fedeghi,

I think your "2" is because there are 2 saddles equally loaded by the longitudinally force FL (which is a global on vessel horizontal force loading the saddles).
As a confirmation of Mr. Delaforce opinion about the convoluted development in Moss book, in fig. 4-56 the original intention was to consider FL/2n instead of FL/(2A)*e=fu*e, but this is just a detail.

I would add that in "M" formula the factor 0.5=1/2 is likely to be due to the guided cantilever behavior assumed for ribs, here it is not related to saddles.

In the end, trying to conclude my understanding of Moss model: the horizontal vessel is loaded longitudinally with force FL and connected to base by- lets say about- n guided cantilevers (ribs in left saddle) + n guided cantilevers (ribs in right saddle) and each guided cantilever on the left side has an identical correspondent in the right side. The transverse rigidity of the assembly of guided cantilevers in left side is identical with those of the assembly on the right side. We can conclude that both assemblies (ribs in saddles) are in bending due to FL/2 and after that we try to calculate the load and stress in each rib- guided cantilever. Make sense for you?

Best regards.