Measurements of the inertia parameters of a keelboat hull using a bifilar suspension are described. Bifilar yaw moment measurement normally entails accurate measurement of the length l and spacing 2d of the suspension, and of Ty the period of pure yaw oscillation1. The primary difficulty with a bifilar suspension is avoiding unwanted modes of oscillation, specifically sway when measuring yaw.
However, for an athwartships suspension the sway motion is that of a simple pendulum of period Ts and observation of the combined motion allows the yaw gyradius to be determined as ky = (Ty/Ts)d. Thus only the ratio of the periods and the suspension spacing need be measured. Measurements of the normal mode periods of the double pendulum motion when the suspension is displaced in heave allows the height of the center of mass and the pitch gyradius kp to be determined. Repeating yaw measurements with the hull tilted, and then with the bifilar suspension fore and aft to measure the roll gyradius kr, allow the angle ψ _of the inertia ellipse principal x axis to the hull x axis to be calculated.