Mast preflexion: what it is, what it is for, and how it is achieved
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The term pre-bending applied to a sailboat’s mast refers to the longitudinal curvature that the profile assumes based primarily on the stresses in the rigging. on boats with quartered spreaders the preflection can be accentuated, while it is more difficult to recreate on boats with in-line spreaders and other expedients are needed. But what is the purpose of tree preflexion and how is it achieved?
Mast preflexion, what is it for?
Mast pre-flexion should be managed according to wind and wave conditions. Concretely, the more curved a mast is, the thinner the sails will be because their fat will be absorbed, and moved forward, by the curvature of the profile. The pre-flexion then needs to be increased gradually according to the intensity of the wind, and also following, of course, the mast manufacturer’s instructions on the stresses applicable to the profile.
Mast preflexion, how to get it
To achieve longitudinal curvature it is mainly necessary to work on the high shrouds, and adjust the intermediate and low shrouds accordingly. The right tension to give to the high shrouds according to various wind situations is something that has to be worked out gradually through progressive trial and error, the perfect balance in mast adjustment is something that in fact is not simply achieved with a day’s work, but has to be developed with patience by testing at sea the adjustments we experience ashore.
In principle, once the tension of the high shrouds has been established, the intermediate and low shrouds should simply be pinned, so that they go into load as soon as the boat heels. It will be normal to see the downwind shrouds, especially the low and intermediate shrouds, just slightly bailing if the adjustment has been made correctly according to the wind strength.
When the crosses are in line
The curvature is more difficult to achieve when the spreaders are in line and the masthead rig. and thus do not have a pull angle that allows the profile to flex sensitively. In these cases, the work of pre-flexing the mast is mainly done by the backstay and, if present, by a bow stay, if any, which will have an anti-pumping function on the mast but will also help to flex it.
Mauro Giuffrè
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