The basic maxim is that a heavy guy needs a stiff mast and a lighter guy a softer mast. However the combination of sideways and fore and aft bend has occupied whole careers! In addition it’s very important that the sail matches the mast bend so it’s normal to buy them together, whether new or second-hand.
Collar wooden masts, made at Oxford, were the basic club level Finn masts, rather stiff and heavy. Bruder masts from Brazil were the very best wooden masts, winner of the Gold Cup many times.
Boyce wooden masts were very successful in the last days before aluminium came in. After 1972 John Boyce at Burnham-on-Crouch also made aluminium masts, using an oval tube, cut and welded at the top to produce the taper. Very successful in the OK class they were never very common in Finns.
Needlespar masts were boldly selected for the Olympic games in 1972 to reduce the variability of the wooden masts supplied to the competitors. (All Finns are supplied for the Olympics by the host country). The early masts were oval tubes at the bottom with a series of reducing diameter tubes for the top mast, glued together and turned down on the outside to provide a smooth taper. Later the bottom section was also round, but with thicker walls at the side than fore and aft. Different bend characteristics could be achieved with shorter or longer top masts, to produce stiffer or softer masts as required. The bend characteristics can therefore be guessed at by measuring the height of the top mast join. Needlespar also produced a few softer Delta masts with a D section bottom tube. These were standard for the Lightening 368 class.
Mast bend characteristics are usually defined by supporting the mast at the base ring and deck ring then hanging a fixed weight from the top. Bend is then measured at the top, half and quarter heights. Both sideways bend and fore and aft can be measured in this way.
In the days of wooden masts it was generally accepted that mast bend could be selected to suit a wide range of helm weights. However masts varied a lot and so much adjusting and swapping of masts took place to get a fast mast.
Aluminium masts introduced much greater reliability and mast selection became much less of an issue. However alloy masts were generally much stiffer and so lighter helms (less than 15 stone) became less competitive.
A claimed advantage of carbon masts is that it has re-introduced the possibility of softer masts for lighter helms.