Single skin fibreglass hulls will almost certainly require stringers, ribs and web floors.   Sandwich hulls may require some stringers and ribs and will require web floors.   Your plans will show you what types of stiffening your hull requires and where it is to be located.   Stringers and ribs have similar construction.   It is normal to install the stringers first and then use an intercostals type of rib.   An intercostals is simply a short length of rib between each stringer, running from the sheer or deck shelf down to the sole stringer or sole shelf.   The transverse webs take over from there in supplying the athwart ships stiffening.  

   There is no reason why you should not lay out the system of ribs and stringers with foam and then apply the laminate simultaneously.   A problem that may occur is that the foam cores of the stringers and ribs are easily damaged and you would need to be very careful until you have installed some fibreglass covering.   A foot in the wrong place and you can do damage.   Electrical wiring and plumbing can be placed in these stringers but if there is ever a problem you will never know where it is coming from, as we once found out to our cost, so it may be best to keep the wiring and plumbing external. Wiring can be set into cored deckheads but you have to know where the lights are going to be and some certification authorities are not keen on this.

   Stringers and ribs are usually foam cored.   Rigid urethane foam of about 2 ½ pounds density can be purchased in sheets of a thickness equal to the depth of your stringers and ribs.   Cut this material into strips on a band saw or other fine bladed saw and, if you are careful, you can even use a handsaw.   Angle the saw to make stringer cores that are wider at the base than the top.   Alternating the cutting angle will ensure there is no wastage.   Stringers and ribs can be various shapes to play special roles in the hull.   For instance, a stringer that will form a deck shelf will be flat on the top to accept the deck panel, but angled underneath.   A sole stringer will be flat on the top and shaped to fit the contours of the hull.  The engine bed stringers may be vertical on the inside and flared outwards on the outer sides.

   All the shapes can be arranged when you cut the foam into strips, so make sure you have the right materials on hand.   The various lengths of foam stringer material are butt joined and placed in the hull where they are quickly fastened into position with a hot mix of resin putty.   A few spoon size lumps of putty set about 6 inches [150mm] apart will hold any foam stringer in place until you are ready to apply the stringer laminate.