the strips are only edge-nailed together which doesn't form a very strong bond between strips. Every strip is then clich-nailed or riveted to every rib. In this case the ribs not only maintain the cross section but are also the medium that ties all the planks together.
Ribbing of another sort can be seen on many fabric/epoxy laminate hulls. Because canoes can have large areas of reasonably flat surface, laminate hulls are often afflicted with "oilcanning". The cure for this is a limited mumber of ribs, wood or 'glass, to reinforce these areas.
Charlie

: Sorry this is basic, I'm building my 1st canoe. From what I have gathered
: they provide rigidity. When a strip of wood is bent it wants to straighten
: out. Is this outward force the critical function?

: I'm debating on using strips of bamboo laminated at 1"-2" by
: .25" by 2-5' for ribs. They provide a good amount of outward pressure
: and are FREE.
: Or buying some oak, hickory, ash, or something.
: Or puting kligicell/divinicell foam core for deck and re-infocing sides with
: more glass.

: Another thought, Do I even need ribs or deck re-inforcement? Can I get by w/
: all glass with out being too heavy?