The invention relates to a roller reefing boom (1 in FIG. 1) for a boom sail set with sail battens, preferably a mainsail (6), of a ship (2) with a partially enclosing sail stowage space (27) of aluminum or Faserplast, in which a largely planar sail (6) can be reefed or stowed by rolling it up, wherein the trimming of the sail (6) in all reefing states is achieved by the roll reefing boom (1) comprising a number of segments (11, 12, 14) which can be swung away horizontally in relation to one another, making the sail (6) that is guided by them adjustable variably according to the strength of the wind by means of internal control devices (37 and 76) with fixed profile characteristics (23 a and 23 b) in the depth of the profile (24).