Anonymous wrote:
I wonder if I might ask a (what may be a quite basic) question of the experts here?
Experts? I probably don't quite qualify on that count...
When I do the calculations for mast breaking strength to calculate the diameter and wall thickness of an aluminium tube for a hybrid mast, does the fact that the boat has 2 masts come into play? Do both masts have to be strong enough to cover the Mb x 2.5 (for example), or can I halve the required mast strength as there are 2 of them, or does some other factor come into play?
A bit late in the game for that? I thought you had already made the partners and mast step based on wood masts (hollow). So I am guessing that the current partners and steps are based on the plans and plans sail area. Therefore, you should be able to take the plans size and wood type and work the formula backwards to see what numbers the designer used for wood and then use those numbers with aluminum.
That's all very well and good but the questions still remain. The moment of the hull is what it is. While I would not suggest going with one mast, it may be worth while calculating out one mast with the whole sail area of both sails and the highest AR out of the two (I think they are the same AR actually). This will give you the largest possible mast diameter, one that is too big. If nothing else, this will make schooner rig masts at least seem small by comparison.
So anyway, back to the question. As above the hull moment "is what it is" and each mast should probably be calculated as if it was the only mast. The stiffness of the hull that the mast has to deal with on a schooner rigged vessel is going to be much higher than on a vessel using a single sail of the same area/AR as one of yours. While it is true that the second sail will start to push the hull over at the same time as the first, there by relieving some of the force on the partner, that does not take into account any resonances or cases where one sail is up and the other is not. Also, any gust may hit one sail before the other.
Having said all that, I will note that these calculations (that you mentioned above) are about the loads on the mast when the wind is on the beam. They do not take into account the hull moment when running down wind when the hull is much stiffer and yet it still seems to work fine even when mast size is calculated for a vessel with lower beam on moment.
My understanding is that you are not planning day sails but rather offshore voyaging. This is partly the reason for a two mast rig, it gives redundancy as well as adjustable balance for easy steering auto or manual. I think this also means being conservative with mast sizing.
I am not sure I have been much help...