The GMt (or transverse metacentric height) is usually measured in metres or feet, but sometimes inches. It can be thought of as a measure of the stiffness of a hullform. Stiffness is a term used to describe how much heeling moment is required to induce a certain angle of heel. A hullform with greater stiffness requires more heeling moment to induce a given heel angle. The GMt has a somewhat counter-intuitive impact on gyro sizing. A yacht with higher stiffness (larger GMt) will heel less when a mass is located near the gunwale. We have all experienced this while using small craft. A boat that you can step onto without causing large roll angle is described as having higher static stability, or being ‘more stable’. However exactly the opposite holds true for yachts operating in the presence of waves.The yacht with high stiffness offers a wave rolling under the yacht a greater purchase with which to apply a roll inducing torque. This yacht will require a more powerful stabiliser installation. This is true for roll fins, gyros or any roll stabilising device.

The Displacement, BWL and GMt at various loading conditions can be found in the stability book, located on the bridge.