Five questions about your tree – First installment

How many words are spent in the boat criticizing, praising, or commenting on the state of the mast. But do you really know him well? So we asked ourselves five questions about the tree (five more will be answered in the next installment).

1. Why are there through masts and supported masts on deck?
The choice of how to mount the mast is made in consultation between the boat designer and the mastmaker, and it can be resting on the deck or passing through it. The most tangible difference between the two systems, with the same boat, is the mast section. The through one can be called more constrained; it is fixed in the keel by screws or interlocking and has a constraint in the coaming (hole in the deck through which the mast passes). A leaning tree, on the contrary, has no constraint. For this simple reason, without going into statics formulas regarding the moment of inertia of a beam, the through-shaft is smaller in cross section because it is inherently stronger due to the constraint points. In contrast, a leaning one has a larger section to “make up” for the structural lack of constraint. For example, imagine a rod and apply a weight at the tip-it will flex. To decrease the deflection either increase the cross section of the rod or decrease its length. For the same profile, a mast in the keel is less stressed than one in the deck.

2. What are the major loads it supports?
The stresses to which a shaft is subjected are many, but the main ones are:
– compression, due to the weight of the mast itself, its structure, and from the tension given to the rigging when the boat is in port. In sailing, the weight of the sails and the tension of the halyards that support them also act in compression. If the boat is heeled, it also affects the windward rigging surge;
– bending, which obviously depends on the material of the shaft; aluminum is “softer” and carbon is stiffer. The other main causes that flex the mast are the weight of the rigging and a series of pushes caused by the boom, spanker, spreaders, and mainsail battens;
– the often underestimated twist, which depends on the thrust of the spinnaker when sailing with the spinnaker and boom.

3. What form can it take?
Again, the choice is made in consultation between the designer and the tree farmer. The former conditions the shape, or rather the section of the mast according to the sail plan he thought of and the location of its seat. Instead, the sapling maker conditions the section according to the materials chosen and the framework that is made. Finally, the type of boat, whether cruising, racing, oceanic, etc., also comes into play. To give practical examples think of purely cruising boats made in series. The cross section of their shafts is usually constant throughout the length of the pole. For boats that are a bit more “racer,” the head, usually from the last spreader onward, is tapered, decreasing its cross section and, consequently, its weight. The shape of the section varies from circular to elliptical, but there is no rule as to whether one is better than the other. The elliptical shape, which is very common today, has a larger section on the fore-aft axis so as to better withstand the stresses that would otherwise be borne only by the forestay and backstay or flyers. Transversely, however, the section is thinner given the presence of shrouds and spreaders.

4. What are the fixed maneuvers?
These are all those used to support the mast, so shrouds, backstays, stays, and flywheels. They differ from running rigging, which instead is rigging that runs through blocks and is used to adjust sails.

5. What does it mean that a tree is fractional?
It means that the forestay attachment is not at the masthead, but slightly lower. The ratio of the distance of the forestay attachment to the masthead, in relation to its total length, defines the type of fractional rigging which can be 3/4, 7/8 , 9/10 etc.