Marine propulsion unit comprising a rotary casing rotatable about a central axis A, and blades extending axially from the rotary casing for rotation with the rotary casing about the central axis A, wherein each blade is mounted for pivotal movement about blade axes B, a blade shaft portion of each blade, which is at least partly surrounded by a blade housing and a blade portion of each blade is outside the blade housing, wherein the blade housing is releasable attached to the rotary casing and wherein each blade is supported in the blade housing by means of bearings (6a and 6b) for said pivotal movement, wherein the rotary casing comprises a lower surface that is configured to be in direct contact with water, wherein the rotary casing comprises mounting seats configured to releasably receive the blade housing of each blade, wherein the mounting seats extend from the lower surface of the rotary casing into the rotary casing and form apertures or recesses in the rotary casing, where the blade housing has an outer surface and the mounting seats have an inner surface, the inner surface together with the outer surface functions as steering surfaces when mounting the blade housing together with the blade at the rotary casing, characterised in that the mounting seats are designed and dimensioned to allow inserting and removal of a pre-assembled module formed by the blade housing having a blade supported in the blade housing by means of bearings (6a and 6b), solely from the side of lower surface of the rotary casing. [end of claim 1, text before claim 2 can be deleted] casing, and by the blade housing has an outer surface and the mounting seat has an inner surface, the inner surface together with the outer surface functions as steering surfaces when mounting the blade housing together with the blade at the rotary casing. a rotary casing rotatable about a central axis A, and blades extending axially from the rotary casing for rotation with the rotary casing about the central axis A, wherein each blade is mounted for pivotal movement about blade axes B, a blade shaft portion of each blade, which is at least partly surrounded by a blade housing and a blade portion of each blade is outside the blade housing, wherein the blade housing is releasable attached to the rotary casing and wherein each blade is supported in the blade housing by means of bearings (6a and 6b) for said pivotal movement, wherein the rotary casing comprises a lower surface that is configured to be in direct contact with water, wherein the rotary casing comprises mounting seats configured to releasably receive the blade housing of each blade, wherein the mounting seats extend from the lower surface of the rotary casing into the rotary casing and form apertures or recesses in the rotary casing, where the blade housing has an outer surface and the mounting seats have an inner surface, the inner surface together with the outer surface functions as steering surfaces when mounting the blade housing together with the blade at the rotary casing, characterised in that the mounting seats are designed and dimensioned to allow inserting and removal of a pre-assembled module formed by the blade housing having a blade supported in the blade housing by means of bearings (6a and 6b), solely from the side of lower surface of the rotary casing. [end of claim 1, text before claim 2 can be deleted] casing, and by the blade housing has an outer surface and the mounting seat has an inner surface, the inner surface together with the outer surface functions as steering surfaces when mounting the blade housing together with the blade at the rotary casing.