Marine propellers, being operated in the wake of a vessel, are subjected to the forces and moments of every kind. This paper deals with the theoretical treatise of this probem with an approximation after the manner of Mr. H. Glauert. Consider a right handed propeller and choose the system of axes shown in Fig. 1, The xis of x is taken forward along the axis of rotation, the axis of y to the left, and the xis fz upwards. Let OP represent one blade of the propeller at an angle ψ from Oy, and let the speed of advance and the angular velocity of the propeller be V and Ω respectively, For a propeller of a twin-screw vessel, the flows as follows must be taken into account besides the uniform axial one.(i) Velocity v along the axis of y......δΩ=usinψ/r, δV=0.(ii) Non-uniform flow of the direction of the axis of x varying along the axis of y, h=w2-w1/2(1-w)V......δΩ=0, δV=hr/Rcosψ(iii) circular flow around the axis of x, {rotational flow, ......δΩ=ω, irrotational flow, ......δΩ=f/r2, where w1 and w2 are the wake fraction at ψ=0 and ψ=π respectively, w is the mean wake fraction over the propeller disc, and r=√y2+z2, f=Γ/2π, Γ is the cyclic constants. Let the components of the inerements of the resultant force and moment experienced by the propeller be δX, δY, δZ and δL, δM, δN respectively, then these values may be expressed as shown in the following table. In this table T and Q are the thrust and torque of the propeller in uniform axial motion respectively, and R is the radius of the propeller.Δ1 and Δ2 denote the differential operators defined by the equations Δ1=1-1/2J∂/∂J, Δ2=J/2∂/∂J respectively, A1 and A2 are the coefficients expressed asA1=lnπ22+J22/J2/1-J2/π2lnπ2+J2/J2, A2=1/{21-J2/π2ln (J2+π2/J2)}-J2/π2and J is speed coefficient.The authors apply the above results to the case of a twin-screw vessel, and obtain the forces and moments experienced by the vessel. Furthermore, they make a numericalr calculation for the case of full speed sailing of the train ferry-steamer “Ishikari-Maru” of the Aomori-Hakodate line as an example.