Originally Posted by brian eiland
Let me ask you a question. Assume an analogous force-beam loading configuration.
Lay that mast (beam) out horizontally as a cantilevered beam with one end stuck in a wall (the deck). Now pull upwards at the tip just as the forestay does. And then in an analogous situation load some distributed weights onto the beam (outwardly from the wall) just as the mainlsail pulls on the mast tube as it rises from the deck.
Now ask yourself whether that beam (mast section on the horizontal) with the those extra weights has to be stronger than one with no added weights (no mainsail)??
Now 'point loading' of the mast tube can be a different situation (where most of the loading is by a single or multiple forestays and backstays). If these are not lined-up in certain manner, then the mast tube itself has to be stronger to resist extra bending loads resulting from 'out-of-direct-alignment of the staying configuration. And if your mast is made of wood (likely for that boat), and desired NOT to be 'bendy' as most in those days were, ....then you need to build it heavier. And likely of a larger sectional shape, ....which would have been detrimental to the smaller mainsail's performance,... and a real drag factor up above the head of the mainsail (bare mast section).
Furthermore, likely that mast section was NOT of a more aerodynamic shape that we get with modern materials, but probably a rectangular section characteristic of wooden mast of that era.
A better analogy would be to stand the mast up in a warehouse, put a huge weight on top, then run a bunch of strings, spaced evenly apart, along its length, to to the walls of the this warehouse. These strings would only be oriented for and aft. Along the side of this mast would be the usual set of shrouds and spreaders.
Basically, the way I see it, is the sail cloth prevents the mast from buckling forward, as its attached to the boom and has some leech tension. So if the mast starts to bend, the sail flattens a bit until it can flatten no more. At this point, the mast is prevented from buckling further in that direction.
The lift that the sail cloth provides arguably helps prevent the mast from buckling aft. But the main reason it doesn't buckle aft is because it is encouraged to buckle forward a bit, in the first place, and once buckling in one direction, it is very unlikely to buckle in the other direction.
The thing about buckling is it takes a relative weak force, perpendicular from it, to stop it in its early stages.
With a jib only, or a large jib and a very small main, The mast must be prevented from buckling at all. Hence it needs sections that are longer than normal, if not deeper too. So the mast begins to resemble a stove pipe, or wing. One possible solution is to run a second set of double lower shrouds up to the second set of spreaders, even if they have to attach to the same chain plates as the original set does.
It is for this reason, IMHO, that mast aft and jib only rigs are never as structurally efficient as fractional ones are.
Structural inefficiency usually means greater weight for the same SA. And this is absolutely fatal in competitive racing, except in situations where the CE of the rig must absolutely be moved aft.