I suppose that wind and water mill work is our speciality.
Our current windmill project in Barbados has arms that were single piece greenheart - 68 feet long 14.5" x 11". The main windshaft is also greenheart octagonal 24" x 24" with the vertical drive shaft the same dimension but in purpleheart.
We have been unable to find replacement single piece arm timbers and so will make up new multi piece arms with a 40 foot centre section and 20 foot outers with 6 foot overlapping scarf joints.
This kind of project is not for the feint of heart. Windmills are very dangerous beasts. People regularly get killed.