WRONG !

The flexing of the beams is determined by the size of the crosssection, the length AND the size of loads placed on them.

The latter two ARE significantly different from the F18 design and therefore the first can be significantly different from the F18 design as well while maitaining the same overall stiffness.

The loads placed on the beams are directly linked to the displacement of the hulls. How else can the boat twist or bend them ? The overall volume of the F16 hulls are (5/5.52)^3 = 75% of that of the F18's. That is then the maximal ratio to what the F16's beams can be subjected too. A similar reasoning is valid for the mast step load.

The flexing of the beams is related to overall length of the beams by the thir power. Or rather to the unsupported span of the beams. This means that a typical F18 flexes by (1.9/1.8)^3 = 118 % when compared to a typical F16 when only looking at beam lengths.

Combining these two ratio's results in the F18 flexing by 158 % when compared to a F16 when identical crossectional beams are used. Quite a difference, I say !

In layman's terms this means that the wallthickness of the F18 beams can be reduced to 63% while maintaining identical flexing during sailing. Of course there are other factors as well that improve this situation further, like the fact that the F16 hulls are shorter and therefor have the beams have to flex more to lift the bows by the same amount. This is felt as additional stiffness.

Grouping all these factors and ending with the same flexing of the bows while sailing (assuming both boats encounter the same height of waves) result in the F16 beams only requiring 55% of stiffness (crossectional) that a F18 needs. Basically, if the wallthickness and shape of both beams is maintained then the F16 beams will weight only 74% of the F18 versions. If all reduction in stiffness is handled by reductions in wallthickness (maintaining identical overall shape and size of the beams) then the F16 beams only need to weight 55% of the F18 versions. The real figure will lay somewhere between these two extremes.

Notice that a pair of bare beams can easily be 10 kg combined; this means a weight saving from the range of 2.6 kg to 4.5 kg is possible while maintaining the same overall effective platform stiffness between the F18 and F16.

That is a difference worth noting, I think.

Replacing the aluminium by carbon will only win you 3 kg over the full set at much increased costs. Designing specilized alu F16 beams while applying basic engineering skills wins you about the same amount for very small costs indeed. See another recent thread about the cost of having an extrusion die made and fabricating/shipping a batch of beams. That is why the last route was taken by Formula catamarans Aus and VectorWorks Marine (now Falcon marine).

Again, similar paths can be given for items such as the daggerboards. Here the F18 boards weight at least 3.0 kg a piece (as per class rule) while the F16 boards (just as dependable, but unregulated by class rules) are between 1.6 and 1.8 kg a board. Rudderstocks, same story. Then add all this up and you'll see that a glass/alu F16 at 111 kg is viable. Replace the superwing by a carbon mast (at minimal tipweight of 6.0 kg) and the ready to sail F16 will be spot on 107 kg and still not cost more then a modern F18.

Now we have also covered the reason why the glass/alu F16's are overweight to the min. class weight by a few kg's. The builders rightly reason that it is not economically attractive to offer the carbon mast upgrade if the weight savings there are matched by lead corrector weights. So they aim for a glass/alu weight that will see the ready to sail weight with a carbon mast upgrade end up right at the class minimum. Any serious racer will buy the carbon mast and any buyer looking for a cheaper F16 will not fret over being 4 kg overweight.

Wouter