In this paper, a new method to determine the manoeuvrability of ships by using some kinds of phase portraits is introduced. When we want to determine the stability indices T ₁ T ₂/ K and ( T ₁+ T ₂) / K by Bech's method, it is much necessary to assess the derivative ψ=ψ ( d ψ/ d ψ) with enough accuracy. However, it is difficult to do so because the yaw rate signal measured on the board containes many noises. On the contrary, this new method makes good use of the areas of limit cycles on the phase portraits instead of the time derivatives (ψ, ψ, ψ), at certain instants. The validity of this method is verified by some trials on the analogue simulator. Moreover this method is applied to the analysis of the modified zigzag manoeuvres of a certain real ship and is proved to be very useful.