Here is a review of Bene construction of the 411 model from BOATS.COM. It describes several aspects of their construction:
"The rudder of the 411 is all composite, even the rudder-post. The blade of the rudder is formed of foam sandwiched between fiberglass layers. The post is a glass-epoxy composite that is both lighter than stainless steel and stronger. Epoxy
rudder shafts have several benefits. First, the blade and the shaft are one inert piece, so water
will not migrate into the foam sandwich where the shaft and blade join. Take a moisture meter to any boatyard and measure the water
in the rudder on the hauled boats. You will see that virtually all of those with steel shafts joined to foam sandwich blades have high moisture readings.
The second benefit is the flex inherent in the epoxy
shaft. The rudder is the most vulnerable point on the boat in heavy weather
or when grounding. When the boat falls off a wave sideways and come up short at the bottom of the trough, there is a huge side strain on the shaft. In rare instances, a steel or aluminum shaft can bend or even break under such loads. An epoxy shaft could brake as well, and every year leading-edge racing
boats lose shafts that have been engineered outside the envelop.
But the 411 is not a racing
boat and has been engineered well within the envelop. The shafts Beneteau uses have tested out in Beneteau’s own tests as being three times as strong as stainless steel, largely because the shaft will flex and deflect the side force. This is not experimental technology anymore; instead, it is rapidly becoming the best way to engineer
The guts of the boat rest on an interior
grid pan that is joined to the inside of the hull with polyester paste and fiberglass tabbing. Integral to the grid are two levels of fore-and-aft stringers, the chainplates, engine
mounts, tank cavities and bulkhead slots. The downside of such interior
pans used by many big production builders lies in the inability of a skipper
to get to and repair a breach in the hull that lies behind the interior liner.
That noted, however, the grid system adds strength to the hull without adding excessive weight and simplifies the building process, thereby keeping costs down. Prefabricated interior modules fit neatly into the interior grid, where they are tabbed into place. The whole boat, once the deck
is on, becomes a virtual monocoque.
The 411′s deck
is a fiberglass, end-grain balsa sandwich. The hull and deck are joined on the flange molded into the hull, glued with polyurethane
mastic, and fixed in place with stainless-steel screws. It used to be the rule
that flange joints should be thru-bolted on 6- or 8-inch centers. Nowadays, the adhesives used in hull-deck joints are so strong and reliable that screws have taken the place of bolts. Beneteau makes a point of noting that none of their boats has ever suffered a hull-deck-joint failure"